Chapter 4 Spinal cord reflex organization of sympathetic systems

G.Holstege, R. Bandler and C.B. Saper (Eds.) Progress in Brain Research, Vol. 107

0 1996 Elsevier Science B.V. All rights reserved.

CHAPTER 4

Spinal cord reflex organization of sympathetic systems Wilfrid Jhig Physiologisches Institut, Christian-Albrechts- Universitdt zu Kid, Olshausenstr. 40, 24098 Kid, Germany

Introduction The peripheral sympathetic nervous system is organized into several functionally different types of pathways which transmit the activity from the spinal cord to the target organs in the periphery of the body. Integration resulting in different patterns of discharge of the sympathetic outflow, and therefore in distinct regulation of organ systems, occurs almost exclusively within the central nervous system, notably in the spinal cord, the brain stem and the hypothalamus. At these sites, excitatory and inhibitory signals arising from many different sources sum and are modulated by the state of the final output neuron, the preganglionic neuron. This state is dependent on integrative processes in the spinal cord and signals generated supraspinally which influence the activity of preganglionic neurons must be seen together with these spinal integrative processes. This article focusses on the spinal cord as an integrative organ in its own right which determines many discharge components of sympathetic neurons. Four topics are discussed: (1) the functional organization of the sympathetic nervous system; (2) reflexes in sympathetic systems which are organized at the level of the spinal cord; (3) residual autonomic functions which are preserved after spinal cord transection; (4)integration of spinal cord circuits and supraspinal pathways during normal neural regulation of autonomic target organs.

Functional organization of the sympathetic nervous system: a short review Studies of the involvement of the autonomic nervous system in the regulation of function of different organs clearly demonstrate the precision by which this occurs in the context of the behavior of the whole organism. This implies that there must be subgroups of pre- and postganglionic autonomic neurons which are distinct with respect to the function they control in their target organs. However, these experimental investigations have not identified the neural mechanisms in the peripheral and central nervous systems by which this specificity is brought about. They also do not explain how activity is transmitted through autonomic ganglia, in particular the paravertebral and prevertebral sympathetic ones in which there is considerable convergence and divergence, so as to guarantee that the central message is transmitted through ganglia and across neuroeffector junctions to the target tissue. The main question which was addressed in these studies is: can individual sympathetic neurons be characterized by way of their reflex discharge pattern in vivo as belonging to one of the sympathetic subsystems (Fig. 1, Table l)? This would result in functional markers for sympathetic neurons which would allow to recognize different functional groups of neurons independently of recording the responses of the target organs. Such functional markers, first, should be able to be interpreted in the context of the target tissue and its

44

Fig. 1. Lumbar sympathetic systems supplying skeletal muscle, skin, distal colon and pelvic organs (systems projecting in the distal lumbar sympathetic trunk, distal to ganglion Lg. and in the lumbar splanchnic nerves and hypogastric nerves). At the base of this organization are the sympathetic target organs which are supplied by the “final common sympathetic pathways”. The “target organs” of some MR neurons are other neurons or the synaptic terminals on these neurons in the pelvic ganglia to the urinary bladder (see de Groat and Booth, 1993a) and in the enteric nervous system of the colon (Furness and Costa, 1987). Impulses in the peripheral sympathetic pathways which are transmitted from pre- to postganglionic neurons innervating somatic tissues, visceral blood vessels and some pelvic organs are transmitted in a relay-like fashion. The nicotinic transmission in vasoconstrictor pathways may be modified by cholinergic muscarinic and non-cholinergic (probably peptidergic) synaptic processes which are also initiated by preganglionic axons, but not by additional synaptic inputs (see Janig, 1995). Some postganglionic MR neurons may integrate synaptic inputs from peripheral afferent neurons and from collaterals of spinal visceral afferent neurons (see Janig, 1995). The stippled areas symbolize “spinal sympathetic programs” which consist of preganglionic neurons, autonomic interneurons and their synaptic connections with somatic and visceral spinal primary afferents (hatched arrows; see Table 2) and with the descending systems. Many neurons of six sympathetic systems (drawn in thick lines) have ongoing activity which is of central origin. The other three systems (right side) are silent and probably largely dependent on the activity in the descending systems. The existence of the cutaneous vasodilator system is not as well established as that of the other systems (see Janig and Kiimmel, 1981; Bell et al., 1985). For abbreviations, see Table 1. Modified from Jiinig (1986).

regulation(s) and, secondly, be correlated with other properties of the neurons, such as its biophysical properties, which determine its firing pattern to the synaptic inputs. To achieve this aim, the experimental strategy has been to select, first, appropriate nerves in which the autonomic axons project only to known targets and, secondly, to use natural (adequate) stimuli to excite afferent receptors which are appropriate for eliciting reflexchanges of activity in the autonomic neuron to be

analyzed. For example, changes of activity in cutaneous vasoconstrictor neurons innervating the skin of the distal extremities should be correlated with changes in blood flow through that skin and in skin temperature; activity in these vasoconstrictor neurons should be inhibited by stimulating warm-sensitive neurons in the hypothalamus (by increasing the temperature in the hypothalamus above body core temperature). Thus these vasoconstrictor neurons should change their activity

45 TABLE 1 Functional classification of sympathetic neurons based on reflex behavior in vivoa Likely function

Vasoconstrictor Muscle (MVC) Cutaneous (CVC)

Location

Target organ

Likely target tissue

Major identifying stimulusb

Ongoing activity

LumbarC Cervicale

Hindlimb muscle Head and neck muscle Hindlimb skin

Resistance vessels Resistance vessels

Baro-inhibitiond Baro-inhibition

Yes Yes

Inhibited by CNS warming Inhibited by CNS warming Baro-inhibition

Yes

Head and neck skin Pelvic viscera

Thermoregulatory vessels Thermoregulatory vessels Resistance vessels

Hypothalamic stimulation ?

No

Vibration (in cat) Hypothalamic stimulation Inspiration

Yes, some No

Inhibition by light

Yes, some

Bladder distention

Yes

Inhibited by bladder distention ?

Yes

Lumbar C eITica1

Visceral (VVC)

Lumbar splanchnic

Vasodilator Muscle (MVD)

Lumbar

Hindlimb muscle

Muscle arteries

Cutaneous (CVD)

Lumbar

Hindlimb skin

Skin vasculature

Sudomotor (SM) Pilomotor (PM)

Lumbar Lumbar

Paw pads Tail

Sweat glands Piloerector muscles

Inspiratory (INSP)

Cervical

Airways?

Pupillo-motor (PUP)

Cervical

Iris

Nasal mucosal vasculature Dilator pupillae muscle

Lumbar splanchnic Lumbar splanchnic Lumbar splanchnic

Hindgut, urinary tract Hindgut, urinary tract Internal reproductive organs

Motility regulating Type 1 (MRl) Type 2 (MR2) Reproduction (REPR)

Visceral smooth muscle Visceral smooth muscle Visceral smooth muscle. other?

Yes Yes

No

Yes

No

aExperimental data from anesthetized cats (see Janig, 1985, 1988; Janig and McLachlan, 1987; Jlnig et al., 1991; Boczek-Funcke et al., 1992b). bExcitation by stimulus unless inhibition specified. CLumbarrepresents preganglionic and postganglionic axons in lumbar outflow. dBaro-inhibition represents inhibition by stimulation of arterial baroreceptors. eCervical represents preganglionic axons in the cervical sympathetic trunk.

according to the functional changes in which they are involved (e.g. thermoregulation, regulation of blood flow as a consequence of noxious events, etc.). In addition, they should exhibit changes in their activity to natural stimuli, such as cutaneous noxious stimuli, visceral stimuli, stimulation of

arterial baroreceptors and chemoreceptors. The responses to these stimuli are not necessarily predictable but also contribute to the functional marker pattern. Therefore these studies focussed on the analysis of (1) the patterns of discharge of individual functionally identified neurons during

46

physiological responses, both in the anesthetized animals and in conscious humans; (2) the integrative properties of preganglionic neurons in vivo (McLachlan and Hirst, 1980; Dembowsky et al., 1985, 1986), and in vitro (Shen et al., 1990; Inokuchi et al., 1992; Weaver and Polosa, 1996); (3) the connectivity and electrophysiological characteristics of ganglion cells in paravertebral (Cassell et al., 1986) and prevertebral ganglia (Szurszewski, 1981; Keef and Kreulen, 1986; Kreulen and Peters, 1986; Janig, 1988; McLachlan and Meckler, 1989) and their behavior in vivo in anesthetized animals (Skok, 1980, 1986; Skok and Ivanov, 1987; for review see Janig, 1995); and (4) the mechanisms by which the discharges of postganglionic neurons produce responses of their targets (Cunnane and Stjame, 1984; Hirst and Edwards, 1989; Luff and McLachlan, 1989; Brock and Cunnane, 1992; Hirst et al., 1992). These studies have found features that distinguish between neurons with respect to their functions and therefore their role in the regulation of different types of normal body functions. The widely held view that the sympathetic system “acts as a whole” in the sympatho-adrenal response to stress of physical and emotional origin (Cannon, 1929, 1939; see Janig and McLachlan, 1992b) ignores this distinct differentiation and is conceptually misleading. Neurons of discrete functional pathways are characterized by distinct rejlex patterns

Using this approach one not only gets information about the functional specificity of neurons, but also about the relation between activity in particular groups of neurons and the responses of the target tissue, as well as about the organization of the central circuits which determine the discharge pattern of these neurons: Up to now, it has not been proved in an in vivo experiment that a particular postganglionic neuron, which displays a typical reflex pattern, innervates a specific target organ. Technically, it is almost impossible to record in vivo from a single postganglionic or pregangiionic neuron with an intact

connection to its target organ, to identify the neuron functionally and to stimulate it selectively in order to observe the response of the target organ. The closest one can get, at present, is to record activity in autonomic neurons and effector responses of the target tissue simultaneously. This enables to obtain a temporal correlation between neuron discharge and target cell response and a relation between the sizes of the neuron discharge and of the target response. The best examples are the correlation between discharges in sudomotor neurons and the skin potential recorded from the hairless skin of the paw pad in the cat (Figs. 8 and 11) (JHnig and Kummel, 1977) and the correlation between discharges in cutaneous vasoconstrictor neurons innervating hairless skin and change of skin temperature (Grewe et al., 1995). The types of reflexes elicited in autonomic neurons by afferent stimuli and the correlation of the activity in these neurons with other centrally generated parameters (e.g . the centrally generated respiratory cycle and the cycle of sleep and wakefulness) depend on the organization of the control systems in spinal cord, brain stem, hypothalamus or higher structures. Thus, the central organization of the nervous system can be investigated by recording autonomic reflexes in appropriately designed experiments (e.g. experiments on animals with transected spinal cords; experiments with central interventions, such as focal lesions and a focal stimulation of populations of neurons). Most of the neurophysiological studies in vivo were performed on anesthetized cats and concentrated on pre- and postganglionic neurons of the lumbar sympathetic system supplying skeletal muscle, skin and pelvic organs and on preganglionic neurons of the thoracic sympathetic system innervating postganglionic neurons in the superior cervical ganglion, which are destined for target organs in the head and upper neck (Janig, 1985, 1988; Janig and McLachlan, 1987, 1992a; Jiinig et al., 1991; Boczek-Funcke et al., 1992a-c, 1993). Some neurophysiological studies were conducted on anesthetized rats and concentrated on postganglionic neurons supplying skin and skeletal muscle (Habler et al., 1993, 1994a). Activity was

47

recorded from isolated cut axons of pre- and postganglionic neurons leaving their central connections intact. Physiological stimuli were applied to well defined afferent receptor populations of the body (mechanoreceptors of the body surface and viscera, chemo- and baroreceptors, central thermosensitive neurons) in order to evoke reflexes in the sympathetic neurons. The conditions under which the discharge in the neurons was measured were probably nearly normal for the sympathetic systems, as judged from the reactions of the effector organs, such as, e.g. skin temperature, systemic blood pressure and blood flows through skeletal muscle, skin and viscera. Therefore both level and pattern of discharge measured in these neurons in the anesthetized animal are probably comparable to those in the unanesthetized animal. The central effect of the anesthesia used, of course, distorts some reflexes. This has been tested for various anesthetics using two well-defined physiological reflexes elicited in sudomotor neurons of the cat (JLnig and Rath, 1980) and it is quite obvious that systems regulations, such as thermoregulation (Bligh, 1966) or regulation of the cardiovascular system, are quantitatively distorted by anesthesias. However, it is important to note that most anesthesias do not change the principles of peripheral and central organization of the sympathetic systems. By the same token it is likely that the discharge pattern in neurons of some sympathetic systems is dominated, in the unanesthetized state, by integrative processes in higher brain centers (e.g. the emotional motor system). Therefore, integration in the spinal cord may not be seen in the discharge pattern of these neurons when the spinal cord is intact. For example, activity in cutaneous vasoconstrictor and sudomotor neurons in conscious humans is dominated by the thermoregulatory and emotional situation of the subject. Arousal and emotional stimuli will activate both sympathetic systems and reflexes which are based on spinal circuits in both systems can barely be seen but they are seen in anesthetized animals (see below). On the basis of this conceptual and methodical approach many sympathetic neurons can now be

functionally characterized and recognized, independently of the responses of the target organs, by way of their distinct reflex patterns. For both the lumbar sympathetic outflow supplying skeletal muscle and skin of the hindlimb and tail of the cat and the lumbar sympathetic outflow supplying the pelvic viscera, the same types of reflex patterns have been found pre- as well as postganglionically (Janig, 1985, 1986, 1988; Janig and McLachlan, 1987). For the sympathetic outflow supplying pelvic viscera it is evident that even the proportions of functionally different types of neurons are similar pre- and postganglionically (Janig et al., 1991). Thus, functionally similar preganglionic and postganglionic neurons must be connected synaptically in sympathetic ganglia, establishing distinct pathways between spinal cord and target organs. Further, for the thoracic sympathetic outflow to the superior cervical ganglion, it has recently been demonstrated that many preganglionic neurons exhibit reflex patterns that are typical for muscle or cutaneous vasoconstrictor neurons in the lumbar sympathetic outflow whereas other types of preganglionic neurons exhibit reflex patterns that are virtually absent in neurons of the lumbar sympathetic outflow (Boczek-Funcke et al., 1992a,b). It should now in principle be possible to use the reflex patterns measured in pre- and postganglionic neurons as “fingerprints” in order to recognize the function of sympathetic neurons without knowing their target organs, at least in the upper thoracic and lumbar sympathetic outflows. Such information is available for eight types of neurons with ongoing discharge and four types of neurons which are silent and exhibit no reflexes in the anesthetized cat (Table 1). Similar reflex patterns have been found in muscle and cutaneous vasoconstrictor neurons supplying the hairy skin and skeletal muscle of the rat hindlimb (Habler et al., 1994a). Sympathetic pathways are chemically coded

The organization of the sympathetic nervous system into functional subunits according to the

