Chapter 7 Pain and the spinal cord

Handbook of Clinical Neurology, Vol. 81 (3rd series) Pain F. Cervero, T.S. Jensen, Editors © 2006 Elsevier B.V. All rights reserved

Spinal cord processing Chapter 7

Pain and the spinal cord FERNANDO CERVERO* Anesthesia Research Unit (Faculty of Medicine), Faculty of Dentistry and McGill Center for Pain Research, McGill University, Montréal, Québec, Canada

7.1. Introduction For over 100 years the spinal dorsal horn and its brain stem equivalent, the spinal nucleus of the trigeminal nerve, have been regarded as important points of processing of pain-related information. The relevance of these regions to pain was identified in the early years of the 20th century and was further enhanced by the gate control theory of pain mechanisms (Melzack and Wall, 1965) which proposed a modulatory mechanism for pain sensations at the first synapse of the nociceptive pathway in the dorsal horn of the spinal cord. The identification of the first synaptic relay as a potential pain modulation site focused the attention of scientists and clinicians on the spinal cord as a target of analgesic procedures. A great deal of work in the last 40 years has been directed to unraveling the anatomical connections of primary afferent terminals and secondorder neurons in the dorsal horn, to the study of the networks of cells, both interneurons and projecting neurons, that receive and process nociceptive information in the cord, to the identification of the neurochemicals contained by afferent fibers and spinal neurons and, more recently, to the role of non-neural elements such as glia in spinal nociceptive processing. Much of the effort directed to the analysis of spinal cord mechanisms of pain processing is based on the notion of “sensitization” of pain pathways whereby intense nociceptive input triggers central changes in spinal neurons that in turn cause an amplification of pain sensations (hyperalgesia) (Treede et al., 1992; Woolf and Salter, 2000). Many studies have focused on the spinal cord as a source of central sensitization, but it is also

known that increases in neuronal excitability can occur throughout the brain, that supraspinal modulation can powerfully influence transmission through the spinal cord and that spinal–supraspinal loops can make a major contribution to an enhanced excitability of the nociceptive pathway (Ossipov et al., 2000; Porreca et al., 2002). The aim of this chapter is to review what we know about dorsal horn anatomy and physiology in the light of what we also know about the relevance of this region to pain processing. Complex pain syndromes, particularly those involving chronic and neuropathic lesions have also been studied by looking mainly at spinal cord processing and the validity of this approach needs to be examined. There is no doubt that pain related information is integrated and modulated at the spinal dorsal horn but the question is if this modulation is sufficiently elaborate to consider this region of the nervous system a major center for pain processing. 7.2. Historical background The so-called Bell–Magendie law, formulated at the beginning of the 19th century, identified the dorsal portion of the spinal cord as the target of sensory nerve fibers and the ventral portion as the origin of the motor output of the cord. Ever since, the dorsal horn of the spinal cord has been known to be the area of projection of primary afferent fibers from skin, muscle and viscera with the spinal nucleus of the trigeminal nerve being the brain stem equivalent for the afferent innervation of the head and face. By the end of the 19th century much detailed information was available about the patterns of termination of primary afferent fibers in the spinal

*Correspondence to: Fernando Cervero, Anesthesia Research Unit, McGill University, McIntyre Medical Building, Room 1207, 3655 Promenade Sir William Osler, Montreal, Quebec H3G 1Y6, Canada. E-mail: [email protected], Tel: +1-514398-5764, Fax: +1-514-398-8241.

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dorsal horn and the different histological types of neurons in this region (Fig. 7.1) (see Cervero and Iggo, 1980, for a detailed review). A series of morphological studies of the spinal dorsal horn by Ranson and colleagues in the mid-1910s (Ranson, 1914; Ranson and von Hess, 1915; Ranson and Billingsley, 1916) established that the superficial layers of the spinal dorsal horn were the area of termination of unmyelinated and small myelinated afferent fibers and that these fibers reached the dorsal horn via Lissauer’s tract, an intraspinal bundle known to contain fine afferent fibers as well as axons of superficial dorsal horn cells (see Cervero and Iggo, 1980, for references). These observations were interpreted in the light of von Frey’s ideas on the specificity of cutaneous sensory receptors which attributed to unmyelinated and small myelinated afferents a function as “pain fibers” (von Frey, 1895). Ranson and Billingsley (1916) and Ranson and von Hess (1915) then showed that sections of the lateral division of the dorsal roots, which carried fine afferent fibers to the superficial dorsal horn, resulted in the abolition of the vasomotor and respiratory reactions to noxious stimuli known as pseudo-affective reflexes

(Sherrington, 1906) and thought to be components of nociceptive processing. From then on until the 1960s the general consensus was that fine afferent fibers from the skin, muscle and viscera that carried “pain” impulses terminated in the superficial dorsal horn where they relayed their information on to second-order projecting neurons whose axons reached the brain via antero-lateral pathways (Earle, 1952). The spinal cord was considered to be an important relay nucleus for pain-related information and for the sensory-motor integration of nociceptive reflexes; however, no special consideration was given to this spinal relay in the modulation of pain sensation. The publication of the gate control theory of pain mechanisms (Melzack and Wall, 1965) shifted the role of the spinal dorsal horn from a simple relay nucleus to a major modulatory center in pain processing. A detailed mechanism of presynaptic interactions between the terminals of large and fine afferents was proposed and the neurons of the superficial layers of the spinal dorsal horn were given a protagonist role in the modulation of pain sensitivity. Although the actual mechanism proposed was the subject of controversy (see Nathan, 1976, for a

Fig. 7.1. A drawing by Ramón y Cajal (1890) of the different cells and fibers seen in a transverse section of the dorsal horn of a newborn dog (Golgi stain). The drawing illustrates marginal (O, Z), limiting (A, F) and central (S, T, H) cells of the superficial dorsal horn as well as larger cells (L, N, Y) in the deeper dorsal horn some of which have axons that project through the white matter.

