Two different mechanisms control inhibition of spike discharge in neurons of cat motor cortex after stimulation of the pyramidal tract.

Intracellular recordings were made from neurons of the motor cortex of awake cats while the pyramidal tract (PT) was stimulated at the level of the facial nucleus. In some neurons IPSPs of 35-120 ms peak latency were recorded that diminished in size or reversed with hyperpolarizing current. During these IPSPs a decrease in input resistance reflective of a conductance increase was measured. More often, however, PT stimulation produced IPSPs with comparable latencies that increased in size with hyperpolarizing current. These IPSPs diminished with depolarizing current, and in some instances they appeared to reverse with strong depolarization. During these IPSPs an increased input resistance reflective of a decreased conductance was measured. The results indicate that two different mechanisms control rapid inhibition of spike discharge in neurons of the motor cortex after PT stimulation.