Ventral Spinocerebellar Tract Neurons Research Articles

Control of mammalian locomotion by ventral spinocerebellar tract neurons

Locomotion is a complex behavior required for animal survival. Vertebrate locomotion depends on spinal interneurons termed the central pattern generator (CPG), which generates activity responsible for the alternation of flexor and extensor muscles and the left and right side of the body. It is unknown whether multiple or a single neuronal type is responsible for the control of mammalian locomotion. Here, we show that ventral spinocerebellar tract neurons (VSCTs) drive generation and maintenance of locomotor behavior in neonatal and adult mice. Using mouse genetics, physiological, anatomical, and behavioral assays, we demonstrate that VSCTs exhibit rhythmogenic properties and neuronal circuit connectivity consistent with their essential role in the locomotor CPG. Importantly, optogenetic activation and chemogenetic silencing reveals that VSCTs are necessary and sufficient for locomotion. These findings identify VSCTs as critical components for mammalian locomotion and provide a paradigm shift in our understanding of neural control of complex behaviors.

Simulating spinal border cells and cerebellar granule cells under locomotion--a case study of spinocerebellar information processing.

The spinocerebellar systems are essential for the brain in the performance of coordinated movements, but our knowledge about the spinocerebellar interactions is very limited. Recently, several crucial pieces of information have been acquired for the spinal border cell (SBC) component of the ventral spinocerebellar tract (VSCT), as well as the effects of SBC mossy fiber activation in granule cells of the cerebellar cortex. SBCs receive monosynaptic input from the reticulospinal tract (RST), which is an important driving system under locomotion, and disynaptic inhibition from Ib muscle afferents. The patterns of activity of RST neurons and Ib afferents under locomotion are known. The activity of VSCT neurons under fictive locomotion, i.e. without sensory feedback, is also known, but there is little information on how these neurons behave under actual locomotion and for cerebellar granule cells receiving SBC input this is completely unknown. But the available information makes it possible to simulate the interactions between the spinal and cerebellar neuronal circuitries with a relatively large set of biological constraints. Using a model of the various neuronal elements and the network they compose, we simulated the modulation of the SBCs and their target granule cells under locomotion and hence generated testable predictions of their general pattern of modulation under this condition. This particular system offers a unique opportunity to simulate these interactions with a limited number of assumptions, which helps making the model biologically plausible. Similar principles of information processing may be expected to apply to all spinocerebellar systems.

Neural circuits in movement control

Neural circuits in movement control

Interactions between spinal interneurons and ventral spinocerebellar tract neurons

Recent evidence indicates that ventral spinocerebellar tract (VSCT) neurons do not merely receive information provided by spinal interneurons but may also modulate the activity of these interneurons. Hence, interactions between them may be mutual. However, while it is well established that spinal interneurons may provide both excitatory and inhibitory input to ascending tract neurons, the functional consequences of the almost exclusively inhibitory input from premotor interneurons to subpopulations of VSCT neurons were only recently addressed. These are discussed in the first part of this review. The second part of the review summarizes evidence that some VSCT neurons may operate both as projection neurons and as spinal interneurons and play a role in spinal circuitry. It outlines the evidence that initial axon collaterals of VSCT neurons target premotor inhibitory interneurons in disynaptic reflex pathways from tendon organs and muscle spindles (group Ia, Ib and/or II muscle afferents) to motoneurons. By activating these interneurons VSCT neurons may evoke disynaptic IPSPs in motoneurons and thus facilitate inhibitory actions of contralateral muscle afferents on motoneurons. In this way they may contribute to the coordination between neuronal networks on both sides of the spinal cord in advance of modulatory actions evoked via the cerebellar control systems.

A survey of spinal collateral actions of feline ventral spinocerebellar tract neurons

