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.     

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.     

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. III. Effects from supraspinal pathways.

Supraspinal effects were investigated in interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as Ia inhibitory interneurones). It was revealed that volleys in the vestibulospinal tract may evoke mono- and disynaptic EPSPs in interneurones monosynaptically excited from extensor muscles, i.e. extensor coupled Ia inhibitory interneurones. Flexor coupled interneurones instead received disynaptic inhibition. Volleys in the rubrospinal tract evoked a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled interneurones. Disynaptic rubrospinal EPSPs and IPSPs were also revealed. The pyramidal tract also gives rise to a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled Ia inhibitory interneurones. Rubrospinal and pyramidal volleys were shown to facilitate transmission in various segmental reflex pathways to the Ia inhibitory interneurones. A detailed comparison reveals a striking parallelism of segmental and supraspinal effects on alpha-motoneurones and Ia inhibitory interneurones connected to the same muscles. This considerably strengthens the hypothesis of an "alpha-gamma-linkage in the reciprocal inhibition".     

Convergence on Interneurones Mediating the Reciprocal Ia Inhibition of Motoneurones III. Effects from supraspinal pathways

Supraspinal effects were investigated in interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as “Ia inhibitory interneurones”). It was revealed that volleys in the vestibulospinal tract may evoke mono- and disynaptic EPSPs in interneurones monosynaptically excited from extensor muscles, i.e. extensor coupled Ia inhibitory interneurones. Flexor coupled interneurones instead received disynaptic inhibition. Volleys in the rubrospinal tract evoked a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled interneurones. Disynaptic rubrospinal EPSPs and IPSPs were also revealed. The pyramidal tract also gives rise to a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled Ia inhibitory interneurones. Rubrospinal and pyramidal volleys were shown to facilitate transmission in various segmental reflex pathways to the Ia inhibitory interneurones. A detailed comparison reveals a striking parallelism of segmental and supraspinal effects on α-motoneurones and Ia inhibitory interneurones connected to the same muscles. This considerably strengthens the hypothesis of an “α–γ-linkage in the reciprocal inhibition”.     

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. II. Effects from segmental flexor reflex pathways.

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. Volleys in ipsilateral and contralateral high threshold muscle afferents, cutaneous and high threshold joint afferents evoked a mixture of polysynaptic excitation and inhibition. These effects were ascribed to pathways activated by flexor reflex afferents (FRA) and in addition a specific ipsilateral low threshold cutaneous pathway. Ia inhibitory interneurones excited monosynaptically from flexor nerves received stronger net excitation by volleys in ipsilateral FRA than did extensor coupled interneurones, while the opposite pattern was seen from the contralateral FRA. These patterns are similar to those found in flexor and extensor motoneurones respectivey. The FRA inhibition in Ia inhibitory interneurones was partly mediated by "opposite" Ia inhibitory interneurones, i.e. those which are mediating the Ia inhibition of Ia inhibitory interneurones. The extent to which the FRA inhibition is transmitted by Ia inhibitory interneurones was roughly estimated by its susceptibility to recurrent depression by antidromic ventral root stimulation. The main conclusion is that most segmental pathways seem to evoke their effects in parallel to motoneurones and Ia inhibitory interneurones which are monosynaptically linked to the same muscle. The functional importance of this conclusion is discussed in a following report.     

Recurrent Control from Motor Axon Collaterals of la Inhibitory Pathways to Ventral Spinocerebellar Tract Neurones

The effects of impulses in recurrent motor axon collaterals on transmission in different inhibitory pathways to ventral spinocerebellar tract (VSCT) neurones were investigated in the cat by conditioning of IPSPs evoked in intracellularly recorded VSCT cells. Disynaptic IPSPs from large muscle spindle (la) afferenls were depressed in many but not all VSCT cells following an antidromic stimulation of ventral roots. The effect was found in VSCT neurones which themselves did not receive recurrent inhibition from motor axon collaterals. In cells with affected la IPSPs also some polysynaptic IPSPs evoked from ipsi- and contralateral group II muscle afferents and low threshold cutaneous afferents were depressed by a ventral root volley as well as disynaptic IPSPs from fibres descending on the ipsilateral side of the spinal cord. In unanesthetized preparations recurrent facilitatory potentials similar to those in motoneurones were evoked in VSCT neurones with la IPSPs. The findings indicate that some VSCT neurones receive collateral connexions from the interneurones which mediate la recipiocal inhibition to motoneurones and support the hypothesis that the VSCT conveys information about transmission in inhibitory reflex pathways to motoneurones (Lundberg 1971).     

