Identification of possible inhibitory neurons in the pericruciate cortex of the cat

Abstract

The extracellular spike discharges recorded from pairs of neighboring cortical pyramidal tract and non-pyramidal tract neurons were examined for evidence which suggested inhibitory synaptic connections. The results are summarized as follows. (1) The firing frequency of some pyramidal and non-pyramidal tract neurons could be decreased by localized intracortical microiontophoretic (glutamate) stimulation of neighboring non-pyramidal tract neurons. The latter neurons, called I-type cells, could not be excited by acetylcholine. (2) In addition to the reciprocal activity patterns observed with intracortical microiontophoretic stimulation, on-line computation of joint scatter diagrams facilitated the search for 10 pairs of cells which demonstrated unidirectional negative cross-correlation functions, and included one instance where the activity of two I-type neurons was negatively correlated with that of the same pyramidal tract neuron. Latency measurements of the trough in the negative cross-correlograms revealed an onset as brief as 1.9 msec (2 pairs of cells) and as long as 7.3 msec (2 pairs of cells), with trough durations that ranged from 36.1 to 60.4 msec. Stimulation of recurrent or afferent inhibitory pathways evoked either excitation or inhibition from I-ype neurons. (3) These findings suggest that some I-type neurons may represent cortical inhibitory neurons.