The aim of this study was to identify spinal target cells of spinocerebellar neurons, in particular the ventral spinocerebellar tract (VSCT) neurons, giving off axon collaterals terminating within the lumbosacral enlargement. Axons of spinocerebellar neurons were stimulated within the cerebellum while searching for most direct synaptic actions on intracellularly recorded hindlimb motoneurons and interneurons. In motoneurons the dominating effects were inhibitory [inhibitory postsynaptic potentials (IPSPs) in 67% and excitatory postsynaptic potentials (EPSPs) in 17% of motoneurons]. Latencies of most IPSPs indicated that they were evoked disynaptically and mutual facilitation between these IPSPs and disynaptic IPSPs evoked by group Ia afferents from antagonist muscles and group Ib and II afferents from synergists indicated that they were relayed by premotor interneurons in reflex pathways from muscle afferents. Monosynaptic EPSPs from the cerebellum were accordingly found in Ia inhibitory interneurons and intermediate zone interneurons with input from group I and II afferents but only oligosynaptic EPSPs in motoneurons. Monosynaptic EPSPs following cerebellar stimulation were also found in some VSCT neurons, indicating coupling between various spinocerebellar neurons. The results are in keeping with the previously demonstrated projections of VSCT neurons to the contralateral ventral horn, showing that VSCT neurons might contribute to motor control at a spinal level. They might thus play a role in modulating spinal activity in advance of any control exerted via the cerebellar loop.

Rhythmic activity of feline dorsal and ventral spinocerebellar tract neurons during fictive motor actions

Neurons of the dorsal spinocerebellar tracts (DSCT) have been described to be rhythmically active during walking on a treadmill in decerebrate cats, but this activity ceased following deafferentation of the hindlimb. This observation supported the hypothesis that DSCT neurons primarily relay the activity of hindlimb afferents during locomotion, but lack input from the spinal central pattern generator. The ventral spinocerebellar tract (VSCT) neurons, on the other hand, were found to be active during actual locomotion (on a treadmill) even after deafferentation, as well as during fictive locomotion (without phasic afferent feedback). In this study, we compared the activity of DSCT and VSCT neurons during fictive rhythmic motor behaviors. We used decerebrate cat preparations in which fictive motor tasks can be evoked while the animal is paralyzed and there is no rhythmic sensory input from hindlimb nerves. Spinocerebellar tract cells with cell bodies located in the lumbar segments were identified by electrophysiological techniques and examined by extra- and intracellular microelectrode recordings. During fictive locomotion, 57/81 DSCT and 30/30 VSCT neurons showed phasic, cycle-related activity. During fictive scratch, 19/29 DSCT neurons showed activity related to the scratch cycle. We provide evidence for the first time that locomotor and scratch drive potentials are present not only in VSCT, but also in the majority of DSCT neurons. These results demonstrate that both spinocerebellar tracts receive input from the central pattern generator circuitry, often sufficient to elicit firing in the absence of sensory input.

Inhibitory inputs to four types of spinocerebellar tract neurons in the cat spinal cord

Spinocerebellar tract neurons are inhibited by various sources of input via pathways activated by descending tracts as well as peripheral afferents. Inhibition may be used to modulate transmission of excitatory information forwarded to the cerebellum. However it may also provide information on the degree of inhibition of motoneurons and on the operation of inhibitory premotor neurons. Our aim was to extend previous comparisons of morphological substrates of excitation of spinocerebellar neurons to inhibitory input. Contacts formed by inhibitory axon terminals were characterised as either GABAergic, glycinergic or both GABAergic/glycinergic by using antibodies against vesicular GABA transporter, glutamic acid decarboxylase and gephyrin. Quantitative analysis revealed the presence of much higher proportions of inhibitory contacts when compared with excitatory contacts on spinal border (SB) neurons. However similar proportions of inhibitory and excitatory contacts were associated with ventral spinocerebellar tract (VSCT) and dorsal spinocerebellar tract neurons located in Clarke’s column (ccDSCT) and the dorsal horn (dhDSCT). In all of the cells, the majority of inhibitory terminals were glycinergic. The density of contacts was higher on somata and proximal versus distal dendrites of SB and VSCT neurons but more evenly distributed in ccDSCT and dhDSCT neurons. Variations in the density and distribution of inhibitory contacts found in this study may reflect differences in information on inhibitory processes forwarded by subtypes of spinocerebellar tract neurons to the cerebellum.

Excitatory inputs to four types of spinocerebellar tract neurons in the cat and the rat thoraco-lumbar spinal cord.

