Discharge patterns of principal cells and interneurones in lateral geniculate nucleus of rat

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Abstract
The cells of the lateral geniculate nucleus of the rat are classified into one of two distinct groups on the basis of their responses to a single shock to optic nerve or visual cortex. The distinctive difference between these two groups is that the P cells (87% of all cells) responded with short bursts of 1-5 spikes, whereas the I cells (13%) responded with long bursts of about 10 spikes. Both groups give recurring bursts at intervals of 100 msec or more for a total dura?tion of up to about 5 sec. The majority of P cells (75%) have a lower threshold for late firing than for early firing. The majority of I cells (62%) have about the same threshold and all I cells have very low thresholds. To stimulation of the optic nerve the mean latency of the first spikes of all P cells is 4.4 msec. The mean latency of the first spikes of all I cells is 5.8 msec. The latency of the earliest spike in an I cell is 0.9 msec longer than the earliest spike in a P cell. To stimulation of the visual cortex the latency of the earliest spike in an I cell is 1.0 msec longer than the earliest spike in a P cell. Most P cells (78%) respond with either very short latency (less than 1.7 msec) or with a wave form characteristic of antidromic invasion or both. No I cell has either of these characteristics. It is concluded that P cells are principal cells projecting to visual cortex and that I cells are interneurones.