SINGLE- AND MULTIPLE-UNIT ANALYSIS OF CORTICAL STAGE OF PYRAMIDAL TRACT ACTIVATION

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Abstract
Electrical responses of the pyramidal tract to cortical shocks were studied in cats and monkeys under Dial or chloralose-tubocurarine anesthesia. Single shocks to contralateral sensorimotor cortex evoked complex responses lasting 15-20 msec. These consisted of a stable early (0.4-1.2 msec.) deflection (D wave) followed by later deflections (I waves) of variable latency and configuration. It is concluded that D wave results from direct activation of pyramidal neurons whereas I waves result from relayed excitation of pyramidal neurons through cortical interneurons because: 1. latency of D wave agreed with conduction time computed from previously determined conduction velocities of large corticospinal axons; 2. D, but not I, waves could be elicited at rates approaching the refractory period of A fibers; 3.I waves were more susceptible to cortical injury and asphyxia than D waves; 4. microstimulation deep in white matter elicited D wave, I waves appeared as stimulating electrode was pulled through gray matter. Single-unit recording indicated that the same pyramidal unit may be fired directly and through cortical interneurons to produce repetitive bursts. Conditioning-testing procedures revealed facilitation (30-40%) of D and I lasting up to 40-50 msec.; facilitation curves sometimes showed peaks corresponding to periodicity of I waves. Responses recorded from the region of pyramidal decussation (Adrian and Moruzzi) were contaminated with spike activity of extrapyramidal (probably bulboreticular) units.

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