The cerebellum receives information from the hindlimbs through several populations of spinocerebellar tract neurons. Although the role of these neurons has been established in electrophysiological experiments, the relative contribution of afferent fibres and central neurons to their excitatory input has only been estimated approximately so far. Taking advantage of differences in the immunohistochemistry of glutamatergic terminals of peripheral afferents and of central neurons (with vesicular glutamate transporters VGLUT1 or VGLUT2, respectively), we compared sources of excitatory input to four populations of spinocerebellar neurons in the thoraco-lumbar spinal cord: dorsal spinocerebellar tract neurons located in Clarke's column (ccDSCT) and in the dorsal horn (dhDSCT) and ventral spinocerebellar tract (VSCT) neurons including spinal border (SB) neurons. This was done on 22 electrophysiologically identified intracellularly labelled neurons in cats and on 80 neurons labelled by retrograde transport of cholera toxin b subunit injected into the cerebellum of rats. In both species distribution of antibodies against VGLUT1 and VGLUT2 on SB neurons (which have dominating inhibitory input from limb muscles), revealed very few VGLUT1 contacts and remarkably high numbers of VGLUT2 contacts. In VSCT neurons with excitatory afferent input, the number of VGLUT1 contacts was relatively high although VGLUT2 contacts likewise dominated, while the proportions of VGLUT1 and VGLUT2 immunoreactive terminals were the reverse on the two populations of DSCT neurons. These findings provide morphological evidence that SB neurons principally receive excitatory inputs from central neurons and provide the cerebellum with information regarding central neuronal activity.

Do spinocerebellar neurones forward information on spinal actions of neurones in the feline red nucleus?

We recently demonstrated that feline ventral spinocerebellar tract (VSCT) neurones monitor descending commands for voluntary movements initiated by pyramidal tract (PT) neurones as well as locomotor movements relayed by reticulospinal (RS) neurones. The aim of the present study was to examine whether VSCT neurones likewise monitor descending commands from the red nucleus (RN). Extracellular records from the spinal border (SB) subpopulation of VSCT neurons revealed that a third (31%) of SB neurones may be discharged by trains of stimuli applied in the RN. Moreover, when RN stimuli failed to discharge SB neurones they facilitated activation of some of these neurones by RS and/or PT neurones, while activation of other SB neurones was depressed. We propose that the facilitation and depression of actions of RS neurones by RN neurones might serve to reflect a higher or lower excitability of motoneurones and therefore a likely higher or lower efficacy of the RS descending commands, prompting the cerebellum to adjust the activation of reticulospinal neurones. Activation of SB neurones by RN stimuli alone would also allow monitoring and adjusting the RN descending commands. Intracellular records from SB neurones revealed both monosynaptic and disynaptic EPSPs and disynaptic IPSPs evoked by RN stimuli. The disynaptic actions remained following transection of axons of reticulospinal neurones within the medullary longitudinal fascicle (MLF) and were therefore taken to be relayed primarily by spinal neurones, in contrast to EPSPs and IPSPs evoked by PT stimuli found to be relayed by reticulospinal rather than spinal neurones.

Processing information related to centrally initiated locomotor and voluntary movements by feline spinocerebellar neurones

Feed-back information on centrally initiated movements is processed at both supraspinal and spinal levels and is forwarded by a variety of neurones. The aim of the present study was to examine how descending commands relayed by reticulospinal neurones are monitored by a population of spinocerebellar tract neurones. Our main question was whether a spinal border (SB) subpopulation of ventral spinocerebellar tract (VSCT) neurones monitor actions of reticulospinal neurones with input from the mesencephalic locomotor region (MLR) as well as from pyramidal tract (PT) neurones. In the majority of intracellularly recorded SB neurons, stimuli applied in the MLR and in the medullary pyramids evoked EPSPs in parallel with EPSPs evoked by stimulation of axons of reticulospinal neurones in the medial longitudinal fascicle (MLF). In extracellularly recorded neurones short trains of stimuli applied in the ipsilateral and contralateral pyramids potently facilitated discharges evoked from the MLF, as well as EPSPs recorded intracellularly. In both cases the facilitation involved the disynaptic but not the monosynaptic actions. These results indicate that reticulospinal neurones activating SB neurones (or more generally VSCT neurones) are co-excited by axon-collaterals of other reticulospinal neurones and by fibres stimulated within the MLR and PTs. The study leads to the conclusion that these spinocerebellar neurones monitor descending commands for centrally initiated voluntary as well as locomotor movements relayed by reticulospinal neurones. Thereby they may provide the cerebellum with feed-back information on the likely outcome of these commands and any corrections needed to avoid errors in the issuing movements.

