Intracellular records of the effects of primary afferent input in lumbar spinoreticular tract neurons in the cat

1. The afferent-evoked synaptic input to lumbar spinal cord (L5-S1) neurons that were activated antidromically from the medial pontomedullary reticular formation (nucleus reticularis gigantocelluaris and vicinity) was investigated with the use of intracellular recordings in pentobarbital sodium-anesthetized cats. 2. Spinoreticular tract (SRT) neurons (n = 33) were categorized into three types ("deep-inhibited," "deep-complex," and "intermediate") on the basis of their locations and of their responses to natural and electrical stimulation. 3. The deep-inhibited-type neurons, located in the medial part of the deeper laminae (approximately VI-VIII), comprised a large component of the sample (20/33). They had no demonstrable excitatory receptive field (RF). However, electrical stimulation of low-threshold cutaneous afferents of hindlimb nerves evoked inhibitory postsynaptic potentials (IPSPs) via an oligosynaptic linkage. High-threshold cutaneous and muscle afferents also evoked IPSPs. 4. In the deep-complex-type neurons (8/33), electrical stimulation of low-threshold cutaneous afferents evoked complex IPSP-excitatory postsynaptic potential (EPSP) sequences. With intense stimuli, long-latency C-fiber-like EPSPs were evoked. Two of these eight neurons were characterized as wide-dynamic-range (WDR) neurons with large, excitatory and inhibitory cutaneous RFs. 5. Intermediate-type neurons (5/33) were concentrated in the lateral spinal gray and relatively superficially (approximately lamina V). These neurons had convergent low- and high-threshold cutaneous inputs (WDR neurons). Electrical stimulation of low-threshold cutaneous afferent fibers from within the excitatory RF evoked mono- or disynaptic EPSPs followed by IPSPs. High-threshold muscle and cutaneous afferents also evoked EPSPs. 6. These results show that SRT neurons have a variety of response characteristics resulting from various degrees of spatial and temporal summation of primary afferent input. Neurons with widespread inhibitory responses but no excitatory drive from the periphery comprise a surprisingly large component of the SRT: the function of these cells is unknown. It is apparent that the spinoreticular projection has considerable functional heterogeneity.     

Reticulospinal inhibition of interneurones

1. The effect of electrical stimulation of the brain stem on interneurones in the dorsal horn and intermediary region has been investigated in decerebrate cats after partial transection of the spinal cord.2. Stimuli that effectively depress reflex transmission without giving a primary afferent depolarization inhibit the discharge evoked from the flexor reflex afferents in interneurones.3. Brain stem stimulation did not give post-synaptic potentials in the great majority of interneurones but effectively depressed the excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs) evoked from the flexor reflex afferents in these interneurones.4. IPSPs were, however, evoked in five of seventy-eight intracellularly recorded interneurones. These five interneurones were monosynaptically activated from primary afferents.5. It is tentatively postulated that a dorsal reticulospinal system inhibits reflex transmission by giving post-synaptic inhibition in first order interneurones. The results are also discussed in relation to effects on interneurones from other descending pathways.     

Do premotor interneurons act in parallel on spinal motoneurons and on dorsal horn spinocerebellar and spinocervical tract neurons in the cat?

It has previously been established that ventral spinocerebellar tract (VSCT) neurons and dorsal spinocerebellar tract neurons located in Clarke's column (CC DSCT neurons) forward information on actions of premotor interneurons in reflex pathways from muscle afferents on α-motoneurons. Whether DSCT neurons located in the dorsal horn (dh DSCT neurons) and spinocervical tract (SCT) neurons are involved in forwarding similar feedback information has not yet been investigated. The aim of the present study was therefore to examine the input from premotor interneurons to these neurons. Electrical stimuli were applied within major hindlimb motor nuclei to activate axon-collaterals of interneurons projecting to these nuclei, and intracellular records were obtained from dh DSCT and SCT neurons. Direct actions of the stimulated interneurons were differentiated from indirect actions by latencies of postsynaptic potentials evoked by intraspinal stimuli and by the absence or presence of temporal facilitation. Direct actions of premotor interneurons were found in a smaller proportion of dh DSCT than of CC DSCT neurons. However, they were evoked by both excitatory and inhibitory interneurons, whereas only inhibitory premotor interneurons were previously found to affect CC DSCT neurons [as indicated by monosynaptic excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in dh DSCT and only IPSPs in CC DSCT neurons]. No effects of premotor interneurons were found in SCT neurons, since monosynaptic EPSPs or IPSPs were only evoked in them by stimuli applied outside motor nuclei. The study thus reveals a considerable differentiation of feedback information provided by different populations of ascending tract neurons.     

