Influence of molecular layer on pyramidal tract neurons.

Activity in layer I increases the excitability of pyramidal tract (PT) neurons, the effect being stronger on slow than on fast PT neurons. Extracellular recordings were made from lateral postcruciate cortex of domestic cats, using antidromic activation from medullary pyramid to identify and classify PT neurons. Their responses to contralateral forepaw (CFP) and direct cortical (Ctx) stimulation, 3 to 4 mm caudal to the recording site, were determined before and after placement of vertical cuts between the Ctx stimulating and recording sites. These cuts had a minor effect on the responses of PT neurons to CFP stimulation, but a strong effect on the responses to Ctx stimulation. Cuts through layers I and II markedly delayed the responses of slow PT neurons, but had no effect on fast PT neurons. After deeper cuts (II/III through V/VI), half the fast and half the slow PT neurons failed to respond to Ctx stimulation. Of those that did, fast PT responses were markedly delayed, but slow PT responses were only mildly affected. The Ctx-CFP interactions showed the familiar facilitation-depression sequence. The period of depression was unaffected by any of the vertical cuts, but disappeared after undercutting the stimulus site below layer VI. The period of facilitation depended primarily on layer I for its production, although deeper layers also contributed to the facilitation of fast PT neurons.