Convergence on Interneurones Mediating the Reciprocal Ia Inhibition of Motoneurones

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as “Ia inhibitory interneurones”) were recorded in the lumbar spinal cord of the cat. Volleys in ipsilateral and contralateral high threshold muscle afferents, cutaneous afferents and high threshold joint afferents evoked a mixture of polysynaptic excitation and inhibition. These effects were ascribed to pathways activated by flexor reflex afferents (FRA) and in addition a specific ipsilateral low threshold cutaneous pathway. Ia inhibitory interneurones excited monosynaptically from flexor nerves received stronger net excitation by volleys in ipsilateral FRA than did extensor coupled interneurones, while the opposite pattern was seen from the contralateral FRA. These patterns are similar to those found in flexor and extensor motoneurones respectively. The FRA inhibition in Ia inhibitory interneurones was partly mediated by “opposite” Ia inhibitory interneurones, i.e. those which are mediating the Ia inhibition of la inhibitory interneurones. The extent to which the FRA inhibition is transmitted by Ia inhibitory interneurones was roughly estimated by its susceptibility to recurrent depression by antidromic ventral root stimulation. The main conclusion is that most segmental pathways seem to evoke their effects in parallel to motoneurones and Ia inhibitory interneurones which are monosynaptically linked to the same muscle. The functional importance of this conclusion is discussed in a following report.     

Effects on the ventral spinocerebellar tract neurones from deiters' nucleus and the medial longitudinal fascicle in the cat.

Effects from the vestibulospinal tract (VST) and from fibres descending in the medial longitudinal fascicle (MLF) on the cells of origin of the ventral spinocerebellar tract (VSCT) have been studied with intracellular recording. Out of 110 VSCT neurones, the VST evoked monosynaptic EPSPs in 27, di- or polysynaptic EPSPs in 56 and disynaptic IPSPs in 26. In 93 tested VSCT cells, MLF stimulation evoked monosynaptic EPSPs in 26, monosynaptic IPSPs in 2, di- or polysynaptic EPSPs in 25 and disynaptic IPSPs in 21. Convergence of monosynaptic EPSPs from VST and MLF was found in a small proportion of cells whereas the two descending pathways evoked reciprocal effects in another small group of neurones. Convergence of monosynaptic EPSPs from VST or MLF and from group I afferents was also modest. In 9 VSCT neurones there was convergence of monosynaptic excitation and disynaptic inhibition from the vestibulospinal tract and the same pattern from MLF was recorded in 9 neurones. The results are discussed in view of the hypothesis that VSCT neurones carry information on the interneuronal ttransmission in the spinal cord.     

Disynaptic inhibition of spinal motoneurones from the motor cortex in the monkey.

1. The neuronal mechanism of disynaptic inhibition of spinal motoneurones by the corticospinal tract was investigated in Macaca irus. Surface stimulation or weak intracortical stimulation was used in order to evoke the inhibition. Intracellular records were taken from motoneurones in lumbar segments. 2. Both the disynaptic i.p.s.p.s evoked from group Ia afferents and the disynaptic i.p.s.p.s evoked from corticospinal fibres were found to be depressed by conditioning stimulation of motor axons to antagonistic muscles. Mutual facilitation of the actions from these two fibre systems occurred when nerve impulses set up in them reached the explored spinal segment synchronously. These observations led to the conclusion that disynaptic i.p.s.p.s from group Ia afferents and from the motor cortex are mediated by common interneurones. 3. No evidence either for or against projections of the same pyramidal tract cells to motoneurones of one motor nucleus and to interneurones interposed between group Ia afferents and motoneurones of an antagonistic muscle could be obtained by comparing cortical areas from which monosynaptic e.p.s.p.s and disynaptic i.p.s.p.s were evoked in the different motor nuclei. 4. The areas from which the disynaptic i.p.s.p.s were evoked in individual motoneurones appeared to be similar in size to the areas of cortical monosynaptic projections to motoneurones and showed similar degrees of overlap, indicating that the projections of pyramidal tract cells to Ia inhibitory interneurones are as extensive as to motoneurones and that they are similarly organized.     

