Electrophysiological study of the perigeniculate region during natural sleep in the cat

Abstract

A technique is described which permits glass micropipet recordings from the thalamus of undrugged, spontaneously breathing cats during waking and natural sleep. An inhibitory extrageniculate feed-back loop on lateral geniculate body (LGB) principal cells has been described from anatomical and electrophysiological experiments in acutely prepared animals. We identified the perigeniculate (PG) neurons and reevaluated their role in free-behaving animals. Four populations of units located above the LGB were classified according to their responses to optic tract (OT) and visual cortex (VC) stimulation and their spontaneous activity. Relay axons responded with a single action potential to OT and VC stimulation. Corticofugal axons responded with a burst of action potentials of longer latency to OT than to VC stimulation. Arguments are presented which suggest that their cell bodies are situated in the visual areas including the Clare and Bishop area. Thalamic reticularis neurons were unresponsive to OT and VC stimulation. Perigeniculate neurons responded with several action potentials to OT and VC stimulation (mean latencies: 2 ± 0.68 and 1.6 ± 0.61 ms, respectively) followed by a long-lasting (200 to 500 ms) negative shift of the membrane potential and a rebound discharge. Although anatomically the PG neurons belong to the thalamic reticularis nucleus they are readily identified by their responses to OT and VC stimulation and their firing patterns during natural sleep. Firing arrest and rebound discharge after OT and VC stimulation were absent in LGB principal cells during paradoxical sleep but not in PG neurons. The firing arrest of PG neurons when stimuli were applied during paradoxical sleep could lead to desinhibition of relay neurons and thus have facilitated transmission of afferent impulses.