48

target organs is now supported by histochemical experiments conducted on neurons of the lumbar outflow to the hindlimb and of the thoracic outflow to the head. For example most postganglionic vasoconstrictor neurons to skin and skeletal muscle contain in addition to noradrenaline neuropeptide Y (see Lundberg and Hokfelt, 1986). In the guinea pig vasoconstrictor neurons to small arteries of the ear contain in addition dynorphin and those to the precapillary sphincters only dynorphin (in addition to noradrenaline). In the same species, pilomotor neurons are noradrenergic and contain calcitonin gene-related peptide (CGRP) but not neuropeptide Y, whereas muscle vasodilator neurons are cholinergic and contain neuropeptide Y, vasoactive intestinal peptide and dynorphin (Gibbins and Morris, 1990; Gibbins, 1991, 1992). Furthermore, noradrenergic postganglionic neurons (“secretomotoneurons”) in the celiac ganglion of the guinea-pig which project to the submucous plexus of the enteric nervous system contain somatostatin, whereas in noradrenergic neurons which project to the myenteric plexus (probably motilityregulating neurons) no peptide has been found so far (see Elfvin et al., 1993). In the cat, noradrenergic muscle vasoconstrictor neurons contain neuropeptide Y and galanin, cholinergic muscle vasodilator neurons vasoactive intestinal peptide (VIP) and sudomotor neurons VIP and calcitonin gene-related peptide (CGRP). This “chemical coding” is also present preganglionically. In the guinea-pig, preganglionic terminals containing substance P are found exclusively on postganglionic vasodilator neurons, but not on postganglionic vasoconstrictor and pilomotor neurons in lumbar paravertebral ganglia; preganglionic terminals containing calcitonin gene-related peptide were found only on postganglionic vasoconstrictor neurons but not on vasodilator and pilomotor neurons (Gibbins, 1992). In the cat, postganglionic muscle vasodilator neurons are innervated by preganglionic neurons containing enkephalin and postganglionic sudomotor neurons by preganglionic neurons containing corticotropin releasing hormone (CRH) (Shafton et al., 1992; see Elfvin et al., 1993). This

shows that the neuropeptides which are colocalized with acetylcholine or noradrenaline in pre- and postganglionic sympathetic neurons are principally correlated with the target organs supplied by these neurons in a particular body domain. However, it is important to note that the concept of chemical coding of functionally distinct sympathetic pathways is not straightforward and the correlation between function and neurochemistry is by no means direct. For example, as well as vasoconstrictor neurons, noradrenergic postganglionic neurons projecting to the vas deferens (see S t j h e and Lundberg, 1986) and many to the iris (Gibbins and Morris, 1987) contain neuropeptide Y. Notably, the peptides expressed differ to a large extent between species for particular functional pathways and possibly even in the same species for neurons with similar functions which innervate different sections of the same organ system (e.g. prevertebra1 neurons innervating the gastrointestinal tract in the guinea pig; Keast et al., 1993). The functions of most neuropeptides are unknown although for a few neuropeptides they are beginning to be revealed (Lundberg, 1981; Furness et al., 1992; Ulman et al., 1992). Thefinal common autonomic pathway The bases of each sympathetic subsystem are sets of preganglionic and postganglionic neurons which are synaptically connected in the autonomic ganglia and which constitute a pathway that transmits the central message to its target tissue. This pathway has been called “final common autonomic pathway” (Janig, 1986). The concept described by this term probably applies to most if not all sympathetic and parasympathetic pathways. For the sympathetic pathways through the paravertebral ganglia to target organs in skeletal muscle and skin, through the inferior mesenteric ganglion to the vasculature and to pelvic organs, neurophysiological investigations have shown that there is functionally probably very little or no “cross-talk’’ between different peripheral pathways (see Table l). Thus each autonomic target organ or set of target organs which is

49

under central autonomic control is in principle supplied by one or more separate “final common autonomic pathways”. This is exemplified for the lumbar sympathetic outflow to skeletal muscle and skin and the cat hindlimb and tail and the lumbar sympathetic outflow to the pelvic viscera in Fig. 1. Whether the separation between the different sympathetic pathways also exist anatomically, awaits final proof. For example, in paravertebral ganglia several preganglionic neurons converge on one postganglionic neuron. The axons of one (sometimes two) of these preganglionic neurons form strong synapses with the postganglionic neuron which always generates a large suprathreshold postsynaptic potential in the postganglionic neuron and determines the discharge pattern of this neuron. The other preganglionic axons form weak synapses with the postganglionic neurons which generate subthreshold postsynaptic potentials. It is unclear whether all converging preganglionic axons are functionally of the same type, e.g. cutaneous vasoconstrictor neurons, muscle vasoconstrictor neurons, etc. This addresses the question whether divergence and convergence in sympathetic ganglia occurs within functionally distinct peripheral autonomic pathways or also between different autonomic pathways (for detailed discussion of transmission of impulses through autonomic ganglia, see Janig, 1995). As far as the transmission of the central message to the target organs is concerned the concept of the “final common autonomic pathway” is similar to that of the “final common motor path” in the somato-motor system, in the sense that it corresponds to the innervation of a skeletal muscle or group of muscles with the same function by alphamotoneurons. The main differences are (1) that the same autonomic target organ can be innervated by more than one “final common autonomic pathway”, (2) that the transmission of the central message may undergo quantitative changes in the autonomic ganglia, in particular in prevertebral ones (see McLachlan, 1995) and (3) that the central neural control of the target organ may interact with other controls at the level of the target organ or the ganglion. These may consist of local neural

influences (e.g. by afferents being excited antidromically and releasing peptides), of remote and local hormonal controls, local metabolic control and endogenous activity of the target organ (e.g. myogenic activity). Such factors vary between target organs.

Reflexes organized in the spinal cord Experiments are described which clearly demonstrate that various distinct reflex pathways are organized at the level of the spinal cord for the MVC, cutaneous vasoconstrictor, sudomotor and motility-regulating system. This has not been shown to exist for the other known sympathetic systems (Table 1); however, it is possible that the same principle of spinal organization applies to them too. These reflex pathways are characterized by their spinal afferent input systems and by the function of the peripheral sympathetic pathway and therefore also the target organs innervated by them. It will be shown, firstly, that each of these sympathetic systems is associated with several distinct spinal reflex pathways and, secondly, that most of the responses in the sympathetic neurons which are mediated by these reflex pathways are principally also seen in animals with intact spinal cord under standardized experimental conditions as well as in chronic spinal animals. When the spinal cord is isolated from the brain stem, almost all reflexes in sympathetic neurons, including those elicited by strong noxious stimuli, disappear acutely for days and weeks in cats and in humans, probably as a consequence of the interruption of the descending control systems (see recovery of functionally distinct reflexes in the sudomotor system after transection of the spinal cord in Fig. 9). This state is generally called “spinal shock” (Guttmann, 1976). Only for reflexes and ongoing activity in some sympathetic systems which project to visceral organs and do not regulate blood vessels, the “spinal shock” seems to be absent; this e.g. applies to some reflexes in lumbar “motility regulating” neurons in the cat (Bartel et al., 1986). In the chronic stage spinal reflexes upon noxious and innocuous somatic and visceral

50

stimuli may be very strong (Horeyseck and Janig, 1974c; Janig and Spilok, 1978; Mathias and Frankel, 1992). In this chapter spinal reflexes in sympathetic systems (with the exception of the motility-regulating system) refer to the chronic state after interruption of the spinal cord.

. l J J . . 1 4

spinal cord L6

Segmental and suprasegmental reflexes elicited in preganglionic sympathetic neurons by electrical stimulation of somatic and visceral afferents

Sympathetic preganglionic neurons exhibit short and long-latency reflexes to electrical stimulation of spinal afferents (e.g. in the dorsal roots and white rami). These reflexes are mediated by spinal and supraspinal pathways. The spinal reflexes are most powerful when the afferents of the same segment are stimulated in which the preganglionic neurons are located. Although there seems to exist a general principle in the organization of the segmental (spinal) and supraspinal organization of reflexes in the sympathetic system it is unclear from these investigations, first, which functional types of afferents are involved in the these reflexes and, secondly, whether different types of sympathetic neuron exhibit functionally distinct types of segmental and suprasegmental reflexes (Sato and Schmidt, 1971, 1973; Sato, 1972). In a recent review Coote (1988) discussed extensively the integration of spinal and supraspinal reflexes elicited in sympathetic preganglionic neurons by electrical stimulation of visceral and somatic afferents. Preganglionic visceral vasoconstrictor neurons which project in the lumbar splanchnic nerves exhibit very powerful reflexes to electrical stimulation of lumbar somatic and visceral afferents (Fig. 2B). The excitatory reflexes consist of several components when classified by way of their latency: Short latency reflexes probably are mediated by the spinal cord and long-latency reflexes by supraspinal reflex pathways. Interestingly, the reflexes with the shortest latency have the lowest threshold and are probably elicited by stimulation of fast conducting somatic and visceral afferents. Preganglionic motility-regulating neurons which

Fig. 2. Segmental and suprasegmental reflex activation generated in a preganglionic visceral vasoconstrictor (VVC) neuron and in a preganglionic motility-regulating (MR) neuron by electrical stimulation, with single pulses, applied to visceral afferents in lumbar white rami (WR) and to somatic afferents in the ventral ramus of lumbar spinal nerves (SN). Activity was recorded from preganglionic axons (identified by electrical stimulation of lumbar white ramus LA (upper record in B, dot) and the lumbar white ramus L3 (C)) which were isolated from a lumbar splanchnic nerve. (B) Visceral vasoconstrictor neuron. In the first trace the preganglionic axon was directly excited (action potential marked by dot). Pulse duration 0.2 ms for the WR and 0.5 ms for the SN. Each trace 10 times superimposed. Note the powerful short latency (probably segmental) and long latency (probably suprasegmental) reflex responses. (C) MR neuron. Pulse duration 0.2 ms for the WR and 0.5 ms for the SN. Each trace 11 times superimposed. Note the weak reflex activity and the short latency of the reflexes in the MR neuron. (B) Modified from Bahr et al. (1981); (C) modified from Bahr et al. (1986~).

project in the lumbar splanchnic nerves also exhibit reflexes to electrical stimulation of lumbar somatic and spinal afferents (Fig. 2C). However, these reflexes are present in only about 60% of the functionally identified motility-regulating neurons, they are much weaker in the motility-regulating neurons than in visceral vasoconstrictor neurons and most of them have short latency (i.e. are

probably mediated by the spinal cord) (Bahr et al., 1986~).Recording intracellularly from preganglionic sympathetic neurons in the spinal segment T3 of the cat Dembowsky et al. (1985) have shown that electrical stimulation of somatic and visceral spinal afferents elicits early and late excitatory postsynaptic potentials (EPSPs) in 280% of the neurons when the neuraxis is intact, early EPSPs in almost all neurons in spinal animals and late EPSPs in 50% of the neurons in spinal animals. The results obtained on functionally identified preganglionic neurons reported by Bahr et al. (1986c) are not at variance with these intracellular measurements. However, they clearly show that spinal and supraspinal reflexes are different in functionally different types of preganglionic neuron. Unfortunately also the results obtained on the functionally identified preganglionic sympathetic neurons give only limited answers to the question which spinal reflexes are associated with which type of sympathetic system because the function of the spinal afferents stimulated are unknown. Muscle vasoconstrictor system

Most muscle vasoconstrictor neurons show excitatory reflexes to noxious stimulation of skin as well as to stimulation of visceral afferents from the pelvic organs in cats with intact spinal cord and in cat after spinalization (Figs. 3A, 5 and 6). These reflexes are more or less generalized and not spatially organized. Stimulation of hair follicle afferents generates short lasting inhibition in some muscle vasoconstrictor neurons both in cats with intact spinal cord and in spinalized cats (Fig. 4A,C). It is most likely that these reflexes have a spinal pathway, in addition to a supraspinal one (see Sato and Schmidt, 1973). The visceral vasoconstrictor pathway which regulates resistance vessels in the visceral domain may be similarly organized at the spinal cord level. Preganglionic neurons of this pathway exhibit short latency reflexes to electrical stimulation of segmental somatic and visceral afferents (see Fig. 2B). Muscle vasoconstrictor neurons show sparse excitatory reflexes to cutaneous stimuli and to

51

-

s t i r rnech noci

MVC

-

cvc

2s

11

-

-

-

B

C I forepaw

c I hindpaw

hlndpaw

401

CVC hindlimb OJ,

0

t-r0 '

2

0

10

I

4

tail

II h z p a w

60 1

I

I

I

s

I

min

C

c I Ldpaw n

--

200

10

s

200

10

s

20

Fig. 3. Reflexes elicited in muscle vasoconstrictor and cutaneous vasoconstrictor neurons by noxious cutaneous stimuli. (A) Stimulation of cutaneous nociceptors by pinching the ipsilateral hindpaw (indicated by bar); excitation of muscle vasoconstrictor neurons (multi-unit bundle isolated from the deep peroneal nerve) with concomitant increase of arterial blood pressure (BP) and inhibition of cutaneous vasoconstrictor neurons (bundle isolated from the superficial peroneal nerve). (B) Reactions of postganglionic cutaneous vasoconstrictor neurons supplying hairy skin of the cat hindlimb (sural nerve) to mechanical noxious stimulation (black bars) of skin of different paws (i.l., ipsilateral; c.l., contralateral). Ordinate scale, impulses per 6 s in three postganglionic neurons. (C) Reaction of a postganglionic cutaneous vasoconstrictor neuron supplying hairy skin of the tail. Ten times superimposed; ordinate, impls. (B) Modified from Janig (1975); (C) modified from Grosse and Janig (1976).

distention of the urinary bladder in humans with transected spinal cord (Stjernberg et al., 1986). Cutaneous vasoconstrictor system Reflexes to noxious cutaneous stimulation Noxious stimulation of skin of the distal parts of

52

the extremities elicits inhibition in many cutaneous vasoconstrictor neurons innervating hairy and hairless skin (Fig. 3A,B). For the cutaneous vasoconstrictor neurons supplying the cat and rat skin of the hindlimb or the head of the cat, this reflex is preferentially elicited by noxious stimulation of the ipsilateral skin, which is innervated by the vasoconstrictor neurons; however, it is weak or absent when the skin on the contralateral body side is stimulated (Fig. 3A,B). When the noxious contralateral stimuli are very strong (e.g. stimulating a whole contralateral distal extremity with water of >48”C) the inhibitory reflex can also be seen in cutaneous vasoconstrictor neurons (Horeyseck and Janig, 1974b; Janig, 1975). Many cutaneous vasoconstrictor neurons innervating the cat tail exhibit inhibition during noxious stimulation of the tail and excitation during noxious stimulation of the hindpaws (Fig. 3C). In the chronic spinal cat cutaneous noxious stimulation also generates inhibition of activity in many cutaneous vasoconstrictor neurons supplying hairy and hairless skin of hindpaw. This inhibition is normally long-lasting and is followed by an increase of blood flow through the skin and, consequently, to a long-lasting increase of temperature on the surface of the skin (Janig and Kiimmel, 1981; Janig, 1985). This long duration of reflex inhibition of cutaneous vasoconstrictor activity following a brief stimulus is linked to the activation of cutaneous nociceptors. Activation of other cutaneous receptors elicits only reflexes of short duration (Fig. 11). The inhibitory nociceptive reflex has, as in animals with an intact neuraxis, a very distinct spatial organization: noxious stimulation of the skin of the contralateral extremity has either no or only a weak effect (Fig. 8B). The inhibitory reflex elicited in cutaneous vasoconstrictor neurons and its spatial organization is most interesting. It was first described by LovCn (1866) for rabbits and is therefore sometimes called “LovCn reflex”. Reflexes in muscle vasoconstrictor neurons and sudomotor neurons (Fig. 8B) do not exhibit this spatial organization when noxious stimuli are applied to the skin of the paws of different extremities. In conscious humans the

inhibitory reflex in cutaneous vasoconstrictor neurons upon noxious stimulation of skin cannot be elicited since noxious stimuli generate arousal reactions and, consequently, an activation of the cutaneous vasoconstrictor neurons (cf. Janig et al., 1983). This is probably equivalent to the defence reaction seen in animals. It is unknown whether these types of inhibitory reflexes in cutaneous vasoconstrictor neurons also exist in chronic tetraor paraplegic patients; up to the present they have not been described (Stjernberg and Wallin, 1983; Wallin and Stjernberg, 1984). Recent investigations on humans have shown that electrical intraneural microstimulation of thin myelinated afferents in skin nerves of an extremity elicits reflex dilatation (increase of blood flow) in skin areas lying adjacent to or in the territory of the stimulated nerve (Blumberg and Wallin, 1987). The dilatation in the skin was abolished by blockade of the conduction in the nerve proximal to the stimulation site. It was largest in the skin of the stimulated hindlimb and smaller on the contralateral hindlimb. Thus, the reflex in humans appears to be very similar to the inhibitory reflex in cutaneous vasoconstrictor neurons elicited by cutaneous noxious stimuli in anesthetized cats and rats although the authors tend to believe that the vasodilation in the skin is generated by reflex activation of cutaneous vasodilator neurons (see Fig. 1). Reflexes to stimulation of innocuous stimulation of skin Stimulation of hair follicle afferents elicits excitation in some cutaneous vasoconstrictor neurons supplying hairy skin in the cat (Fig. 4B). In the spinal preparation this stimulus may elicit brief inhibition of the activity in some cutaneous vasoconstrictor neurons (Fig. 4D). Stimulation of Pacinian corpuscles in the cat paw also elicits depression of activity in some cutaneous vasoconstrictor neurons supplying the paw, in cats with intact spinal cord as well in cats after spinalization (Fig. 11B). This reflex is reciprocal to the reflex activity elicited in sudomotor neurons in both types of preparations by stimulation of Pacinian corpuscles (Fig. 11A).

