Activity of vestibulospinal neurons during locomotion

BRAIN RESEARCH

85

ACTIVITY OF VESTIBULOSPINAL NEURONS DURING LOCOMOTION

G. N. ORLOVSKY Interfaculty Laboratory of Mathematical Methods in Biology, M. V. Lomonosov Moscow State University, Moscow (U.S.S.R.}

(Accepted May 2nd, 1972)

INTRODUCTION

The vestibulospinal tract originating from the lateral vestibular nucleus of Deiters exerts an excitatory influence on extensor muscles. Destruction of this pathway results in reduction of extensor rigidity of the ipsilateral limbs in decerebrate cats (for references see ref. 5), while electrical stimulation of Deiters' nucleus enhances the activity of extensor muscles30,a9 and increases the reflex excitability of extensor motoneuronsS,32,3L With stimulation of Deiters' nucleus both monosynaptic and polysynaptic EPSPs can be obtained in most extensor alpha- and gamma-motoneuronsl0,11. Excitatory vestibulospinal influences on extensor muscles can be observed not only at rest, but also during locomotion. The present author has demonstrated2a that stimulation of Deiters' nucleus during locomotion in thalamic and mesencephalic cats enhances the activity of extensors during the stance phase of the step. Stimulation does not influence these muscles during the swing phase, when flexors are active. Destruction of Deiters' nucleus results either in disappearance of the stepping movements of the ipsilateral hindlimb, or in decrease of extensor activity during locomotion. Deiters' nucleus is an important efferent output of the cerebellumS, al. Purkinje ceils from the cerebellar vermis influence the neurons of Deiters' nucleus both directlyl4, 4° and indirectly through the fastigial nucleusal. The afferent projection in the cerebellar cortex, the cerebellar projection in Deiters' nucleus, and the vestibulospinal projection are somatotopically organized (for references see refs. 5, 6, 31). Deiters' nucleus is apparently closely connected with the cerebellum but very little is known about the function of this system in the control of movements. This paper describes the activity of Deiters' neurons during locomotion in thalamic and mesencephalic cats. Locomotion in these cats in many respects resembles that in intact animalslg,aT. The activity of Deiters' neurons was recorded both in cats with intact cerebellum and in decerebellate cats, in an attempt to estimate the role of the cerebellum in the discharge of these neurons during locomotion. The discharge pattern of Deiters' neurons was correlated with the activity of extensor muscles during locomotion. Such a correlation appears to be simple and Brain Research, 46 (1972) 85-98

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reasonable since the patterns of activity of most hindlimb extensors are similar: the muscles are active during most of the stance phase 7. An earlier study in this series deals with the activity of reticulospinal neurons during locomotion20-2L METtiODS

Thalamic and mesencephalic cats capable of locomotion (i.e., walking and running on the treadmill) were used in these experiments. Transection of the brain stem was performed under ether anesthesia at approximately the A5 level (mesencephalic cats) and A12 level (thalamic cats). A laminectomy was performed at the level of the first lumbar vertebra. EMG electrodes (thin copper wires) were inserted into the right lateral gastrocnemius muscle. The cat was then fixed in a stereotaxic device, and the first and second cervical vertebrae, the first lumbar vertebra and the pelvis were fixed. The legs of the cat were lowered to make contact with the treadmill which could move at a speed of up to 5 km/h. Locomotion was evoked in mesencephalic cats by repetitive stimulation (30 pulses/sec, t msec pulse duration, 10-20 V, bipolar electrode) of the 'locomotor region' of the mesencephalon37, 3s, and in thalamic cats by stimulation of the posterior hypothalamus tg, or by exteroceptive stimulation (usually by the movement of the treadmill); sometimes, in thalamic cats, locomotion began spontaneously. In some experiments the entire cerebellum was removed. Locomotion in thalamic cats after removal of the cerebellum was satisfactory, although movements are then apparently less well coordinated; this effect is more marked in mesencephalic preparations 2~. In most experiments the forelimbs were motionless during weak stimulation of the brain stem19, 37. If not, they were deliberately fixed. It was thus possible to minimize brain displacement during hindlimb movements. Extracellular recording of vestibulospinal (VS) neurones projecting to the lumbosacral spinal cord was performed with platinum microelectrodes (tip diameter, 10-20 #m; resistance, 100-200 k ~ ) inserted stereotaxically into the right Deiters' nucleus either through the cerebellum or, in decerebellate cats, through the cerebellar peduncle. Location of Deiters' nucleus was aided by recording the characteristic antidromic field potential evoked from the L1 segment. VS neurons were also identified by their antidromic activation from the L1 segment. The method was similar to that described by other authors lz. Activity of VS neuron, movement of the right hip in the sagittal plane, EMG from gastrocnemius lateralis, speed of the treadmill, and brain stem stimulation were recorded simultaneously by a loop-oscillograph (mirror-galvanometer) and an inkwriter. In the latter case a peak-detector was used to transform bipolar spikes into monopolar impulses with a duration of 15-20 msec. This method permitted recording, with the inkwriter, of neuronal activity up to 50 pulses/sec. Some details concerning methods of work are also given. For a detailed description of the experimental methods see refs. 21, 23. Brain Research, 46 (1972) 85-98