Collateral Actions of Premotor Interneurons on Ventral Spinocerebellar Tract Neurons in the Cat

Strong evidence that premotor interneurons provide ventral spinocerebellar tract (VSCT) neurons with feedback information on their actions on motoneurons was previously found for Ia inhibitory interneurons and Renshaw cells, while indications for similar actions of other premotor interneurons were weaker and indirect. Therefore the aim of the present study was to reexamine this possibility with respect to interneurons relaying actions of group Ib afferents from tendon organs and group II afferents from muscle spindles. In all, 133 VSCT neurons in the L3-L5 segments (including 41 spinal border neurons) were recorded from intracellularly in deeply anesthetized cats to verify that stimuli applied in motor nuclei evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) attributable to stimulation of axon collaterals of premotor interneurons. IPSPs were found in over two thirds of the investigated neurons. When intraspinal stimuli were preceded by stimuli applied to a muscle nerve at critical intervals, IPSPs evoked from motor nuclei were considerably reduced, indicating a collision of nerve volleys in axons of interneurons activated by group I and group II afferents. In individual VSCT neurons monosynaptic IPSPs were evoked from both biceps-semitendinosus and gastrocnemius-soleus motor nuclei, in parallel with disynaptic IPSPs from group Ib and group II as well as group Ia afferents. These observations indicate that individual VSCT neurons may monitor the degree of inhibition of both flexor and extensor motoneurons by premotor interneurons in inhibitory pathways from group Ib and group II afferents to motoneurons. They may thus be providing the cerebellum with feedback information on actions of these premotor interneurons on motoneurons.

Modulation of responses of feline ventral spinocerebellar tract neurons by monoamines.

Ventral spinocerebellar tract neurons located in laminae V-VII of cat lumbar spinal cord were tested for the effects of ionophoretically applied monoamines and receptor selective agonists. Extracellularly recorded responses, monosynaptically evoked by group I afferents in a muscle nerve, were compared before, during, and after ionophoresis. They were analyzed with respect to changes in the number of evoked spikes and in the latency. Both serotonin (5-HT) and noradrenaline (NA) were found to facilitate responses of all neurons tested. Ionophoresis of three serotonin subtype receptor agonists (5-carboxamidotryptamine maleate, 5 methoxytryptamine HCl, and alpha-methyl 5-hydroxytryptamine) and of two NA receptor agonists (phenylephrine and isoproterenol) likewise had a facilitatory effect. However, three other 5-HT receptor agonists (8-hydroxy-dipropylaminotetraline hydrobromide), 2-methyl 5-hydroxytryptamine, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl and two NA receptor agonists (tizanidine and clonidine) had the opposite effect because they depressed responses of the tested neurons. These results show that information forwarded by means of the ventral spinocerebellar tract may be modulated by monoamines and that several receptor subtypes, located pre- or postsynaptically, may be involved. The results also demonstrate that transmission by means of group I muscle afferents may not only be facilitated by monoamines but also depressed by selective receptor subtype activation.

Nociceptive input to ascending tract neurones forwarding information from low threshold cutaneous and muscle afferents in cats.

Effects of noxious skin stimulation (central foot pad and foot dorsum) by radiant heat were tested on neurones of ascending tracts with a main input from non-nociceptors. The dominating effect on ventral spinocerebellar tract neurones was a depression (mainly from the pad). Responses of spinocervical tract neurones were either facilitated (predominantly from the foot dorsum) or depressed (predominantly from the pad). The dominating effect on neurones tentatively classified as dorsal horn dorsal spinocerebellar tract neurones was facilitatory from both skin areas. Similar effects were evoked by selective actions of C-fibres when A-delta fibres were blocked by TTX.

Functional properties of crossed spinocerebellar tract neurones with cell bodies in the S1 segment

Functional properties of a crossed spinocerebellar tract with cell bodies located in laminae VII and IX of the S1 segment were investigated using intracellular recording. The neurones were found to be excited by group I and II muscle afferents and afferents of skin, joint and interosseus nerves. Volleys from group II muscle afferents and cutaneous afferents evoked inhibition in these cells. It is concluded that S1 spinocerebellar neurones convey a similar type of information to that of dorsal spinocerebellar tract and ventral spinocerebellar tract neurones but integrate it in a different way.

