- Email: [email protected]
New Observations on Neuronal Organization of Reflexes from Tendon Organ Afferents and their Relation to Reflexes Evoked from Muscle Spindle Afferents
New Observations on Neuronal Organization of Reflexes from Tendon Organ Afferents and their Relation to Reflexes Evoked from Muscle Spindle Afferents E. JANKOWSKA Department of Physiology, University of Goteborg, S-400 33 Goteborg (Sweden)
Spinal reflexes from group Ia muscle spindle afferents and from group Ib tendon organ afferents have usually been analysed as if they were mediated by separate neuronal channels. Such an approach turned out to be justified in the case of the Ia reciprocal inhibition because interneurones which mediate it (Hultborn et al., 1971; Jankowska and Roberts, 1972) lack any input from Ib afferents. The neuronal circuitry of polysynaptic Ia excitatory actions (for references, see Hultborn and Wigstrom 1979) is still unknown and not much can be said about properties of the interposed neurones. With regard to reflexes evoked from Ib afferents there is, on the other hand, accumulating evidence that they may be mediated largely by interneurones used in common by Ia and Ib reflex pathways. Two groups of observations leading to this conclusion will be summarized in this report. LAMINAE V-VI INTERNEURONES AS TARGET CELLS OF BOTH Ia AND Ib AFFERENTS The first systematic intracellular study of interneurones with group I input by Eccles et al. (1960) suggested a selective monosynaptic input from either Ia or Ib afferents to two subgroups of these interneurones denoted A and B. Subsequent investigations by Hongo et al. (1966, 1972) showed, however, that Ia and Ib afferents converge onto a number of laminae V-VI interneurones; monosynaptic EPSPs were found to be evoked in individual interneurones by lowest threshold afferents in one hindlimb nerve and by higher threshold group I afferents in another, or the same nerve. For technical reasons the analysis had to be limited primarily to synaptic actions from knee flexor posterior biceps and semintendinosus (PBSt) and knee extensor quadriceps (Q), whose Ia and Ib afferents show clearest differences in threshold to electrical stimuli and in conduction velocity (Bradley and Eccles, 1953; Laporte and Bessou, 1957; Lundberg and Eccles, 1957a; Coppin et al. 1969). There were nevertheless indications for convergence of Ia and Ib afferents from other nerves as well. In addition, disynaptic excitation by Ib afferents was found to be combined with monosynaptic excitation by Ia afferents (Eccles et al., 1960) and disynaptic inhibition from either Ia or Ib afferents with excitation. We have recently reinvestigated cpntribution of muscle spindle and tendon organ afferents to excitation and inhibition of laminae V-VI interneurones using
30 intracellular recording (Czarkowska et al., 1979; Jankowska et al., 1979). One of our aims was to analyse contribution of Ia and Ib afferents from a number of different muscles. Those from knee flexors and extensors were stimulated electrically while Ia muscle spindle afferents from ankle extensors (medial gastrocnemius, MG; lateral gastrocnemius and soleus, LGS; and plantaris, P1) were activated by adequate stimuli (brief muscle stretches with 1.5-2.0 msec rise time and 30-35 pm amplitude at initial tension 5 N). Additional effects of larger stretches were considered to be due to either Ib or higher threshold Ia afferents, because under our experimental conditions a certain proportion of Ib afferents was excited by stretches of 40-50pm, while activation of some 20-30% of Ia afferents required stretches of up to about 60 pm. We therefore attributed to Ib afferents only the difference between postsynaptic potentials evoked by electrical stimuli maximal for group I afferents in a given nerve, and the 60 pm stretches. Convergence of Ia and Ib afferents of PBSt and Q was found in 22% of 33 interneurones excited from these muscles (Czarkowska et al., 1979) and in a majority of interneurones excitation from one or another subgroup of group I afferents from A internecrone offermt volley stretch
stretch
0
stretch
’: .I
.-c 60 )rm
b -t--
7 E
MG
C
f
h
P‘
interneurn offerent volley stretch
60 pm
moioneumne offermt d k y stretch
‘K
7
50 pm
i
2mv
I
I
I ms
7
Fig. 1. Excitation by Ia afferents and excitation combined with inhibition from Ib afferents in a laminae V-VI interneurone. Extracellular (A-B, upper traces) and intracellular (C-F, upper traces) records from
the same interneurone to be compared with records from a plantaris motoneurone (G-I) penetrated during the same experiment. Middle traces, records of afferent volleys from the surface of the spinal cord near L7 dorsal root entry zone. Lower traces, records of changes in muscle length (increase in length downwards). Left column (A, D, G), effect of muscle stretches submaximal for Ia afferents and subthreshold for Ib afferents. Middle column (B, E, H), effects of larger muscle stretches, near maximal for Ia afferents and subthreshold for Ib afferents. Right column (C, F, I), effects of electrical stimulation of medial gastrocnemius (MG) and plantaris (Pl) nerves. Note excitation of the interneurone by small muscle stretches (A, B), an increase of excitation and appearance of IPSPs cutting short the EPSPs with larger stretches (cf. amplitude of the EPSPs in D and E and time course of the EPSPs in E with that of EPSPs in D and H), and much larger EPSPs and IPSPs evoked from the nerves. Note also that 25 p m stretches were practically maximal for Ia afferents in this cat as judged from EPSPs in H and I; only minimal heteronymous EPSPs were evoked in this motoneurone. (From Jankowska, Johannisson and Lipski, unpublished observations.)