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detailed review) the concept of “gating” of afferent inputs in the spinal dorsal horn continues to form the basis of current interpretations of the spinal cord as a center for pain modulation (see Cervero, 2005, for a recent discussion). The gate theory was originally proposed as a thesis against the specificity of a pain pathway and in favor of a pattern interpretation of pain perception. Much of the debate between specificity and pattern is now outdated as both specific and non-specific neurons are known to coexist in nociceptive pathways (Cervero and Laird, 1991). Nevertheless, an enduring legacy of the gate theory has been the prominence given to the spinal cord as a center for modulation of pain related information. The gate theory also stimulated new pain therapies based on targeting the spinal cord. Peripheral nerve and dorsal column electrical stimulators were used in an attempt at “closing the gate” by activating large afferent fibers. Some of these stimulators have evolved into implantable permanent devices used to control or modulate several forms of chronic pain. Pharmacological tools have also been used by injecting intrathecally a variety of compounds, from local anesthetics to opioids. In some cases these injections are made semi-permanent by the use of implantable pumps that can be regulated by the patients themselves. Yet the efficacy of electrical stimulators and of intrathecal long-term treatment in the management of chronic pain is still not clear. Whether the beneficial effects of spinal procedures are mainly due to a direct interference with a spinal pain system is also unclear. The original mechanism proposed by the gate theory has been shown to be only a component of a more complicated interaction of afferent inputs and descending outputs and the neurochemical analysis of the first synaptic relay in the cord has produced a very long list of potential neurotransmitters and modulators. The idea of a spinal site of modulation of pain sensitivity remains in force but the experimental data have not produced a simple or even a uniform picture. 7.3. Dorsal horn anatomy There are many regions of the central nervous system (CNS) that show an orderly and almost geometrical internal organization (the cerebellar cortex, the hippocampus, the cerebral cortex) but the dorsal horn of the spinal cord is not one of them. We have known for a long time that this is the area of termination of most primary sensory afferents and the location of the cell bodies of many ascending neurons. However, the organization of the connections between these two elements does not follow a clear and orderly pattern and there are many interneurons in this region that establish

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complex networks between primary afferents and projection neurons. There is also a lack of correlation between morphology and physiology with similar neurons having very different functional properties and highly dissimilar cells responding uniformly to peripheral inputs. If we consider the distribution of the neurochemical contents of primary afferents and dorsal horn cells then the picture gets even more complex and less organized. There are, however, a few general principles of organization in the spinal dorsal horn. The superficial layers of the gray matter (lamina I or marginal zone and lamina II or substantia gelatinosa) are easily identifiable and receive projections mainly from small afferent fibers. The center of the dorsal horn (laminas III, IV and V, also known as the nucleus proprius) is less organized and many different types of neuron and of primary afferent terminal, from large as well as small afferents, can be identified there. As for three-dimensional organization, all attempts at finding spatial correlations between neurons and afferents in the dorsal horn – longitudinally as well as transversally – have failed. The introduction of techniques based on intracellular recording of the responses of neurons to peripheral stimuli and the subsequent injection of tracers into these cells produced, in the 1980s, a fairly complete picture of the morphological and functional classes of afferent terminal and spinal neurons in the dorsal horn (Fig. 7.2) (Brown, 1981; Willis and Coggeshall, 1991). Large myelinated (A-β) fibers connected to low-threshold cutaneous mechanoreceptors terminate in some or all of laminas III, IV, V and the dorsal part of lamina VI (Fig. 7.2) (Brown, 1981). There is essentially no input from these axons to laminas I or II. Similarly, lowthreshold cutaneous mechanoreceptors with fine myelinated (A-δ) afferent fibers terminate in lamina III and its border with lamina II (Light and Perl, 1979). Nociceptors in general are connected to unmyelinated or fine myelinated axons. Cutaneous high-threshold mechanoreceptors with A-δ axons terminate in lamina I, and most have branches terminating in lamina V. Some also have terminals in the mid-line and contralaterally in laminas I and V (Light and Perl, 1979). Cutaneous nociceptors with unmyelinated (C) fibers terminate predominantly in lamina II, although some also have terminals in laminas I and/or III–IV (Sugiura et al., 1986). Nociceptive afferents from deep tissues (muscle and joints) terminate in laminas I, V–VI and the dorsal layers of lamina VII (Mense, 1986). There are no projections to laminas II, III or IV from deep tissues. Fine afferents from viscera project to lamina I and bilaterally to laminas V and X (Cervero and Tattersall, 1986; Sugiura et al., 1989, 1993; Mizumura et al., 1993). Some unmyelinated visceral afferents also have

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Fig. 7.2. Schematic diagram of the neuronal organization of, and afferent input to, the first four layers of the dorsal horn. The following types of neuron are illustrated (from top to bottom): a marginal cell, a limiting cell, two central cells and two neurons of the deeper dorsal horn. (From Cervero and Iggo, 1980.)

a few collaterals in lamina II, but this lamina seems to be mostly concerned with the processing of input from skin nociceptors with unmyelinated afferent fibers. The general patterns of termination of afferents within the trigeminal nucleus caudalis are similar to those seen in the dorsal horn of the spinal cord (Dubner and Bennett, 1983). Thus, large myelinated afferents terminate in laminas III–VI and fine myelinated and unmyelinated afferents terminate in the superficial layers. Some nociceptive trigeminal afferent fibers also terminate in the nucleus interpolaris, a more rostral structure in the trigeminal complex. Afferents from the specialized structures of the head innervated by the trigeminal nerve, such as the cornea and teeth, terminate in discrete areas of the trigeminal nucleus caudalis and interpolaris. As for second-order neurons, several well-defined morphological types have been identified but once again, there is no correlation between morphological types and functional responses. Lamina I contains large neurons (Waldeyer cells) as well as some smaller interneurons (limiting or stalked cells – see Cervero and Iggo, 1980; and Cervero, 1986, for details). The substantia gelatinosa (lamina II) contains large numbers of very small neurons described by Ramón y Cajal (1909) as “central cells”. The deeper layers contain

