Post-synaptic effects of cortical stimulation on forelimb motoneurones in the baboon.

1. The arm area of the baboon's precentral motor cortex was stimulated by brief surface-anodal pulses, and the post-synaptic potentials elicited in contralateral forelimb motoneurones were studied by intracellular recording.2. Strong cortical stimuli elicited a rapid series of excitatory and, in some cells, inhibitory post-synaptic potentials (EPSPs and IPSPs respectively). Comparisons with the simultaneously recorded response of the pyramidal tract indicated that these post-synaptic potentials were due to a repetitive discharge of fast pyramidal fibres. Thus, the later synaptic events were mostly due to a repetition of the early monosynaptic EPSP and early IPSP respectively.3. Inhibition was seen more often in cells whose monosynaptic EPSP had a small maximal size than in those whose monosynaptic EPSP was larger. The net depolarization produced by a strong cortical stimulus was related to the maximal size of the early monosynaptic EPSP.4. In the Discussion, an interpretation is suggested for previous findings concerning the spinal distribution of late synaptic effects elicited by cortical stimulation.