The lateral reticular nucleus in the cat

The afferent paths from the spinal cord and from trigeminal afferents to the lateral reticular nucleus (LRN) were investigated by intracellular recording from 204 LRN neurones in preparations with a spinal cord lesion at C3 that spared only the ipsilateral ventral quadrant. Stimulation of nerves in the limbs evoked EPSPs and JPSPs in 201 of 204 tested LRN neurones. The strongest input was from the ipsilateral forelimb (iF) which evoked EPSPs in 49% and IPSPs in 73% of the LRN neurones. Each of the other limbs evoked EPSPs in approximately 20% and IPSPs in approximately 25% of the neurones. Stimulation of the ipsilateral trigeminal nerve (iTrig) evoked EPSPs in 32% and IPSPs in 46% of the neurones. The shortest latencies of the EPSPs and IPSPs indicated a disynaptic connection between primary afferents in the iF and iTrig and the LRN. The most direct pathways for excitatory and inhibitory responses from the other limbs were trisynaptic. Stimulation of the ventral part of the ipsilateral funiculus (iVLF) at C3 (C3iVLF) evoked monosynaptic responses in 189 of 201 tested LRN neurones. Monosynaptic EPSPs were recorded in 104 neurones and monosynaptic IPSPs in 126 neurones. Monosynaptic EPSPs and IPSPs were encountered in all parts of the LRN. Stimulation of the iVLF at L1 (L1iVLF) evoked monosynaptic EPSPs and IPSPs in the ventrolateral part of the LRN. The termination areas of excitatory and inhibitory fibres appeared to be the same. LRN neurones without monosynaptic EPSPs or IPSPs from the L1iVLF were located mainly in the dorsal part of the magnocellular division. Stimulation of the dorsal funiculi (DF) at C2 and the ipsilateral trigeminal nerve (iTrig) evoked excitatory and inhibitory responses in the LRN. The shortest latencies of EPSPs and IPSPs indicated disynaptic connections.     

Same spinal interneurons mediate reflex actions of group Ib and group II afferents and crossed reticulospinal actions

The aim of the study was to analyze interactions between neuronal networks mediating centrally initiated movements and reflex reactions evoked by peripheral afferents; specifically whether interneurons in pathways from group Ib afferents and from group II muscle afferents mediate actions of reticulospinal neurons on spinal motoneurons by contralaterally located commissural interneurons. To this end reticulospinal tract fibers were stimulated in the contralateral medial longitudinal fascicle (MLF) in chloralose-anesthetized cats in which the ipsilateral half of the spinal cord was transected rostral to the lumbosacral enlargement. In the majority of interneurons mediating reflex actions of group Ib and group II afferents, MLF stimuli evoked either excitatory or inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) or both EPSPs and IPSPs attributable to disynaptic actions by commissural interneurons. In addition, in some interneurons EPSPs were evoked at latencies compatible with monosynaptic actions of crossed axon collaterals of MLF fibers. Intracellular records from motoneurons demonstrated that both excitation and inhibition from group Ib and group II afferents are modulated by contralaterally descending reticulospinal neurons. The results lead to the conclusion that commissural interneurons activated by reticulospinal neurons affect motoneurons not only directly, but also by enhancing or weakening activation of premotor interneurons in pathways from group Ib and group II afferents. The results also show that both excitatory and inhibitory premotor interneurons are affected in this way and that commissural interneurons may assist in the selection of reflex actions of group Ib and group II afferents during centrally initiated movements.     

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.     

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.     

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.     

Recurrent inhibition from motor axon collaterals of transmission in the Ia inhibitory pathway to motoneurones

1. The effects of impulses in recurrent motor axon collaterals on reflex transmission from different types of primary afferents to motoneurones were investigated in the cat by conditioning of PSPs evoked in motoneurones.

2. IPSPs evoked by volleys in large muscle spindle (Ia) afferents were effectively decreased when preceded by an antidromic stimulation of ventral roots. Some IPSPs from group II muscle afferents and low threshold cutaneous afferents were also slightly depressed, while other PSPs were unaffected.

3. The depression of the IPSPs could be evoked by antidromic volleys, which produced neither conductance changes in the motoneurones nor depolarization of Ia afferent terminals.

4. The effect on the Ia IPSPs is most likely due to post-synaptic inhibition of the Ia inhibitory interneurones, evoked through α-motor axon collaterals and Renshaw cells. The depression of some IPSPs from flexor reflex afferents is explained by a convergence of excitatory effects from these afferents on the Ia inhibitory interneurones.

5. The results indicate a selective recurrent control from motor axon collaterals of the interneurones in the reciprocal Ia inhibitory pathway to 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.     

