Neurotransmitters: central and peripheral mechanisms

Reflexive erection initiated by recruitment of penile afferents, involves both autonomic and somatic efferents. The reflex is mediated at the spinal cord level, modulated by supraspinal influences, and may use several transmitters. Dopamine, acetylcholine, nitric oxide, and peptides, such as oxytocin and ACTH/alpha-MSH, seem to have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. Peripherally, the balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa, and determines the functional state of the penis. Noradrenaline contracts both corpus cavernosum and penile vessels via stimulation of alpha(1)-adrenoceptors. The role of endothelins in the control of penile smooth muscle tone is presently unclear. Neurogenic nitric oxide (NO) is considered the most important factor for relaxation of penile vessels and corpus cavernosum. The role of other mediators, released from nerves or endothelium has not been definitely established.