larger neurons without specific morphological characteristics, some of which show extensive dendritic trees with a dorsal orientation (Brown, 1981). There is also no correlation between cell types and the presence of long axonal projections to supraspinal regions. Projection neurons are intermingled with interneurons and lack specific morphological characteristics that would make them easily identifiable as neurons with long ascending axons. The situation becomes more complex if we take into account the transmitters and other neurochemicals identified in primary afferent terminals and secondorder neurons. The reader is referred to Chapter 6 by A. J. Todd, which contains a detailed description of the various types of dorsal horn neurons and primary afferent terminals and their neurochemical contents. Although a few patterns of organization have been identified it is true to say that the dorsal horn crossroads between primary afferents and second-order neurons lacks the precise and well-organized patterns shown by other regions of the CNS. 7.4. Dorsal horn electrophysiology There are two major classes of spinal neuron concerned with the processing of nociceptive information: wide

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dynamic range (WDR) and nociceptor specific (NS) cells. Although there is a general consensus about the existence of these two classes of dorsal horn neuron there remain some open questions regarding their functional role in pain processing. WDR neurons (also known in the literature as Class 2 cells, multireceptive cells and convergent neurons – see Cervero, 1986, for details) were the first to be identified (Kolmodin and Skoglund, 1960; Wall, 1960) and their unspecific responses to a variety of peripheral stimuli made them the protagonists of the spinal mechanism proposed by the gate theory (labeled then as “T” cells). They respond to the activation of all classes of peripheral sensory receptor: nociceptors, low-threshold mechanoreceptors and thermoreceptors (Fig. 7.3). They have larger receptive fields in the skin and also receive inputs from muscle and visceral receptors. They show a great deal of plasticity in their responses and are subject to considerable descending modulation. They are the most numerous class of dorsal horn neuron and, although they have not been associated with any specific morphological type, they are generally large multipolar cells with extensive dendritic trees (Cervero, 1986). Their high numbers (and their large size, which makes them relatively easy to record from) have placed WDR cells at the center of most interpretations of pain processing in the dorsal horn. The other nociceptive dorsal horn neurons are those known as nociceptor-specific, Class 3 cells or highthreshold neurons (Cervero, 1986). They were first

described by Christensen and Perl (1970) and their properties have been studied in detail by several laboratories (i.e. Cervero et al., 1976; Light et al., 1987; Craig et al., 2001). They have functional responses restricted to the activation of peripheral nociceptors from the skin, muscle and viscera (Fig. 7.3). They have smaller receptive fields and show less plasticity than WDR cells. They are abundant in the superficial layers of the dorsal horn, mainly in lamina I although they can also be found in smaller numbers in the deeper dorsal horn. They have been identified with the morphological type known as Waldeyer’s neuron or marginal cell although there is some debate as to whether all Waldeyer’s cells are NS and vice versa (Lima and Coimbra, 1986; Zhang et al., 1996). Their small size and their relatively low numbers have made recording from these cells more difficult than from WDR neurons, which goes some way to explain the relative protagonism of the latter in interpretations of pain processing. There are other functional types of dorsal horn neuron generally regarded as not immediately concerned with nociceptive processing. Some have inputs exclusively from low-threshold mechanoreceptors in the skin (low-threshold cells or Class 1 neurons) and others are driven by muscle proprioceptors and other sensory receptors concerned with locomotion and motor control. Finally, consideration should also be given to non-neuronal elements present in the dorsal horn (astrocytes, neuroglia and microglia) which have

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Fig. 7.3. Oscilloscope traces showing the responses of two different dorsal horn neurons in the spinal cord of a rat to natural stimulation of their cutaneous receptive fields. Upper trace: a WDR (Class 2) neuron responding to both innocuous and noxious stimuli. Lower trace: a NS (Class 3) neuron responding only to noxious stimuli. (From Cervero and Laird, 1996a.)

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recently been implicated in nociceptive processing (Watkins et al., 2001; Tsuda et al., 2003). The relatively simple classification of nociceptive dorsal horn neurons into WDR and NS cells becomes more complicated when considering subclasses according to the source of peripheral input (skin, muscle or viscera) or other properties such as the presence of inhibitory receptive fields. This is particularly important when considering nociceptive processing since neuronal inhibition can play a role as important as that of excitation. Inhibitory responses were given a prominent role in the interpretation of the functional role of neurons in the substantia gelatinosa (see Cervero, 2005, for a recent review) although the specific functions of substantia gelatinosa neurons in pain processing remain unclear (Lu and Perl, 2003; Kato et al., 2004). Both WDR and NS neurons are found in most ascending tracts projecting to supraspinal sites as well as among many classes of spinal interneuron (see Ch. 8). This shows, once again, the difficulties in trying to establish patterns of association between the functional responses of a given neuron, the destination of its axon and its possible functional role. The multiplicity of functional roles of WDR and NS neurons was shown in a study of the ability of dorsal horn cells to encode small changes in the intensity of a noxious mechanical stimulus (Laird and Cervero, 1991). Whether the neurons were WDR or NS did not influence, positively or negatively, their ability to encode these noxious events, which demonstrates that neurons with similar input properties and location are not necessarily a homogeneous group in terms of their processing of nociceptive stimuli.

central components (Woolf and Salter, 2000). The hyperexcitability shown by peripheral nociceptors after an injury has been called “nociceptor sensitization” and is thought to be responsible for the hyperalgesia found at the site of injury (primary hyperalgesia) (Treede et al., 1992). By analogy to the process of peripheral sensitization a similar mechanism of “central sensitization” has been proposed to operate in the CNS. Central sensitization would thus be a mechanism of enhanced excitability of central nociceptive neurons triggered and maintained by a peripheral injury or neuropathic lesion and responsible for the increased pain sensitivity that develops in areas distant from the site of injury (secondary hyperalgesia). This notion, which in one form or another has been with us for almost 100 years (MacKenzie, 1909; Cervero, 2000), has been applied to the study of nociceptive systems in the dorsal horn with the aim of identifying which mechanisms could be responsible for the enhanced excitability of spinal neurons. In some cases the object of study has not been the sensory output of the spinal cord but its motor outflow, analyzed in the form of withdrawal reflexes. Conclusions have been extracted about sensitization of pain processing based on the study of spinal motor outflow even though the two processes do not necessarily share a common mechanism (Schouenborg et al., 1995). Those studies that have directly examined the responses of dorsal horn neurons have focused on several parameters of spinal cord excitability which have been identified as evidence of central sensitization in the dorsal horn: (i) the presence of “wind-up”,