Long-term change in synaptic transmission in CA3 circuits followed by spontaneous rhythmic activity in rat hippocampal slices

The relevance of long-term potentiation (LTP) at excitatory synapses in CA3 circuits to generation of spontaneous epileptiform bursts in CA3 was investigated using rat hippocampal slices. CA3 pyramidal cells were antidromically stimulated through Schaffer collaterals. Evoked field potentials were extracellularly recorded from the stratum pyramidale and the stratum radiatum in CA3. Therefore, field potentials reflecting recurrent excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs) were positive at the stratum pyramidale and negative at the stratum radiatum. First, we tested how the amplitude of the evoked field potentials depends on a γ-aminobutyric acid (GABA_A) antagonist. Both of the positive and negative field potential peaks reduced in the medium containing penicillin (2 mM) or bicuculline (20 μM). This suggests that unmasked EPSPs due to suppression of IPSPs do not result in an increase in the evoked potentials. Second, CA3 pyramidal cells were antidromically stimulated by tetanic stimulation of Schaffer collaterals in order to induce LTP at synapses in CA3 circuits. Both of the positive and negative field potentials increased, suggesting that recurrent EPSPs were enhanced by tetanic stimulation. Induction of LTP at recurrent excitatory synapses was followed by spontaneous epileptiform bursts which persisted throughout experiments (1.5 h), while LTP of afferent synaptic potential evoked by hilar test stimulation was not induced. These results suggest that LTP at the afferent synapses is not necessary to spontaneous epileptiform bursts in CA3, but LTP at excitatory synapses between CA3 pyramidal cells contribute to spontaneous epileptiform bursts.     

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.     

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.     

The vestibulo-ocular reflex arc in the newborn kitten

Summary Field potentials and postsynaptic potentials were recorded in the vestibular and abducens nuclei and neurons following vestibular nerve stimulation in anesthetized newborn kittens (within 72 h after birth). Stimulation of the ipsilateral vestibular nerve evoked an initial P wave and an N₁ field potential in the vestibular nuclei. No N₂ potential was evoked. Latencies of the peak of the P wave, the onset and the peak of the N₁ potential were 0.99±0.16 ms, 1.66±0.18 ms, and 2.51±0.23 ms, respectively. Ipsilateral vestibular nerve stimulation evoked monosynaptic excitatory postsynaptic potentials (EPSPs) and polysynaptic inhibitory postsynaptic potentials (IPSPs) in vestibular nuclear neurons. Stimulation of the contralateral vestibular nerve evoked polysynaptic IPSPs in vestibular nuclear neurons. In abducens motoneurons, ipsilateral vestibular nerve stimulation evoked monosynaptic EPSPs and disynaptic IPSPs; contralateral vestibular nerve stimulation produced disynaptic EPSPs. We conclude that short circuit pathways of the vestibul-ovestibular and vestibulo-ocular reflex arc are present in the kitten already at birth.Supported by the Japanese Ministry of Education, Science, and Culture Grants-in-Aid for Scientific Research nos. 572 140 30 and 575 700 53     

Neuronal activity in the lateral vestibular nucleus of the cat

The synaptic input to Deiters neurones evoked by stimulation of peripheral somatic nerves was measured by intracellular recordings. EPSPs with broad receptive fields and latencies which indicate polysynaptic connexions were commonly evoked from the FRA. In other cells, low threshold cutaneous afferents were effective at rather short latencies suggesting oligosynaptic connexions from fast ascending fibres. One example was found of EPSPs due to low threshold muscle afferents. IPSPs due to climging fibre activation of Purkinje cells as observed in most of the neurones were evoked by cutaneous volleys above 1.5-2.0T and muscle volleys above 5T (above 3-3.5T in case of Q). Often, IPSPs were evoked by stimulation of nerves, to the segmental level of which the the vestibulospinal neurone under investigation projected. A small proportion of cells received short latency IPSPs involving direct fast mossy fibre tracts, which were evoked from low threshold cutaneous afferents. IPSPs due to polysynaptic mossy fibre activation of Purkinje cells were evoked from the FRA bilaterally and from ipsilateral cutaneous afferents at 1.5-2.0T ("prolonged inhibition"). Prolonged excitatory/inhibitory events mediated by mossy fibre pathways may be involved in quadruped locomotion or other processes making use of a broad motor integration.     

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).     