Premotor interneurones contributing to actions of feline pyramidal tract neurones on ipsilateral hindlimb motoneurones

The aim of the study was to analyse the potential contribution of excitatory and inhibitory premotor interneurones in reflex pathways from muscle afferents to actions of pyramidal tract (PT) neurones on ipsilateral hindlimb motoneurones. Disynaptic EPSPs and IPSPs evoked in motoneurones in deeply anaesthetized cats by group Ia, Ib and II muscle afferents were found to be facilitated by stimulation of the ipsilateral, as well as of contralateral, PT. The ipsilateral actions were evoked by either uncrossed or double-crossed pathways. The results show that interneurones mediating reflex actions of muscle afferents may be activated strongly enough by PT stimulation to contribute to movements initiated by ipsilateral PT neurones and that PT actions relayed by them might be enhanced by muscle stretches and/or contractions. However, in some motoneurones disynaptic IPSPs and EPSPs evoked from group Ib or II afferents were depressed by PT stimulation. In order to analyse the basis of this depression, the transmitter content in terminals of 11 intracellularly labelled interneurones excited by PT stimulation was defined immunohistochemically and their axonal projections were reconstructed. The interneurones included 9 glycinergic and 2 glutamatergic neurones. All but one of these neurones were mono- or disynaptically excited by group I and/or II afferents. Several projected to motor nuclei and formed contacts with motoneurones. However, all had terminal projections to areas outside the motor nuclei. Therefore both inhibitory and excitatory interneurones could modulate responses of other premotor interneurones in parallel with direct actions on motoneurones.     

Effects on the Ventral Spinocerebellar Tract Neurones from Deiters' Nucleus and the Medial Longitudinal Fascicle in the Cat

Effects from the vestibulospinal tract (VST) and from fibres descending in the medial longitudinal fascicle (MLF) on the cells of origin of the ventral spinocerebellar tract (VSCT) have been studied with intracellular recording. Out of 110 VSCT neurones, the VST evoked monosynaptic EPSPs in 27, di- or polysynaptic EPSPs in 56 and disynaptic lPSPs in 26. In 93 tested VSCT cells, MLF stimulation evoked monosynaptic EPSPs in 26, monosynaptic IPSPs in 2, di- or polysynaptic EPSPs in 25 and disynaptic IPSPs in 21, Convergence of monosynaptic EPSPs from VST and MLF was found in a small proportion of cells whereas the two descending pathways evoked reciprocal effects in another small group of neurones. Convergence of monosynaptic EPSPs from VST or MLF and from group 1 afferents was also modest. In 9 VSCT neurones there was convergence of monosynaptic excitation and disynaptic inhibition from the vestibulospinal tract and the same pattern from MLF was recorded in 9 neurones. The results are discussed in view of the hypothesis that VSCT neurones carry information on the interneuronal transmission in the spinal cord.     

Effects of Volleys in Cortico-spinal Tract Fibres on Ventral Spino-cerebellar Tract Cells in the Cat

Both excitation and inhibition has been found in cells of origin of the ventral spino-cerebellar tract (VSCT) to be evoked by volleys in cortico-spinal fibres. The earliest EPSPs and IPSPs had features of disynaptically evoked postsynaptic potentials; these were, however, found only in a small proportion of cells and polysynaptic EPSPs and IPSPs were dominating. Postsynaptic potentials evoked in VSCT cells from primary afferents were effectively facilitated by cortico-spinal volleys. The cortico-spinal effects on VSCT cells may thus well be mediated by the same interneurones which mediate their excitation or inhibition from the periphery and which could evoke similar postsynaptic potentials in motoneurones. Generally all the observations are in keeping with the hypothesis (Lundberg 1971) that VSCT cells monitor transmission through interneurones interposed in various reflex paths to motoneurones.     

Integration in descending motor pathways controlling the forelimb in the cat

A further analysis has been made of inhibitory pathways to motoneurones via C3-C4 propriospinal neurones (PNs). Intracellular recording was made from triceps brachi motoneurones and effects from higher centres and forelimb afferents on corticospinal IPSPs were investigated after transection of the corticospinal tract at the C5/C6 border. The shortest latencies of the IPSPs evoked by stimulation of the pyramid were as brief as those of the pyramidal EPSPs (Illert et al. 1977). It is postulated that the minimal linkage of the pyramidal IPSPs is disynaptic via inhibitory C3-C4 PNs projecting directly to motoneurones. It was confirmed that pyramidal IPSPs usually are depressed by volleys in forelimb motor axon collaterals (Illert and Tanaka 1978). A quantitative comparison was made of the recurrent depression of pyramidal IPSPs and of IPSPs caused by activation of the Ia inhibitory interneurones. The result support the hypothesis of two parallel inhibitory cortico-motoneuronal pathways via C3-C4 PNs, one disynaptic via the inhibitory PNs and the other trisynaptic via excitatory PNs and Ia inhibitory interneurones. Pyramidal volleys also evoked late IPSPs which in some cases were not depressed from forelimb motor axon collaterals. It is postulated that the late IPSPs are partly due to activation of inhibitory C3-C4 PNs. Disynaptic pyramidal IPSPs were effectively facilitated by volleys in rubro-, tecto- and reticulospinal fibres - but not from vestibulospinal fibres - showing a convergence from the former descending tracts on common inhibitory C3-C4 PNs. Projection from forelimb afferents and corticospinal fibres on common inhibitory C3-C4 PNs was revealed by strong facilitation of disynaptic pyramidal IPSPs from cutaneous forelimb afferents. No corresponding effect was evoked from C2 neck afferents. Stimulation in the lateral reticular nucleus (LRN) evoked monosynaptic IPSPs in some motoneurones. The results of threshold mapping in and around the LRN suggest that the IPSPs are caused by antidromic stimulation of ascending collaterals of inhibitory neurones also projecting to motoneurones, possibly the inhibitory C3-C4 PNs.     