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Fig. 1. Activity of a VS neuron during locomotion (intact cerebellum). A, Activity of an extracellularly recorded vestibulospinal neuron (VS) together with EMG activity of the gastrocnemius lateralis (GL) and movement in the hip joint (H, flexion, i.e., swing phase: up, extension, i.e., stance phase: down), as well as speed of the treadmill (V, the interval between two vertical marks = 0.5 m). Locomotion was evoked by stimulation (S) of the brain stem (pulses 1 msec, 30/sec, 15 V). The position indicated by an interrupted horizontal line in A is shown in B with an enlarged time scale, stance phases being indicated by horizontal lines. C, Effect of the increase (marked by arrow) of stimulation of the brain stem. Antidromic invasion (from the lumbar spinal cord, L1) of the VS neuron is shown in D (single stimulus) and in E (400/sec). RESULTS

Activity o f VS neurons in cats with intact cerebellum One hundred and nine VS neurons were recorded during locomotion o f cats with intact cerebellum (11 mesencephalic and 2 thalamic). The results obtained in the two types o f preparations are similar. Latencies o f antidromic invasion o f VS neurons range from 1.8 to 5.0 msec with a mean o f 2.6 msec; the majority o f units (90~o) have latencies ranging from 2.0 to 3.0 msec, corresponding to conduction velocities o f 75-100 m/sec. A t rest, when there is no locomotion, most neurons (75 ~ ) are spontaneously active with a discharge frequency o f 5-30 pulses/sec (occasionally as m u c h as 50 pulses/sec); mean value, with silent neurons included is 16.6 zL 16.0 (SD) pulses/sec. D u r i n g locomotion (evoked by brain stem stimulation, by exteroceptive stimulation or arising spontaneously) almost all VS neurons show increased activity. A typical pattern o f discharge during locomotion (intact cerebellum) is shown in Fig. 1 A - C (identification, cf. Fig. 1D and E). After stimulation is started, the discharge frequency o f the n e u r o n increases markedly. The cat then starts stepping movements and the discharge frequency begins to alternate according to the rhythm o f

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Fig. 2. Examples of discharge of 7 VS neurons during locomotion (intact cerebellum). Schematic representations of 7 original recordings are shown here. Stance phases are marked by horizontal lines. A, Activity of the neuron: maximal at the beginning of the stance phase and minimal at the end of this phase. B, The neuron discharges by 1-2 spikes at the beginning of the stance phase. C, The neuron is active in the middle of the stance phase. D, The neuron has two bursts of activity, in the swing and in the stance phases. E and F, Activity of the neuron during weak (E) and forceful (F) locomotion: maximal activity has moved from the swing to the stance phase. G, The non-modulated neuron: soon after beginning stimulation of the brain stem (marked by the arrow) activity of the neuron increases and continues constant during locomotion. locomotion (cf. H and VS in Fig. 1A, B). If the discharge is compared with that of the extensor gastrocnemius lateralis recorded simultaneously (GL) it is evident that the maximal activity of the VS neuron coincides with the muscle activity (gastrocnemius is active in the stance phase of the step; stance phases are marked by horizontal lines in Fig. 1B). However, the neuronal activity appears somewhat earlier than the muscle activity, i.e., in the swing phase. After stimulation is switched off, locomotion stops and the discharge frequency drops to its initial level. Fig. 1C shows the activity of the same VS neuron when the strength of stimulation of the mesencephalon was increased (indicated by arrow). With stronger stimulation maximal discharge frequency increases 4 times, extensor activity also increases and movements of the limb become more forceful. The activity of this VS neuron increases with stimulation of the brain stem, before locomotion begins. However, a few recorded neurons were not influenced until locomotion started. In 73 of 109 (67%) VS neurons in cats with intact cerebellum it was possible to demonstrate periodic frequency changes in the locomotor rhythm ('modulated' Brain Research, 46 (1972) 85-98