31 knee flexors and extensors was combined with excitatory actions from group1 afferents from other nerves. Convergence of Ia and Ib afferents of MG, LGS and P1 appeared in a much higher proportion of 70 interneurones with input from these muscles (Jankowska et al., 1979). More than 40% were co-excited by Ia afferents (Fig. l A , D) and by Ib afferents (see difference between effects of stimulation of all group I afferents and of stimulation maximal for group Ia afferents, Fig. l E , F). For interneurones with only excitatory input from ankle extensors the proportion was even higher (Fig. 2). Since in many interneurones excitation was combined with inhibition from Ia, Ib or both Ia and Ib afferents (Fig. 2) only about one-third of interneurones with excitatory input from Ib afferents was found not to be affected in one way or another (excited or inhibited) by Ia afferents (Fig. 2). Thus intracellular records gave a different picture of the input to laminae V-VI interneurones than that based on extracellular records (Lucas and Willis, 1974). We found no evidence for laminae V-VI interneurones selectively excited by Ia afferents from GS and PI. For nearly 50 interneurones with the above described input we were able to define their axonal projections, by classifying them to one of the 6 previously differentiated types of projections (Czarkowska et al., 1976). The comparison of the synaptic input with axonal projections showed that co-excitation, co-inhibition or excitation by one subgroup and inhibition by another subgroup of group I afferents occurs in
stretch 5 pm
MG 2 x
I a + I b EPSPs
I a + I b IPSR
-u -&--25 pm
LG-S 2 x
- --Ll b
I a + Ib EPSPs Ia IPSPs 1111 I a EPSPs I 0 IPSPs I b EPSPs
I b IPSR -lr7TTT7rll
I ms
ImS
Fig. 2. Excitation from Ib afferents combined with inhibition from Ia afferents in a laminae V-VI interneurone. Upper traces, intracellular records. Lower traces, records of change in muscle length in the left column and records of afferent volleys in the right column. Effects of muscle stretches with increasing amplitudes to the left and of electrical stimulation of medial gastrocnemius (MG), lateral gastrocnemius (L.GS) and plantaris ( P l ) nerves to the right. Note that IPSPs of practically the same amplitude were evoked by all muscle stretches and that EPSPs appeared only to electrical stimulation of two of the three stimulated nerves, evidencing Ia origin of IPSPs and Ib origin of EPSPs. Somewhat longer IPSPs following MG and LGS stimuli might indicate a certain contribution of Ib afferents to the inhibition of the interneurone. Upper diagrams show relative proportions of interneurones: excited by only Ib afferents (dotted) and co-excited by Ia and Ib afferents (hatched and dotted), and inhibited by only Ia afferents (hatched) or Ib afferents (dotted) and co-inhibited by Ia and Ib afferents (hatched and dotted). Lower diagram shows proportions of interneurones with input from only Ib (dotted) afferents and with Ib excitation combined with Ia excitation and/or inhibition (hatched and dotted). (From Jankowska, Johannisson and Lipski, unpubl. obs.)
32 interneurones projecting to motor nuclei as well as in those which apparently terminate only on other interneurones. It was a feature of interneurones with ipsilateral as well as of those with crossed projections. Generally one may, therefore, conclude that a variety of spinal reflexes evoked from group I afferents should depend on activation of both muscle spindle and tendon organ afferents, and be mediated by common rather than separate interneurones. As a particular consequence of co-excitation of the same interneurones by Ia and Ib afferents, one may further expect some similar reflex actions to be evoked by activation of these afferents and a mutual facilitation of their effects. We have now evidence that this is the case for autogenetic inhibition of motoneurones, or more exactly, for inhibition of motoneurones from homonymous and synergistic muscles. ORIGIN OF AUTOGENETIC INHIBITION OF MOTONEURONES FROM Ia AS WELL AS FROM Ib AFFERENTS Recent series of experiments (Fetz et al., 1979) revealed that inhibition of motoneurones may be evoked from Ia muscle spindle afferents as well as from Ib tendon organ afferents of the homonymous and synergistic muscles. The Ia inhibition of such an origin was demonstrated primarily for motoneurones of medial and lateral gastrocnemius and plantaris on brief stretches of these muscles. The minimal effective stretches were 10-20 p m in amplitude and much below threshold for Ib afferents (above 40 pm). The inhibition was demonstrated under several different experimental conditions as exemplified in Figs. 3 and 4. In motoneurones of Fig. 3 it was evoked while the nerves to their homonymous muscles were intact. The IPSPs were therefore visualized as a hump on the decay phase of the monosynaptic EPSPs after their reversal by hyperpolarization of the motoneurones and chloride injection and as cutting short the decay phase of the EPSPs after membrane depolarization.
PI rnotoneurone
LG- S motoneurone
- PI ~5 pm
~ G S , M G , P 28 I prn ImS
Fig. 3. IPSPs evoked by small stretches of homonymous muscles and by homonymous plus synergistic muscles. Averaged records from plantaris (PI) and lateral gastrocnemius (LGS) motoneurones (upper traces), and from the surface of the spinal cord (middle traces, with higher amplification to the right) and records of changes in muscle length (lower traces). Photographically superimposed intracellular records taken during hyperpolarization (40 nA) and depolarization (20 nA). (Modified from Fig. 3 of Fetz et al., 1979.)
33
MG,LG-S stretch 30pm intracell.
aff.volley -.\ stretch extracell.
A
B
LG-S 2xthr:
-- -~
C
I
_ 1 ,
,, , , , , ,
1
1
ImS
Fig. 4. Comparison of IPSPs evoked from Ia, and from all group I afferents of synergistic muscles. Intracellular records from a PI motoneurone. From top to bottom: intracellular records taken during depolarization of the motoneurones (50 nA), afferent volleys recorded from L7 dorsal root entry zone, changes in muscle length and extracellular field potential to the same stretch recorded just outside the motoneurone. A and B) Averaged and single superimposed records of IPSPs evoked by stretches of triceps surae estimated to activate about 80% of Ia afferents from these muscles. C) IPSPs evoked by electrical stimuli supramaximal for group I afferents in lateral gastrocnemius and soleus. (From Fetz, Jankowska and Lipski, unpublished observations.)