53 Spinal c o r d # n l a c l

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5

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spinal c o r d transecled

0

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S

?

Fig. 4. Reflexes in postganglionic muscle vasoconstrictor and cutaneous vasoconstrictor neurons elicited by stimulation of hair follicle afferents in the cat. Bundles with muscle vasoconstrictor axons isolated from the nerve to the lateral gastrocnemius muscle; bundles with cutaneous vasoconstrictor axons isolated from the superficial peroneal nerve. Hair follicle afferents were stimulated by air jets of about 0.5 s duration which were oriented tangentially to the skin of the lateral surface of the ipsilateral and contralateral thigh. (A,B) Cat with intact spinal cord; same experiment. (C,D) Chronic spinal cats 48 days (C) and 96 days (D) after transection of the spinal cord at the thoracic level T8. (A), (B) and (D) were obtained from 50 successive trials and (C) from 25 successive trials at a stimulus repetition rate of 1 per 6 s. Ordinate scales, impulses per 50 ms. From Horeyseck and Janig (1974%~).

Reflexes to stimulation of visceral receptors Most cutaneous vasoconstrictor neurons are inhibited in their activity by stimulation of afferents from pelvic viscera (e.g. by distention or isovolumetric contraction of urinary bladder or colon) in the anesthetized cat with intact spinal cord (Fig. 5 ; Habler et al., 1992). This inhibition is dependent on the excitation of sacral visceral afferents. After cutting these sacral afferents (leaving the lumbar visceral afferent supply intact) the reflex inhibition cannot any longer be elicited from the urinary bladder (Michaelis et al., 1996). In the chronic stage, weeks after interruption of the spinal cord at the thoracic level, isovolumetric contractions and distentions of urinary bladder and colon lead to reflex excitation of cutaneous vasoconstrictor neurons which are synchronous to the reflex excitation of other sympathetic neurons (e.g.

muscle vasoconstrictor and sudomotor neurons; Fig. 6). The activation of the cutaneous vasoconstrictor neurons by stimulation of sacral visceral afferents is the reverse of the inhibitory reflex in cutaneous vasoconstrictor neurons in cats with an intact neuraxis. So far known, is this the main example of a change of a reflex elicited by excitation of spinal afferents (from inhibition to excitation) after interruption of the spinal cord. Chronic spinal (tetra- or high-paraplegic) patients exhibit similar reflexes upon strong visceral stimulation. Contraction and distention of the urinary bladder is followed by an increase of diastolic and systolic blood pressure (constriction of resistance vessels), by vasoconstriction in skin and probably by vasoconstriction of blood vessels in the viscera (Guttmann, 1976). Microneurographic recordings illustrate that distention of the urinary bladder in tetra- and paraplegic patients elicits activation of

intravesical pressure

60 //\\,

rnrn Hg

I20

0

I

OJ

Fig. 5. Reflexes in muscle vasoconstrictor and cutaneous vasoconstrictor neurons to isovolumetric contraction of the urinary bladder in the cat. Multi-unit activity recorded from postganglionic cutaneous vasoconstrictor (hairy skin) and muscle vasoconstrictor axons. Note that the increase of mean arterial blood pressure (MAP) followed the reactions of the neurons with a delay of several seconds. From Habler et al. (1992).

54

SM

j:i

MVC

n

23.4

j

skin temp

cvc

imp

colon

pressure

mm 10

OJ

-

bladder pressure

lmin

Fig. 6. Reactions of cutaneous vasoconstrictor (CVC), sudomotor (SM, skin potential) and muscle vasoconstrictor neurons (MVC) during isovolumetric contractions of urinary bladder and colon in a chronic spinal cat 135 days after interruption of the spinal cord at spinal level T10. The postganglionic activities were recorded from multiunit bundles which were isolated from the superficial peroneal nerve (hairy skin) and from a muscle branch of the deep peroneal nerve. For the SM activity, the skin potential was recorded from the surface of the central pad of the ipsilateral hindpaw (see JInig and Kiimmel, 1977). The skin temperature was recorded on the surface of the central pad of the left hindpaw (effector response for the cutaneous vasoconstrictor neurons supplying hairless skin). The urinary bladder was filled with 20ml saline and the intravesical pressure measured through the urethral catheter by a transducer. The colon was filled with about 40 ml saline in a flexible balloon and the intracolonic pressure recorded through an anal catheter was connected with the balloon and a transducer. Note that cutaneous vasoconstrictor, sudomotor and muscle vasoconstrictor neurons are synchronously excited during isovolumetric contraction of the urinary bladder and colon; that the discharges in the cutaneous vasoconstrictor neurons are followed by a decrease in skin temperature; that urinary bladder and colon contract reciprocally; and that the reflex excitation of muscle vasoconstrictor neurons is not particularly strong in comparison to the reflex excitation of cutaneous vasoconstrictor and SM neurons. Ordinate scale for activity in muscle vasoconstrictor and cutaneous vasoconstrictor neurons, number of impulses per 0.5 s (Janig and Kummel, unpublished).

cutaneous vasoconstrictor neurons although the size of activation appears to be rather small (Wallin and Stjernberg, 1984). Reactions to spinal cord warming and cooling Functionally very specific afferent receptors which are associated with the cutaneous vasoconstrictor system are central thermoreceptors. In cats with intact spinal cord, cutaneous vasoconstrictor neurons supplying hairy and hairless skin are inhibited in their activity by warming of the spinal cord. These responses are graded. Spinal cord cooling elicits weak activation of the cutaneous vasoconstrictor neurons. These responses are specific for cutaneous vasoconstrictor neurons. sudomotor neurons are not activated by spinal cord warming but sometime activated by spinal cord cooling (Gregor et al., 1976; Grewe et al., 1995). In chronic spinal cats, spinal cord warming also leads to decrease of activity in cutaneous vasoconstrictor neurons. Cooling of the spinal cord generates transient activation of some cutaneous vasoconstrictor neurons (Fig. 7) (Janig and Kummel, 1981). Sudomotor neurons do either not react to these spinal thermal stimulation or do only exhibit weak transient activations to both stimuli. Sudomotor system

The time course of recovery, after transection of the spinal cord at the segmental level of the lower thoracic spinal cord, has been best studied quantitatively for the nociceptive reflexes and the reflexes to stimulation of Pacinian corpuscles in the sudomotor system (Janig and Spilok, 1978). Both types of reflexes take about 10 weeks or longer to recover to values which are similar to those seen in animals with intact spinal cord (Fig. 9). Similar time courses of recovery after transection of the spinal cord do also occur for the resting activity and spinal reflexes in muscle vasoconstrictor and cutaneous vasoconstrictor neurons (Horeyseck and Janig, 1974c). Reflexes to noxious cutaneous stimulation In spinal cats, the reflex activation of the sudomotor neurons elicited by noxious cutaneous

55

Fig. 7. Effect of spinal cord warming (A) and cooling (B) on postganglionic cutaneous vasoconstrictor neurons supplying the cat hindpaw in a chronic spinal cat (120 days after transection of the spinal cord between T8 and T10). Upper records in A and B recording from a strand with about three postganglionic cutaneous vasoconstrictor axons (see D)which was isolated from the medial plantar nerve. Lower records in (A) and (B) indicate the perfusion temperature of the water measured at the beginning of a Ushaped tubing which was positioned extradurally dorsal to the spinal cord distal to about T10. The tubing was inserted between the vertebrae S1 and L7 (for details see Gregor et al., 1976). (C) Inhibition of the cutaneous vasoconstrictor activity during noxious mechanical stimulation of the skin of a toe of the ipsilateral hindpaw. Modified from Janig and Kiimmel(1981).

stimulation is followed by an enhancement of the “ongoing” activity in these neurons (Fig. 8A) which may last for up to 10 min following a stimulus of 10 s duration. This enhancement is reflected in an increase of size and frequency of the transient skin potentials recorded from the surface of the hairless skin (Fig. 10A). It is present in the sudomotor system >30 days after interruption of the spinal cord (Fig. 10B); it occurs after brief noxious stimuli of about 10 s duration applied to the skin and is not related to sensitization or afterdischarges in nociceptors; it does not occur after reflex activation of the sudomotor neurons by stimulation of the Pacinian corpuscles in spinal animals (Fig. 11A); and it is not seen in cats with intact spinal cord. Thus, this enhancement (and probably also the long-lasting inhibitory nociceptive reflexes in cutaneous vasoconstrictor neurons

in spinal animals; Fig. 8B, see above) depends on neuronal long-term potentiating mechanisms in the spinal cord which is isolated from supraspinal influences. This potentiation depends on the activation of nociceptive afferents (Janig and Spilok, 1978; Janig and Kummel, 1981). It is unknown whether these types of exaggerated reflexes also exist in chronic tetra- or paraplegic patients; up to the present they have not been described (Stjernberg and Wallin, 1983; Wallin and Stjernberg, 1984). Reflexes to stimulation of Pacinian corpuscles A unique and powerful activation of the sudomotor neurons is attained by stimulation of Pacinian corpuscles in the cat paws. Any stimulus which excites the afferent neurons innervating the Pacinian corpuscles (such as vibration, air jet

56

flex activation of the sudomotor neurons (Fig. 11C). This “vibration” reflex is mediated through the spinal cord; it is also present in chronic spinal animals (Fig. 11A). Sweat glands in the hands of hyperhydrotic patients (who suffer from uninhibited profuse sweating) can be activated reflexly by stimulation of low-threshold mechanoreceptors (probably

Fig. 8. Reflexes in postganglionic cutaneous vasoconstrictor neurons and SM neurons supplying hairless skin of the hindpaw in chronic spinal cats elicited by mechanical noxious stimulation of the skin of the ipsilateral and contralateral hindpaw. (A) Activity in a single SM neuron. 91 days after spinalization at the lower thoracic level. (B) Activity in cutaneous vasoconstrictor neurons (multi-unit activity). 108 days after spinalization at the lower thoracic level. The stimuli were applied to one of the toes of the left or right hindfoot by a toothed forceps. The activity was recorded from bundles isolated from the medial plantar nerve. Lower records in (A) and (B) skin potentials, indicating activation of SM neurons, recorded simultaneously to the neural activity from the central pad of the ipsilateral hindpaw. Skin potential was recorded from the surface of the hairless skin of the cat hindpaw with an AgAgCI-electroded via a saline bridge against a reference AgAgCI-electrode which was positioned below the hairy skin nearby. Note the long-lasting after-responses. From Janig and Spilok (1978).

stimuli applied to the hairy skin of the paws, tapping on the experimental frame) will lead to this type of specific reflex. Tangential stimulation of hair follicle afferents of the thigh or trunk which does not activate the afferents from the Pacinian corpuscles in the paws but generates strong activation of hair follicle afferents, does not lead to re-

Fig. 9. Time course of recovery of nociceptive and nonnociceptive reflexes in the sudomotor system (skin potential recorded from the central pad) after transection of the spinal cord between segmental levels T1 and L1. Stimulation of Pacinian corpuscles in the paw by vibration stimuli (elicited by tapping on the paw holder for 13.4 s); stimulation of cutaneous nociceptors by mechanical noxious stimuli (by pinching of a toe of the ipsilateral hindpaw for 13.4 s or by radiant heat at 55°C surface temperature applied to a toe for 13.4 s). Average size of reflexes (mV.s; mean f ISEM) for the time periods of 7-25 days, 2 6 5 0 days, and 5 1-75 days after spinalization. Each value is the average of 50-60 individual recordings. The average values were formed by pooling 5-6 measurements chosen randomly from each of 10-12 experiments. The data were obtained in 32 experiments on 11 animals. The size of the reflexes was evaluated by measuring the area below the potential curves (see inset). Statistical significance at *PcO.O5 and **PcO.Ol compared to the preceding time period after spinalization (Student’s t-test for means with different variances). From Janig and Spilok (1978).

57

radiantheat

A

31 days

--

_I 2

72 days

m ~

15s

% change of Dotenlial size

B

0

0

300

0 0

200

O

T

P

x)O

0

-

0

0

-

100

0

0

0 0

.

o

l -

0

0 . p

0

-

0 0 .

0 -

0

I

50

0

100 davs after spinalizatibn

0

Fig. 10. Resting activity of the sudomotor neurons before and after noxious stimulation of skin (ipsilateral toes) with radiant heat (55°C) for 13.4 s in spinal cats 7-86 days after interrupting the spinal cord between T7 and T13. The skin potential was recorded from the central pad of the ipsilateral hindpaw. (A) Experiments 31, 44 and 72 days after spinalization. The resting activity on the left was recorded just before the noxious stimulus and at least 14 min after the last noxious stimulus. The activity on the right was recorded in the time period of 2 4 min 14 s after the end of the noxious stimulus. The potential scale of 2 mV in the lower left applies only to the resting activity at 72 days after spinalization before the stimulus was applied. The potential scale on the left applies to all other records. (B) Percentage change of size of transient resting skin potentials after noxious stimulation of skin 7-86 days after spinalization. All single transient potentials larger than 1 mV were measured in time periods of 3 min just before the noxious stimuli and after the end of the noxious stimuli, respectively. The increase of the mean potential size was expressed in per cent of the mean resting potential size before the stimulation (ordinate scale). Data from 25 experiments (2 measurements per experiment). The open squares and vertical bars indicate means 2 lSEM for time periods of 7-25 days, 26-50 days, and 51-86 days after spinalization. From Janig and Spilok (1978).

