Simply stated, normal male sexual function can be divided into five events, each of which is under diverse regulation: libido, erection, ejaculation, orgasm, and detu-mescence.
The first, sexual desire or libido, is regulated by psychic factors and by testicular androgens. Castration produces a decline in libido that can be restored by treatment with testosterone.
The second phase, erection, is primarily a neurologic event that results in modification of the vascular supply to the penis, causing it to become engorged with blood. The neurologic aspect of erection is controlled by both reflex and psychic stimuli. The sensory portion begins with fibers that originate in pacinian corpuscles of the penis and pass via the pudendal nerve to the S2-S4 dorsal root .ganglia. The efferent limb begins with parasympathetic preganglionic fibers from S2-S4 which synapse in the perivesicular, prostatic, and cavernous plexuses. From there, postganglionic fibers pass to blood vessels of the corpora cavernosa. Efferent fibers from S3-S4 also travel in the pudendal nerve to the ischiocavernosus and bulbocav-ernosus muscles. Sympathetic innervation of the male genitalia originates in fibers from the lateral columns of T12 and LI, the so-called thoracolumbar erection center, that synapse in the pelvic and perivesicular plexuses. Postganglionic fibers innervate the smooth muscle of the vas deferens, seminal vesicle, and internal sphincter of the bladder. Sympathetic innervation can act synergistically with the sacral parasympathetics to mediate erection initiated by psychic stimuli but is not mandatory for erection, because most men have normal potency after bilateral complete sympathectomy. The central nervous system modulates erectile response via pathways thought to descend in the lateral columns of the spinal cord. The effect of the central nervous system on erection can either be stimulatory or inhibitory, thus the importance of psychic factors for erection.
While erection is controlled by the parasympathetic nervous system, the transformation of the penis from a flaccid to an erect state is a vascular phenomenon. Blood reaches the penis via terminal branches of the right and left internal pudendal arteries. The erectile tissue of the penis consists of two corpora cavernosa lying side by side on the dorsal aspect of the penis and the corpus spongiosum that surrounds the urethra. This erectile tissue consists of an irregular spongelike system of vascular spaces interspersed between arteries and veins.
Erection is initiated by a decrease in arterial resistance resulting in increased arterial blood flow with a subsequent decrease in venous outflow. The neurotransmitter responsible for these events has not been identified, but vascular or cavernosal smooth muscle relaxation may result from mediation by beta-adrenergic, cholinergic, or vasoactive intestinal polypeptide (VIP) mechanisms. Furthermore, alpha-adrenergic antagonists can cause increased blood flow to the corpus and hence erection.
The third phase, ejaculation, is under control of the sympathetic nervous system and consists of two processes, seminal emission and true ejaculation. Emission results from the contraction of the vas deferens, prostate, and seminal vesicles which causes seminal fluid to enter the urethra. True ejaculation results from contraction of the muscles of the pelvic floor including the bulbocavernosus and ischiocavernosus muscles. Retrograde ejaculation into the bladder is prevented by partial bladder neck closure mediated by the sympathetic nerves.
The fourth phase, orgasm, is a cortical sensory phenomenon in which the rhythmic contraction of the bulbocavernosus and ischiocavernosus muscles is perceived as pleasurable. It is purely psychic. The fact that orgasm can occur without either erection, ejaculation, or bladder neck closure explains why some drugs that prevent erection or ejaculation do not interfere with orgasm.
Detumescence after orgasm and ejaculation may be the result of vasoconstriction of the arterioles supplying blood to the erectile tissue, thus allowing venous drainage to empty the sinuses and the penis to become flaccid. Following orgasm, there is a refractory period that varies with age, physical condition, and psychic factors during which erection and ejaculation are inhibited.