Abstract
1. Neurones in the Group I projection area of the first sensori-motor cortex were investigated with extra- and intracellular technique.2. The majority of the neurones influenced by volleys in Group I afferents of contralateral forelimb nerves received exclusively excitation, but some received exclusively inhibition, and some mixed excitation and inhibition.3. The Group I influenced cells were usually found 500-1500 mu beneath the cortical surface.4. The EPSPs and IPSPs evoked from Group I afferents had a steep rising phase 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.5. Latency measurements indicate that the majority of the Group I activated neurones were monosynaptically excited from the thalamic fibres, and hence constitute the fourth-order neurones in the Group I projection system. The latencies of the IPSPs suggest a disynaptic linkage with thalamic fibres. Hence, the exclusively inhibited cells would constitute fifth-order neurones. It is suggested that most or all of the fourth-order neurones are inhibitory.6. 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.7. The neurones influenced from Group I afferents were not antidromically activated on stimulation of the pyramidal tract.