Neural control of penile erection.

Filling of the sinusoidal spaces with blood due to smooth muscle relaxation results from parasympathetic neural pathway activation and probably simultaneous inhibition of sympathetic outflow. The final common pathway for proerectile fibers is represented by the cavernous nerves and fibers controlling detumescence and flaccidity originating in the sympathetic chain. The hypogastric nerve could represent an accessory proerectile pathway unmasked by a sacral spinal cord lesion. Nitric oxide, which can be colocalized with VIP and acetylcholine, is the main proerectile neurotransmitter and noradrenaline is considered to be the major antierectile agent. Reflexive erection elicited by recruitment of penile afferents involves both autonomic and somatic efferents. This reflex is mediated at the spinal cord level and modulated by supraspinal influences. Serotonergic pathways originating in the raphe nuclei may mediate inhibitory control on reflexive erections. The hypothalamic medial preoptic area is an important integrating center and dopamine may regulate penile erection at this level. Neuroendocrine regulation may vary depending on the context in which erection occurs, for example, coitus, in response to extrinsic or psychogenic stimuli, and rapid eye movement sleep.