In the plantaris motoneurone of Fig. 4A the Ia IPSP was evoked only from synergistic muscles from triceps surae, and therefore without any preceding EPSP (cf. Eccles et al., 1957b). Control experiments confirmed that the observed inhibition disappeared after cutting the nerves of the stretched muscles, and was therefore not evoked from some other, unintentionally activated receptors. Moreover, stretches used to evoke the inhibition were below threshold for discharging motoneurones; it was thus not evoked secondarily to activation of Renshaw cells and could be differentiated from the recurrent inhibition of motoneurones. When compared to IPSPs evoked by selective electrical stimulation of Ib afferents using method of Coppin et al. (1970) the stretch-evoked l a IPSPs appeared with similar latencies (di- and trisynaptically) but had much smaller amplitudes. Ia IPSPs were 16-35% of the amplitudes of Ib IPSPs, although the latter were evoked by a smaller proportion, probably less than 50% as compared to about 80% of Ia afferents of the same muscles. The Ia and Ib IPSPs constituted about 10% and 25-66% of IPSPs evoked from all group I afferents, respectively. Autogenetic inhibition could also be evoked in a few posterior bicepssemitendinosus motoneurones by near threshold (1.1-1.2 times threshold) electrical stimulation of the nerves (Fetz, Jankowska ana Lipski, unpublished observations) (Fig. 5). In view of observations of Laporte and Bessou (1957) and Coppin et al. (1969) on thresholds of group I afferents in biceps and semitendinosus, this effect could be likewise attributed to the muscle spindle afferents. These results are in general agreement with some previous observations which left open the possibility that Ia afferents may contribute to the inhibition evoked from group I afferents from homonymous and synergistic muscles. Inhibition from low threshold group I afferents stimulated electrically, was in fact already reported in one of the first studies on group I inhibitory actions on motoneurones (Eccles et al., 1 9 5 7 ~but ) in view of a possible overlap between the thresholds of Ia and Ib afferents to electrical stimuli, it was considered as due to Ib rather than to Ia afferents. Lundberg et al. (1977) found similarly low, or even lower thresholds for evoking IPSPs from quadriceps in ankle and toe extensors. These were sometimes (during facilitation by
34
Tlx B
A
PBSt 1.08
A
C
PBSt 1.15
50
0 0 1.1 1.2 13 1.4 1.5
x threshold
Ims B C
lmsD
Fig. 5 . Autogenetic inhibition from Ia afferents of posterior biceps and semitendinosus. A) Amplitudes of the first (Ia) and the second (Ib) components of the afferent volleys, evoked by the first and the second of two stimuli applied to the PBSt nerve, as a function of stimulus intensities; sample records are shown in E. Intervals between the two stimuli were shorter than the refractory period of the tested afferents. The second stimulus was maximal for group I afferents and intensity of the first was increased as indicated by multiples of threshold above the records. Decrease of the second component of the afferent volley evoked by the second stimulus defined threshold for Ib afferents. B, C) Postsynaptic potentials evoked in a PBSt motoneurone by stimulation of the PBSt nerve with intensities 1.08 and 1.15 x threshold. From top to bottom: potentials recorded in the motoneurone during hyperpolarization (50 PA), potentials recorded during depolarization (20 PA) and afferent volleys. (From Fetz, Jankowska and Lipski, unpublished observations.)
cutaneous afferents) lowered to 1.l-1.15 times threshold, and much below threshold for the Ib component of the afferent incoming volley. A facilitation of IPSPs evoked from Ib afferents from G-S by conditioning volleys in Ia afferents from Q was occasionally observed by these authors (Lundberg et al., 1977) and found also between Ia and Ib actions from triceps surae and plantaris (Fetz, Jankowska and Lipski, unpublished observations). It might be recalled in this context that the autogenetic inhibition does not represent the only known combined reflex di- or polysynaptic action of muscle spindle and tendon organ afferents. Both these groups of afferents contribute to the crossed reflexes from group I afferents (Perl, 1959; Holmqvist, 1961; Baxendale and Rosenberg, 1976, 1977). Both contribute to the presynaptic depolarization of group I afferents from flexors as well as from extensors (for reference, see Schmidt 1973), and the information from both is jointly forwarded by some of the ventral spinocerebellar tract cells (Lundberg and Weight, 1971). CO-EXCITATION
OF
MUSCLE SPINDLE AFFERENTS
AND
TENDON
ORGAN
In view of the conclusion that reflexes from tendon organ afferents and some reflexes from group I muscle spindle afferents may be mediated by common neuronal
35 pathways, the co-excitation of these afferents by various stimuli becomes of particular interest. It has long been known (cf. Granit, 1955; Matthews, 1972) that due to the y-system the Ia and Ib afferents may be excited in parallel even during muscle contractions, which should otherwise unload muscle spindles. Recent studies on skeletofusimotor p-fibres (Emonet-Dtnand and Laporte, 1975; Laporte and Emonet-Dtnand, 1976) revive the problem by showing that &fibres may as effectively activate Ia as Ib afferents, that they innervate a considerable number of muscle spindles and constitute a high proportion of all the extrafusal motor fibres. There are thus good peripheral conditions for combined reflex actions of la muscle spindle and Ib tendon organ afferents. There is no doubt that some of the reflex actions of Ia afferents would utilize separate neuronal pathways (e.g., via monosynaptic connexions with motoneurones and via interneurones mediating Ia reciprocal inhibition of antagonists). To what extent other Ia and Ib actions are subserved by common or separate neurones and are jointly or independently controlled by various segmental and supraspinal neuronal systems remains to be established. SUMMARY Input from group la muscle spindle and group Ib tendon organ afferents to laminae V-VI interneurones has been reinvestigated with both electrical and adequate stimulation of these afferents. The reported observations show that a great proportion (about two-thirds) of interneurones excited by Ib afferents are co-excited and/or inhibited by Ia afferents from the same, synergistic or other muscles. These interneurones are thus shared by Ia and Ib reflex pathways. Ia autogenetic inhibition was revealed as a particular case of similar reflex actions of Ia and Ib afferents from a given group of muscles. It was demonstrated in triceps surae and plantaris motoneurones on stretches of their homonymous or synergistic muscles, subthreshold for Ib afferents. ACKNOWLEDGEMENTS I wish to express my thanks to Drs. Czarkowska, Fetz, Johannisson, Lipski and Sybirska for their permission to present some of our unpublished materials. The reported studies were supported by the Swedish Medical Research Council (project No. 94). REFERENCES Baxendale, R.H. and Rosenberg, J.R. (1976) Crossed reflexes evoked by selective activation of muscle spindle primary endings in the decerebrate cat. Bruin Res., 115: 324-327. Baxendale, R.H. and Rosenberg, J.R. (1977) Crossed reflexes evoked by selective activation of tendon organ afferent axons in the decerebrate cat. Bruin Rex, 127: 323-326. Bradley, K. and Eccles, J.C. (1953) Analysis of the fast afferent impulses from thigh muscles. J. Physiol. (Lond.), 122: 462-473. Coppin, C.M.L., Jack, J.J.B. and McIntyre, A.K. (1969) Properties of group I afferent fibres from semitendinosus muscle in the cat. J. Physiol. (Lond.), 203: 4546P. Coppin, C.M.L., Jack, J.J.B. and MacLennan, C.R. (1970) A method for the selective activation of tendon organ afferent fibres from the cat soleus muscle. J. Physiol. (Lond.), 210: 18-20.
