Reactions of the geniculate cells to extraocular proprioceptive activation in rabbits.

Abstract

The aim of these investigations was to advance our knowledge of the influence of extrinsic ocular muscle (EOM) stretching on cell excitability of the retinocortical pathway. The initial goal was the lateral geniculate; however, histologic analysis indicated that most cells that responded to stretching were located near its superior edge, in an area analogous to the perigeniculate. Rabbits were anaesthetized and prepared for single-cell recordings. The extraocular muscles were detached from the eye and attached to a rigid tungsten hook. The hook was soldered to the pivot of a galvanometer that was controlled by a waveform generator. Precise and repeated length changes were obtained with extension and relaxation ramps of constant velocity. Thirty percent of the units (N = 250) responded to EOM stretching. These stimuli evoked either an excitation or a decrease of the units' spontaneous activity. Several experimental controls provided evidence that the signals that evoked the geniculate responses originated from proprioceptors belonging to the EOM. Most responding cells had their receptive fields located eccentrically (greater than or equal to 50 degrees). Also, cells that reacted to EOM stretching responded to optic nerve stimulation with a significantly shorter latency than cells that were unresponsive. Pairing the EOM stretching with light stimuli produced the following results. In about half of the units the light-evoked responses augmented, but in 28% of the cells the light-evoked discharges decreased. These effects were obtained even though EOM stretching delivered singly failed to elicit a reaction from the cell. Histologic reconstructions indicated that cells were distributed in a discrete region lying rostral and dorsomedial to the lateral geniculate nucleus. This area has been associated with the perigeniculate nucleus in cats and rats.