7.5. The dorsal horn as a pain modulation center

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Unlike all other sensory processes, pain is a non-adaptative sensation. If we are subjected to a prolonged visual, auditory or tactile stimulus of constant intensity, the stimulus will be progressively perceived as less and less intense to the point that, after a while, we cease to perceive it. We simply adapt to it. Pain is the only sensory process which not only does not adapt to a prolonged stimulus but becomes progressively more intense as the stimulus continues. An intense noxious stimulus leading to the production of an injury evokes sensory changes characterized by an amplification of the painful sensation, such that innocuous stimuli now evoke pain (allodynia) and noxious stimuli evoke a more intense pain than before (hyperalgesia) (Fig. 7.4). This amplification of pain sensitivity evoked by an injury has been interpreted in neurobiological terms as the consequence of the sensitization of the nociceptive pathway, either of its peripheral receptors or of its

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Fig. 7.4. Diagram illustrating the changes in pain sensation induced by injury. The normal relationship between stimulus intensity and the magnitude of pain sensation is represented by the curve at the right-hand side of the figure. Pain sensation is only evoked by stimulus intensities in the noxious range (the vertical dotted line indicates the pain threshold). Injury provokes a leftward shift in the curve relating stimulus intensity to pain sensation. Under these conditions, innocuous stimuli evoke pain (allodynia). (From Cervero and Laird, 1996b.)

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(ii) increases in receptive field size of dorsal horn neurons and (iii) plasticity of the peripheral inputs to dorsal horn cells.

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7.5.1. “Wind-up” of dorsal horn neurons The phenomenon known as “wind-up” (Mendell, 1966) describes a progressive increase in the C-fiber mediated responses of dorsal horn neurons on repetitive electrical stimulation of afferent nerves. Dorsal horn neurons can be divided into those whose afferent input is mediated exclusively by A-fibers and those with a mixture of A and C afferent fibers (Fig. 7.5). When afferent nerves are stimulated electrically at frequencies greater than 0.5 Hz the responses of dorsal horn neurons to activation of A-fibers remains stable whereas their responses to stimulation of C-fibers increases progressively during the first 10 to 15 stimuli (Fig. 7.6). This progressive increase in responsiveness to C afferent volleys of dorsal horn neurons is the original “wind-up” although the term has also been applied to similar increases in excitability shown by motoneurons and even by withdrawal reflexes (Herrero et al., 2000). Not all neurons with a C-fiber input show “wind-up”. WDR cells are more likely to express “wind-up” than NS neurons, the latter often showing a brief, time-locked and stable response to C-fiber stimulation (Fig. 7.5). “Wind-up” has often been regarded as an indication of central sensitization although the process itself is rather short lived and does not imply a prolonged increase in neuronal excitability (Laird et al., 1995; Woolf, 1996). The sensitivity of “wind-up” to pharmacological manipulation of transmitters associated with central sensitization (like NMDA receptors and tachykinins,

Fig. 7.5. Oscilloscope traces showing the responses of two different dorsal horn neurons in the spinal cord of a rat to electrical stimulation of their receptive fields. Left: a WDR (Class 2) neuron responding to stimulation of both A and C afferent fibers. Right: a NS (Class 3) neuron also responding to stimulation of A and C afferent fibers. Note the differences in the C-fiber responses of both neurons. Time marks: 10 ms for the top traces and 100 ms for the bottom traces. (Unpublished data from the laboratory of F. Cervero and J. M. A. Laird.)

i.e. Woolf and Thompson, 1991) added support to interpretations of “wind-up” as an indicator of the earlier phases of sensitization. Strictly speaking “wind-up” is purely an electrophysiological phenomenon that demonstrates the plasticity of the C-fiber input to some – but not all – dorsal horn cells. It shows that the excitability of these neurons can be increased by repetitive stimulation of nociceptive afferents. This may be due to the properties of the neuronal network, to the neurotransmitters involved or to both. Beyond that it is difficult to attribute a specific function to this phenomenon in the perception of pain or in the development of hyperalgesia. It remains a useful marker of spinal cord excitability and an index of increased responsiveness of some central neurons to noxious stimuli but

"Wind-up" of a nociceptive dorsal horn neuron C-volley

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Fig. 7.6. “Wind-up” of a WDR neuron in the dorsal horn of the spinal cord of the rat. The graph on the left shows a raster display of successive electrical stimuli (applied at 1 Hz) to the sciatic nerve at an intensity maximal for A and C afferent fibers. Note the progressive increase in the number of spikes contained in the late (C-fiber mediated) response. The graph on the right shows the quantitative data for the raster data shown on the left. (Unpublished data from the laboratory of F. Cervero and J. M. A. Laird.)

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should not be considered a direct correlate of increased pain perception in chronic pain states (Woolf, 1996).