36 Czarkowska, J., Jankowska, E. and Sybirska, E. (1976) Axonal projections of spinal interneurones excited by group I afferents in the cat, revealed by intracellular staining with horseradish peroxidase. Brain Res., 118: 115-118. Czarkowska, J., Jankowska, E. and Sybirska, E. (1979) Common interneurones in reflex pathways from group Ia muscle spindle and group Ib tendon organ afferents. I. Interneurones with input from knee flexors and extensors. In preparation. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1957a) Synaptic actions on motoneurones in relation to the two components of the group I muscle afferent volley. J . Physiol. (Lond.), 136: 527-546. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1957b) The convergence. of monosynaptic excitatory afferents on to many different species of alpha motoneurones. J. Physiol. (Lond.), 137: 22-50. ) actions motoneurones caused by impulses in Eccles, J.C., Eccles, R.M. and Lundberg, A. ( 1 9 5 7 ~Synaptic Golgi tendon organ afferents. J. Physiol. (Lond.), 138: 227-252. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1960) Types of neurone in and around the intermediate nucleus of the lumbosacral cord. J. Physwl. (Lond.), 154: 89-114. Emonet-DCnand, F. and Laporte, Y. (1975) Proportion of muscle spindles supplied by skeletofusimotor axons (p-axons) in peroneus brevis muscle of the cat. J. Neuroph$iol., 38: 1390-1394. Fetz, E., Jankowska, E., Johannisson, T. and Lipski, J. (1979) Autogenetic inhibition of motoneurones by impulses in group Ia muscle spindle afferents. J. Physwl. (Lond.), in press. Granit, R. (1955) Receptors and Sensory Perception. Yale University Press, New Haven. Holmqvist, B. (1971) Crossed spinal reflex actions evoked by volleys in somatic afferents. Acta physiol. scand., 52, Suppl. 181. Hongo, T., Jankowska, E. and Lundberg, A. (1966) Convergence of excitatory and inhibitory action on interneurones in the lumbosacral cord. Exp. Brain Res., 1: 338-358. Hongo, T., Jankowska, E. and Lundberg, A. (1969) The rubrospinal tract. 11. Facilitation of interneuronal transmission in reflex paths to motoneurones. Exp. Brain Res., 7: 365-391. Hultborn, H. and Wigstrom, H. (1978) Motor response with long latency and maintained duration evoked by activity in la afferents. Progr. in Clin. Neurophysiol., Vol. 8, J. Desmedt (Ed.) Karger, Basel. Hultborn, H., Jankowska, E. and Lindstrom, S. (1971) Recurrent inhibition of interneurones monosynaptically activated from group Ia afferents. J. Physiol. (Lond.), 215: 613-636. Jankowska, E., Johannisson, T. and Lipski, J. (1979) Common interneurones in reflex pathways from group Ia muscle spindle and group Ib tendon organ afferents. 11. Interneurones with input from ankle extensors. In preparation. Laporte, Y. and Bessou, P. (1957) Distribution dans les sous-groupes rapide et lent du groupe I des fibres Ia d'origine fusoriale et des fibres Ib d'origine golgienne. J. Physwl. (Paris), 49: 252-253. Laporte, Y.and Emonent-Dtnand, J. (1976) The skeleto-fusimotor innervation of cat muscle spindle. In Progress in Brain Research, Vol. 44, Understanding the Stretch Reflex, S . Homma (Ed.) Elsevier, Amsterdam, pp. 99-106. Lucas, M.E. and Willis, W.D. (1974) Identification of muscle afferents which activate interneurons in the intermediate nucleus. J. Neurophysiol., 37: 282-293. Lundberg, A., Malmgren, K. and Schomburg, E.D. (1977) Cutaneous facilitation of transmission in reflex pathways from Ib afferents to motoneurones. J. Physiol. (Lond.), 265: 763-780. Lundberg, A. and Weight, F. (1971) Functional organization of connexions to the ventral spinocerebellar tract. Exp. Brain Res., 12: 295-316. Matthews, P.B.C. (1972) Mammalian Muscle Receptors and their Central Actions. Edward Arnold, London. Perl, E.R. (1959) Effects of muscle stretch on excitability of contralateral motoneurones. J. Physiol. (Lond.), 14.5: 193-203. Schmidt, R.F. (1973) Control of the access of afferent activity to somatosensory pathways. In: Handbook of Sensory Physiology. Vol. It, Somatosensory System. A. Iggo (Ed.), Springer-Verlag, Berlin, pp. 151-206.