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VESTIBULOSPINAL NEURONS DURING LOCOMOTION

neurons). Some patterns o f discharge d u r i n g l o c o m o t i o n are shown in Fig. 2 A - F . All m o d u l a t e d n e u r o n s are divided into groups (Fig. 3A, I - I V ) c o r r e s p o n d i n g to the p a t t e r n o f discharge in relation to the limb m o v e m e n t , i.e., to the phase o f the step. A g r o u p o f n e u r o n s with m a x i m a l activity at the end o f the swing phase a n d the b e g i n n i n g o f the stance phase (the hatched zone in Fig. 3A, I) will be considered first. This g r o u p I

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Fig. 3. Relationship between the activity of VS neurons and the phase of the step of the ipsilateral hindlimb. A, Scheme of the activities of different neuron groups during locomotion of cats with intact cerebellum. Phases of maximal activity of the relevant groups of neurons are hatched. The typical pattern of discharge is also given for each group. Maximal activity occurs in group I: at the end of the swing phase or at the beginning of the stance phase; in II: in the middle of the stance phase; in III: two maxima, one in each phase; in IV: at the end of the stance phase or at the beginning of the swing phase; V: non-modulated neurons. The relative numbers in these groups are shown in B. C, Activity of an 'average' VS neuron during locomotion in cats with intact cerebellum (black dots) and decerebellar cats (open dots) (for details see text); on the right is shown the average frequency of the resting discharge ('rest') for both types of preparation; at the bottom is shown the schematic pattern of extensor activity of the ipsilateral hindlimb.

Brain Research, 46 (1972) 85-98

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G. N. ORLOVSKY

is particularly interesting since the extensor muscles become active at precisely this moment 7, and the vestibulospinal tract also exerts maximal influence on their activity at this time '~3. The pattern of discharge of such a neuron is shown in Fig. I. Activity of neurons of this group usually increases in the swing phase of the ipsilateral hindlimb, achieves a maximum at the end of the swing phase (that is at the moment when the extensors are activated), and continues high during the greater part of the duration of the stance phase, decreasing at the end of this phase. The maximum discharge frequency is usually 50-100 pulses/sec, but extreme values range from a few pulses/step (Fig. 2B) to a peak of 200 pulses/sec. The minimal discharge frequency of many neurons is zero; that is, they are silent at the end of the stance phase (Figs. 1, 5B, 6A and 6B). However, weak activity during this phase may be seen in some neurons (Fig. 2A and F). Fifty-seven of 73 modulated neurons, i.e., 52 ~o of the total number of recorded VS neurons, show this pattern of discharge (with maximal activity at the end of the swing phase or at the beginning of the stance phase and minimal activity at the end of the stance phase). Fig. 3A, I shows this pattern of activity schematically; this group of neurons is called group I. VS neurons in group II (4 units, 4~o) have their maximal activity in the middle of the stance phase (the hatched zone in Fig. 3A, II). Examples of this type of discharge are given in Figs. 2C and 5A. VS neurons in group III (5 units, 5 ~ , cf Fig. 2D) have two maxima, one in the stance phase and another in the swing phase (the hatched zones in Fig. 3A, Ill). Finally, VS neurons in group IV (7 units, 6 ~ ) have their maximal activity right at the end of the stance phase or at the beginning of the swing phase (the hatched zone in Fig. 3A, IV, cf. Fig. 2E). The positions of the maximal and minimal activity within the locomotor cycle are usually fairly stable, but in several cases variations could be observed. The VS neuron of Fig. 2E was active during the swing phase in weak locomotion, but in more forceful locomotion (Fig. 2F) activity was seen in the stance phase. Sometimes modulation disappeared, to reappear some steps later or in the next test. The neurons of groups I-IV are all modulated during locomotion, but in onethird of the VS neurons rhythmic changes of activity could not be revealed (group V). These neurons increase their discharge frequencies during locomotion up to an average of 60 pulses/sec (cf. Fig. 2G). The spontaneous activity of these neurons is much higher than that of the modulated neurons; average frequencies for nonmodulated and modulated neurons are 24.2 ~ 17.0 (SD) pulses/sec and 12.8 14.1 (SD) pulses/sec respectively. The difference is statistically significant (Student's t-test, P <. 0.01). In the 'non-modulated' group only 4 units (11 ~o) show no spontaneous activity; in the 'modulated' group this was true of 25 units (34~). No correlation was found between conduction velocity of VS neurons and their behavior during locomotion: mean latencies of antidromic invasion for modulated and nonmodulated groups were equal to one another (2.6 msec). Fig. 3A summarizes the discharge pattern from the different groups and Fig. 3B shows the relative size of these groups. Obviously the dominating pattern of discharge of vestibulospinal neurons during locomotion is determined by groups I and V.

