Organization of neurones in the cat cerebral cortex that are influenced from Group I muscle afferents

Abstract

Neurones in the Group I projection area of the 1st sensori-motor cortex were investigated with extra-and intracellular technique. The majority of the neurones influenced by volleys in Group I afferents of contralateral forelimb nerves received exclusively excitation, but some received exclusively inhibition, andsomemixedexcitationandinhibition. The Group I influenced cells were usually found 500-1500 [mu] beneath the cortical surface. The EPSPs [excitatory post- synaptic potentials] and IPSPs [inhibitory post-synaptic potentials] evoked from Group I afferents had a steep risingphase and a slow, approximately exponential decay. The duration was sometimes more than 50 msec. The EPSP evoked by a maximal Group I volley was often formed by a small number of large unitary EPSPs. Latency measurements indicate that the majority of the Group I activated neurones were monosynaptically excited from the thalamic fibers, and hence constitute the 4th-order neurones in the Group I projection system. The latencies of the IPSPs suggest a disynaptic linkage with thalamic fibers. Hence, the exclusively inhibited cells would constitute 5th-order neurones. It is suggested that most or all of the 4th-order neurones are inhibitory. The convergence of excitation and/or inhibition to individual cells was usually extensive and included effects not only from muscle groups working at different joints but also effects from antagonistic groups at the same joint. In addition cutaneous afferents contributed synaptic actions which had a longer latency than the synaptic actions from Group I afferents. The neurones influenced from Group I afferents were not antidromically activated on stimulation of the pyramidal tract.