Corticocortical synaptic influences on morphologically identified pyramidal neurones in the motor cortex of the monkey.

1. Corticocortical synaptic influences on pyramidal neurones in the precentral motor cortex of monkeys were examined using intracellular recordings. Corticocortical afferents from the postarcuate premotor area and the somatic sensory cortical areas were activated by bifocal stimulation of the cortical surface. Neurones that were found to respond orthodromically to such stimuli were labelled by intracellular ionophoresis of horseradish peroxidase. 2. Almost all neurones that were penetrated satisfactorily and labelled successfully were found to be pyramidal neurones located in lamina III or lamina V. Some labelled neurones in lamina V were also characterized as pyramidal tract neurones (PTNs) by antidromic activation from the cerebral peduncles or medullary pyramids. 3. Pyramidal neurones located in lamina III and lamina V (including PTNs) were excited at short latency by stimulation of the premotor cortex (1.1-4.0 ms) and somatosensory cortex (1.1-6.5 ms). There were no statistical differences in the distributions of latencies of corticocortical EPSPs between those evoked in lamina III neurones and those recorded in lamina V neurones, or between corticocortical EPSPs evoked from the premotor cortex in comparison with those from the somatosensory cortex. Excitatory responses to stimulation of the premotor area were usually more difficult to evoke and smaller in amplitude than those produced by stimulation of the somatosensory areas. 4. Corticocortical EPSPs were often followed by IPSPs. The amplitudes of the EPSPs and IPSPs could be increased by increasing the stimulus intensity. In a few neurones IPSPs that were not preceded by EPSPs were recorded.