The lateral reticular nucleus in the cat

Summary

Intracellular recordings were obtained from 204 neurones in the lateral reticular nucleus (LRN). LRN neurones contacted by the bVFRT were identified by the responses evoked on stimulation of descending fibres in the contralateral ventral quadrant of the spinal cord (cVQ) at cervical (C5cVQ) and lumbar (L2cVQ) levels. Stimulation of the cVQ evoked excitatory or inhibitory responses in 124 of the 204 LRN neurones. EPSPs were evoked in 45, IPSPs in 52 and both EPSPs and IPSPs in 27 LRN neurones. The shortest latencies of the responses evoked from the cVQ indicated that both EPSPs and IPSPs were disynaptic. This finding was confirmed by direct stimulation of the ascending fibres in the ipsilateral ventrolateral funiculus at C3 (C3iVLF) or LI (LliVLF). In most LRN neurones activated or inhibited from the cVQ, stimulation of the iVLF evoked similar responses at a monosynaptic latency. These results indicate that the bVFRT consists of roughly equally large groups of excitatory and inhibitory neurones monosynaptically connected with the LRN. Excitatory and inhibitory bVFRT neurones had similar peripheral receptive fields and termination areas in the LRN. LRN neurones were divided into those contacted by cervical bVFRT neurones and lumbar bVFRT neurones. The former group consisted of LRN neurones responding to C5cVQ stimulation at latencies below 5 ms, whereas the latter group contained LRN neurones responding to stimulation of the L2cVQ. Cervical bVFRT neurones projected to most parts of the LRN whereas the projection of lumbar bVFRT neurones were confined to the ventrolateral part of the nucleus. Excitatory and inhibitory bVFRT neurones of each group had similar termination areas. About half of the LRN neurones contacted by cervical bVFRT neurones did not respond to stimulation of the contralateral forelimb (cF) nerve. These bVFRT neurones formed a separate group which terminated in the dorsomedial part of the LRN. Cervical bVFRT neurones activated by the cF terminated in the ventrolateral part of the nucleus. The conduction velocity between the L1 and C3 segments was determined for axons of lumbar bVFRT neurones. The velocities ranged between 55–137 m/s (mean 90.6 m/s) for excitatory neurones and between 35–120 (mean 87.5 m/s) for inhibitory neurones. Monosynaptic responses, particularly EPSPs, were frequently evoked from the L1iVLF in LRN neurones with a bVFRT input from cervical segments only. The results suggest that many excitatory cervical bVFRT neurones have fast conducting descending axon branches projecting to the lumber cord. Long descending axon branches seemed to be less common among inhibitory cervical bVFRT neurones.