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

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Abstract
Male rats were given 3 weekly tests for penile reflexes, which were evoked from supine animals by extending and then maintaining the glans penis outside its sheath. Prior to tests 2 and 3 (6 days), 5 groups (5-7) of males were subjected to spinal cord (SC) transection (at the 9th thoracic vertebra), dorsal penile nerve (DPN) transection, both operations (1 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. SC transection prevented most of the depressive effects of subsequent DPN transection. During the reflex latency of intact males, DPN-mediated tonic stimulation acts to reduce the suprasegmental inhibition of penile reflexes and following the reduction in supraspinal 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.