Spinal reflexes from group Ia muscle spindle afferents and from group Ib tendon organ afferents have usually been analysed as if they were mediated by separate neuronal channels. Such an approach turned out to be justified in the case of the Ia reciprocal inhibition because interneurones which mediate it (Hultborn et al., 1971; Jankowska and Roberts, 1972) lack any input from Ib afferents. The neuronal circuitry of polysynaptic Ia excitatory actions (for references, see Hultborn and Wigstrom 1979) is still unknown and not much can be said about properties of the interposed neurones. With regard to reflexes evoked from Ib afferents there is, on the other hand, accumulating evidence that they may be mediated largely by interneurones used in common by Ia and Ib reflex pathways. Two groups of observations leading to this conclusion will be summarized in this report. LAMINAE V-VI INTERNEURONES AS TARGET CELLS OF BOTH Ia AND Ib AFFERENTS The first systematic intracellular study of interneurones with group I input by Eccles et al. (1960) suggested a selective monosynaptic input from either Ia or Ib afferents to two subgroups of these interneurones denoted A and B. Subsequent investigations by Hongo et al. (1966, 1972) showed, however, that Ia and Ib afferents converge onto a number of laminae V-VI interneurones; monosynaptic EPSPs were found to be evoked in individual interneurones by lowest threshold afferents in one hindlimb nerve and by higher threshold group I afferents in another, or the same nerve. For technical reasons the analysis had to be limited primarily to synaptic actions from knee flexor posterior biceps and semintendinosus (PBSt) and knee extensor quadriceps (Q), whose Ia and Ib afferents show clearest differences in threshold to electrical stimuli and in conduction velocity (Bradley and Eccles, 1953; Laporte and Bessou, 1957; Lundberg and Eccles, 1957a; Coppin et al. 1969). There were nevertheless indications for convergence of Ia and Ib afferents from other nerves as well. In addition, disynaptic excitation by Ib afferents was found to be combined with monosynaptic excitation by Ia afferents (Eccles et al., 1960) and disynaptic inhibition from either Ia or Ib afferents with excitation. We have recently reinvestigated cpntribution of muscle spindle and tendon organ afferents to excitation and inhibition of laminae V-VI interneurones using
30 intracellular recording (Czarkowska et al., 1979; Jankowska et al., 1979). One of our aims was to analyse contribution of Ia and Ib afferents from a number of different muscles. Those from knee flexors and extensors were stimulated electrically while Ia muscle spindle afferents from ankle extensors (medial gastrocnemius, MG; lateral gastrocnemius and soleus, LGS; and plantaris, P1) were activated by adequate stimuli (brief muscle stretches with 1.5-2.0 msec rise time and 30-35 pm amplitude at initial tension 5 N). Additional effects of larger stretches were considered to be due to either Ib or higher threshold Ia afferents, because under our experimental conditions a certain proportion of Ib afferents was excited by stretches of 40-50pm, while activation of some 20-30% of Ia afferents required stretches of up to about 60 pm. We therefore attributed to Ib afferents only the difference between postsynaptic potentials evoked by electrical stimuli maximal for group I afferents in a given nerve, and the 60 pm stretches. Convergence of Ia and Ib afferents of PBSt and Q was found in 22% of 33 interneurones excited from these muscles (Czarkowska et al., 1979) and in a majority of interneurones excitation from one or another subgroup of group I afferents from A internecrone offermt volley stretch
stretch
0
stretch
’: .I
.-c 60 )rm
b -t--
7 E
MG
C
f
h
P‘
interneurn offerent volley stretch
60 pm
moioneumne offermt d k y stretch
‘K
7
50 pm
i
2mv
I
I
I ms
7
Fig. 1. Excitation by Ia afferents and excitation combined with inhibition from Ib afferents in a laminae V-VI interneurone. Extracellular (A-B, upper traces) and intracellular (C-F, upper traces) records from
the same interneurone to be compared with records from a plantaris motoneurone (G-I) penetrated during the same experiment. Middle traces, records of afferent volleys from the surface of the spinal cord near L7 dorsal root entry zone. Lower traces, records of changes in muscle length (increase in length downwards). Left column (A, D, G), effect of muscle stretches submaximal for Ia afferents and subthreshold for Ib afferents. Middle column (B, E, H), effects of larger muscle stretches, near maximal for Ia afferents and subthreshold for Ib afferents. Right column (C, F, I), effects of electrical stimulation of medial gastrocnemius (MG) and plantaris (Pl) nerves. Note excitation of the interneurone by small muscle stretches (A, B), an increase of excitation and appearance of IPSPs cutting short the EPSPs with larger stretches (cf. amplitude of the EPSPs in D and E and time course of the EPSPs in E with that of EPSPs in D and H), and much larger EPSPs and IPSPs evoked from the nerves. Note also that 25 p m stretches were practically maximal for Ia afferents in this cat as judged from EPSPs in H and I; only minimal heteronymous EPSPs were evoked in this motoneurone. (From Jankowska, Johannisson and Lipski, unpublished observations.)