58

vibration

B

CVC

vibration

Y

10s

C

sudomotor reflex (air jet stimulation)

A

15mv

-

u

hindpaw 10s

- -

thigh

back

- -

----..-

foreleg

tail

Fig. 11. Reflexes in postganglionic sudomotor (SM) neurons (A) and a postganglionic cutaneous vasoconstrictor neuron (B) supplying hairless skin of the hindpaw in a chronic spinal cat (about 130 days after spinalization at the lower thoracic level) elicited by stimulation of Pacinian corpuscles in the hindpaws by vibration (of the holder fixating the hindpaw). The neuronal activity was recorded from postganglionic axons which were isolated from the medial plantar nerve. Lower traces: skin potential recorded with AgAgCI-electrodes from the central pad. Note the absence of afterdischarges in the SM neurons. (C) Reflex activation of sudomotor neurons (recording of skin potential from the central pad of the left hindpaw of a cat) elicited by stimulation of the Pacinian corpuscles in the left hindpaw by airjets. Tangential stimulation of hair follicle afferents on the left thigh, back, left foreleg and tail did not activate the SM neurons. (A,B) From Kiimmel (1983); (C) from Janig and Rath (1977).

Pacinian corpuscles) in the hand. Microneurographic recordings and blood flow recording show that sympathetic axons are activated in this way without change of blood flow (Culp, Marchetini, Ochoa, Torebjork, pers. commun.) indicating that sudomotor neurons are reflexly activated. This

reflex is similar to the vibration reflex in sudomotor neurons of cats and may also be mediated by a spinal pathway. Under healthy conditions this reflex activation of sudomotor neurons is probably depressed in humans; it may be activated during distinct motor tasks leading to sensory exploration with the hand and may keep the glabrous skin flexible for optimal sensory discrimination. Effects of anesthetics on spinal reflexes in sudomotor neurons Spinal reflexes elicited by stimulation of cutaneous nociceptors and of Pacinian corpuscles in sudomotor neurons exhibit characteristic changes to systemic applications of anesthetics in cats with intact spinal cord. Ketamine (a “dissociative” anesthetic) suppresses the reflex excitation elicited in sudomotor neurons by cutaneous noxious stimuli and enhances the reflexes elicited in the same neurons by stimulation of Pacinian corpuscles in the paws. Small doses of methohexital (a short acting barbiturate) have the reverse effect (enhancement of the nociceptive reflex and depression of the non-nociceptive reflex). Both types of anesthetic reduce the spontaneous activity in the sudomotor neurons. Other anesthetics have other effects on this system. In chronic spinal cats, both ketamine and methohexital have only a small effect on the resting activity in the sudomotor neurons and almost no effect on both types of distinct reflexes in sudomotor neurons (Janig and Rath, 1980). These results strongly suggest that the anesthetics do not act directly on the spinal reflex circuits but on supraspinal circuits which control the spinal sudomotor circuits, depressing and/or enhancing the activity in the descending systems. The reciprocal effects of both anesthetics suggest furthermore that there are two such descending systems which operate on the spinal sudomotor circuits. Reflexes to stimulation of visceral receptors Stimulation of visceral afferents from pelvic organs (generated by distention or contraction of the organs) lead to reflex activation of the sudomotor neurons in animals with intact spinal cord as well

59

stim anus

I I

bladder pressure

Fig. 12. Effects of passive distention of the urinary bladder and of the colon and of mechanical stimulation of the mucosal skin of the anus (black bars) on two preganglionic neurons (in the same bundle) that projected through a lumbar splanchnic nerve to the inferior mesenteric ganglion, in a cat which was spinalized at the segment TI0 about 1 h before the experiment. Upper histogram: activity in the two neurons. ’?, excitation produced by filling of the bladder; &, depression of activity produced by filling of the colon; double up arrow, excitation following release of pressure in the colon; double down arrow, decrease of activity following release of intravesical pressure; stim anal, gentle mechanical shearing stimuli applied to the mucosal skin of the anus. Ordinate scale, impulses per 2 s. Middle histogram: intravesical pressure, measured through a catheter in the urethra. The bladder was filled successively three times with 10 ml fluid each (arrows). Lower histogram: pressure in a large flexible ballon in the distal colon which was connected to a catheter inserted through the anal canal. The colon balloon was filled six times with 10 ml saline each (arrows). From Bartel et al. (1986).

as in spinal cats (Fig. 6). In para- and tetraplegic humans visceral stimuli lead to reflex activation of sudomotor neurons and to sweating. “Motility-regulating ” system

Preganglionic neurons which project in the lumbar splanchnic nerves consist of visceral vasoconstrictor neurons and different types of motilityregulating neurons (Bahr et al., 1986a-c; Janig and McLachlan, 1987; see Table 1). Motility-regulating neurons are characterized by distinct excitatory and inhibitory reflexes elicited by stimulation of sacral visceral afferents from the pelvic organs (urinary bladder, hindgut, anal canal). These reflexes are specific for the different types of visceral afferents, they are mediated by sacro-lumbar spinal reflex pathways and little affected after acute interruption of the thoracic spinal cord. This applies to the reflex excitation elicited by stimula-

tion of mechanoreceptive afferents from the anal canal, to the reflex excitation elicited from the urinary bladder and for the reflex inhibition elicited from the colon. Fig. 12 illustrates a complete reflex pattern in two preganglionic motility-regulating type 1 neurons (recorded simultaneously) one hour after spinalization. Interestingly, the inhibitory reflex elicited from the hindgut seems to be enhanced after spinalization. Inhibitory reflexes elicited from the anal canal (see Fig. 13B) and from the urinary bladder (as they occur in motility-regulating type 2 neurons) are reduced after spinalization; however they are not absent (Bartel et al., 1986). The excitation elicited in the motility-regulating neurons by stimulation of the anal canal very often show powerful afterdischarges which lasted for 1-12 min (Fig, 13A,C). After acute spinalization at the thoracic level, both reflex excitation and afterdischarge are still present in many motility-regulating

60

A

anal

perigenital

-1

5s

50p~

B

anal

aka I

C

lLs 1 min

Fig. 13. Reactions of preganglionic “motility-regulating” (MR) neurons which project into the lumbar splanchnic nerves to mechanical stimulation of the mucosa of the anal canal and of the perigenital hairy skin. The anal stimuli consisted of light shearing stimuli applied at a moving frequency of about 0.5-1 Hz with a spatula to the anal mucosa. (A) Activation and afterdischarge to anal stimulation and no response to perigenital stimulation. (B) Inhibition to anal stimulation. (C) Long-lasting afterdischarge following reflex activation to anal stimulation. Modified from Bahr et al. (1986a).

neurons which indicates that both are mediated by spinal reflex pathways (Bartel et al., 1986). Other functional types of sympathetic neurons

For other functional types of sympathetic neurons in the lumbar sympathetic outflow to skin, skeletal muscle and pelvic viscera and in the thoracic sympathetic outflow to head and neck we have no idea whether spinal circuits exist and whether they are of any functional significance. The neurons of most of these sympathetic systems (pilomotor, muscle vasodilator, cutaneous vasodilator, internal reproductive organs; see Table 1) are

silent under normal conditions and probably only activated in special behavioural conditions from special supraspinal centers. This may apply to the pilomotor and muscle vasodilator neurons during defence behavior, to the cutaneous vasodilator neurons during themoregulatory behavior (Grewe et al., 1995) and to the neurons innervating the internal reproductive organs during copulation. For pupillomotorneurons and inspiration-type neurons, both being spontaneously active (see Table l), we have also no idea about spinal pathways which are associated with these sympathetic systems. Both are under predominant supraspinal control: inspiration-type neurons discharge only in inspiration and cease firing when the respiratory drive is reduced. Activity in pupillomotorneurons may be dependent on mesencephalic and hypothalamic circuits. These neurons are inhibited in their activity during illumination of the retina (BoczekFuncke et al., 1992a). Motility-regulating neurons which regulate internal reproductive organs are normally silent. They can only be activated in special behavioural conditions. Specific afferent stimuli leading to their reflex activation in the anesthetized state have not been found so far.

Integration of spinal and supraspinal circuits The results reported above suggest that the autonomic circuits in the spinal cord are important for integrating information from the periphery and from supraspinal brain structures for the sympathetic outflows. They clearly demonstrate that the spinal cord contains neuronal circuits which consist of preganglionic neurons, putative interneurons and the synaptic connections of these neurons with the afferent inflow from the periphery and with the descending spinal pathways. Different types of spinal sympathetic system may be connected to different spinal pathways which are characterized by functionally distinct spinal afferent inputs (Fig. 14, Table 2) and one sympathetic system may be associated with several spinal pathways: e.g. each motility-regulating system is linked to at least three different spinal sacro-

61

1-

SYMPATHETIC SYSTEM

6

A AFFERENT SYSTEMS

nociceptor Skin visceral spinal hairfollicle receptor

MVC (VVCI

barOreCeDlOr. chemorecepror. 0.0

SYMPATHETIC SYSTEM

SM

AFFERENT SYSTEMS

1

nociceptor skin visceral spinal Pacinian c paw

urinary bladder (51 colon

(sl

anal canal

cvc

-

(sl

urinary bladder Ls) colon Lsl anal canal Lsl

Fig. 14. Spinal reflex pathways, the afferent input systems of these pathways and supraspinal systems with their afferent input systems for preganglionic muscle vasoconstrictor (and possibly visceral vasoconstrictor) neurons (MVC, CVC), cutaneous vasoconstrictor (CVC) neurons innervating skin of the distal extremities, sudomotor (SM) neurons and motility-regulating (MR) neurons in the cat. Most neurons of these sympathetic systems have ongoing activity. Some of them have also ongoing activity in the chronic state after transection of the spinal cord. Open symbols (>, 0)excitatory; closed symbols (+, 0 ) inhibitory. Summary scheme.

lumbar reflex pathways that mediate excitation from the urinary bladder, inhibition from the colon and excitation from the anal canal; the cutaneous vasoconstrictor system to the distal hindlimb skin is linked to at least four or five reflex pathways that mediate excitation and inhibition from cutaneous nociceptive afferents, excitation from sacral visceral afferents, inhibition from low-threshold cutaneous receptors, inhibition from spinal warm receptors etc. Idea about general principle of organization

Some functional characteristics of the discharge pattern in neurons of the sympathetic systems (Table 1) are dependent on spinal reflex circuits and some on supraspinal mechanisms (Janig, 1985, 1986, 1988). For example, discharge pattern and spontaneous activity (“tone”) in vasoconstrictor neurons regulating resistance vessels (e.g. muscle vasoconstrictor and visceral vasoconstrictor neurons, renal vasoconstrictor neurons) are largely dependent on activity generated in the medulla oblongata, on reflexes associated with cardiovascular afferents (notably baroreceptor and chemoreceptor

afferents) (Guyenet, 1990; Koshiya et al., 1993; see Chapter 8, this volume) and on the coupling between respiratory neurons and “presympathetic” neurons in the medulla oblongata (McAllen, 1987; Richter and Spyer, 1990; Guyenet and Koshiya, 1992; Habler et al., 1994b; Habler and Janig, 1995); the discharge pattern and spontaneous activity in most cutaneous vasoconstrictor neurons is mainly dependent on the hypothalamus (thermoregulation), on the spinal circuits (nociceptive and non-nociceptive somato-sympathetic and viscero-sympathetic reflexes, thermoregulatory reflexes) and on the medulla oblongata (cardiovascular reflexes, coupling to central respiratory generator); the discharge pattern and spontaneous activity in motility-regulating neurons is dependent on sacro-lumbar reflex pathways and relatively independent of brain stem and hypothalamus. This does not rule out, first, that the spinal circuits are important for the regulation of activity in sympathetic systems which are under predominant supraspinal control and, secondly, that the motility-regulating systems are under supraspinal control. Details about the neural mechanisms of the spi-

62

nal reflex circuits and the integration of these reflex arcs with the multiple descending influences are unknown. Histological and neurophysiological investigations show that somatic and visceral spinal afferents do not form monosynaptic connections with sympathetic preganglionic neurons (for review see Coote, 1988). Intracellular measurements in these neurons show furthermore that most synaptic connections of descending axons in the dorsolateral funiculus are probably also not monosynaptic (Dembowsky et al., 1985). From these observations and from the analysis of the reflex pattern in the sympathetic systems (Tables 1 and 2) several functionally distinct types of spinal interneurons must be postulated for the spinal circuits, if it is assumed that the spinal afferents do not form monosynaptic connections with the preganglionic neurons, and several functionally distinct descending systems: Spinal autonomic interneurons are possibly located in laminae V, VII and X of the spinal cord as well as in the intercalated and central autonomic nuclei of the intermediate zone (Strack et al., 1989a; see Chapter 3, this volume). These interneurons may be segmental and propriospinal. The segmental interneurons may be associated with the preganglionic neurons in the same and adjacent segments of a particular sympathetic outflow (e.g. the lumbar outflow to the pelvic organs, or the lumbar outflow to skin and skeletal muscle of the hindlimb). The propriospinal autonomic interneurons may be important for the communication between different spinal autonomic outflows and may project over many segments. For example, sacro-lumbar reflexes elicited in motilityregulating neurons of the lumbar sympathetic outflow to pelvic organs and colon are mediated by propriospinal neurons. Or many spinal somatosympathetic reflexes which are associated with the different types of cutaneous afferents are mediated by propriospinal intemeurons. Candidates are lamina I neurons of the dorsal horn. These neurons may specifically project to the thoracolumbar sympathetic nuclei (Craig, 1993; see Chapter 13, this volume). The interneurons are probably important for the integration of spinal and supraspinal

TABLE 2 Reflex pathways in the spinal cord associated with lumbar sympathetic systems ~~

Sympathetic Afferent system neurone

Reactiona

Ref.

MVC

Excitation Inhibition Excitation Inhibition i.Lb Excitation c.1. InhibitionC

38 3 58 2-8

Excitationd Inhibition excitation) Excitationb Excitation Excitation Excitation Inhibitione Excitationf Inhibition Excitation Excitationf

5.8 6 6 7,8 7,8 53 1 1 1 1 1 I

cvc

SM MRl MR2g

Nox. cutaneous Hair follicle Viscera, sacral Nox. cutaneous Low threshold mech. (Pacinian c., hair follicle) Viscera, sacral Warm,spinal cord Cold, spinal cord Nox. cutaneous Pacinian corp. paw Viscera, sacral Urinary bladder, sacral Colon, sacral Anus, sacral Urinary bladder Colon, sacral Anus, sacral

2,4

3,8

All data obtained on chronic spinal (MVC, CVC, SM) or acute spinal (MR) cats. Spinal cord transected between segmental level 'I7 and TI3 30 to 135 days before the experiments; cJ., contralateral; i.l., ipsilateral. For abbreviations of MVC, CVC, SM and MR see Table 1. References: 1, Bartel et al. (1986); 2, Grosse and JSLnig (1976); 3, Horeyseck and Jlnig (1974~);4, Janig (1975); 5 , Janig (1985); 6, Jlnig and KUmme1 (1981); 7, Jlnig and Spilok (1978); 8, Kiimmel(1983). aAIl reflexes which are present in the Sympathetic neurones after transection of the spinal cord are also present, with two exceptions (see c*d),in animals with intact spinal cord under standardized experimental conditions. bInhibition and excitation outlast stimulus. 'In cats with intact spinal cord, stimulation of hair follicle receptors elicits excitation mostly followed by depression of activity in many CVC neurones (Horeyseck and Jlnig, 1974a; Grosse and Jlnig, 1976). cats with intact spinal cord, inhibition in most CVC neurones (see HPbler et al., 1992). eProbably more pronounced in spinal preparation (acute) than in the intact preparation. fAfterdischarge sometimes present, but shorter in duration than in intact preparation (Bahr et al., 1986a); sometimes also inhibition present in acute spinal preparation; sometimes also inhibition to mechanical stimulation of the anal canal. gThis pattern seem to be rare.