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Fig. 4. Activity of a VS neuron during locomotion (decerebellatecat). Activity of a VS neuron together with EMG from gastrocnemius lateralis (GL) and movement in the hip joint (H) are shown. Locomotion of this thalamic cat was evoked by exteroceptive stimulation. The pattern of discharge of the small groups II and III is somewhat similar to that of group I, since they are active simultaneously in some phases of the step. Group IV, however, is active at the end of the stance phase and the beginning of the swing phase, i.e., it acts reciprocally with the neurons of group I. Fig. 3C shows the discharge frequency as a function of the phase of the step for an 'average' VS neuron in cats with intact cerebellum (curve with black dots). To obtain this curve the locomotor cycle was divided into 7 intervals (3 in the swing phase and 4 in the stance phase). One part of the recording containing regular steps was selected for each VS neuron and the average frequencies were counted for all 7 intervals. The average frequencies for every interval were then averaged for all 109 VS neurons. The results represent only an approximation of VS tract activity, for the neuronal activity depends on the strength of the brain stem stimulation (see Fig. I C). Further, the large groups I and V exerted undue influence on the averaging. Nevertheless the curve characterizes, to some extent, the 'flow' of impulses from the Deiters' nucleus to the lumbosacral spinal cord. It is evident that the 'flow' is strongly dependent on the phase of the step, and the maximal value of the 'flow' coincides with the extensor activity. The average frequency of spontaneous activity ('rest') is also given in Fig. 3C, from which it is evident that the activity of the VS tract is considerably increased during locomotion. Activity o f V S neurons in decerebellate cats

Fifty-two VS neurons were recorded during locomotion in decerebellate cats (5 thalamic and 2 mesencephalic). At rest most neurons (80%) are spontaneously active with a discharge frequency of 20-60 pulses/sec, the average frequency is 29.3 :~ 23.6 (SD) pulses/sec, which is greater than in cats with intact cerebellum (16.6 ± 16.0 (SD) pulses/sec). The difference is statistically significant (Student's t-test, P < 0.01). Increased activity of VS neurons is usually accompanied by extensor rigidity which, in some cases, is quite considerable. The pattern of discharge of VS neurons during locomotion in the decerebellate cat is shown in Fig. 4. The locomotion of this thalamic cat was evoked by exteroceptive stimulation (by moving the treadmill). During locomotion the discharge frequency increases initially from 50 to 90 pulses/sec, at which level it remains until the end of the testing period, when it decreases to the stationary level. Most neurons increased their discharge frequency during locomotion and only a few neurons (10%) did not change their discharge or were inhibited. Not one of 52 VS neurons showed any sign of periodic frequency modulation; this figure should be compared with 67% in cats Brain Research, 46 (1972) 85-98

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Fig. 5. Influence of hindlimb movements on the discharge of VS neurons (intact cerebellum). Movement of the treadmill was stopped during locomotion (A, arrest marked by a horizontal line); the stepping movements of both ipsilateral (Hwsl) and contralateral (Heontra) hindlimbs and the rhythmic modulation of the VS neuron and of the EMG from gastrocnemius lateralis (GL) were consequently interrupted. B, Movement of the ipsilateral hindlimb was stopped by hand during locomotion (marked by a horizontal line), with consequent decreased activity of the VS neuron; however, slight changes of activity related to contralateral limb movement persisted. C, Passive movements of hindlimbs modulate activity of the VS neuron: during flexion in all joints of the ipsilateral limb reflex activation of the GL can be seen, correlating with the increase of activity of the VS neuron; the neuron is also weakly activated during passive extension of the contralateral limb.

with i n t a c t cerebellum. Fig. 3C shows the discharge o f an ' a v e r a g e ' VS n e u r o n in decerebellate cats d u r i n g l o c o m o t i o n (curve with o p e n dots). The discharge frequency o f the n e u r o n increased from 29.3 ( ' r e s t ' ) to 60.0 pulses/sec. Hence decerebellation

not only abolishes the cyclic modulation of VS neurons during locomotion but it also enhances the mean frequency of VS neurons as compared with that in cats with intact cerebellum. Relation between different kinds of hindlimb movements and activity of VS neurons Discharge p a t t e r n o f VS n e u r o n s d u r i n g l o c o m o t i o n has been described in the preceding sections. The results show t h a t m o d u l a t i o n occurs only when the cerebellum is intact. I n o r d e r to find to w h a t extent this m o d u l a t i o n is influenced by p e r i p h e r a l events the following e x p e r i m e n t s have been c a r r i e d out. I f the m o v e m e n t s o f b o t h h i n d l i m b s a r e s t o p p e d by force d u r i n g l o c o m o t i o n , p e r i o d i c m o d u l a t i o n o f the discharge frequency is a b o l i s h e d (Fig. 5A). I f the m o v e m e n t s o f the ipsilateral hindl i m b only a r e s t o p p e d , the m o d u l a t i o n usually d i s a p p e a r s , a l t h o u g h the c o n t r a l a t e r a l h i n d l i m b continues to r u n o n the treadmill. H o w e v e r , in several cases a very weak