31 knee flexors and extensors was combined with excitatory actions from group1 afferents from other nerves. Convergence of Ia and Ib afferents of MG, LGS and P1 appeared in a much higher proportion of 70 interneurones with input from these muscles (Jankowska et al., 1979). More than 40% were co-excited by Ia afferents (Fig. l A , D) and by Ib afferents (see difference between effects of stimulation of all group I afferents and of stimulation maximal for group Ia afferents, Fig. l E , F). For interneurones with only excitatory input from ankle extensors the proportion was even higher (Fig. 2). Since in many interneurones excitation was combined with inhibition from Ia, Ib or both Ia and Ib afferents (Fig. 2) only about one-third of interneurones with excitatory input from Ib afferents was found not to be affected in one way or another (excited or inhibited) by Ia afferents (Fig. 2). Thus intracellular records gave a different picture of the input to laminae V-VI interneurones than that based on extracellular records (Lucas and Willis, 1974). We found no evidence for laminae V-VI interneurones selectively excited by Ia afferents from GS and PI. For nearly 50 interneurones with the above described input we were able to define their axonal projections, by classifying them to one of the 6 previously differentiated types of projections (Czarkowska et al., 1976). The comparison of the synaptic input with axonal projections showed that co-excitation, co-inhibition or excitation by one subgroup and inhibition by another subgroup of group I afferents occurs in
stretch 5 pm
MG 2 x
I a + I b EPSPs
I a + I b IPSR
-u -&--25 pm
LG-S 2 x
- --Ll b
I a + Ib EPSPs Ia IPSPs 1111 I a EPSPs I 0 IPSPs I b EPSPs
I b IPSR -lr7TTT7rll
I ms
ImS
Fig. 2. Excitation from Ib afferents combined with inhibition from Ia afferents in a laminae V-VI interneurone. Upper traces, intracellular records. Lower traces, records of change in muscle length in the left column and records of afferent volleys in the right column. Effects of muscle stretches with increasing amplitudes to the left and of electrical stimulation of medial gastrocnemius (MG), lateral gastrocnemius (L.GS) and plantaris ( P l ) nerves to the right. Note that IPSPs of practically the same amplitude were evoked by all muscle stretches and that EPSPs appeared only to electrical stimulation of two of the three stimulated nerves, evidencing Ia origin of IPSPs and Ib origin of EPSPs. Somewhat longer IPSPs following MG and LGS stimuli might indicate a certain contribution of Ib afferents to the inhibition of the interneurone. Upper diagrams show relative proportions of interneurones: excited by only Ib afferents (dotted) and co-excited by Ia and Ib afferents (hatched and dotted), and inhibited by only Ia afferents (hatched) or Ib afferents (dotted) and co-inhibited by Ia and Ib afferents (hatched and dotted). Lower diagram shows proportions of interneurones with input from only Ib (dotted) afferents and with Ib excitation combined with Ia excitation and/or inhibition (hatched and dotted). (From Jankowska, Johannisson and Lipski, unpubl. obs.)
32 interneurones projecting to motor nuclei as well as in those which apparently terminate only on other interneurones. It was a feature of interneurones with ipsilateral as well as of those with crossed projections. Generally one may, therefore, conclude that a variety of spinal reflexes evoked from group I afferents should depend on activation of both muscle spindle and tendon organ afferents, and be mediated by common rather than separate interneurones. As a particular consequence of co-excitation of the same interneurones by Ia and Ib afferents, one may further expect some similar reflex actions to be evoked by activation of these afferents and a mutual facilitation of their effects. We have now evidence that this is the case for autogenetic inhibition of motoneurones, or more exactly, for inhibition of motoneurones from homonymous and synergistic muscles. ORIGIN OF AUTOGENETIC INHIBITION OF MOTONEURONES FROM Ia AS WELL AS FROM Ib AFFERENTS Recent series of experiments (Fetz et al., 1979) revealed that inhibition of motoneurones may be evoked from Ia muscle spindle afferents as well as from Ib tendon organ afferents of the homonymous and synergistic muscles. The Ia inhibition of such an origin was demonstrated primarily for motoneurones of medial and lateral gastrocnemius and plantaris on brief stretches of these muscles. The minimal effective stretches were 10-20 p m in amplitude and much below threshold for Ib afferents (above 40 pm). The inhibition was demonstrated under several different experimental conditions as exemplified in Figs. 3 and 4. In motoneurones of Fig. 3 it was evoked while the nerves to their homonymous muscles were intact. The IPSPs were therefore visualized as a hump on the decay phase of the monosynaptic EPSPs after their reversal by hyperpolarization of the motoneurones and chloride injection and as cutting short the decay phase of the EPSPs after membrane depolarization.
PI rnotoneurone
LG- S motoneurone
- PI ~5 pm
~ G S , M G , P 28 I prn ImS
Fig. 3. IPSPs evoked by small stretches of homonymous muscles and by homonymous plus synergistic muscles. Averaged records from plantaris (PI) and lateral gastrocnemius (LGS) motoneurones (upper traces), and from the surface of the spinal cord (middle traces, with higher amplification to the right) and records of changes in muscle length (lower traces). Photographically superimposed intracellular records taken during hyperpolarization (40 nA) and depolarization (20 nA). (Modified from Fig. 3 of Fetz et al., 1979.)
33
MG,LG-S stretch 30pm intracell.
aff.volley -.\ stretch extracell.
A
B
LG-S 2xthr:
-- -~
C
I
_ 1 ,
,, , , , , ,
1
1
ImS
Fig. 4. Comparison of IPSPs evoked from Ia, and from all group I afferents of synergistic muscles. Intracellular records from a PI motoneurone. From top to bottom: intracellular records taken during depolarization of the motoneurones (50 nA), afferent volleys recorded from L7 dorsal root entry zone, changes in muscle length and extracellular field potential to the same stretch recorded just outside the motoneurone. A and B) Averaged and single superimposed records of IPSPs evoked by stretches of triceps surae estimated to activate about 80% of Ia afferents from these muscles. C) IPSPs evoked by electrical stimuli supramaximal for group I afferents in lateral gastrocnemius and soleus. (From Fetz, Jankowska and Lipski, unpublished observations.)