Uncrossed actions of feline corticospinal tract neurones on lumbar interneurones evoked via ipsilaterally descending pathways

Effects of stimulation of ipsilateral pyramidal tract (PT) fibres were analysed in interneurones in midlumbar segments of the cat spinal cord in search of interneurones mediating disynaptic actions of uncrossed PT fibres on hindlimb motoneurones. The sample included 44 intermediate zone and ventral horn interneurones, most with monosynaptic input from group I and/or group II muscle afferents and likely to be premotor interneurones. Monosynaptic EPSPs evoked by stimulation of the ipsilateral PT were found in 12 of the 44 (27%) interneurones, while disynaptic or trisynaptic EPSPs were evoked in more than 75%. Both appeared at latencies that were either longer or within the same range as those of disynaptic EPSPs and IPSPs evoked by PT stimuli in motoneurones, making it unlikely that premotor interneurones in pathways from group I and/or II afferents relay the earliest actions of uncrossed PT fibres on motoneurones. These interneurones might nevertheless contribute to PT actions at longer latencies. Uncrossed PT actions on interneurones were to a great extent relayed via reticulospinal neurones with axons in the ipsilateral medial longitudinal fascicle (MLF), as indicated by occlusion and mutual facilitation of actions evoked by PT and MLF stimulation. However, PT actions were also relayed by other supraspinal or spinal neurones, as some remained after MLF lesions. Mutual facilitation and occlusion of actions evoked from the ipsilateral and contralateral PTs lead to the conclusion that the same midlumbar interneurones in pathways from group I or II muscle afferents may relay uncrossed and crossed PT actions.     

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. I. Disynaptic Ia inhibition of Ia inhibitory interneurones.

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. It was revealed that the Ia inhibitory interneurones themselves receive disynaptic Ia inhibition. The muscles from which this inhibition is evoked are strictly antagonistic to those supplying their Ia excitation. Similar to the Ia inhibition in motoneurones the Ia inhibition in the Ia inhibitory interneurones is decreased when preceded by an antidromic stimulation of ventral roots. Furthermore, transmission of Ia inhibition to the Ia inhibitory interneurones is facilitated from ipsilateral and contralateral primary afferents as well as several supraspinal pathways analogous to earlier findings for the Ia inhibition of motoneurones. The pattern and control of the Ia inhibition of motoneurones and of Ia inhibitory interneurones display so striking similarities that it is suggested that identical interneurones are responsible. The conclusion thus emerges that "opposite" Ia inhibitory interneurones (i.e. interneurones monosynaptically connected to antagonistic muscles) are mutually inhibiting each other. The functional significance of this organization is discussed.     

Convergence on Interneurones Mediating the Reciprocal Ia Inhibition of Motoneurones I. Disynaptic Ia Inhibition of Ia Inhibitory Interneurones

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as “Ia inhibitory interneurones”) were recorded in the lumbar spinal cord of the cat. It was revealed that the Ia inhibitory interneurones themselves receive disynaptic Ia inhibition. The muscles from which this inhibition is evoked are strictly antagonistic to those supplying their Ia excitation. Similar to the Ia inhibition in motoneurones the Ia inhibition in the Ia inhibitory interneurones is decreased when preceded by an antidromic stimulation of ventral roots. Furthermore, transmission of Ia inhibition to the Ia inhibitory interneurones is facilitated from ipsilateral and contralateral primary afferents as well as several supraspinal pathways analogous to earlier findings for the Ia inhibition of motoneurones. The pattern and control of the Ia inhibition of motoneurones and of Ia inhibitory interneurones display so striking similarities that it is suggested that identical interneurones are responsible. The conclusion thus emerges that “opposite” Ia inhibitory interneurones (i.e. interneurones monosynaptically connected to antagonistic muscles) are mutually inhibiting each other. The functional significance of this organization is discussed.     