Differential presynaptic inhibition of actions of group II afferents in di- and polysynaptic pathways to feline motoneurones

The aim of this study was to investigate differences in the effects of presynaptic inhibition of transmission from group II muscle afferents to neurones in the dorsal horn and in the intermediate zone and the consequences of these differences for reflex actions of group II afferents upon α-motoneurones. The degree of presynaptic inhibition was estimated from the degree of depression of monosynaptic components of population EPSPs (field potentials) evoked by group II muscle afferents in deeply anaesthetized cats. The decrease in the area of field potentials was considerably larger and longer lasting in the intermediate zone, where they were often obliterated, than in the dorsal horn, where they were reduced to about two-thirds. Presynaptic inhibition of field potentials evoked by other afferents at the same locations was much weaker. Intracellular records from α-motoneurones revealed that short latency EPSPs and IPSPs evoked from group II afferents are considerably reduced by conditioning stimuli that effectively depress intermediate zone field potentials of group II origin. The results of this study lead to the conclusion that strong presynaptic inhibition of transmission to intermediate zone interneurones allows a selective depression of disynaptic actions of group II muscle afferents on α- and γ-motoneurones, mediated by these interneurones, and favours polysynaptic actions of these afferents.     

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.     

Pyramidal actions in identified radial motornuclei of the cat

This study aimed to establish the projection from the corticospinal tract (CST) to the motoneurones innervating the deep radial (DR) forelimb muscles. In the anaesthetized cat stimulation of the contralateral pyramid and intracellular recording from identified forelimb motoneurones was used. A train of pyramidal stimuli evoked disynaptic EPSPs in DR motoneurones. The effects were very similar in the different nuclei. Pyramidal IPSPs had a slightly longer latency and occurred in most cases together with disynaptic EPSPs. It is suggested that the inhibitory actions to the distal forelimb are predominantly relayed in a trisynaptic pathway, but that a disynaptic linkage seems possible as well. The disynaptic pyramidal EPSPs remained after CST transection in C5. They were abolished after CST transections in C2. It is concluded that disynaptic corticospinal excitation of distal DR motornuclei is relayed in a short midcervical propriospinal system. Transection experiments at different cervical levels suggest that the majority of the propriospinal neurones is located in C3-C4. The CST facilitated a variety of reflex pathways to motoneurones innervating distal forelimb muscles. Disynaptic excitatory and inhibitory effects from cutaneous and low threshold group I muscle afferents were common. They were present in all investigated nuclei and powerfully facilitated from the CST. It is suggested that this allows the brain to adapt the reflex mechanisms of the distal forelimb to the synergistic-antagonistic relations between the muscles, which are changing according to the performed movement.     

Synaptic organization of the tectal-facial pathways in the cat. I. Synaptic potentials following collicular stimulation

1. The synaptic pathways underlying tectal influence over pinna movements were studied using an acute electrophysiological approach. Under pentobarbital anesthesia, postsynaptic potentials were recorded intracellularly in antidromically identified, cat facial motoneurons following electrical stimulation of the superior colliculus. How collicular topography is reflected in these synaptic potentials was examined using multiple stimulation sites. The pathways responsible for tectally evoked synaptic potentials were studied by making acute brain stem lesions and by intra-axonal horseradish peroxidase (HRP) staining. 2. Monosynaptic excitatory potentials (EPSPs) with latencies ranging from 0.7 to 1.1 ms and amplitudes that were always less than 1 mV were recorded in motoneurons following stimulation of the contralateral superior colliculus. Larger disynaptic EPSPs ranging in latency from 1.2 to 2.0 ms were recorded both in isolation and in association with monosynaptic EPSPs. In addition, disynaptic inhibitory synaptic potentials (IPSPs) with latencies ranging from 1.5 to 2.5 ms were observed, often in combination with monosynaptic EPSPs. Both disynaptic EPSPs and IPSPs were graded, augmented by multiple stimuli and found in all categories of motoneurons. 3. Stimulation of the ipsilateral superior colliculus produced nearly the same spectrum of potentials and latencies as did contralateral tectal stimulation. Occlusion between ipsi- and contralaterally evoked IPSPs suggests there might be a common element in the inhibitory disynaptic pathways. 4. More discrete populations of facial motoneurons were investigated. Specifically, motoneurons innervating the platysma and orbicularis oculi muscles, the intrinsic ear muscles, and muscles that move the vibrissae all displayed tectally elicited mono- and di-synaptic potentials. Collicular input was not restricted to motoneurons involved in orienting the pinnae. 5. The presence, polarity, and amplitude of the synaptic potentials evoked in individual facial motoneurons exhibited variations that were related to the site of stimulation in either the ipsi- or contralateral colliculus. These variations are compatible with the idea that the collicular input to facial motoneurons is topographically organized. 6. Acute lesions at the level of the superior olive indicated that the pathway producing the contralateral monosynaptic EPSPs runs, near the midline, ipsilateral to the target facial nucleus, whereas the contralateral disynaptic and the ipsilateral mono- and disynaptic pathways lie further lateral.(ABSTRACT TRUNCATED AT 400 WORDS)