Brain Research, 46 (1972) 85-98

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Fig. 6. Influence of conditions during the stance phase on the discharge of VS neurons (intact cerebellum). A and B, Activity of the VS neuron during locomotion at speeds of 4.9 km/h and 2.7 km/h

respectively (speed determined by movement of the treadmill). C, Result of an experiment in which the treadmill was raised by 2-3 cm during locomotion (L shows the level of the belt of the treadmill).

modulation in the rhythm of the contralateral leg persisted (Fig. 5B). It can therefore be concluded that without locomotion of the ipsilateral hindlimb there is no or very weak cyclical activity only of VS neurons. It should be noted, however, that the forelimbs were not usually active in the present experiments (see Methods). Passive limb movements can also influence the discharge of VS neurons. In 14 of 34 tested neurons passive flexion of all joints of the ipsilateral hindlimb (i.e., stretching the extensor muscles) causes a marked increase in discharge frequency (Fig. 5C); activation also occurs, though to a much smaller extent, when the contralateral hindlimb is extended. Opposite reactions (activation on extension of the ipsilateral hindlimb or flexion of the contralateral) were recorded in 5 units. The response of the remainder (15 neurons) was hard to define since these units had a very low threshold for activation and were influenced by slight movements of the limbs or in some cases by touch alone. The discharge frequencies observed on passive movements were usually much lower than those observed during locomotion. Nevertheless, in a number of cases, frequencies comparable to the values found during locomotion were observed. In decerebellate cats, however, passive limb movements never modulated the activity of VS neurons (25 were tested). As mentioned above, VS neurons are usually active during the major part of the stance phase. In the following experiments, the duration of the stance phase was varied. Fig. 6 shows recordings from a VS neuron at running speeds of 4.9 (A) and 2.7 (B) km/h (the speed is determined by the speed of the treadmill). It can be seen that at a slower velocity the stance phase and the extensor activity are prolonged as is

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the burst of activity of the VS neuron. In both cases the neuron is silent at the end of the stance phase. In order to investigate the effect of changing the load on the limbs during the stance phase, the experiments shown in Fig. 6C (ef. scheme) were carried out. During locomotion the treadmill was raised, this being equivalent to an extra load on the back (the cat's pelvis being fixed). Fig. 6C shows that the increase of the load causes increased extensor activity; the discharge frequency of the VS neuron also increases. This kind of response (increase in frequency of 30-70%) was observed in 9 of 13 neurons tested. The primary reason for increased muscular activity in these experiments is probably the stretch reflex. The above results thus show that the cyclic modulation of VS neurons is closely linked with the limb movements. DISCUSSION

The present study has shown that the modulation of vestibulospinal neurons with locomotor rhythm occurs only when the cerebellum is intact. Corrresponding findings have been obtained for rubro- and reticulospinal neurons21, zz,24. However, since locomotion also occurs in decerebellate cats, the modulation of these bulbospinal neurons is not essential for the generation of locomotion, although it should be noted that locomotion in the decerebellate cat is not well coordinated z2. It must be recognized that with the microelectrode technique recording is mostly from large neurons and the occurrence of unrecorded modulation in some small fiber systems cannot be excluded; it seems, however, more probable that stepping movements in decerebellate cats are generated by a spinal mechanism, which is 'switched on' through descending effects evoked from the 'locomotor region' of the brain stem (cf. ref. 4). It is probable that this is the same mechanism that is responsible for spinal stepping 36 and the 'stepping' that occurs in spinal cats after injection of DOPA 9, very likely due to activation of descending noradrenergic fibers (cfi refs. 1, 15).