63

circuits in the regulation of autonomic functions. So far no autonomic interneuron has been positively identified and functionally characterized in the thoracolumbar spinal cord. Candidates for descending systems which are distinct with respect to their origin and histochemistry (monaminergic, peptidergic) and which project to the sympathetic preganglionic neurons have been described (Strack et al., 1989a,b; see Loewy and Spyer, 1990; Dampney, 1994): they originate in the rostral ventrolateral medulla, the rostral ventromedial medulla, the caudal raphe nuclei, the pontine A5 area, the midbrain periaqueductal gray, the paraventricular hypothalamic nuclei, the lateral hypothalamic nuclei and the zona incerta. For example, for the different types of vasoconstrictor neurons (muscle, visceral, renal, cutaneous vasoconstrictor neurons) distinct descending pathways from the rostoventrolateral medulla, in which presympathetic neurons for these sympathetic systems are situated (Dampney and McAllen, 1988; McAllen and May, 1994a) and which mediate baroreceptor, chemoreceptor and other cardiovascular reflex activities and components of coupling with the central respiratory generator (see Habler and Janig, 1994; Habler et al., 1994b) have been postulated. However, the functions of the other descending systems which project to the preganglionic sympathetic neurons are unknown. It is hypothesized that integration of spinal and supraspinal circuits, which leads to the characteristic discharge patterns in the sympathetic neurons, occurs principally in the same way as in the somatomotor system (see Baldissera et al., 1981). Spinal autonomic circuits are integrative mechanisms for the control of autonomic target organs. These may be called “spinal subroutines” or “spinal autonomic motor programs”. Higher (supraspinal) centers use these spinal integrative mechanisms in their control of the peripheral sympathetic pathways. The supraspinal signals are finally shaped in this way by the spinal circuits before they are channelled into the peripheral sympathetic pathways. This shaping by the spinal autonomic programs certainly varies between different sympathetic systems, and there are

certainly also many monosynaptic connections between descending systems and preganglionic sympathetic neurons (Strack et al., 1989b; Morrison et al., 1991; Zagon and Smith, 1993; McAllen et al., 1994). Although this spinal component is not readily visible in the regulation of several autonomic target organs (e.g. cardiovascular regulation, thermoregulation of blood flow through skin, regulation of evacuative organs) it is probably very important for the understanding of these regulations. Spinal circuits may set the gain for supraspinal reflexes or supraspinal systems may regulate the sensitivity of spinal reflex circuits (e.g. the spinal nociceptive and non-nociceptive reflexes in sudomotor and cutaneous vasoconstrictor neurons and the viscero-visceral reflexes in motility-regulating neurons). For example, activation of sympathetic cardiomotor neurons during exercise from supraspinal systems may be enhanced and maintained by the spinal afferent feedback from the heart; inhibition of cutaneous vasoconstrictor neurons during warming of the hypothalamus is enhanced by spinal thermosensitive circuits (Grewe et al., 1995); nociceptive and non-nociceptive spinal reflexes in sudomotor neurons are enhanced by supraspinal systems, etc. Preganglionic neurons, autonomic interneurons, spinal afferent neurons and descending systems

Fig. 15. Hypothetical configuration of spinal autonomic circuits and their connection with spinal afferents ( 1 4 ) and descending systems (a-c). Preganglionic neuron shaded; inhibitory interneurons and their synapses in black; excitatory interneurons and their synapses open.

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may be synaptically connected in a similar way as the spinal interneurons in the somatomotor system (see Baldissera et al., 1981; Jankowska and Lundberg, 1981; Schomburg, 1990). Two hypothetical examples are illustrated in Fig. 15. The neuronal regulation of target organs by sympathetic systems would obtain considerable flexibility by using this type of organization. We have no idea about synaptic connections of autonomic interneurons with each other, in analogy to the synaptic connections of Renshaw interneurons, Ia interneurons, Ib interneurons, or interneurons associated with the flexor-reflex pathways constituting “spinal motor units” which are important in the regulation of the activity in alpha- and gamma-motoneurons (Baldissera et al., 1981; Schomburg, 1990). However, it can be assumed that conceptually similar circuits do exist for the spinal sympathetic systems, e.g. for the cutaneous vasoconstrictor and sudomotor system in the coordinated regulation of sweating and blood flow through hairless skin, or for the motility-regulating systems in the coordinated regulation of evacuative organs (lower urinary tract and hindgut). Recently, Coote (1988) has discussed the reflex pathways within the spinal cord for regulation of cardiovascular target organs. In the following some (hypothetical) examples of integration of spinal and supraspinal circuits are discussed. Vasoconstrictorpathways and rostra1 ventrolateral medulla (RVLM)

The most thoroughly studied excitatory descending pathways to preganglionic sympathetic neurons have their origin in the RVLM. Polson et al. (1992) have shown, in the cat, that these neurons are concentrated within a longitudinal cell column of the RVLM which is called the subretrofacial nucleus. Many neurons in this area project to preganglionic vasoconstrictor neurons, preganglionic cardiomotor neurons and preganglionic neurons innervating the adrenal medulla and are probably presympathetic neurons which are involved in cardiovascular regulation; thus these neurons are essential for phasic and tonic regula-

tion of the activity in vasoconstrictor neurons innervating resistance vessels, heart and adrenal medulla and therefore for control of blood pressure. They mediate several types of well-defined cardiovascular reflexes (e.g. from arterial baro- and chemoreceptors, see Guyenet, 1990; Koshiya et al., 1993; and Chapter 8, this volume) and other reflexes, which are typical for systemic regulation of arterial blood pressure, to muscle vasoconstrictor neurons, visceral vasoconstrictor neurons, renal VC neurons, sympathetic cardiomotoneurons, preganglionic neurons associated with the adrenal medulla and to cutaneous vasoconstrictor neurons (see below). Furthermore they probably mediate the coupling between cardiovascular neurons and respiratory neurons (see below) and integrate many supramedullary afferent inputs (for review see Dampney, 1994). The synaptic connections of the spinobulbar neurons in the RVLM to the preganglionic neurons are probably monosynaptic as well as di- or polysynaptic. However, with a few exceptions showing monosynaptic connections, positive experimental evidence that the RVLM neurons influence preganglionic neurons via polysynaptic routes does not exist (Strack et al., 1989b; Momson et al., 1991; Zagon and Smith, 1993; McAllen et al., 1994). Based on experiments in which activation of postganglionic neurons in various peripheral nerves was measured during localized microinjection of glutamate, McAllen and co-workers suggested (McAllen, 1986a; Dampney and McAllen, 1988; McAllen and Dampney, 1989; McAllen and May, 1994a,b; McAllen et al., 1995) that presympathetic vasoconstrictor neurons to different vascular beds have a distinct topographical representation in the subretrofacial nucleus of the cat. Renal presympathetic vasoconstrictor neurons are located most rostrally and presympathetic muscle vasoconstrictor-like neurons most caudally, with presympathetic visceral vasoconstrictor neurons (which regulate the resistance in the mesenteric vascular bed) being located somewhat more medial to renal presympathetic vasoconstrictor neurons and presympathetic cutaneous vasoconstrictor neurons medial to presympathetic muscle vaso-

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Fig. 16. Viscerotopic organization of presympathetic neurons in the subretrofacial cell column of the rostroventrolateral medulla of the cat. Ventral view of the cat hemi-medulla, showing surface structures, inferior olive (10) and the facial nucleus (VII). The approximate locations of sympathetic premotor neuron pools are shown as indicated. CVC, cutaneous vasoconstrictor; MVC, muscle vasoconstrictor; RVC, renal vasoconstrictor; VVC, visceral vasoconstrictor (mesenteric vascular bed; see Lovick (1 987); SM, sudomotor. Modified from McAllen et al. (1995) with permission.

constrictor neurons (Fig. 16). Lovick (1987) described a similar organization of the topographical representation of renal, mesenteric and muscle vascular beds as well as of heart and adrenal medulla in the nucleus paragigantocellularis of the cat (which is equivalent to the RVLM and includes the SRF nucleus), the presympathetic neurons for latter organs being located most rostrally. The subpopulations of presympathetic vasoconstrictor neurons in the SRF nucleus are supposed to exhibit distinct projections to the respective functional types of preganglionic neurons although overlapping of projections are possible, particularly to renal, visceral vasoconstrictor and muscle vasoconstrictor preganglionic neurons, all three being similar in their discharge patterns although not identical. For example, chemoreceptor

reflexes and coupling to the central respiration generator seem to be weaker in visceral vasoconstrictor neurons projecting to pelvic organs and colon than in muscle vasoconstrictor neurons (Bahr et al., 1986b; Dorward et al., 1987; BoczekFuncke et al., 1992~).The anatomical and functional separation of presympathetic neurons in the SRF nucleus is supported by two further lines of evidence: Firstly, putative presympathetic cutaneous vasoconstrictor neurons were found which were inhibited by stimulation of arterial chemoreceptors (McAllen, 1992) or inhibited by warming of the hypothalamus (McAllen and May, 1994a). Secondly, there are rostrocaudal differences in morphology and transmitter content of the SRF neurons so that at least four subgroups of SRF neurons could be defined by these criteria (Polson et al., 1992). It is clear that presympathetic vasoconstrictor neurons in the RVLM and preganglionic vasoconstrictor neurons do have only those functional characteristics in common which are dependent on neural circuits in the medulla oblongata and relayed to the preganglionic neurons via the RVLM, yet not those which are dependent on spinal circuits and on supramedullary circuits which project via other descending pathways to the spinal circuits and bypass the RVLM (see Strack et al., 1989b). Therefore it is expected that preganglionic sympathetic vasoconstrictor neurons have a wider spectrum of functional characteristics than the corresponding presympathetic neurons and this may particularly apply to cutaneous vasoconstrictor neurons. Furthermore, sympathetic neurons of other types probably have no representation in the RVLM and therefore no common functional characteristics with neurons in the RVLM, although McAllen has shown in the cat that presympathetic sudomotor neurons are present somewhat medial and rostra1 to the subretrofacial cell column (see Fig. 16) (McAllen, 1986b; Shafton and McAllen, 1994). The finding that sympathetic outflows which regulate different cardiovascular target organs are driven by separate populations of presympathetic neurons in the RVLM has considerable implica-

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tions. It shows that the functional differentiation which is seen in the pre- and postganglionic sympathetic neurons is already present in this population of presympathetic neurons and that the assumption that a general “neural vasomotor tone” is generated in the RVLM (Guyenet et al., 1989) may not be correct. It is possible that the background activity in the different types of sympathetic cardiovascular neurons is generated by different sets of neurons in the RVLM and elsewhere (for discussion of this point, see McAllen and May (1994b). The neurons in the RVLM would then be presympathetic in the sense that they project to neurons of one of the sympathetic pathways which are associated with the regulation of cardiovascular target organs. By the same token it may turn out in future that these neurons are presympathetic for particular functional properties of a sympathetic pathway but not for other properties. Alternatively it may turn out that a set of bulbospinal neurons in RVLM determines functional properties of several types of sympathetic systems. For example, excitatory chemoreceptor reflexes and inhibitory baroreceptor reflexes in muscle, visceral and renal vasoconstrictor neurons may be generated by the same functional type of bulbospinal RVLM neuron. Sympathetic pathways and respiration

Neural regulation of cardiovascular organs and regulation of lungs are precisely coordinated from moment-to-moment which leads to precise adjustment of respiration, cardiac output and peripheral vascular resistance for the transport of oxygen and carbon dioxide. By the same token does thermoregulation, regulation of extracellular fluid volume and probably other autonomic regulations require a similar coordination between neural regulation of autonomic target organs and regulation of respiration. An expression of this coordination is the respiratory profile of activity in sympathetic neurons which is seen in the activity of many sympathetic neurons and which is due to a central coupling between neurons of the central respiratory generator and neurons of autonomic

pathways and the connections of both with various types of afferents. The respiratory profile of the activity in sympathetic neurons is not uniform but varies with the function of a neuron, i.e. its target tissue. This is schematically illustrated in Fig. 17 for the different types of sympathetic neurons with spontaneous activity of the lumbar sympathetic outflow to skin, skeletal muscle and pelvic viscera and for the thoracic sympathetic outflow to head and neck in the cat: Neurons supplying resistance vessels such as muscle vasoconstrictor neurons and visceral vasoconstrictor neurons are activated during inspiration and suppressed in postinspiration and sometimes in early inspiration (see arrows in upper panel of Fig. 17). The respiratory rhythmicity seems to be weaker in visceral vasoconstrictor neurons than in muscle vasoconstrictor neurons (Boczek-Funcke et al., 1992~).This activity pattern in muscle vasoconstrictor and visceral vasoconstrictor neurons which is seen when vagus nerves and carotid sinus nerves are cut is normally masked by the inhibitory effects of the arterial baroreceptor activity. In cats with intact vagus nerves, ventilated artificially at a rate of 20/min, central respiration and artificial ventilation are synchronized: the arterial blood pressure decreases during expiration leading to unloading of arterial baroreceptors; during central inspiration the arterial blood pressure increases leading to activation of arterial baroreceptors. This generates a reflex activation of muscle vasoconstrictor and visceral vasoconstrictor neurons during expiration (see * in upper histogram of Fig. 17) and masks the activation during central inspiration. With increased respiratory drive (induced by raising the arterial C0,-pressure) is the activation during inspiration visible (Boczek-Funcke et al., 1992b,c). In humans the changes of activity in muscle vasoconstrictor neurons is fully dominated by the rhythmic changes of the activity in arterial baroreceptors (Eckberg et al., 1985, 1988; Fritsch et al., 1991; see Habler et al., 1994b). Most cutaneous vasoconstrictor neurons show no respiratory modulation in their activity, some are inhibited during inspiration and activated during expiration, and others exhibit a peak during

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Fig. 17. Synopsis of the different patterns of respiratory modulation in sympathetic neurons with different functions in the cat. Bottom: Activity profile in phrenic nerve indicating the phases of inspiration (I), postinspiration (PI) and phase I1 of expiration (EII). (A) Pattern present in almost all neurons which exhibit a high degree of cardiac rhythmicity in their activity, e.g. muscle vasoconstrictor and visceral vasoconstrictor neurons. The central respiratory modulation consists of a drive-dependent inspiratory (I) peak with a small and variable depression of activity in early I and a more pronounced one in postinspiration (p-I) (see interrupted arrows). In addition to the central respiratory modulation there is a baroreceptor mediated peak of activity (*) due to the second order blood pressure waves. (B) An I peak of activity is exhibited by some cutaneous vasoconstrictor neurons (mainly supplying hairless skin). (C) Neurons discharging only in I are present in the thoracic preganglionic outflow projecting into the cervical sympathetic trunk but not in other sympathetic outflows. (D) Respiratory modulation is absent in the majority of cutaneous vasoconstrictor neurons and MR neurons. (E) Expiratory pattern of activity with a depression during I and activation during PI extending into phase I1 of expiration (E 11) is shown by some cutaneous vasoconstrictor and MR neurons and in SM neurons. For abbreviations, see Table 1. Modified from HSibler et al. (1994b).