Most neurons in sensory-motor pathways show subliminal fringes in their peripheral receptive fields expressed as changes in receptive field size, depending on their level of excitability. This is regarded as a demonstration of their ability to alter their responsiveness in the context of what is happening in adjacent peripheral inputs. Surprisingly, one of the first reports of the afferent properties of dorsal horn cells denied the existence of such subliminal fringes in the dorsal horn and reported that the receptive field areas of dorsal horn neurons were unaffected by post-tetanic potentiation, strychnine, asphyxia, small doses of barbiturate or temperature changes (Wall, 1960). However, some years later it was shown that the receptive fields of dorsal horn neurons were indeed plastic and that this plasticity could be modified by administration of amino acid neurotransmitters in the vicinity of the cell (Zieglgänsberger and Herz, 1971). From then on, the descriptions of changes in receptive field size of dorsal horn neurons have been intimately linked to the analysis of central sensitization in the nociceptive pathway. The size of the receptive fields of dorsal horn neurons has been shown to increase following repetitive electrical stimulation of afferent C-fibers, natural stimulation of nociceptors inside and outside the receptive fields, inflammation of the skin, muscle, joints and viscera and in animals with neuropathic lesions (i.e. Cervero et al., 1984; Cook et al., 1987; Schaible et al., 1987; Hylden, 1989; Hoheisel and Mense, 1990; Woolf and King, 1990; Cervero et al., 1992; Hoheisel et al., 1993; Suzuki et al., 2000). This phenomenon is not restricted to neurons with inputs from afferent C-fibers although it is particularly prevalent in them. On the other hand, some dorsal horn neurons of the NS type and with a clear C-fiber input do not show alterations in the size of their receptive fields after noxious stimulation (Fig. 7.7) (Laird and Cervero, 1989; Garcia-Nicas et al., 2003). The existence of subliminal fringes in the receptive fields of dorsal horn neurons appears to be linked to processes other than having a C-fiber input or even a direct activation by the sensitizing stimulus. Like “wind-up” the increases in receptive field size are sensitive to NMDA receptor blockers and to local or systemic administration of takykinin antagonists (i.e. Woolf and Thompson, 1991; Laird et al., 2000, 2001) which suggests that the two processes are mechanistically linked. Receptive field size increases are also related to increased responsiveness to peripheral inputs and it

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Fig. 7.7. Changes in receptive field size of WDR (Class 2) and NS (Class 3) neurons (from a barbiturate anesthetized rat) over time during a series of noxious pinches (P1, P2 and P3) applied to the receptive field. Note the large changes in the WDR cells and the absence of changes in the NS neuron. (Reproduced from Laird and Cervero, 1989, with permission from The American Physiological Society.)

is often reported that a peripheral injury or inflammation evokes not only an increase in receptive field size but also an enhanced response of the neuron to stimulation of its peripheral drives. The expression of receptive field changes is probably the best indicator of increased excitability of a nociceptive neuron and a good marker of sensitization of the nociceptive pathway. It indicates the removal of inhibition from the cell and the strengthening of the synaptic connections of a neuron with its afferent inputs. A great deal of current work is aimed at the analysis of the cellular and molecular mechanisms of dorsal horn sensitization and several chapters in this volume discuss these studies in detail. The emphasis is on the identification of the transmitters responsible for the initiation of the sensitization process and the molecular cascades and kinases involved in the generation and maintenance of the enhanced excitability of the cells. There are also studies on the role of non-neuronal elements (such as glia) in the generation of sensitization (Watkins et al., 2001; Tsuda et al., 2003; Watkins et al., this volume, Ch. 22) and on the trafficking of glutamate receptors from the cytosol to the membrane of spinal neurons as a way of enhancing synaptic transmission in nociceptive cells (Galan et al., 2004). 7.5.3. Plasticity of afferent inputs to the dorsal horn A popular model for a possible mechanism of touchevoked pain has been the acquisition by NS neurons in the spinal dorsal horn of a novel input from low-threshold mechanoreceptors. The underlying implications of this model are that NS cells play a key role in the perception

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of pain and that much of the processing leading to touch-evoked pain takes place in the spinal dorsal horn. Notwithstanding the simplicity of both assumptions there is evidence that the inputs to dorsal horn neurons are plastic, not only in their strength – and thus in the determination of the size of a receptive field – but also in their quality, i.e. dorsal horn neurons can express different input modalities depending on their level of excitability. The possibility that a NS neuron could respond, following a peripheral injury or a naturopathic lesion, to activation of low-threshold afferents has been taken as evidence that NS and WDR neurons might only be time-dependent expressions of a homogeneous class of dorsal horn cell that responds to noxious or innocuous stimuli depending on the circumstances (Woolf et al., 1994). When it comes to addressing this issue there are essentially three possibilities: (i) all dorsal horn neurons receive inputs from low- and high-threshold receptors and they express them differentially or all together depending on their levels of excitability; (ii) there are some neurons whose peripheral inputs are predominantly from peripheral nociceptors but also include subliminal or weak or indirect inputs from low-threshold afferents that can be expressed after an injury or prolonged noxious stimulation; and (iii) NS neurons do not normally have inputs from lowthreshold mechanoreceptors but can acquire them in

chronic pain states by anatomical reorganization of low-threshold afferent terminals that can then reach and activate these cells. The first possibility is an extreme interpretation of subliminal fringes and implies that all dorsal horn cells are essentially similar. There is considerable evidence showing that the dorsal horn contains several distinct classes of neurons with differential responses to A and C afferent fibers, to various forms of natural stimulation of the periphery and expressing different levels of excitability and modulation (Cervero, 1995). It is therefore not possible to consider all nociceptive cells in the spinal cord as a homogeneous group. However, the second possibility, i.e. that NS cells have a “weak” low-threshold input under normal circumstances that can be enhanced in states of central sensitization has been defended and supported in several studies (Woolf et al., 1994). In a study from our laboratory we have shown that low-threshold inputs to NS cells can be revealed after a peripheral injection of capsaicin in or close to the cutaneous receptive field of the cell and that this process is likely to be mediated by a presynaptic link between low- and high-threshold afferents (Fig. 7.8) (Garcia Nicas et al., 2003). This mechanism gives low-threshold afferents access to the nociceptive channel, which could account for the initial processing of touch-evoked pain in hyperalgesic states (Cervero and Laird, 1996b).