Origin of modulation in VS neurons Since the cyclic modulation of VS neurons during locomotion was shown in this study to be closely linked with limb movements and entirely dependent on the cerebellum, it seems likely that this modulation is determined by signals coming through the spinocerebellar pathways and the cerebellum. Deiters' neurons are known to be under direct control of the Purkinje cells from the cerebellar vermis14,4°; these cells can also influence the Deiters' neurons through the fastigial nucleus 41. The finding in the present work that the tonic discharge rate of the VS neurons is significantly higher after decerebellation (cf. ref. 43) can be explained by disinhibition following abolition of tonic inhibition from the Purkinje axons14; the situation could however be more complex. Those VS neurons in cats with intact cerebellum (33 %) with higher spontaneous activity and no modulation are apparently less affected by the inhibitory control from Purkinje cells. It is somewhat surprising that no modulation occurs in the decerebellate cats although the VS neurons can still be influenced

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both by collaterals of the spinocerebellar pathways and by the direct spinovestibular projectionslZ, 4e. Information regarding the hindlimbs can reach the cerebellar vermis via the ventral spinocerebellar tract (VSCT) as well as via the spino-reticulo-cerebellar and spino-olivary-cerebellar pathways 27,~a. These channels have been thought to transmit information concerning not only peripheral events but also events within the spinal cord 16,18,z9. This hypothesis has been confirmed while recording the activity of VSCT neurons during locomotion: these neurons are phasically modulated even after complete deafferentation of the hindlimbs 3, in contrast to the dorsal spinocerebellar tract in which deafferentation results in a cessation of the periodic modulationL It therefore seems probable that the VSCT carries information to the cerebellum concerning the activity of the spinal mechanism generating stepping movements z, which, in its turn, depends on the segmental afferent inflow (cf. refs. 17, 25). It therefore seems possible that peripheral events influence the VS neurons not directly but act mainly through changes in the activity of the spinal mechanism generating stepping movements. Role of the vestibulospinal tract during locomotion The vestibulospinal tract excites hindlimb extensor alpha- and gamma-motoneurons by mono- and polysynaptic pathways10,11. It has been shown that stimulation of the Deiters' nucleus during locomotion does not influence the timing of the locomotor cycle, i.e., the beginning and end of muscular activity, but increases the level of extensor activity during the stance phase 23. Extensor activity in the stance phase is correspondingly greatly reduced following an ipsilateral lesion of the Deiters' nucleus 23. The present data show that the majority of the vestibulospinal neurons transmitting impulses to the lumbosacral spinal cord are modulated in phase with the locomotor cycle, with maximal discharge at the end of the swing phase or in the early stance phase, i.e. at the time when the extensor activity is commencing. Extensor alpha- and gamma-motoneurons would be expected to be influenced by this descending activity in parallel during the stance phase (cf. refs. 33, 34). What is the role of the vestibulospinal tract during locomotion? The increase of activity in this tract, with stimulation of the 'locomotor region' of the brain stem, which is in parallel with the increase of activity in other descending pathways (e.g., the reticulospina121,2z and the rubrospina124) apparently causes the spinal stepping mechanism to be 'switched on'. This mechanism generates reciprocal activity of flexors and extensors (cf. refs. 17, 23, 25) and, at the same time sends information to the cerebellum, via the spinocerebellar pathways, concerning the phase of the step and the level of locomotor activity. The cerebellar output, e.g., VS neurons, are thus phasically modulated in relation to the locomotor cycle, with maximal activity at the precise moment when extensors are activated by the spinal mechanism. The 'flow' of impulses descending in the VS tract may thus adjust the level of extensor activity precisely in accordance with the phase of the step. This 'feedback loop' apparently Brain Research, 46 (1972) 85-98

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helps to coordinate movements. Indeed, locomotion in the decerebellate cat is not well coordinated, the vestibulospinal tract and other descending tracts in this preparation are not modulated21,z2,z 4. However it must be recognized that decerebellation results not only in cessation of the periodic modulation, but also in considerable change in the mean level of the activity of the VS, reticulospinal'~l, 22 and rubrospina124 tracts, which may also be responsible for uncoordinated movements. Deiters' neurons are also known to be an 'output' of the vestibular systemL It might therefore be supposed that the VS tract would participate in the control of equilibrium during locomotion. However, study of the response of VS neurons to tilt has shown that this response is strongly inhibited during locomotion 26, i.e., the VS tract does not transmit (or transmits to a very small extent) vestibular influences to the spinal stepping mechanism. SUMMARY The activity of vestibulospinal neurons giving axons to the lumbosacral spinal cord was recorded during locomotion (walking and running on the treadmill) in mesencephalic and thalamic cats. The overall activity of most neurons increases to a considerable degree during locomotion, and periodic alternations of this activity in relation to the locomotor cycle (modulation) were observed in cats with intact cerebellum. The peak discharge usually occurs at the beginning of the stance phase of the ipsilateral hindlimb, i.e., when the extensor muscles are activated. Phasic modulation disappears when the limbs are stopped by force. There is no modulation in decerebellate cats. ACKNOWLEDGEMENT I wish to thank Dr. S. Grillner for helpful discussion as well as for assistance during preparation of the manuscript.