inspiration (mainly represented in cutaneous vasoconstrictor neurons innervating hairless skin) under normal ventilation and respiration. The percentage of cutaneous vasoconstrictor neurons which are depressed in their activity during inspiration and/or activated during expiration increases when the respiratory drive is increased. The baroreceptor mediated component is absent or barely detectable in the respiratory profile of the activity in the cutaneous vasoconstrictor neurons. Sudomotor neurons are preferentially active during postinspiration. This pattern is enhanced during increased respiratory drive. Most motility-regulating neurons exhibit no respiratory rhythmicity in their activity; a few motility-regulatingneurons show a peak of activity during expiration. “Inspiratory-type” neurons are only active during inspiration and cease to fire when the activity in the phrenic nerve is abolished. There are some species differences in the respiratory rhythmicity of sympathetic activity between the rat and larger mammals, such as cats, dogs, rabbits and piglets (see Habler et al., 1994b). The pattern of respiratory modulation which is most frequently found in the rat is a depression of activity during inspiration and an activation during postinspiration (Darnall and Guyenet, 1990; Habler et al., 1993, 1994b), whereas in larger animals the majority of sympathetic neurons shows the reverse. On the contrary to what has been observed in cats, in rats the patterns of respiratory modulation are similar in the activity of postganglionic muscle vasoconstrictor and cutaneous vasoconstrictor neurons of the hindlimb (Habler et al., 1993). Therefore, respiratory modulation cannot be used as a functional marker for these sympathetic neurons in the rat. Experimental evidence suggests that the patterns of respiratory modulation observed so far in muscle and skin postganglionic activity of conscious humans are fully compatible with those exhibited by muscle vasoconstrictor and cutaneous vasoconstrictor neurons of anesthetized cats (Hagbarth et al., 1972; Eckberg et al., 1985; for review, see Habler et al., 1994b). The synaptic interaction between respiratory

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neurons and “presympathetic” neurons probably takes place mainly at the level of the medulla oblongata in bulbospinal RVLM neurons which are thought to represent a major excitatory pathway onto preganglionic vasoconstrictor and probably sudomotor neurons (McAllen, 1987; Haselton and Guyenet, 1989; Guyenet, 1990; Guyenet et al., 1990; Guyenet and Koshiya, 1992). It is unclear whether coupling between respiratory neurons and sympathetic neurons occurs additionally at the level of the spinal cord and elsewhere. Likewise, it remains to be established by intracellular recordings from “presympathetic” neurons in the RVLM and other areas of the medulla oblongata which types of respiratory neuron are directly involved in shaping the activity in the sympathetic pathways and in which way both systems are synaptically coupled (Fig. 18). Based on a working model published recently by Richter and Spyer (Richter and Spyer, 1990; Richter et al., 1991) it is proposed that this coupling may occur, at least for the muscle vasoconstrictor and visceral vasoconstrictor neurons, via ramp-inspiratory and postinspiratory neurons. However, in order to account for the various patterns of respiratory modulation seen in the activity of functionally different types of sympathetic neurons, in the cat, the nature of the coupling between neurons of the central respiratory generator (CRG) and the sympathetic systems must vary according to their function. Finally, cardiovascular afferent systems probably can influence “presympathetic” neurons independently of the effects they exert on the respiratory network, indicating that the central respiratory generator and autonomic systems are separate in the medulla oblongata. Thus, the neural circuits which mediate reflexes elicited in different types of sympathetic neurons by stimulation of arterial baroreceptors, of arterial chemoreceptors and of other types of cardiopulmonary receptors are distinct from neural circuits which are involved in adjustment of respiration elicited by the same types of cardiopulmonary afferents (Fig. 18) Guyenet, 1990; Guyenet and Koshiya, 1992; Koshiya et al., 1993; see Hgibler et al., 1994b).

Lung

Laryngeal

1

stem motor

sympathetic preganglionic neurons

respiratory motoneurons

Fig. 18. Idea about how respiratory modulation is generated in the activity of sympathetic muscle vasoconstrictor, visceral vasoconstrictor, cutaneous vasoconstrictor and INS neurons (see Table 1). The neurons controlling sympathetic preganglionic neurons are located in the rostra1 ventrolateral medulla (RVLM) and probably elsewhere. RVLM neurons have been shown to receive input from inspiratory (I) or postinspiratory (PI) neurons of the central respiratory generator (CRG) resulting in a respiratory modulation of their activity (McAllen, 1987; Haselton and Guyenet, 1989; Richter and Spyer, 1990; Hlbler et al., 1994b). Reflexes from respiratory and cardiovascular afferents are very likely also integrated in the RVLM and elsewhere, independent of their connections with neurons of the CRG (Guyenet and Koshiya, 1992). It is clear that only some types of “presympathetic neurons” which are antecedent to the spinal autonomic circuits (particularly those being associated with vasoconstrictor neurons) are represented in the RVLM. Other types of “presympathetic neurons” to the same functional types of spinal autonomic circuits may have their origin elsewhere in brain stem and hypothalamus. Therefore, respiratory modulation of activity in sympathetic neurons may also be dependent on descending systems other than those from the RVLM. CB, excitation; 8 , depression of activity; 0, excitation; 0 , inhibition.

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Cutaneous vasoconstrictor system: integration in spinal cord, RVLM and hypothalamus

The discharge pattern in neurons of the cutaneous vasoconstrictor pathways is complex and variable and consists of several components. This complexity is expressed in the different types of reflexes (and their changes induced by various types of experimental interventions) and is dependent on the organization of the cutaneous vasoconstrictor system in the spinal cord (Table 2, Fig. 14), in the medulla oblongata (reflexes to stimulation of cardiovascular afferents, coupling to central respiratory generator), in the hypothalamus (e.g. thermoregulatory reflexes) and elsewhere. Given the size and differentiation of the skin surface and its vascular bed, it is not far-fetched to assume that the cutaneous vasoconstrictor system has a high central representation and is differentiated into several subsystems. The following points are worth mentioning: Most reflexes which can be evoked in cutaneous vasoconstrictor neurons by adequate stimulation of somatic and visceral receptors and which are organized at the different levels of the neuraxis are inhibitory in comparison to the reflexes in muscle vasoconstricter, sudomotor, inspiratorytype or other sympathetic neurons (Janig, 1985; Boczek-Funcke et al., 1992a). The respiratory profile in the activity of cutaneous vasoconstrictor neurons is variable and much less stereotyped than in the activity of muscle vasoconstrictor, visceral vasoconstrictor and inspiratory-type neurons (Fig. 17). This indicates that coupling between the central respiratory generator and the cutaneous vasoconstrictor system is variable too and probably dependent on the function of cutaneous vasoconstrictor neurons (Boczek-Funcke et al., 1992b,c; see Habler et al., 1994b). Different sections of the cutaneous vascular bed (e.g. capacitance vessels, resistance vessels and arterio-venous anastomoses in the hairless skin), different types of skin (hairy skin, hairless skin) and different areas of skin (skin of the distal extremities, of the proximal extremities and trunk, of

the face) may be innervated by different types of cutaneous vasoconstrictor neurons. After experimental interventions, such as chronic nerve lesions (Blumberg and Janig, 1985; Janig and Koltzenburg, 1991) or acute decerebration (Gregor and Janig, 1977), typical reflex patterns which are normally seen in cutaneous vasoconstrictor neurons may disappear and change into a reflex pattern observed in muscle vasoconstrictor neurons (excitatory chemoreceptor reflex, strong phasic baroreceptor inhibition, excitation to stimulation of cutaneous nociceptors). It is suggested that a more basic reflex pattern which is similar to that seen in muscle vasoconstrictor neurons and dependent on the neural circuits in the medulla oblongata (and probably connected to the RVLM) is normally masked in cutaneous vasoconstrictor neurons by a reflex pattern which is characterized by inhibitory reflexes elicited by stimulation of arterial chemoreceptors, central warm receptors and nociceptors and by the absence of or by small inhibitory baroreceptor reflexes (Janig, 1975; Gregor and Janig, 1977; Blumberg and Janig, 1985; Janig and Koltzenburg, 1991). At present it is not possible to describe where in the neuraxis this change in reflex pattern occurs; however, it may be related to the hypothalamus which is important for the thermoregulatory functions of the cutaneous vasoconstrictor system. Cutaneous vasoconstrictor neurons innervating hairless and hairy skin of the cat hindlimb are inhibited by warming of the spinal cord and of the anterior hypothalamus and some are weakly excited by cooling of both structures of the neuraxis. Activity in muscle vasoconstrictor neurons does either not change or increases slightly during hypothalamic warming. This increase of activity in muscle vasoconstrictor neurons is always correlated with a decrease of arterial blood pressure (when the resistance in the cutaneous vascular bed decreases) and therefore subsequently an unloading of the arterial baroreceptors. sudomotor neurons (in the cat) are not activated during hypothalamic warming but weakly activated during hypothalamic and spinal cord cooling. The responses in the cutaneous vasoconstrictor neurons to warming

of anterior hypothalamus and spinal cord are graded and interact with each other in a multiplicative way as predicted from investigations of thermoregulation (Grewe et al., 1995; see Simon, 1974; Simon et al., 1986). McAllen and May (1 994a) have recently shown that putative presympathetic cutaneous vasoconstrictor neurons in the RVLM (see Fig. 16) are also inhibited in their activity during hypothalamic warming in a graded manner. These results, in conjunction with those obtained on postganglionic cutaneous vasoconstrictor neurons, strongly support the notion that there exists a distinct pathway from the hypothalamus to the preganglionic cutaneous vasoconstrictor neurons. Neurons in the hypothalamus which mediate central thermoregulatory information to cutaneous sympathetic pathways may project to spinal cutaneous vasoconstrictor circuits in two ways: via presympathetic cutaneous vasoconstrictor neurons in the RVLM and possibly elsewhere as well as directly. At the level of the RVLM the descending pathway from the hypothalamus is integrated with cardiovascular and other reflex pathways as well as with information from the central respiratory generator. Also these may be specific for presympathetic cutaneous vasoconstrictor neurons (when compared with those in putative presympathetic muscle vasoconstrictor neurons) insofar as cutaneous vasoconstrictor neurons exhibit distinct respiratory profiles in their activity (see Habler et al., 1994b) and some presympathetic cutaneous vasoconstrictor neurons are probably inhibited to stimulation of arterial chemoreceptors (McAllen, 1992). At the spinal level the descending projections from hypothalamus and RVLM are integrated with specific spinal reflex pathways (see Fig. 14). These distinct responses to hypothalamic and spinal cord warming could be used in further analysis to unravel the central organization of the cutaneous vasoconstrictor system. Functioning of the isolated spinal cord

In the cat, spontaneous activity in vasoconstrictor neurons (muscle, visceral, cutaneous vasocon-

strictor) and sudomotor neurons is dependent on supraspinal inputs to the spinal autonomic circuits. This spontaneous activity disappears following transection of the spinal cord and slowly recovers over weeks and months. It may reach rates of about 0.5-1.5 imp/s in the vasoconstrictor neurons and low rates of 0.1-0.5 imp/s in sudomotor neurons under thermoneutral resting conditions with normal blood pressure after recovery from "spinal shock" (Horeyseck and Janig, 1974c; Janig and Spilok, 1978; Janig and Kiimmel, 1981). These rates are similar to those found in animals with intact spinal cord (Janig, 1985, 1988). However, it is unknown whether the percentage of silent postganglionic neurons increases after spinalization. The mechanisms by which this resting activity is generated in the spinal cord and whether it is an intrinsic property of the preganglionic neurons or of the autonomic interneurons or both is unclear. Resting activity in preganglionic motilityregulating neurons which project to the inferior mesenteric ganglion changes very little after acute spinalization (Bartel et al., 1986). As already emphasized, many functionally specific reflexes elicited in sympathetic systems by stimulation of somatic and visceral afferents are preserved chronically after interruption of the spinal cord (Table 2, Fig. 14). This is the basis for several residual autonomic functions of the spinal cord which are related to thermoregulation (Simon, 1974; Hensel, 1981, 1982), to regulation of evacuative organs (lower urinary tract and colon; de Groat et al., 1993), to regulation of sexual organs (de Groat and Booth, 1993b), to regulation of blood vessels (e.g. activation of vasoconstrictor neurons during tilting of tetraplegic patients, possibly by hypoxia of the spinal cord due to decreased perfusion pressure; see Mathias and Frankel, 1992), and for spinal intestino-intestinal reflexes, sacro-lumbar reflexes in motilityregulating neurons (Bartel et al., 1986) and possibly other spinally mediated reflexes (e.g. those associated with heart and kidney). Spinal autonomic programs obviously continue to function in a biologically meaningful way after they have been isolated from the supraspinal centers. The different

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functions are less well or not coordinated and the range of regulations are smaller than normal. Whatever reorganization may happen in the spinal cord with time after interruption of its connections with supraspinal brain structures, the investigation of the spinal autonomic functions suggest that the organization of the synaptic connections between primary afferents, autonomic interneurons and sympathetic preganglionic neurons are, at least qualitatively, preserved and that the spinal autonomic systems do not get into disorder and do not disintegrate. Thus, primary afferents and axons of the autonomic interneurons which may sprout after supraspinal denervation do not do so randomly. Visceral afferents from pelvic organs and probably other visceral organs seem to have particularly strong reflex effects on sympathetic vasoconstrictor systems after spinalization. For the cutaneous vasoconstrictor system the viscero-sympathetic reflexes are reversed and stimulation of visceral afferents, notably from the urinary bladder, now elicits most powerful reflex vasoconstrictions in all vascular beds, leading to large increases of arterial blood pressure in animals and humans chronically after interruption of the spinal cord rostral to the vasoconstrictor outflow to the viscera (above the thoracic segment T3; see Mathias and Frankel, 1992). Interestingly, reflex activations of muscle vasoconstrictor neurons is not particularly large and it is more likely that the increase of arterial blood pressure is the result of strong vasoconstrictions in the viscera and the skin (see Fig. 6 ) . Recently it has been shown in cats which were spinalized 15-37 days before at the segmental level T3 that activation of visceral afferents from the urinary bladder and electrical stimulation of the sciatic nerve leads to strong reflex activation of the adrenal medulla with large increase of circulating adrenaline and noradrenaline (Stoddard et al., 1988, 1992). This could explain the large increase of arterial blood pressure after interruption of the spinal cord at segmental levels rostral to T3. However, in humans a large increase of adrenaline in the circulation during activation of visceral afferents induced by distention and contraction of the

urinary bladder has not been observed so far (Mathias and Frankel, 1992). The spinal mechanisms which are responsible for these strong activations of vasoconstrictor and other sympathetic pathways by stimulation of spinal visceral afferents are unknown.

Synopsis The spinal cord is an integrative organ in its own right for the response pattern of the sympathetic neurons. Several functionally distinct reflex pathways are postulated for the different types of sympathetic systems (Fig. 14, Table 2). The structure of these reflex pathways and their integration with supraspinal autonomic circuits are almost unexplored. This integration has to be studied systematically using neurophysiological, morphological, histochemical and pharmacological methods. The spinal circuits are probably the basic building blocks for all homeostatic regulations in which the sympathetic systems are involved and which are represented in brain stem and hypothalamus. The limbic system and neocortex, notably the amygdala and its connections with the neocortical sensory association cortices, the orbitofrontal cortex and the hippocampal formation, which contains the representations of the emotions, is using the circuits in the neuraxis during expression of the emotions by the autonomic reactions. The autonomic reaction patterns are probably specific for the different types of emotions and are biologically adaptive responses. An important crossroad, between limbic system structures and the autonomic circuits which are involved in homeostatic autonomic regulations, is probably the midbrain periaqueductal gray. This structure represents in its lateral and ventrolateral rostro-caudal longitudinal columns the basic response patterns which are important for the survival of the organism, such as confrontational defense, flight, freezing, etc. and may represent the basic neural building blocks for active and passive coping strategies. These patterns are elicited by particular external and internal stimuli and consist of coordinated motor, sensory and autonomic

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components. The periaqueductal gray has multiple reciprocal (afferent and efferent) connections with cortical and subcortical forebrain structures and structures in the neuraxis which are involved in the regulation of autonomic systems (Bandler et al., 1991; Bandler and Shipley, 1994; see Chapter 16, this volume).