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A C Fig. 7.8. A: Responses of a nociceptor-specific (NS) neuron to innocuous (top) and noxious (bottom) stimulation of its receptive field (shown in C). Also shown in C is the location of the NS neuron in the superficial dorsal horn of the spinal cord. This neuron did not respond to innocuous (c: cotton bud and b: brushing) stimulation of the receptive field in the control situation but did so after intradermal injection of capsaicin in its receptive field (injection site shown as CAP in C). Each stimulation period is 20s long. The response of the neuron to pinch (p in bottom diagrams of A, also 20 s long) was enhanced after capsaicin injection. B shows the response of the neuron to the capsaicin injection. (From Garcia Nicas et al., 2003.)

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The third possibility – anatomical reorganization in chronic pain states – has also been explored and proposed (Woolf et al., 1992) but the accuracy of the methods used in this study has been challenged (Hughes et al., 2003; Shehab et al., 2004) and the question remains open. In any case there is a strong current of opinion supporting the idea that an access by low-threshold afferents to NS neurons in the spinal cord could explain touch-evoked pain. Whether this phenomenon needs to be explained by a mechanism at the first synaptic relay is open to interpretation. 7.6. Afferent and descending modulation in the dorsal horn The potential of the dorsal horn for modulating nociceptive information is based on what Sherrington (1906) called “integration”. This is the ability of neural networks to generate output signals that are not just a mere relay of their inputs but are also influenced by the past and present history of the network so that the final message can be different depending on the level of afferent excitability. This can be achieved by the actions of neurotransmitters, by the cellular properties of individual neurons, by the properties of the neuronal network or, more likely, by a combination of all these factors. In the case of pain processing, the integrative functions of the dorsal horn are related to the inhibition of nociceptive input in some cases (reduction of pain sensations during stress) and to the enhancement of pain sensitivity in others (sensitization leading to persistent pain and hyperalgesia). If evidence of integration can be found in the dorsal horn then its role beyond that of a simple relay nucleus would be established. Traditionally, modulation in the dorsal horn has been divided into two main areas depending on whether the source of integration is limited to local mechanisms (segmental or local modulation) or involves supraspinal elements (descending modulation). 7.6.1. Segmental and local interactions It is a common experience that the pain of an acute injury can be relieved by tactile stimulation of the area adjacent to the injury site. This observation has produced everyday expressions such as “licking one’s wounds” or “rubbing it better”. The gate theory (Melzack and Wall, 1965) proposed a mechanism in the spinal cord that could explain this observation, based on presynaptic local interactions between the terminals of thick and thin afferents and involving the neurons of the substantia gelatinosa. Since then much evidence has been obtained supporting the idea of interactions between the spinal terminals of low-threshold mechanoreceptors and of nociceptors either pre- or post-synaptically and involving

dorsal horn neurons. These data are generally grouped under the heading of local or segmental modulation of the nociceptive input to the CNS and implies a certain amount of processing of pain-related signals at the level of the first synapse. Segmental modulation of nociceptive afferent impulses is usually interpreted as due to postsynaptic inhibitory mechanisms acting on second-order neurons and involving inhibitory neurotransmitters such as GABA or glycine (Game and Lodge, 1975; Duggan, 1982). Postsynaptic inhibition is well documented and it is an obvious mechanism for the reduction of activity in nociceptive spinal neurons. This can be triggered by the activation of other afferent fibers, either low or high threshold, or by activity in descending pathways. But in addition to existing postsynaptic mechanisms another interpretation has been proposed based on presynaptic interactions between low-threshold mechanoreceptors and nociceptors that could explain not only the reduction in acute pain induced by low-threshold afferents but also the enhancement of pain sensitivity produced by these afferent fibers in hyperalgesic states (touch-evoked pain) (Cervero and Laird, 1996b). It has been reported that an experimental inflammation of the joints induces an enhancement of dorsal root reflexes (DRRs) (Rees et al., 1994, 1996; Sluka et al., 1995; Westlund, this volume, Chapter 9). DRRs were originally described in the 1940s and were known to be an over-expression of primary afferent depolarization (PAD), whereby an intense depolarization of the primary afferent leads to spiking activity conducted antidromically. Therefore, if the DRRs generated by the inflammatory stimulus in the nociceptive afferent were conducted orthodromically the DRR mechanism could provide access to the nociceptive channel to lowthreshold afferents capable of generating PAD in fine afferent fibers. Cervero and Laird (1996b) proposed that tactile allodynia from areas of secondary hyperalgesia could be mediated by a presynaptic link between low- and highthreshold afferents. It is known that impulses in lowthreshold mechanoreceptors with A-β fibers, evoke PAD of nociceptive afferents via a GABAergic presynaptic link between these two kinds of afferent fiber (Schmidt, 1971; Rudomin and Schmidt, 1999) that contains at least one interneuron. Thus, in normal conditions, activation of low-threshold mechanoreceptors with A-β afferents evokes presynaptic inhibition of nociceptive afferents (Calvillo, 1978) and therefore produces a reduction in pain sensation (Fig. 7.9A). However, following an injury the nociceptors in the area close to the lesion are activated and sensitized (Treede et al., 1992). This produces an initial nociceptive discharge induced by the injury as well as persistent

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Pain Relief

A

Axon Reflex (Antidromic) Allodynia (Orthodromic)

B Fig. 7.9. The diagrams proposed by Cervero and Laird (1996b) to illustrate their proposed model for the mechanisms of touch-evoked pain. A: normal skin; B: skin; after an injury. Two types of afferent fiber are illustrated: large caliber, connected to low-threshold mechanoreceptors and fine fibers connected to nociceptors (and showing an axon reflex arrangement). Key: LT = low-threshold cells; N = nociceptive cells; PAD = primary afferent depolarization; DRR = dorsal root reflex. In normal skin, stimulation of A-β afferents evokes PAD in C-fibers and pain inhibition; in hyperalgesic skin the interneurons are sensitized (black neuron) by the nociceptive barrage, and A-β stimulation evokes antidromic DRRs (and flare) and orthodromic activation of the C-fiber terminals (and allodynia) (Reproduced with permission by the International Association for the Study of Pain.)

