Abstract

1. Methods have been described by which neurones in the motor cortex of cats have been successfully impaled by intracellular microelectrodes. 2. The majority of the cells thus entered have shown large stable resting potentials of ‐ 60 to ‐ 90 mV, without natural subthreshold voltage fluctuation or any discharge of impulses. Electrical stimulation of the overlying cortex did not excite them, even when it was strong enough to cause movements of the limbs. 3. Twenty‐six cells impaled in 69 experiments were proved, by their response to antidromic stimulation of the medullary pyramid, to be cells of origin of pyramidal axones (loosely described as Betz cells throughout this paper). 4. These cells were naturally active from time to time under the light anæsthesia employed. Their membrane potentials oscillated below the threshold for firing, and, whenever this level was reached, impulses were discharged, sporadically or in regular trains at frequencies up to 50 c./s. 5. Stronger stimulation of the pyramidal tract caused delayed excitatory depolarizations and inhibitory hyperpolarizations to follow the antidromic impulses which invaded the Betz cells.