Penile erection is initiated by neuropsychological stimuli that ultimately produce vasodilation of the sinusoidal spaces and arteries within the paired corpora cavernosa. Erection is normally preceded by sexual desire (or libido), which is regulated in part by androgen-dependent psychological factors. Although nocturnal and diurnal spontaneous erections are suppressed in men with androgen deficiency, erections may continue for long periods in response to erotic stimuli. Thus, the continuing action of testicular androgens appears to be required for normal libido but not for the erectile mechanism itself.
The penis is innervated by sympathetic, parasympathetic, and somatic fibers. Somatic fibers in the dorsal nerve of the penis form the afferent limb of the erectile reflex by transmitting sensory impulses from the penile skin and glans to the S2-S4 dorsal root ganglia via the pudendal nerve. Unlike the corpuscular-type endings in the penile shaft skin, most afferents in the glans terminate in free nerve endings. The efferent limb begins with parasympathetic preganglionic fibers from S2-S4 that pass in the pelvic nerves to the pelvic plexus. Sympathetic fibers emerging from the intermediolateral gray areas of T11-L2 travel through the paravertebral sympathetic chain ganglia, superior hypogastric plexus, and hypogastric nerves to enter the pelvic plexus along with parasympathetic fibers. Somatic efferent fibers from S3-S4 that travel in the pudendal nerve to the ischiocavernosus and bulbocavernosus muscles and postganglionic sympathetic fibers that innervate the smooth muscle of the epididymis, vas deferens, seminal vesicle, and internal sphincter of the bladder mediate rhythmic contraction of these structures at the time of ejaculation.
Autonomic nerve impulses, integrated in the pelvic plexus, project to the penis through the cavernous nerves that course along the posterolateral aspect of the prostate before penetrating the pelvic floor muscles immediately lateral to the urethra. Distal to the membranous urethra, some fibers enter the corpus spongiosum, whereas the remainder enter the corpora cavernosa along with the terminal branches of the pudendal artery and exiting cavernous veins. If disruption of the cavernous nerves occurs following pelvic trauma or surgery, erectile impotence may ensue.
The brain exerts an important modulatory influence over spinal reflex pathways that control penile function. A variety of visual, auditory, olfactory, and imaginative stimuli elicit erectile responses that involve cortical, thalamic, rhinencephalic, and limbic input to the medial preoptic-anterior hypothalamic area, which acts as an integrating center. Other areas of the brain, such as the amygdaloid complex, may inhibit sexual function.
Although the parasympathetic nervous system is the primary effector of erection, the transformation of the penis to an erect organ is a vascular phenomenon. In the flaccid state the arteries, arterioles, and sinusoidal spaces within the corpora cavernosa are constricted due to sympathetic-mediated contraction of smooth muscle in the walls of these structures. The venules between the sinusoids and the densetanica albuginea surrounding the cavernosa open freely to the emissary veins. Erection begins when relaxation of the sinusoidal smooth muscles leads to dilation of the sinusoids and a decrease in peripheral resistance, causing a rapid increase in arterial blood flow through internal pudendal and cavernosa arteries. Expansion of the sinusoidal system compresses the venules against the interior surface of the tunica albuginea, resulting in venous occlusion. The increase in intracorporeal pressure leads to rigidity; less than complete expansion of the sinusoidal spaces leads to less than complete rigidity.
Erection occurs when adrenergic-induced sinusoid tone is antagonized by sacral parasympathetic stimulation that produces sinusoidal relaxation primarily by synthesis and release of the nonadrenergic-noncholinergic (NANC) neurotransmitter nitric oxide (NO). The contribution of acetylcholine-dependent release of nitric oxide from the vascular endothelium is uncertain. In vitro electrical stimulation of isolated corpus cavernosum strips (with or without endothelium) produces sinusoidal relaxation by release of neurotransmitters within nerve terminals that is resistant to adrenergic and cholinergic blockers. Inhibitors of the synthesis of nitric oxide or of guanosine monophosphate (GMP), as well as nitric oxide scavengers, block sinusoidal relaxation. A variety of neuropeptides found in corporal tissues, including vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP), produce tumescence when injected into the penis but have uncertain physiological roles. Norepinephrine plays an important role in the adrenergic mechanism of detumescence.
Seminal emission and ejaculation are under control of the sympathetic nervous system. Emission results from alpha-adrenergic-mediated contraction of the epididymis, vas deferens, seminal vesicles, and prostate, which causes seminal fluid to enter the prostatic urethra. Concomitant closure of the bladder neck prevents retrograde flow of semen into the bladder, and antegrade ejaculation results from contraction of the muscles of the pelvic floor including the bulbocavernosus and ischiocavernosus muscles.
Orgasm is a psychosensory phenomenon in which the rhythmic contraction of the pelvic muscles is perceived as pleasurable. Orgasm can occur without either erection or ejaculation or in the presence of retrograde ejaculation.