REFERENCES 1 ANDEN,N.-E., JUKES,M. G. M., ANDLUNDBERG,A., The effect of DOPA on the spinal cord. 11. A pharmacological analysis, Acta physiol, scand., 67 (1966) 387-397. 2 ARSHAVSKY,Ju. J., BERKINBLIT,M. B., FUXON, O. J., GELFAND,J. M., AND ORLOVSKY,G. N., Recordings of neurons of the dorsal spinocerebellar tract during locomotion, Brain Research 43 (1972) 272-275. 3 ARSrfAVSKY,Ju. J., BERKINBLIT,M, B., FUXON,O. J., GELFAND,J. M., AND ORLOVSKY,G. N., Origin of modulation in neurons of the ventral spinocerebellar tract during locomotion, Brain Research, 43 (1972) 276-279. 4 BERNSTEIN,N. m., The Coordination and Regulation of Movements, Pergamon Press, Oxford, 1967, 5 BRODAL,A., POMPEtANO,O., AND WALBERG,F., The Vestibular Nuclei and Their Connections, Anatomy and Functional Correlations, Thomas, Springfield, 111., 1962. 6 ECCLES,J. C., ITO, M., ANDSZENT~OOTHAI,J., 7he Cerebellum as a Neuronal Machine, Springer, Berlin, 1967. 7 ENGBERG,I., ANDLUNDBERG,A., An electromyographic analysis of muscular activity in the hindlimb of the cat during unrestrained locomotion, Acta physiol, scand., 75 (1968) 614-630. Brain Research, 46 (1972) 85-98

VESTIBULOSPINALNEURONS DURING LOCOMOTION

97

8 GERNANDT, B. E., AND THULIN, C.-A., Reciprocal effects upon spinal motoneurons from stimulation of bulbar reticular formation, J. Neurophysiol., 18 (1955) 113-129. 9 GRILLNER,S., Supraspinal and segmental control of static and dynamic ),-motoneurones in cat, ActaphysioL scand., 77, Suppl. 327 (1969) 1-34. 10 GRILLNER,S., HONGO,T., ANDLUND, S., Descending monosynaptic and reflex control of 7-motoneurons, Acta physiol, scand., 75 (1968) 592-613. 11 GRILLNER,S., HONGO,T., AND LUND, S., The vestibulospinal tract. Effects on alpha-motoneurons in the lumbosacral spinal cord in the cat, Exp. Brain Res., 10 (1970) 94-120. 12 ITO, M., HONGO, T., YOSHIDA, M., OKADA, Y., AND OBATA, K., Intracellularly recorded antidromic responses of Deiters' neurones, Experientia (Basel), 20 (1964) 295-296. 13 ITO, M., KAWAI,N., UDO, M., AND MANO,N., Axon reflex activation of Deiters' neurones from the cerebellar cortex through collaterals of the cerebellar afferents, Exp. Brain Res., 8 (1969) 249-268. 14 ITO, M., AND YOSHIDA,M., The cerebellar evoked monosynaptic inhibition of Deiters' neurons, Experientia (Basel), 20 (1964) 515-516. 15 JANKOWSKA,E., JUKES, M. G. M., LUND, S., AND LUNDBERG,A., The effect of DOPA on the spinal cord. V. Reciprocal organization of pathways transmitting excitatory action to alpha motoneurones of flexors and extensors, Acta physiol, scand., 70 (1967) 369-388. 16 LUNDBERG, A., Integrative significance of patterns of connections made by muscle afferents in the spinal cord, Syrup. XXI Int. physiol. Congr., Buenos Aires, (1959) 1-5. 17 LUNDBERG,A., Reflex Control of Stepping. The Nansen Memorial Zecture, Universitesforlaget, Oslo, 1969. 18 LUNDBERG,A., Function of the ventral spinocerebellar tract. A new hypothesis, Exp. Brain Res., 12 (1971) 317-330. 19 ORLOVSKY,G. N., Spontaneous and evoked locomotion of the thalamic cat, Biofizika, 14 (1969) 1095-1102. 20 ORLOVSKY,G. N., On relations between reticulo-spinal neurons and 'locomotor regions' of the brain stem, Biofizika, 15 (1970) 171-178. 21 ORLOVSKY,G. N., Activity of reticulo-spinal neurons during locomotion, Biofizika, 15 (1970) 728-737. 22 ORLOVSKY, G. N., Cerebellar influences on the reticulospinal neurons during locomotion, Biofizika, 15 (1970) 894-901. 23 ORLOVSKY, G. N., The effect of different descending systems on flexor and extensor activity during locomotion, Brain Research 40 (1972) 359-372. 24 ORLOVSKY,G. N., Activity of rubrospinal neurons during locomotion, Brain Research, 46 (1972) 99-112. 25 ORLOVSKY,G. N., AND FELDMAN,A. G., On the role of afferent activity for the generation of stepping movements, Neurophysiology, 4 (1972) 401-409. 26 ORLOVSKY,G. N., AND PAVLOVA,G. A., Response of Deiters' neurons to tilt during locomotion, Brain Research 42 (1972) 212-214. 27 OSCARSSON,O., Functional organization of the spino- and cuneocerebellar tracts, Physiol. Rev., 45 (1965) 495-522. 28 OSCARSSON,O., Functional significance of information cannales from the spinal cord to the cerebellum. In D. P. PURPURAAND M. O. YAHR (Eds.), Neurophysiological Basis of Normal and Abnormal Motor Activities, Academic Press, New York, 1967, pp. 93-117. 29 OSCARSSON,O., The sagittal organization of the cerebellar anterior lobe as revealed by the projection patterns of the climbingfibre system. In R. LLIN,~$(Ed.), Neurobiology of Cerebellar Evolution and Development, Chicago, 1969, pp. 525-537. 30 POMPEIANO,O., Organizzazione somatotopica delle risposte posturali alla stimolazione elettrica del nucleo di Deiters nel Gatto decerebrato, Arch. Sci. Biol., 44 (1960) 497-511. 31 POMPEIANO, O., Functional organization of the cerebellar projections to the spinal cord. In Progress in Brain Research, Vol. 25, Elsevier, Amsterdam, 1967, pp. 282-321. 32 SASAKI,K., TANAKA,T., AND MORt, K., Effects of stimulation of pontine and bulbar reticular formation upon motoneurons of the cat, Jap. J. Physiol., 12 (1962) 45-62. 33 SEVERIN,F. V., On the role of gamma-motor system for extensor alpha-motoneurons activation during the controlled locomotion, Biofizika, 15 (1970) 1096-1102. 34 SEVERtN,F. V., ORLOVSKY,G. N., AND SHIK, M. L., Work of the muscle receptors during controlled locomotion, Biofizika, 12 (1967) 502-511.