Inputs to spinocerebellar tract neurones located in Stilling's nucleus in the sacral segments of the rat spinal cord

Spinocerebellar neurones located in the sacral segments of the rat spinal cord have been identified electrophysiologically. The neurones studied were located 0.7-1.1 mm deep to the cord dorsum, lateral and dorsal to the central canal in the medial part of lamina VII. Neurones were identified as spinocerebellar by antidromic activation from the cerebellar surface, the lowest threshold stimulation sites being near the midline on the posterior lobe vermis (lobule VIII). Estimates of conduction velocities of the axons ranged from 15-32 m/s (mean 22.8 m/s) and are directly comparable to velocities of presumed ventral spinocerebellar tract neurones recorded in the same animals. In intact animals, activity was most strongly influenced by passive movement of the tail. Activation by proprioceptors was confirmed with nerve stimulation: all of the neurones studied were discharged by stimulation of nerves which innervate ipsilateral tail muscles. In many cases responses appeared close to the threshold of the nerve, indicating that the largest, fastest conducting afferents (group Ia muscle spindle primary afferents) were responsible for them. Latencies of EPSPs or spikes were brief and in many cases indicative of a monosynaptic connection. We conclude that this group of neurones is powerfully and monosynaptically excited by group I muscle afferents and thus resemble the cells of Clarke's column and cells of the central cervical nucleus, both of which occupy a similar location in the grey matter of more rostral segments.

Comparison between ventral spincocerebellar and rubrospinal activities during locomotion in the cat

During fictive locomotion of the thalamic cat, rhythmic activity related to the efferent discharges in hindlimb nerves was found in rubrospinal neurons (Arshavsky et al., this issue). Since the movements were abolished by curarization, this modulation could not result from rhythmic peripheral inputs and had therefore a central origin. Taking into account the existence of spinal generators, it was suggested that ascending pathways transmit rhythmic activity from these spinal centers to the supraspinal ones. Preliminary results have been obtained for neurons of the ventral spinocerebellar tract (VSCT) recorded during fictive locomotion: (1) their discharge is rhythmically modulated at the periodicity of the locomotor rhythm; (2) their discharge pattern can be complex and variable in relation with the complexity and variability of the pattern of efferent activity in various muscle nerves of the ipsilateral hindlimb; (3) their responses to phasic afferent stimulation of the ipsilateral hindlimb are modulated in parallel with their locomotor-related activity. These results show that VSCT neurons convey information on the central spinal activity during locomotion, and suggest that these neurons contribute to the activity of lumbar-projecting rubrospinal neurons which have similar characteristics.

Comparison of response properties of dorsal and ventral spinocerebellar tract neurons to a physiological stimulus

The response characteristics of dorsal spinocerebellar tract (DSCT) neurons and ventral spinocerebellar tract (VSCT) neurons to the cutaneous inputs applied to footpads were studied in the cat. Three different wave forms were used: (1) step displacement of varying amplitudes (0.01–3.5 mm); (2) constant amplitude ramps with different slopes (5–120 mm/s); and (3) constant amplitude sinusoidal displacements of varying frequencies (1–20 Hz). Both DSCT and VSCT neurons responded phasically to cutaneous stimuli of different wave forms. The phasic responses were related to both the amplitude and velocity of the peripheral stimulus. However, the responses of DSCT neurons were graded over only a very narrow, low range of stimulus intensities, whereas the responses of VSCT neurons were graded over a larger range of skin indentation up to 3 mm. Only the DSCT neurons exhibited some length sensitivity to ramp stimuli, and only DSCT neurons were activated repetitively by periodic stimuli. These results suggest both DSCT and VSCT can transmit exteroceptive information but respond selectively to different features of these stimuli.

Messages conveyed by spinocerebellar pathways during scratching in the cat. II. Activity of neurons of the ventral spinocerebellar tract