activity in the sensitized nociceptors and thus a continuous afferent barrage in these fibers. This afferent barrage converges on to, amongst other places, the spinal substrate that mediates the presynaptic link between low-threshold mechanoreceptors and nociceptors. As a consequence of the increased and persistent barrage driving this system, excitability is increased such that, when activated by low-threshold mechanoreceptors from areas surrounding the injury site, it produces a much more intense PAD in the nociceptive afferent terminals reaching a depolarization level capable of

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generating spike activity. This activation would be conducted antidromically in the form of DRRs but would also be conducted forward, activating the second-order neurons normally driven by nociceptors. The sensory consequence of this mechanism is pain evoked by the activation of low-threshold mechanoreceptors from an area of secondary hyperalgesia, that is, allodynia (Fig. 7.9B). A possible mechanism to explain the shift from PAD to DRRs would be modulation of the chloride concentration inside the primary afferent terminal. Primary afferent neurons maintain a high internal chloride concentration because they express a chloride co-transporter (Na–K–2Cl transporter) that transports chloride into the cell using the energy of the sodium gradient created by the Na–K–ATPase pump. Thus, when the chloride channel of the GABA-A receptor opens, chloride flows out and the membrane depolarizes. A small depolarization of the membrane, or PAD, produces presynaptic inhibition because when an action potential arrives from the periphery along the axon of the primary afferent, the terminal is already depolarized and the shift in membrane potential is reduced, which leads to less transmitter release at the synapse between the primary afferent terminal and the second order neurons (Alvarez-Leefmans et al., 1998). Results using knock-out mice for the Na–K–2Cl transporter have shown that these animals have reduced or abolished tactile allodynia in areas of secondary hyperalgesia (Laird et al., 2004). It has also been shown that the Na–K–2Cl transporter is phosphorylated in the spinal cord shortly after the application of a peripheral noxious stimulus and there is evidence for a membrane enrichment or trafficking of this co-transporter in hyperalgesic states (Galan and Cervero, 2005). These observations support a role for a presynaptic inhibitory mechanism in the generation of hyperalgesia. It remains to be seen if this is only a mechanism for a short-term induction of tactile allodynia or if a similar process could also operate for the longer-lasting phases of allodynia characteristic of neuropathic and chronic inflammatory pain states. 7.6.2. Descending control The second form of modulation of nociceptive transmission through the spinal cord is mediated by descending pathways from supraspinal nuclei. Although this kind of descending modulation was initially thought to be only inhibitory and related to the reduction of pain sensitivity (Basbaum and Fields, 1978), it is now known that supraspinal excitatory influences can also contribute to pain modulation by

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enhancing pain sensitivity in hyperalgesic states (Tattersall et al., 1986a,b; Ossipov et al., 2000; Porreca et al., 2002). There is a large body of literature showing that the responses of nociceptive spinal cord neurons can be inhibited by the stimulation of various supraspinal structures. Similarly, stimulation of brainstem areas, in particular the periaqueductal gray (PAG) matter of the mid-brain in awake animals can evoke stimulationproduced analgesia (SPA), a state in which severe noxious stimuli produce no external signs of discomfort. It has been proposed that the analgesic effect of electrical stimulation of the PAG result from the activation of excitatory connections between the PAG and the nucleus raphe magnus (NRM) of the brain stem. In turn, NRM neurons project to the dorsal horn of the spinal cord via the dorsolateral funiculus (DLF) and inhibit nociceptive neurons of the spinothalamic and spinoreticular pathways, thus reducing transmission of nociceptive information. The role of endogenous opioids and other putative neurotransmitters in this paincontrol mechanism is discussed in Chapters 13–16. Another form of descending inhibition acting on nociceptive dorsal horn neurons is tonic descending inhibition (TDI), which is revealed as increased excitability when descending supraspinal messages are interrupted by spinalization (Duggan, 1985). However, the supraspinal sources of TDI and of SPA do not coincide, so it would appear that the two types of descending inhibition are mediated by separate neural systems. In fact the various forms of descending inhibition are not selective for nociceptive messages and it could be that they represent a generalized inhibitory control of most inputs to the somatosensory system. The system known as diffuse noxious inhibitory controls (DNIC) provides a neural substrate for the observation that pain inhibits pain. It is based on the observation that a noxious stimulus applied to one part of the body will inhibit the activity of nociceptive neurons in the spinal cord and trigeminal system with receptive fields outside the stimulated area (LeBars et al., 1979a,b). This system operates as a widespread and nonsomatotopic inhibitory mechanism and it acts mainly on the responses of WDR neurons. The strength of the inhibitory effect depends on the intensity of the conditioning stimulus; a strong noxious stimulus produces inhibition greater than that of a weak one. The pathways mediating DNIC involve a supraspinal component since this type of inhibition is much reduced after spinalization (LeBars et al., 1984). The supraspinal control of sensory transmission through the spinal cord also involves the descending excitation of spinal cord neurons and not only

their inhibition. It has been demonstrated that some nociceptive neurons in the spinal cord are excited by stimulation of brainstem sites and there are also reports of spinal nociceptive neurons under tonic descending excitation from supraspinal systems (Tattersall et al., 1986a,b). This type of descending excitation will enhance the activity of sensory neurons and generate a positive feed-back loop between the spinal cord and the brain stem which would maintain central activity beyond the time of application of the stimulus (Cervero and Wolstencroft, 1984). Descending excitation of nociceptive pathways accounts for the high degree of central excitability and arousal that often follows a painful sensory experience. Some of these increases in excitability are expressed as enhanced motor and autonomic reflexes as well as expansions of painful sensations beyond the original area of injury. Overall, descending control of nociceptive transmission through the spinal cord includes a mixture of excitatory and inhibitory controls whose final balance determines the magnitude and extent of the pain sensation and of the accompanying nociceptive reactions. 7.7. Pain and the spinal cord It is often useful to look back at what we have done over the last 20 or 30 years to assess the strength of the evidence which we use to support our current assumptions. In the preceding pages only a small fraction of the work that has been carried out on the role of the dorsal horn on pain processing has been reviewed. The aim was to extract from the multitude of reports those observations that form the basis of contemporary thought. So, what is the role of the spinal cord in pain processing? We have a great deal of detailed information about the functional properties of the afferent input to the spinal cord. Sensory information from the skin, muscles and viscera is conveyed to the CNS by separate categories of afferent fiber that transmit information about innocuous and noxious events. Peripheral injuries alter the properties of some of these sensory receptors so that nociceptors can become sensitized, and thus send to the CNS increased afferent discharges, and new categories of previously insensitive or “sleeping” nociceptors can be activated by inflammatory stimuli. There are still many unknowns regarding the details of the transduction process in peripheral sensory receptors but we do have a fairly accurate picture of the organization of the sensory input to the spinal cord. This orderly and distributed organization of the afferent input to the spinal cord is substantially changed