Brain Research, 46 (1972) 85-98

98

(;. N. ORLOVSKY

35 SHAPOVALOV,A. l., Excitation and inhibition of spinal neurones during supraspinal stimulation. In R. GRANIT (Ed.), Muscular Afferents and Motor Control, Almqvist and Wiksell, Stockholm, 1966, pp. 331-348. 36 SHERRINGTON,C. S., Flexion-reflex of the limb, crossed extension reflex, and reflex stepping and standing, J. Physiol. (Lond.), 40 (1910) 28-121. 37 SrtiK, M. L., SEVERIN,F. V., AND ORLOVSI~Y,G. N., The control of locomotion by means of electrical stimulation of the brain stem, Biofizika, 11 (1966) 659-666. 38 SHIK, M. L., SEVERIN, F. V., AND ORLOVSKY,G. N., Structures of the brain stem responsible for evoked locomotion, Sechenov Physiol. J. U.S.S.R., 53 (1967) 1125-1132. 39 SPRA6UE, J. M., SCHREINER, L. H., LINDSLEY, D. B., AND MAGOUN, H. W., Reticulo-spinal influences on stretch reflexes, J. Neurophysiol., 11 (1948) 501-507. 40 WALBERG,F., AND JANSEN,I., Cerebellar corticovestibular fibers in the cat, Exp. Neurol., 3 (1961) 32-52. 41 WALBERG,F., POMPE1ANO,O., BRODAL, A., AND JANSEN,J., The fastigo-vestibular projection in the cat. An experimental study with silver impregnation methods, J. comp. Neurol., i18 (1962) 49-76. 42 WILSOr~, V. J., KATO, M., THOMAS, R. C., AND PETERSON,B. W., Excitation of lateral vestibular neurones by peripheral afferent fibres, J. Neurophysiol., 29 (1966) 508-529. 43 WYLIE, R. M., AND FELPEL, L. P., The influence of the cerebellum and peripheral somatic nerves on the activity of Deiters' cells in the cat, Exp. Brain Res., 12 (1971) 528-546.

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