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

1. 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 duration of up to about 5 sec.2. 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.3. 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.4. 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 characterstics.5. It is concluded that P cells are principal cells projecting to visual cortex and that I cells are interneurones.