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in the dorsal horn. The profound alteration in the way that sensory signals are processed through the cord is perhaps the main reason why we regard the dorsal horn as an important center for modulation of pain-related information. Some of the specificity of the peripheral inputs is preserved in the cord but the most striking observation is that second-order neurons respond to a variety of sensory modalities, innocuous as well as noxious, so that the distributed organization of the afferent input is not preserved after the first synaptic relay. In addition, evidence of modulation in the spinal dorsal horn is shown by the plasticity of the afferent input to spinal neurons and by the changes in excitability induced by afferent as well as by descending activity. Sensory signals arrive at the cord through discrete channels – nociceptors, mechanoreceptors, thermoreceptors, “sleeping” nociceptors – but the output of the dorsal horn to higher CNS centers is mediated in a large measure by multireceptive neurons whose responses are under considerable modulation. Some of the output is mediated by more specific second-order cells – nociceptor specific – but their low numbers and their timelocked responses suggest that their role may be to signal the initiation of a nociceptive event rather than maintain a chronic pain state. Information related to noxious events leaves the spinal cord through many different pathways that project to a variety of brain regions. We cannot recognize a single “pain pathway” in the same way that we can identify a visual or auditory pathway. Of course, nociceptive information is needed for a variety of functions – motor control, autonomic regulation, behavior – and not only for the perception of pain. This could explain the highly divergent distribution of nociceptive signals but does not help to establish which of these many pathways is concerned with the sensory aspects of pain perception or even if there is such thing as a pathway concerned with pain perception. The substantial descending control of nociceptive transmission through the spinal cord adds to the feeling that persistent and chronic pain states require the involvement of several brain regions and cannot be explained by the activation of a single pain pathway. We must therefore conclude that the dorsal horn of the spinal cord represents a major site for modulation of pain-related signals and of distribution of these signals to a variety of systems – sensory, motor and autonomic – all of which take part in the behavioral response to pain. This is not to say that spinal-cord processing explains all properties of pain sensation. Unfortunately, there has been a trend in the literature to try to explain complex aspects of pain perception by reference to the properties of spinal neurons and networks. The challenge

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is to understand the way in which the spinal cord deals with incoming afferent signals and to establish the limits of what can (and cannot) be done in the first synaptic relay. Ultimately, pain perception is a brain function but the understanding of the preprocessing of information by the spinal cord will tell us what kind of pain-related information is received by the brain. References Alvarez-Leefmans FJ, Nani A, Marquez S (1998). Chloride transport, osmotic balance and presynaptic inhibition. In: Rudomin P, Romo R, Mendell LM (Eds.) Presynaptic Inhibition and Neural Control. Oxford University Press, New York, pp. 50–79. Basbaum AI, Fields HL (1978). Endogenous pain control mechanisms: review and hypothesis. Ann Neurol 4: 451–462. Brown AG (1981). Organization in the Spinal Cord. SpringerVerlag, Berlin, Heidelberg, New York. Calvillo O (1978). Primary afferent depolarization of C-fibers in the spinal cord of the cat. Can J Physiol Pharmacol 56: 154–157. Cervero F (1986). Dorsal horn neurones and their sensory inputs. In: Yaksh TL (Ed.) Spinal Afferent Processing. Plenum Press, New York, pp. 197–216. Cervero F (1995). What is a nociceptor-specific (class 3) cell. Pain 62: 123–124. Cervero F (2000). Visceral hyperalgesia revisited. Lancet 356: 1127–1128. Cervero F (2005). The Gate Theory then and now. In: Merskey H, Loeser JD, Dubner R, (Eds.) The Paths of Pain. IASP Press, Seattle, pp. 33–48. Cervero F, Iggo A (1980). The substantia gelatinosa of the spinal cord: a critical review. Brain 103: 717–772. Cervero F, Laird JMA (1991). One pain or many pains?: a new look at pain mechanisms. News Physiol Sci 6: 268–273. Cervero F, Laird JMA (1996a). The neurophysiology of pain. In: Prys-Roberts C, Brown BR, jr (Eds.) International Practice of Anesthesia. Butterworth Heinemann, Oxford, UK, pp. 1-2-1–1-2-21. Cervero F, Laird JMA (1996b). Mechanisms of touch-evoked pain (allodynia): a new model. Pain 68: 13–23. Cervero F, Tattersall JEH (1986). Somatic and visceral sensory integration in the thoracic spinal cord. In: Cervero F, Morrison JFB (Eds.) Visceral Sensation. Progress in Brain Research, Vol. 67. Elsevier, Amsterdam, pp. 189–205. Cervero F, Wolstencroft JH (1984). A positive feedback loop between spinal cord nociceptive pathways and antinociceptive areas of the cat’s brain stem. Pain 20: 125–138. Cervero F, Iggo A, Ogawa H (1976). Nociceptor-driven dorsal horn neurones in the lumbar spinal cord of the cat. Pain 2: 5–24. Cervero F, Schouenborg J, Sjolund BH, Waddell PJ (1984). Cutaneous inputs to dorsal horn neurones in adult rats treated at birth with capsaicin. Brain Res 301: 47–57.

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