Summary The sympathetic nervous system consist of several subsystems which are organized with respect to their target organs and therefore with respect to their function. This has been shown for the lumbar sympathetic outflow to skin, skeletal muscle and pelvic viscera and for the sympathetic outflow to the superior cervical ganglion. Twelve different types of sympathetic subsystems have so far been identified in these sympathetic outflows. Integration resulting in the patterns of discharge which are seen in the different types of sympathetic neuron, and therefore in distinct regulation of organ systems, occurs almost exclusively within the central nervous system, notably in the spinal cord, the brain stem and the hypothalamus. These central messages are faithfully transmitted to the autonomic target organs. They are not qualitatively changed, yet they may be quantitatively modified, in the peripheral sympathetic ganglia (notably the prevertebral ones) and at the neuroeffector junction by other (neural, local and remote hormonal, and non-neural) processes. The spinal cord is an integrative organ in its own right which determines many components of the discharge pattern in the sympathetic neurons. Muscle vasoconstrictor, cutaneous vasoconstrictor, sudomotor and motility-regulating systems generate distinct reflexes on stimulation of different types of afferent receptors in skin and viscera and on thermal stimulation of the spinal cord. Most of these responses are also present when the spinal cord is isolated from the brain stem. These reflexes are based on a distinct organization of the sympathetic systems in the spinal cord. Visceral afferents (notably from pelvic organs) have strong uniform excitatory reflex effects on vasoconstrictor neurons

of all vascular beds chronically after interrupting the connections between the spinal sympathetic systems and their supraspinal control centers. Analysis of the discharge patterns in sympathetic neurons under standardized experimental conditions leads to the following conclusions: (1) The spinal cord contains distinct autonomic reflex pathways which are integrated with supraspinal reflex pathways during normal regulation of the autonomic target organs. (2) The discharge pattern in the different types of sympathetic neuron consist of components which are associated with integration in the spinal cord and integration in lower brain stem, upper brain stem and hypothalamus. (3) Reflex integration in the spinal cord is related to distinct afferent inputs from skin, viscera, spinal cord (thermoreceptors) and other structures. (4) Supraspinal reflex integration is related to distinct inputs, e.g. from cardiopulmonary receptors, respiratory neurons, thermosensitive structures (particularly in the hypothalamus). ( 5 ) Signals in supraspinal systems are integrated with spinal circuits, leading to reciprocal amplification of gain. ( 6 ) The isolated spinal cord is capable to regulate residual autonomic functions and it is the preservation of these discrete spinal autonomic functions after interruption of the spinal cord which is remarkable rather the uniformity of the discharge of neurons of functionally different sympathetic pathways. In conclusion, spinal circuits, spinal afferent inflows and descending influences from brain stem and hypothalamus always work together in the integrative activity of the preganglionic sympathetic neurons. The systems may either be under predominant control of the lower brain stem (e.g. muscle and visceral vasoconstrictor neurons innervating resistance vessels), of the hypothalamus (e.g. cutaneous vasoconstrictor neurons) or of the circuits in the spinal cord (e.g. motility-regulating neurons). However, in all sympathetic systems the spinal component may be essential for this integration because it may set the excitability of the preganglionic neurons and/or shapes the supraspinal signals according to the structure of the “spinal autonomic programs”. Experimental approaches which aim to clarify the central neural circuitry of

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the sympathetic nervous system should start at the level of the spinal cord and use functionally identified neurons as reference.

Acknowledgements This study was supported by the Deutsche Forschungsgeneinschaft.

References Bahr, R., Blumberg, H. and Janig, W. (1981) Do dichotomizing afferent fibers exist which supply visceral organs as well as somatic structures? A contribution to the problem or referred pain. Neurosci. Lett., 24: 25-28. Bahr, R., Bartel, B., Blumberg, H. and Janig, W. (1986a) Functional characterization of preganglionic neurons projecting in the lumbar splanchnic nerves: neurons regulating motility. J. Auton. Nerv. System, 15: 109-130. Bahr, R., Bartel, B., Blumberg, H. and Janig, W. (1986b) Functional characterization of preganglionic neurons projecting in the lumbar splanchnic nerves: vasoconstrictor neurons. J . Auton. Nerv. Sysrem, 15: 131-140. Bahr, R., Bartel, B., Blumberg, H. and Janig, W. (1986~) Secondary functional properties of lumbar visceral preganglionic neurons. J . Auton. Nerv. System, 15: 141-152. Baldissera, F., Hultborn, H. and Illert, M. (1981) Integration in spinal neuronal systems. In: V.B. Brooks (Ed.), Moror Control, Part I, Handbook of Physiology, Section I , The Nervous System. American Physiological Society, Bethesda, MD, pp. 509-595. Bandler, R. and Shipley, M.T. (1994) Columnar organization in the midbrain periaqueductal grey: modules for emotional expression. Trends Neurosci., 17: 379-389. Bandler, R., Carrive, P. and Zhang, S.P. (1991) Integration of somatic and autonomic reactions within the midbrain periaqueductal grey: viscerotopic, somatotopic and functional organization. Prog. Brain Res., 87: 269-305. Bartel, B., Blumberg, H. and Janig, W. (1986) Discharge patterns of motility-regulating neurons projecting in the lumbar splanchnic nerves to visceral stimuli in spinal cats. J. Auton. Nerv. System, 15: 153-163. Bell, C., Janig, W., Kiimmel, H. and Xu, H. (1985) Differentiation of vasodilator and sudomotor responses in the cat paw pad to preganglionic sympathetic stimulation. J. Physiol. (London), 364: 93-1 04. Bligh, J. (1966) The thermosensitivity of the hypothalamus and thermoregulation in mammals. Biol. Rev. Carnb. Philos. SOC., 41: 317-368. Blumberg, H. and Janig, W. (1985) Reflex patterns in postganglionic vasoconstrictor neurons following chronic nerve lesions. J. Auton. Nerv. System, 14: 157-180.

Blumberg, H. and Wallin, B.G. (1987) Direct evidence of neurally mediated vasodilatation in hairy skin of the human foot. J. Physiol. (London), 382: 105-121. Boczek-Funcke, A., Dembowsky, K., Habler, H.-J., Janig, W., McAllen, R.M. and Michaelis, M. (1992a) Classification of preganglionic neurones projecting into the cat cervical sympathetic trunk. J. Physiol. (London),453: 319-339. Boczek-Funcke, A., Dembowsky, K., Habler, H.-J., Jlnig, W. and Michaelis, M. (1992b) Respiratory related activity patterns in preganglionic neurones projecting into the cat cervical sympathetic trunk. J. Physiol. (London), 457: 277296. Boczek-Funcke, A., Habler, H.-J., Janig, W. and Michaelis, M. ( 1 992c) Respiratory modulation of the activity in sympathetic neurones supplying muscle, skin and pelvic organs in the cat. J. Physiol. (London),449: 333-361. Boczek-Funcke, A., Dembowsky, K., Habler, H.-J., Janig, W. and Michaelis, M. (1993) Spontaneous activity, conduction velocity and segmental origin of different classes of thoracic preganglionic neurons projecting into the cat cervical sympathetic trunk. J. Auton. Nerv. System, 43: 189-200. Brock, J.A. and Cunnane, T.C. (1992) Impulse conduction in sympathetic nerve terminals in the guinea-pig vas deferens and the role of the pelvic ganglia. Neuroscience, 47: 185196. Cannon, W.B. (1929) Organization for physiological homeostasis. Physiol. Rev., 9: 3 9 9 4 3 1. Cannon, W.B. (1939) The Wisdom of the Body. Norton, New

York.

Cassell, J.F., Clark, A.L. and E.M. McLachlan, E.M. (1986) Characteristics of phasic and tonic sympathetic ganglion cells of the guinea-pig. J. Physiol. (London),372: 457483. Craig, A.D. (1993) Propriospinal input to thoracolumbar sympathetic nuclei from cervical and lumbar lamina I neurons in the cat and the monkey. J. Comp. Neurol., 331: 517530. Cunnane, T.C. and Stjtirne, L. (1984) Frequency dependent intermittency and ionic basis of impulse conduction in postganglionic sympathetic fibres of guinea-pig vas deferens. Neuroscience, 11: 21 1-229. Dampney, R.A. (1994) Functional organization of central pathways regulating the cardiovascular system. Physiol. Rev., 74: 323-364. Dampney, R.A. and McAllen, R.M. (1988) Differential control of sympathetic fibres supplying hindlimb skin and muscle by subretrofacial neurones in the cat. J. Physiol. (London), 395: 41-56. Darnall, R.A. and Guyenet, P. (1990) Respiratory modulation of pre- and postganglionic lumbar vasomotor sympathetic neurons in the rat. Neurosci. Lett., 119: 148-152. De Groat, W.C. and Booth, A.M. (1993a) Synaptic transmission in pelvic ganglia. In: C.A. Maggi (Ed.), Nervous Control of the Urogenital System. Harwood, Chur, Switzerland, pp. 291-348.

74 De Groat, W.C. and Booth, A.M. (1993b) Neural control of penile erection. In: C.A. Maggi (Ed.), Nervous Control of the Urogenital System. Harwood, Chur, Switzerland, pp. 467-524. De Groat, W.C., Booth, A.M. and Yoshimura, N. (1993) Neurophysiology of micturition and its modification in animal models in human disease. In: C.A. Maggi (Ed.), Nervous Control of the Urogenital System. Harwood, Chur, Switzerland, pp. 227-290. Dembowsky, K., Czachurski, J. and Seller, H. (1985) An intracellular study of the synaptic input to sympathetic preganglionic neurones in the third thoracic segment of the cat. J. Auton. Nerv. System, 13: 201-244. Dembowsky, K., Czachurski, J. and Seller, H. (1986) Three types of sympathetic preganglionic neurones with different electrophysiological properties are identified by intracellular recordings in the cat. P’iigers Arch., 406: 112-120. Dorward, P.K., Burke, S.L., Janig, W. and Cassell, J. (1987) Reflex responses to baroreceptor, chemoreceptor and nociceptor inputs in single renal sympathetic neurones in the rabbit and the effects of anaesthesia on them. J. Auton. Nerv. System, 18: 39-54. Eckberg, D.L., Nerhed, C. and Wallin, B.G. (1985) Respiratory modulation of muscle sympathetic and vagal cardiac outflow in man. J . Physiol. (London), 365: 181-196. Eckberg, D.L., Rea, R.F., Andersson, O.K., Hedner, T., Pernow, J., Lundberg, J.M. and Wallin, B.G. (1988) Baroreflex modulation of sympathetic activity and sympathetic neurotransmitters in humans. Acta Physiol. Scand., 133: 221231. Elfvin, L.-G., Lindh, B. and Hokfelt, T. (1993) The chemical neuroanatomy of sympathetic ganglia. Annu. Rev. Neurosci., 16: 471-517. Fritsch, J.M., Smith, M.L., Simmons, D.T. and Eckberg, D.L. (1 991) Differential baroreflex modulation of human vagal and sympathetic activity. Am. J. Physiol., 260: R635-R641. Furness, J.B. and Costa, M. (1987) The Enteric Nervous System. Churchill Livingston, London. Furness, J.B., Bornstein, J.C., Murphy, R. and Pompolo, S . (1992) Roles of peptides in transmission in the enteric nervous system. Trends Neurosci., 15: 66-71. Gibbins, I.L. (1991) Vasomotor, pilomotor and secretomotor neurons distinguished by size and neuropeptide content in superior cervical ganglia of mice. J. Auton. Nerv. System, 34: 171-183. Gibbins, I.L. (1992) Vasoconstrictor, vasodilator and pilomotor pathways in sympathetic ganglia of guinea-pigs. Neuroscience, 47: 657-672. Gibbins, I.L. and Morris, J.L. (1987) Co-existence of neuropeptides in sympathetic, cranial autonomic and sensory neurons innervating the iris of the guinea-pig. J. Auton. Nerv. System, 21 : 67-82. Gibbins, I.L. and Morris, J.L. (1990) Sympathetic noradrenergic neurons containing dynorphin but not neuropeptide Y

innervate small cutaneous blood vessels of guinea-pigs. J. Auton. Nerv. System, 29: 137-149. Gregor, M. and Janig, W. (1977) Effects of systemic hypoxia and hypercapnia on cutaneous and muscle vasoconstrictor neurones to the cat’s hindlimb. P’ugers Arch., 368: 71-81. Gregor, M., Janig, W. and Riedel, W. (1976) Response pattern of cutaneous postganglionic neurones to the hindlimb on spinal cord heating and cooling in the cat. Pflugers Arch., 363: 135-140. Grewe, J., Janig, W. and Kiimmel, H. (1995) Effects of hypothalamic thermal stimuli on sympathetic neurones innervating skin and skeletal muscle of the cat hindlimb. J. Physiol. (London),488: 139-152. Grosse, M. and Janig, W. (1976) Vasoconstrictor and pilomotor fibres in skin nerves to the cat’s tail. Pfugers Arch., 361: 221-229. Guttmann, L. (1976) Spinal Cord Injuries. Comprehensive Management and Research. Blackwell, Oxford. Guyenet, P.G. (1990) Role of the ventral medulla oblongata in blood pressure regulation. In: A.D. Loewy and K.M. Spyer (Eds.), Central Regulation of Autonomic Functions. Oxford University Press, New York, pp. 145-167. Guyenet, P.G. and Koshiya, N. (1992) Respiratorysympathetic integration in the medulla oblongata. In: G. Kunos and J. Ciriello (Eds.), Central Neural Mechanisms in Cardiovascular Regulation. Birkhauser, Boston, MA, pp. 226-247. Guyenet, P.G., Haselton, J.R. and Sun, M.-K. (1989) Sympathoexcitatory neurons of the rostroventrolateral medulla and the origin of sympathetic vasomotor tone. Prog. Brain Res., 81: 105-1 16. Guyenet, P.G., Darnall, R.A. and Riley, T.A. (1990) Rostra1 ventrolateral medulla and sympathorespiratory integration in rats. Am. J. Physiol., 259: R1063-Rl074. Habler, H.4. and Janig, W. (1994) Coordination of sympathetic and respiratory systems: neurophysiological experiments. Clin. Exp. Hyper., 17: 223-235. Habler, H.-J., Hilbers, K., Janig, W., Koltzenburg, M., Kummel, H. and Lobenberg-Khosravi, M. (1992) Viscerosympathetic responses to mechanical stimulation of pelvic viscera in the cat. J. Auton. Nerv. System, 38: 147-158. Habler, H.-J., Janig, W., Krummel, M. and Peters, O.A. (1993) Respiratory modulation of the activity in postganglionic neurons supplying skeletal muscle and skin of the rat hindlimb. J. Neurophysiol., 70: 920-930. Habler, H.-J., Janig, W., Krummel, M. and Peters, O.A. (1994a) Reflex patterns in postganglionic neurones supplying skin and skeletal muscle of the rat hindlimb. J. Neurophysiol., 72: 2222-2236. Habler, HA., Janig, W. and Michaelis, M. (1994b) Respiratory modulation of activity in sympathetic neurones. Prog. Neurobiol., 43: 567-606. Hagbarth, K.E., Hallin, R.G., Hongell, A,, Torebjork, H.E. and Wallin, B.G. (1972) General characteristics of sympa-

