ACTIONS OF STIMULATION TO MEDULLARY PYRAMID ON SINGLE NEURONS IN CAT'S MOTOR CORTEX By

Abstract

Summary The electrical activities of single neurons in the anterior sigmoid gyrus of 45 adult cats were picked up intracellularly or extracellularly by means of microelectrode technique. The response patterns elicited by single shock applied to the medullary pyramid were recorded. The neurons which showed antidromic spikes with short and constant latency in response to the shock were located in cortical layer V and were identified as pyramidal tract neurons. The latencies of the spikes were 0.45–4.2 msec., and the calculated conduction velocities were within the range of 10.0–93.3 m/sec., they tended to be distributed in groups of three or four. Among 17 pyramidal tract neurons which were recorded intracellularly, 8 showed waves of hyperpolarization with latencies of 2.4–4.2 msec., these waves were supposed to be antidromic IPSPs generated by way of antidromic inhibitory pathway. Some neurons showed response of a wave of depolarization with a latency of 2.3–7.0 msec., spike firing being discernible with various latencies, but no antidromic spike firing. Such neurons were distributed chiefly in cortical layers III and IV, and the wave of depolarization thereof was regarded as the EPSPs caused by the current spread of the stimulation to the medial lemniscus, and by way of ascending sensory pathway. Then there were other neurons which responded with a wave of hyperpolarization with a latency of 4.0–11.5 msec. and of the time course of a few score msec. These lay chiefly in the cortical layers III and IV. Their latencies were evidently longer than that of the antidromic IPSPs of the pyramidal tract neurons, so these were supposed to be the IPSPs generated by the stimulus impulses travelling along the ascending sensory pathway. Records were also taken from the neurons, although there were only 3, in which both EPSPs and IPSPs were elicited. Some neurons showed in the extracellular recording the high frequency spike discharges of several hundred cycles per second. They were widely distributed in the cortical layers except in layer I. From their characteristic response type, which resembles that of an inhibitory interneuron in spinal cord, they were supposed to be cortical inhibitory interneurons. Among them, those responses of short latency were considered to constitute, together with the recurrent collaterals of the pyramidal tract neurons, the antidromic inhibitory pathway, while those with longer latencies were thought to participate in generation of the IPSPs in the pyramidal cells in the cortical layers III and IV. The antidromic spikes as well as the postsynaptic potentials of the above 3 kinds were compared with each component of the surface evoked potential, and the correlation between the two was discussed. It was asserted that the initial positive spike and the 2 or 3 spike-like small potentials following it chiefly represent the antidromic spikes of pyramidal tract neurons lying in layer V, while the indistinct initial negative wave represents the antidromic IPSPs and the surface positive slow wave, the EPSPs of the pyramids lying in layers III and IV, and the surface slow negative wave chiefly represents the IPSPs of the pyramids.