Coupled Spike Activity in Micropopulations of Motor Cortex Neurons in Rats

Using eight-channel metal microelectrodes (diameter of a separate channel 12 mu m), we extracellularly recorded the impulse activity of 186 single neurons or their small groups (usually, pairs) localized in the motor cortex of rats anesthetized with ketamine. In 60 cases (32.3%), action potentials (APs) of two single neurons were generated in a parallel manner and demonstrated fixed time relations with each other. This is interpreted as being a result of excitation of two neighboring functionally connected (coupled) cells. These AP pairs could be recorded via one and the same or two neighboring microelectrode channels. Second APs in the pair were elicited exclusively in the case where an AP was preliminarily generated by another neuron, while APs of the latter in some cases could arrive independently. Therefore, "leading" and "accompanying" cells could be identified in such neuronal pairs. The coupling coefficient in the generation of APs by an accompanying unit with respect to APs generated by a leading cell was close to 100%, with no dependence on the discharge frequency in the latter. Intervals between APs of two neurons in different coupled pairs varied from about 1.0 to 22-23 msec. In the case of minimum values of these interspike intervals, APs generated by coupled neurons overlapped each other; this resulted in the formation of spikes looking like "complex APs." Within some time intervals, interspike intervals could increase, and such APs began to be decomposed. The above-described data are considered electrophysiological proof of the existence of tight functional coupling between a significant part of cortical neurons spatially close to each other, i.e., members of a micropopulation, which was obtained in an in vivo experiment.