Lateral spread of neuronal activity within the motor cortex investigated with intracellular responses to distant epicortical stimulation.

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Abstract
Lateral spread of activity within the motor cortex was examined by means of analyses ofthe direct cortical responses (DCRs) and intracellular responses to distant epicortical stimulation (EPICS) using cat encephale isole preparations. DCRs to the EPICS at a distance of 1.5-6.5 mm consisted of initial small positive (Pd) and subsequent negative waves (Nd). The reversal of polarity in depths occurred at 400-550 .mu.m for Pd and at 150-250 .mu.m for Nd as well as for the initial negative wave elicted by near EPICS. Intracellular responses to distant EPICS consisted of excitatory (EPSP) and inhibitory postsynaptic potentials (IPSP), disfacilitation (DF), and disinhibition (DI). Depth distributions of cells with EPSP at 2 peaks in laminae II and V-VI, and IPSP mainly in lamina III, and with DF or DI in laminae V-VI were the same with those by near EPICS. The inhibitory effects of distant EPICS on middle layer cells were much greater than those by near EPICS. No linear relations of the latency of EPSP or IPSP to the depth were seen for distant EPICS. Instead, the latency increased in proportion to the lateral distance in EPSP at a slower rate than in IPSP compared in superficial and middle layer cells. Several routes for lateral spread of activity were postulated. Most conspicuous are the excitatory route via horizontal axons in lamina I and the inhibitory route via laterally running axons in laminae II-III, which produce overall excitation of superficial layer cells of depression of middle and deep layer cells. Their possible role in phasic cortical arousal was discussed.

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