Excitation of pyramidal tract cells by intracortical microstimulation: effective extent of stimulating current.

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Abstract
Thresholds for direct and synaptic activation of pyramidal tract cells by intracortical microstimulation (ICMS) were examined in cat motorsen-sory cortex. A total of 39 cells was studied by determining threshold for direct excitation by current pulses applied through a microelectrode which was used for both stimulation and recording at different points in their extracellular fields. Threshold current varies as a parabolic function of the distance from the cell. The minimum threshold point coincides with the depth that antidromic spike amplitude recorded by the stimulating electrode reaches its maximum value. Cathodal current pulses are 3-12 times more effective than anodal pulses for direct excitation. Direct excitation produced by weak ICMS is restricted to neurons in the immediate vicinity of the stimulating microelectrode: a 10 [mu]A cathodal current pulse (0.2 msec duration) directly excites only those neurons located within a sphere of 80-90 microns radius from the electrode tip. Long latency (0.8-6.0 msec), synaptic excitation could be elicited with currents of 10 [mu]A or less from points as far as 600 microns from the cell under study.