Hypothetical spinal pacemaker regulating penile reflexes in rats: evidence from transection of spinal cord and dorsal penile nerves.

Male rats were given three weekly tests for penile reflexes, which were evoked from supine animals by extending, and then maintaining, the glans penis outside its sheath. Six days prior to Tests 2 and 3, five groups (n = 5--7) of males were subjected to spinal cord (SC) transection (at the ninth thoracic vertebra), dorsal penile nerve (DPN) transection, both operations (one after Test 1, the other after Test 2), or no surgery. Penile reflex potential was drastically depressed in males receiving DPN surgery after Test 1 and much enhanced in males in which the SC was cut. The depression in reflexes following DPN transection was reversed by subsequent SC transection. Conversely, SC transection prevented most of the depressive effects of subsequent DPN transection. It is inferred that (a) during the reflex latency of intact males, DPN-mediated tonic stimulation acts to reduce the suprasegmental inhibition of penile reflexes and (b) following the reduction in suprasegmental inhibition, whether functionally by DPN-mediated stimulation or surgically by spinal transection, a spinal pacemaker regulates the rate at which clusters of penile reflexes occur, irrespective of further DPN-mediated afference. The reflex pacemaker may also contribute to the pacing of the male's attempts to copulate, but the pacemaker must have considerable latitude for entrainment during copulation by stimuli extrinsic and intrinsic to the male.