In the plantaris motoneurone of Fig. 4A the Ia IPSP was evoked only from synergistic muscles from triceps surae, and therefore without any preceding EPSP (cf. Eccles et al., 1957b). Control experiments confirmed that the observed inhibition disappeared after cutting the nerves of the stretched muscles, and was therefore not evoked from some other, unintentionally activated receptors. Moreover, stretches used to evoke the inhibition were below threshold for discharging motoneurones; it was thus not evoked secondarily to activation of Renshaw cells and could be differentiated from the recurrent inhibition of motoneurones. When compared to IPSPs evoked by selective electrical stimulation of Ib afferents using method of Coppin et al. (1970) the stretch-evoked l a IPSPs appeared with similar latencies (di- and trisynaptically) but had much smaller amplitudes. Ia IPSPs were 16-35% of the amplitudes of Ib IPSPs, although the latter were evoked by a smaller proportion, probably less than 50% as compared to about 80% of Ia afferents of the same muscles. The Ia and Ib IPSPs constituted about 10% and 25-66% of IPSPs evoked from all group I afferents, respectively. Autogenetic inhibition could also be evoked in a few posterior bicepssemitendinosus motoneurones by near threshold (1.1-1.2 times threshold) electrical stimulation of the nerves (Fetz, Jankowska ana Lipski, unpublished observations) (Fig. 5). In view of observations of Laporte and Bessou (1957) and Coppin et al. (1969) on thresholds of group I afferents in biceps and semitendinosus, this effect could be likewise attributed to the muscle spindle afferents. These results are in general agreement with some previous observations which left open the possibility that Ia afferents may contribute to the inhibition evoked from group I afferents from homonymous and synergistic muscles. Inhibition from low threshold group I afferents stimulated electrically, was in fact already reported in one of the first studies on group I inhibitory actions on motoneurones (Eccles et al., 1 9 5 7 ~but ) in view of a possible overlap between the thresholds of Ia and Ib afferents to electrical stimuli, it was considered as due to Ib rather than to Ia afferents. Lundberg et al. (1977) found similarly low, or even lower thresholds for evoking IPSPs from quadriceps in ankle and toe extensors. These were sometimes (during facilitation by
34
Tlx B
A
PBSt 1.08
A
C
PBSt 1.15
50
0 0 1.1 1.2 13 1.4 1.5
x threshold
Ims B C
lmsD
Fig. 5 . Autogenetic inhibition from Ia afferents of posterior biceps and semitendinosus. A) Amplitudes of the first (Ia) and the second (Ib) components of the afferent volleys, evoked by the first and the second of two stimuli applied to the PBSt nerve, as a function of stimulus intensities; sample records are shown in E. Intervals between the two stimuli were shorter than the refractory period of the tested afferents. The second stimulus was maximal for group I afferents and intensity of the first was increased as indicated by multiples of threshold above the records. Decrease of the second component of the afferent volley evoked by the second stimulus defined threshold for Ib afferents. B, C) Postsynaptic potentials evoked in a PBSt motoneurone by stimulation of the PBSt nerve with intensities 1.08 and 1.15 x threshold. From top to bottom: potentials recorded in the motoneurone during hyperpolarization (50 PA), potentials recorded during depolarization (20 PA) and afferent volleys. (From Fetz, Jankowska and Lipski, unpublished observations.)
cutaneous afferents) lowered to 1.l-1.15 times threshold, and much below threshold for the Ib component of the afferent incoming volley. A facilitation of IPSPs evoked from Ib afferents from G-S by conditioning volleys in Ia afferents from Q was occasionally observed by these authors (Lundberg et al., 1977) and found also between Ia and Ib actions from triceps surae and plantaris (Fetz, Jankowska and Lipski, unpublished observations). It might be recalled in this context that the autogenetic inhibition does not represent the only known combined reflex di- or polysynaptic action of muscle spindle and tendon organ afferents. Both these groups of afferents contribute to the crossed reflexes from group I afferents (Perl, 1959; Holmqvist, 1961; Baxendale and Rosenberg, 1976, 1977). Both contribute to the presynaptic depolarization of group I afferents from flexors as well as from extensors (for reference, see Schmidt 1973), and the information from both is jointly forwarded by some of the ventral spinocerebellar tract cells (Lundberg and Weight, 1971). CO-EXCITATION
OF
MUSCLE SPINDLE AFFERENTS
AND
TENDON
ORGAN
In view of the conclusion that reflexes from tendon organ afferents and some reflexes from group I muscle spindle afferents may be mediated by common neuronal
35 pathways, the co-excitation of these afferents by various stimuli becomes of particular interest. It has long been known (cf. Granit, 1955; Matthews, 1972) that due to the y-system the Ia and Ib afferents may be excited in parallel even during muscle contractions, which should otherwise unload muscle spindles. Recent studies on skeletofusimotor p-fibres (Emonet-Dtnand and Laporte, 1975; Laporte and Emonet-Dtnand, 1976) revive the problem by showing that &fibres may as effectively activate Ia as Ib afferents, that they innervate a considerable number of muscle spindles and constitute a high proportion of all the extrafusal motor fibres. There are thus good peripheral conditions for combined reflex actions of la muscle spindle and Ib tendon organ afferents. There is no doubt that some of the reflex actions of Ia afferents would utilize separate neuronal pathways (e.g., via monosynaptic connexions with motoneurones and via interneurones mediating Ia reciprocal inhibition of antagonists). To what extent other Ia and Ib actions are subserved by common or separate neurones and are jointly or independently controlled by various segmental and supraspinal neuronal systems remains to be established. SUMMARY Input from group la muscle spindle and group Ib tendon organ afferents to laminae V-VI interneurones has been reinvestigated with both electrical and adequate stimulation of these afferents. The reported observations show that a great proportion (about two-thirds) of interneurones excited by Ib afferents are co-excited and/or inhibited by Ia afferents from the same, synergistic or other muscles. These interneurones are thus shared by Ia and Ib reflex pathways. Ia autogenetic inhibition was revealed as a particular case of similar reflex actions of Ia and Ib afferents from a given group of muscles. It was demonstrated in triceps surae and plantaris motoneurones on stretches of their homonymous or synergistic muscles, subthreshold for Ib afferents. ACKNOWLEDGEMENTS I wish to express my thanks to Drs. Czarkowska, Fetz, Johannisson, Lipski and Sybirska for their permission to present some of our unpublished materials. The reported studies were supported by the Swedish Medical Research Council (project No. 94). REFERENCES Baxendale, R.H. and Rosenberg, J.R. (1976) Crossed reflexes evoked by selective activation of muscle spindle primary endings in the decerebrate cat. Bruin Res., 115: 324-327. Baxendale, R.H. and Rosenberg, J.R. (1977) Crossed reflexes evoked by selective activation of tendon organ afferent axons in the decerebrate cat. Bruin Rex, 127: 323-326. Bradley, K. and Eccles, J.C. (1953) Analysis of the fast afferent impulses from thigh muscles. J. Physiol. (Lond.), 122: 462-473. Coppin, C.M.L., Jack, J.J.B. and McIntyre, A.K. (1969) Properties of group I afferent fibres from semitendinosus muscle in the cat. J. Physiol. (Lond.), 203: 4546P. Coppin, C.M.L., Jack, J.J.B. and MacLennan, C.R. (1970) A method for the selective activation of tendon organ afferent fibres from the cat soleus muscle. J. Physiol. (Lond.), 210: 18-20.
