The nictitating membrane response: an electrophysiological study of the abducens nerve and nucleus and the accessory abducens nucleus in rabbit.

The nictitating membrane response to periocular electrostimulation was investigated in anesthetized and paralyzed rabbits. Recordings from the abducens nerve, which carries the fibers innervating retractor bulbi muscles that are primarily involved in this reflex, showed two distinct volleys to effective stimulation: a short duration volley with a minimum latency of approximately 4 ms and a longer duration volley beginning approximately 10 ms after stimulus onset and lasting up to 25 ms. Recordings of antidromically evoked field potentials via microelectrodes indicated large responses to abducens nerve stimulation in the vicinity of the accessory abducens nucleus. Single unit recordings from the accessory abducens nucleus produced spike trains with minimal latencies of 3.7-5 ms to eyeshock. The latency of spike discharge was inversely related to stimulus current. Units in the abducens nucleus did not show stimulus-elicited spiking, suggesting that the accessory abducens, but not the abducens, is primarily involved in the reflex pathway. Transverse knife cuts which separated caudal areas of the sensory trigeminal complex from the accessory abducens nucleus did not attenuate the efferent volley to suprathreshold stimulation, suggesting that more rostral components of the trigeminal complex are primarily involved in the reflex pathway.