75 thetic activity in human skin nerves. Acta Physiol. Scand., 84: 164-176. Haselton, J.R. and Guyenet, P.G. (1989) Central respiratory modulation of medullary sympathoexcitatory neurons in rat. Am. J. Physiol., 256: R739-R750. Hensel, H. (1981) Thermal Sensations and Thermoreceptors in Man. Charles C Thomas, Springfield, IL. Hensel, H. (1982) Thermoreception and Temperature Regulation. Academic Press, London. Hirst, G.D. and Edwards, F.R. (1989) Sympathetic neuroeffector transmission in arteries and arterioles. Physiol. Rev., 69: 54-04. Hirst, G.D., Bramich, N.J., Edwards, F.R. and Klemm, M. (1992) Transmission at autonomic neuroeffector junctions. Trends Neurosci., 15: 4 W 6 . Horeyseck, G. and Janig, W. (1974a) Reflexes in postganglionic fibres within skin and muscle nerves after mechanical non-noxious stimulation of skin. Exp. Brain Res., 20: 115123. Horeyseck, G. and Janig, W. (1974b) Reflexes in postganglionic fibres within skin and muscle nerves after noxious stimulation of skin. Exp. Brain Res., 20: 125-134. Horeyseck, G. and Janig, W. (1974~)Reflex activity in postganglionic fibres within skin and muscle nerves elicited by somatic stimuli in chronic spinal cats. Exp. Brain Res., 21: 155-168. Inokuchi, H., Yoshimura, M., Yamada, S., Polosa, C. and Nishi, S. (1992) Fast excitatory postsynaptic potentials and the responses to excitant aminoacids of sympathetic preganglionic neurons in the slice of the cat spinal cord. Neuroscience, 46: 657467. Janig, W. (1975) Central organization of somatosympathetic reflexes in vasoconstrictor neurones. Brain Res., 87: 305312. Janig, W. (1985) Organization of the lumbar sympathetic outflow to skeletal muscle and skin of the cat hindlimb and tail. Rev. Physiol. Biochem. Pharmacol., 102: 119-213. Janig, W. (1986) Spinal cord integration of visceral sensory systems and sympathetic nervous system reflexes. Prog. Brain Res., 67: 255-277. Janig, W. (1988) Pre- and postganglionic vasoconstrictor neurons: differentiation, types, and discharge properties. Annu. Rev. Physiol., 50: 525-539. Janig, W. (1995) Ganglionic transmission in vivo. In: E.M. McLachlan (Ed.), Autonomic Ganglia. The Autonomic Nervous System. Harwood, Chur, Switzerland, pp. 349-395. Janig, W. and Koltzenburg, M. (1991) Plasticity of sympathetic reflex organization following cross-union of inappropriate nerves in the adult cat. J . Physiol. (London), 436: 309-323. Janig, W. and Kiimmel, H. (1977) Functional discrimination of postganglionic neurons to the cat’s hindpaw with respect to the skin potentials recorded from the hairless skin. Pfliigers Arch., 371: 217-225.

Janig, W. and Kiimmel, H. (1981) Organization of the sympathetic innervation supplying the hairless skin of the cat’s paw. J. Auton. Nerv. System, 3: 215-230. Janig, W. and McLachlan, E.M. (1987) Organization of lumbar spinal outflow to distal colon and pelvic organs. Physiol. Rev., 67: 1332-1404. Janig, W. and McLachlan, E.M. (1992a) Characteristics of function-specific pathways in the sympathetic nervous system. Trends Neurosci., 15: 475-48 1. Janig, W. and McLachlan, E.M. (1992b) Specialized functional pathways are the building blocks of the autonomic nervous system. J. Auton. Nerv. System, 41: 3-14. Janig, W. and RLth, B. (1977) Electrodermal reflexes in the cat’s paws elicited by natural stimulation of skin. Pfliigers Arch., 369: 27-32. Janig, W. and Rath, B. (1980) Effects of anaesthetics on reflexes elicited in the sudomotor system by stimulation of Pacinian corpuscles and of cutaneous nociceptors. J. Auron. Nerv. System, 2: 1-14. Janig, W. and Spilok, N. (1978) Functional organization of the sympathetic innervation supplying the hairless skin of the hindpaws in chronic spinal cats. Pfliigers Arch., 377: 25-3 1. Janig, W., Schmidt, M., Schnitzler, A. and Wesselmann, U. (1991) Differentiation of sympathetic neurones projecting in the hypogastric nerves in terms of their discharge patterns in cats. J. Physiol. (London), 437: 157-179. Janig, W., Sundlaf, G. and Wallin, B.G. (1983) Discharge patterns of sympathetic neurons supplying skeletal muscle and skin in man and cat. J. Auton. Nerv. System, 7: 239256. Jankowska, E. and Lundberg, A. (1981) Interneurones in the spinal cord. Trends Neurosci., 4: 230-233. Keast, J.R., McLachlan, E.M. and Meckler, R.L. (1993) Relation between electrophysiological class and neuropeptide content of guinea pig sympathetic prevertebral neurons. J. Neurophysiol., 69: 384-394. Keef, K.D. and Kreulen, D.L. (1986) Venous mechanoreceptor input to neurones in the inferior mesenteric ganglion of the guinea-pig. J. Physiol. (London), 377: 49-59. Koshiya, N., Huangfu, D. and Guyenet, P.G. (1993) Ventrolateral medulla and sympathetic chemoreflex in the rat. Brain Res., 609: 174-184. Kreulen, D.L. and Peters, S. (1986) Non-cholinergic transmission in a sympathetic ganglion of the guinea-pig. J. Physiol. (London), 374: 315-334. Kiimmel, H. (1983) Activity in sympathetic neurons supplying skin and skeletal muscle in spinal cats. J. Auton. Nerv. System, 7: 319-327. Loewy, A.D. and. Spyer, K.M. (1990) Central Regulation of Autonomic Functions. University Press, New York. LovCn, C. (1866) Uber die Erweiterung von Arterien in Folge einer Nervenerregung. Ber. Verh. k8nigL-suchs. Ges. Wiss.: Math.-phys. Classe, 18: 85-1 10.

76 Lovick, T.A. (1987) Differential control of cardiac and vasomotor activity by neurones in nucleus paragigantocellularis lateralis in the cat. J. Physiol. (London), 389: 23-35. Luff, S.E. and McLachlan, E.M. (1989) Frequency of neuromuscular junctions on arteries of different dimensions in the rabbit, guinea pig and rat. Blood Vessels, 26: 95-106. Lundberg, J.M. (1981) Evidence for coexistence of vasoactive intestinal polypeptide (VIP) and acetylcholine in neurons of cat exocrine glands. Morphological, biochemical and functional studies. Acra Physiol. Scand. Suppl., 496: 1-57. Lundberg, J.M. and HGkfelt, T. (1986) Multiple co-existence of peptides and classical transmitters in peripheral autonomic and sensory neurons: functional and pharmacological implications. Prog. Brain Res., 68: 241-262. Mathias, C.J. and Frankel, H.L. (1992) Autonomic disturbances in spinal cord lesions. In: R. Bannister and C.J. Mathias (Eds.), Autonomic Failure. Oxford University Press, Oxford, pp. 839-881. McAllen, R.M. (1986a) Identification and properties of subretrofacial bulbospinal neurones: a descending cardiovascular pathway in the cat. J. Aufon. Nerv. System, 17: 151-164. McAllen, R.M. (1986b) Action and specificity of ventral medullary vasopressor neurones in the cat. Neuroscience, 18: 51-59. McAllen, R.M. (1987) Central respiratory modulation of subretrofacial bulbospinal neurones in the cat. J. Physiol. (London), 388: 533-545. McAllen, R.M. (1992) Actions of chemoreceptors on subretrofacial bulbospinal neurons in the cat. J. Auton. Nerv. System, 40: 181-188. McAllen, R.M. and Dampney, R.A. (1989) The selectivity of descending vasomotor control by subretrofacial neurons. Prog. Brain Res., 81: 233-242. McAllen, R.M. and May, C.N. (1994a) Effects of preoptic warming on subretrofacial and cutaneous vasoconstrictor neurons in anaesthetized cats. J. Physiol. (London), 481: 719-730. McAllen, R.M. and May, C.N. (1994b) Differential drives from rostra1 ventrolateral medullary neurons to three identified sympathetic outflows. Am. J. Physiol., 277: R935R944. McAllen, R.M., Habler, H.-J., Michaelis, M., Peters, O.A. and Janig, W. (1994) Monosynaptic excitation of preganglionic vasomotor neurons by subretrofacial (RVLM) neurons. Brain Res., 634: 227-234. McAllen, R.M., May, C.N. and Shafton, A.D. (1995) Functional anatomy of sympathetic premotor cell groups in the medulla. Clin. Exp. Hyper., 17: 209-221. McLachlan, E.M. (1995) Ganglionic Transmission. The Autonomic Nervous System, Vol. 6, Harwood, Chur, Switzerland. McLachlan, E.M. and Hirst, G.D. (1980) Some properties of preganglionic neurons in upper thoracic spinal cord of the cat. J. Neurophysiol., 43: 1251-1265.

McLachlan, E.M. and Meckler, R.L. (1989) Characteristics of synaptic input to three classes of sympathetic neurone in the coeliac ganglion of the guinea-pig. J. Physiol. (London), 415: 109-129. Michaelis, M., Boczek-Funcke, A., Habler, H.-J. and Janig, W. (1996) Vesico-sympathetic reflexes in muscle and skin hindlimb nerves depend entirely on the intact sacral bladder innervation. Neurosci. Lett., submitted. Morrison, S.F., Callaway, J., Milner, T.A. and Reis, D.J. (1991) Rostra1 ventrolateral medulla: a source of the glutamatergic innervation of the sympathetic intermediolateral nucleus. Brain Res., 562: 126-135. Polson, J.W., Halliday, G.M., McAllen, R.M., Coleman, M.J. and Dampney, R.A.L. (1992) Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus. J. Auton. Nerv. System, 38: 117-138. Richter, D.W. and Spyer, K.M. (1990) Cardiorespiratory control. In: A.D. Loewy and K.M. Spyer ( U s . ) , Central regulation of Autonomic Functions. Oxford University Press, New York, pp. 189-207. Richter, D.W., Spyer, K.M., Gilbey, M.P., Lawson, E.E., Bainton, C.R. and Wilhelm, Z. (1991) On the existence of a common cardiorespiratory network. In: H.P. Koepchen and T. Huopaniemi (EMS.), Cardiorespiratory and Motor Coordination. Springer, Berlin, pp. 118-130. Sato, A. (1972) The relative involvement of different reflex pathways in somato-sympathetic reflexes, analyzed in spontaneously active single preganglionic sympathetic units. Pfliigers Arch., 333: 70-81. Sato, A. and Schmidt, R.F. (1971) Spinal and supraspinal components of the reflex discharges into lumbar and thoracic white rami. J. Physiol. (London), 212: 839-850. Sato, A. and Schmidt, R.F. (1973) Somatosympathetic reflexes: afferent fibers, central pathways, discharge characteristics. Physiol. Rev., 53: 916-947. Schomburg, E.D. (1990) Spinal sensorimotor systems and their supraspinal control. Neurosci. Res., 7: 265-340. Shafton, A.D. and McAllen, R.M. (1994) Premotor neurons of the cat sudomotor pathway. Proc. Aust. Neurosci. SOC., 5 : 183. Shafton, A.D., Oldfield, B.J. and McAllen, R.M. (1992) CRFlike immunoreactivity selectively labels preganglionic sudomotor neurons in cat. Brain Res., 599: 253-260. Shen, E., Mo. N. and Dun, N.J. (1990) APV-sensitive dorsal root afferent transmission to neonate rat sympathetic preganglionic neurons in vitro. J. Neurophysiol., 64: 991999. Simon, E. (1974) Temperature regulation: the spinal cord as a site of extrahypothalamic thermoregulatory functions. Rev. Physiol. Biochem. Pharmacol., 71: 1-76. Simon, E., Pierau, F.K. and Taylor, D.C. (1986) Central and peripheral thermal control of effectors in homeothermic temperature regulation. Physiol. Rev., 66: 235-300.

77 Skok, V.I. (1980) Ganglionic transmission: morphology and physiology. In: D.A. Kharkevich (Ed.), Pharmacology of Ganglionic Transmission. Springer-Verlag, Berlin, pp. 939. Skok, V.I. (1986) Spontaneous and reflex activities: general characteristics. In: A.G. Karczmar, K. Koketsu and S. Nishi (Eds.), Autonomic and Enteric Ganglia. Plenum Press, New York, pp. 425-438. Skok, V.I. and Ivanov, A.Y. (1987) Organization of presynaptic input to neurones of a sympathetic ganglion. In: J. Ciriello, F.R. Calaresu, L.P. Renaud and C. Polosa (Eds.), Organization of the Autonomic Nervous System: Central and Peripheral Mechanisms. Alan Liss, New York, pp. 3746. Stjbne, L. and Lundberg, J.M. (1986) On the possible roles of noradrenaline, adenosine 5’-triphosphate and neuropeptide Y as sympathetic cotransrnitters in the mouse vas deferens. In: T. Hokfelt, K. Fuxe and B. Pernow (Eds.), Coexistence of Neuronal Messengers: a New Principle in Chemical Transmission. Progress in Bruin Research, Vol. 68. Elsevier, Amsterdam, pp. 263-278. Stjernberg, L. and Wallin, B.G. (1983) Sympathetic neural outflow in spinal man. A preliminary report. J. Auton. Nerv. System, 7: 313-318. Stjernberg, L., Blumberg, H. and Wallin, B.G. (1986) Sympathetic activity in man after spinal cord injury. Outflow to muscle below the lesion. Bruin, 109: 695-715. Stoddard, S.L., Tyce, G.M., Carmichael, S.W., Gaumann, D.M. and Yaksh, T.L. (1988) Effect of acute and chronic spinal transection on evoked secretion of adrenal medullary catecholamines in the cat. J. Auton. Nerv. System, 23: 175179.

Stoddard, S.L., Tyce, G.M., Cook, J.A., Gaumann D.M. and Yaksh, T.L. (1992) Adrenal medullary secretions with splanchnic stimulation in spinal cats. J. Auton. New. System, 38: 105-1 16. Strack, A.M., Sawyer, W.B., Hughes, J.H., Platt, K.B. and Loewy, A.D. (1989a) A general pattern of CNS innervation of the sympathetic outflow demonstrated by transneuronal pseudorabies viral infections. Brain Res., 491: 156-162. Strack, A.M., Sawyer, W.B., Platt, K.B. and Loewy, A.D. (1989b) CNS cell groups regulating the sympathetic outflow to adrenal gland as revealed by transneuronal cell body labeling with pseudorabies virus. Bruin Res., 491: 274-296. Szurszewski, J.H. (1981) Physiology of mammalian prevertebra1 ganglia. Annu. Rev. Physiol., 43: 53-68. Ulman, L.G., Potter, E.K. and McCloskey, D.I. (1992) Effects of sympathetic activity and galanin on cardiac vagal action in anaesthetized cats. J. Physiol. (London), 448: 225-235. Wallin, B.G. and Stjernberg, L. (1984) Sympathetic activity in man after spinal cord injury. Outflow to skin below the lesion. Brain, 107: 183-198. Weaver, L. and Polosa, C. (1996) Spinal cord circuits providing control of sympathetic preganglionic neurones. In: D. Jordan (Ed.), Central Nervous Control of Autonomic Function. Harwood, Chur, Switzerland. Zagon, A. and Smith, A.D. (1993) Monosynaptic projections from the rostra1 ventrolateral medulla oblongata to identified sympathetic preganglionic neurons. Neuroscience, 54: 729-743.