36 Czarkowska, J., Jankowska, E. and Sybirska, E. (1976) Axonal projections of spinal interneurones excited by group I afferents in the cat, revealed by intracellular staining with horseradish peroxidase. Brain Res., 118: 115-118. Czarkowska, J., Jankowska, E. and Sybirska, E. (1979) Common interneurones in reflex pathways from group Ia muscle spindle and group Ib tendon organ afferents. I. Interneurones with input from knee flexors and extensors. In preparation. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1957a) Synaptic actions on motoneurones in relation to the two components of the group I muscle afferent volley. J . Physiol. (Lond.), 136: 527-546. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1957b) The convergence. of monosynaptic excitatory afferents on to many different species of alpha motoneurones. J. Physiol. (Lond.), 137: 22-50. ) actions motoneurones caused by impulses in Eccles, J.C., Eccles, R.M. and Lundberg, A. ( 1 9 5 7 ~Synaptic Golgi tendon organ afferents. J. Physiol. (Lond.), 138: 227-252. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1960) Types of neurone in and around the intermediate nucleus of the lumbosacral cord. J. Physwl. (Lond.), 154: 89-114. Emonet-DCnand, F. and Laporte, Y. (1975) Proportion of muscle spindles supplied by skeletofusimotor axons (p-axons) in peroneus brevis muscle of the cat. J. Neuroph$iol., 38: 1390-1394. Fetz, E., Jankowska, E., Johannisson, T. and Lipski, J. (1979) Autogenetic inhibition of motoneurones by impulses in group Ia muscle spindle afferents. J. Physwl. (Lond.), in press. Granit, R. (1955) Receptors and Sensory Perception. Yale University Press, New Haven. Holmqvist, B. (1971) Crossed spinal reflex actions evoked by volleys in somatic afferents. Acta physiol. scand., 52, Suppl. 181. Hongo, T., Jankowska, E. and Lundberg, A. (1966) Convergence of excitatory and inhibitory action on interneurones in the lumbosacral cord. Exp. Brain Res., 1: 338-358. Hongo, T., Jankowska, E. and Lundberg, A. (1969) The rubrospinal tract. 11. Facilitation of interneuronal transmission in reflex paths to motoneurones. Exp. Brain Res., 7: 365-391. Hultborn, H. and Wigstrom, H. (1978) Motor response with long latency and maintained duration evoked by activity in la afferents. Progr. in Clin. Neurophysiol., Vol. 8, J. Desmedt (Ed.) Karger, Basel. Hultborn, H., Jankowska, E. and Lindstrom, S. (1971) Recurrent inhibition of interneurones monosynaptically activated from group Ia afferents. J. Physiol. (Lond.), 215: 613-636. Jankowska, E., Johannisson, T. and Lipski, J. (1979) Common interneurones in reflex pathways from group Ia muscle spindle and group Ib tendon organ afferents. 11. Interneurones with input from ankle extensors. In preparation. Laporte, Y. and Bessou, P. (1957) Distribution dans les sous-groupes rapide et lent du groupe I des fibres Ia d'origine fusoriale et des fibres Ib d'origine golgienne. J. Physwl. (Paris), 49: 252-253. Laporte, Y.and Emonent-Dtnand, J. (1976) The skeleto-fusimotor innervation of cat muscle spindle. In Progress in Brain Research, Vol. 44, Understanding the Stretch Reflex, S . Homma (Ed.) Elsevier, Amsterdam, pp. 99-106. Lucas, M.E. and Willis, W.D. (1974) Identification of muscle afferents which activate interneurons in the intermediate nucleus. J. Neurophysiol., 37: 282-293. Lundberg, A., Malmgren, K. and Schomburg, E.D. (1977) Cutaneous facilitation of transmission in reflex pathways from Ib afferents to motoneurones. J. Physiol. (Lond.), 265: 763-780. Lundberg, A. and Weight, F. (1971) Functional organization of connexions to the ventral spinocerebellar tract. Exp. Brain Res., 12: 295-316. Matthews, P.B.C. (1972) Mammalian Muscle Receptors and their Central Actions. Edward Arnold, London. Perl, E.R. (1959) Effects of muscle stretch on excitability of contralateral motoneurones. J. Physiol. (Lond.), 14.5: 193-203. Schmidt, R.F. (1973) Control of the access of afferent activity to somatosensory pathways. In: Handbook of Sensory Physiology. Vol. It, Somatosensory System. A. Iggo (Ed.), Springer-Verlag, Berlin, pp. 151-206.