(1) The activity of neurons of the ventral spinocerebellar tract (VSCT) during scratching was studied in thalamic and decapitate cats. The neurons were identified antidromically either by stimulation of the hindlimb area in the anterior lobe of the cerebellum (in thalamic cats) or by stimulation of the contralateral ventrolateral funiculus of the spinal cord (in decapitate cats). The scratch reflex was elicited by stimulation of either the pinna (in thalamic cats) or the cervical spinal cord (in decapitate cats). In most experiments, animals were immobilized and the activity of VSCT neurons was recorded during fictitious scratching 13. (2) During both actual and fictitious scratching, the discharge of VSCT neurons was rhythmically modulated in relation with the scratch cycle: neurons fired in bursts separated with periods of silence. Phases of activity of different neurons were unevenly distributed over the scratch cycle: most neurons fired within the limits of the flexor phase of the cycle. (3) The firing pattern of VSCT neurons during fictitious scratching was similar to that during actual scratching. Therefore, rhythmical burst firing of VSCT neurons is determined mainly by central mechanisms and not by a rhythmical sensory input. (4) The firing pattern of VSCT neurons in decapitate cats was similar to that in thalamic cats. Therefore, rhythmical burst firing of VSCT neurons is determined mainly by the central spinal mechanism and not by supraspinal motor centers. (5) The VSCT neurons which fired in long bursts during the greater part of the flexor phase were usually activated during the latent period of scratching, while those firing later in the cycle were usually either inhibited or not affected during this period. (6) The antidromic response in most VSCT neurons could be evoked from a large number of points in the hindlimb area of the cerebellar anterior lobe, both in the vermis and in the pars intermedia. Due to such extensive branching of axons, each point of the cortex receives signals from neurons firing in different phases of the cycle. But axons of VSCT neurons firing in long bursts during the greater part of the flexor phase terminate more extensively in the pars intermedia, while axons of neurons firing later in the cycle terminate more extensively in the vermis. (7) The functoning of the VSCT is essentially similar to that of the spinoreticulocerebellar pathway (SRCP) 5. Both pathways convey messages about activity of the central spinal mechanism generating the motor output pattern of scratching, but the VSCT is active mainly in the flexor phase of the scratch cycle and the SRCP in the extensor one. A hypothesis is advanced that these pathways monitor activity of different groups of spinal interneurons.

Effects from the Vestibulospinal Tract on Transmission from Primary Afferents to Ventral Spino‐cerebellar Tract Neurones

Convergence of vestibulospinal and segmental effects onto spinal interneurones which project to the ventral spino-cerebellar tract (VSCT) neurones has been studied by intracellular recording in VSCT cells. The disynaptic Ia IPSPs evoked in a group of VSCT neurones from the quadriceps nerve are monosynaptically facilitated by the vestibulospinal tract while there was no facilitation of Ia IPSP evoked from a flexor nerve. These results support the view that Ia inhibition to VSCT cells and motoneurones is mediated by common interneurones. The disynaptic inhibition evoked in other VSCT cells from the vestibulospinal tract is facilitated by volleys in the contralateral flexor reflex afferents (FRA) or bilaterally from the FRA. It is postulated that these actions are mediated by collaterals of the interneurones responsible for the analogous effects in motoneurones. Findings are reported suggesting that the monosynaptic vestibulospinal EPSP in VSCT cells in most cases is collateral to the excitatory input to the last order interneurones of reflex pathways from the FRA to motoneurones and only exceptionally to the corresponding input to Ia inhibitory interneurones. In many VSCT cells the vestibulospinal tract evoked disynaptic EPSPs which are facilitated from the FRA; the functional significance of this action is uncertain. The results are consistent with the hypothesis that VSCT neurones signal information on interneuronal transmission to motoneurones.

Rubrospinal Effects on Ventral Spinocerebellar Tract Neurones

Stimulation of the contralateral red nucleus evoked monosynaptic EPSPs in 14 of 82 ventral spinocerebellar tract neurones. In some of these cells the monosynaptic EPSP was followed by a disynaptic IPSP. The remaining cell population received di- or polysynaptic PSPs from the rubrospinal tract, either EPSPs or IPSPs or both. Convergence of the rubrospinal tract onto interneurones of the segmental pathways projecting to VSCT cells was demonstrated. Rubrospinal volleys facilitated disynaptic Ia IPSPs evoked in VSCT neurones from both flexors and extensors, as well as disynaptic Ib IPSPs. Facilitation of the Ia interneurones was disynaptic whereas facilitation of Ib interneurones was monosynaptic. Disynaptic rubrospinal EPSPs and IPSPs were facilitated by volleys in ipsi- as well as in contralateral cutaneous and high threshold muscle afferents. The complex pattern of projections from the rubrospinal tract onto VSCT neurones and the related reflex pathways gives further support to the hypothesis that these tract cells convey information on transmission through interneurones of the spinal segmental mechanisms.