Integration in descending motor pathways controlling the forelimb in the cat. 4. Corticospinal inhibition of forelimb motoneurones mediated by short propriospinal neurones

Summary A previously described inhibitory trisynaptic cortico-motoneuronal pathway (Illert et al., 1976a) was analysed in order to identify the two relay stations. Intracellular recording was made from motoneurones to elbow muscles. Corticospinal fibres were stimulated in the contralateral medullary pyramid.Pyramidal IPSPs were abolished by a transection of the Corticospinal tract in C2 but remained after a corresponding lesion in C5.After a C5 lesion pyramidal volleys facilitated transmission in the Ia inhibitory pathway with a time course suggesting disynaptic excitatory action on the Ia inhibitory interneurones.The trisynaptic pyramidal IPSPs were depressed by volleys in the appropriate recurrent motor axon collaterals as would be expected if these IPSPs were mediated by Ia inhibitory interneurones.It is concluded that trisynaptic cortico-motoneuronal inhibition is evoked by consecutive activation of propriospinal neurones in C3-C4 and segmental Ia inhibitory interneurones.Supported by the Deutsche ForschungsgemeinschaftIBRO/UNESCO Fellow     

Projection from excitatory C3-C4 propriospinal neurones to lamina VII and VIII neurones in the C6-Th1 segments of the cat

Intracellular recording and injection of horseradish peroxidase (HRP) were made in neurones located medially in lamina VII and in lamina VIII of the forelimb segments (C6-Th1). The cells received disynaptic excitation from the contralateral pyramid after corticospinal transection in C5/C6 and monosynaptic excitation from the ipsilateral lateral reticular nucleus. The pyramidal excitation was facilitated by a conditioning volley evoked from the contralateral nucleus ruber, which suggests convergence of cortico- and rubrospinal fibres on the intercalated neurones. It is proposed that laminae VII and VIII neurones receive a collateral input from the same excitatory C3-C4 propriospinal neurones which project to motoneurones and/or Ia inhibitory interneurones. Reconstruction of HRP-stained lamina VII and VIII neurones revealed ipsi- and contralateral ascending and/or descending axonal projections and termination in laminae VII and VIII in the forelimb segments.     

Cutaneous facilitation of transmission in reflex pathways from Ib afferents to motoneurones.

1. The effect of volleys in low threshold cutaneous afferents upon transmission of synaptic action from Ib afferents to motoneurones has been investigated with intracellular recording from alpha motoneurones to hind limb muscles. 2. There was facilitation from cutaneous afferents of transmission in excitatory and inhibitory reflex pathways from Ib afferents without any evidence for difference in effect on di- and trisynaptic pathways. It is postulated that volleys in cutaneous afferents evoke excitatory action in interneurones of these reflex pathways. 3. The time course of the facilitation suggest that cutaneous afferents have disynaptic excitatory connexions with the interneurones intercalated in the disynaptic Ib inhibitory pathways to motoneurones. 4. Some observations are reported suggesting that interneuronal transmission in Ib inhibitory pathways to motoneurones might be facilitated from Ia afferents. 5. The findings are discussed in relation to the presumed role of Ib reflex action in regulating muscle tension.     

Recurrent inhibition of interneurones monosynaptically activated from group Ia afferents

1. Interneurones monosynaptically excited from large muscle spindle (Ia) afferents and inhibited from motor axon collaterals were searched for in the lumbar spinal cord of the cat.2. Monosynaptic Ia excitation was found in sixty-seven of sixty-nine interneurones inhibited by antidromic volleys. These interneurones were excited from Ia afferents from one or a few muscles (mainly close synergists). Volleys in high threshold muscle and skin afferents (FRA) evoked polysynaptic excitation or inhibition. Weak inhibition from Ia afferents (from antagonists to those giving Ia excitation) was seen in a few cells. Monosynaptic excitation was evoked from the ventral quadrant of the spinal cord and polysynaptic excitation from the dorsal quadrant.3. Inhibition from motor axon collaterals was evoked with a latency (1.2-2.0 msec) suggesting a disynaptic linkage and had the same time course as in motoneurones. It prevented synaptic activation of 60% of interneurones and decreased the firing index and delayed generation of spikes in the remaining.4. The interneurones with convergence of monosynaptic Ia excitation and inhibition from motor axon collaterals were found in the ventral horn dorsomedial to motor nuclei. No inhibition by antidromic volleys could be detected in interneurones located in intermediate nucleus and activated monosynaptically from Ia, Ib, group I or cutaneous afferents.5. It was concluded that the ventral Ia interneurones inhibited by volleys in recurrent motor axon collaterals mediate the reciprocal Ia inhibition to motoneurones.