The arousal of libido in men and women may result from a variety of stimuli, some purely psychic. Neocortical influences referable to sex involve the limbic system and are transmitted to the hypothalamus and spinal centers. The suprasegmental pathways traverse the lateral funiculi of the spinal cord near the corticospinal tracts to reach sympathetic and parasympathetic segmental centers. Penile erection is effected through sacral parasympathetic motor neurons (S3 and S4), the nervi erigentes, and pudendal nerves. There is some evidence also that a sympathetic outflow from thoracolumbar segments (originating in T12-L1) via the inferior mesenteric and hypogastric plexuses can mediate psychogenic erections in patients with complete sacral cord destruction. Activation from these segmental centers opens vascular channels between arteriolar branches of the pudendal arteries and the vascular spaces of the corpora cavernosa and corpus spongiosum (erectile tissues), resulting in tumescence. Detumescence occurs when venous channels open widely. Ejaculation involves rhythmic contractions of the prostate, compressor (sphincter) urethrae, and bulbocavernosus and ischiocavernosus muscles, which are under the control of both the sympathetic and parasympathetic centers. Afferent segmental influences arise in the glans penis and reach parasympathetic centers at S3 and S4 (reflexogenic erections). Figure 25-7 shows the organization of this neural system and the locations of lesions that can abolish normal erectile function. Similar neural arrangements exist in females.
The different aspects of sexual function may be affected separately. Loss of libido may depend on both psychic and somatic factors. It may be complete, as in old age or in medical and endocrine diseases, or it may occur only in certain circumstances or in relation to certain situations. In the latter case, which is a result of psychologic factors, reflex penile erection during rapid eye movement (REM) sleep, and even emission of semen, may occur.
Sexual desire can be altered in the opposite direction, that is, it may be excessive. This, too, is usually psychologic or psychiatric in origin, as in manic states, but sometimes it occurs with neurologic disease, such as encephalitis and tumors that affect the diencephalon, septal region, and temporal lobes; with the dementias; and as a result of certain medications such as l-dopa, as discussed in
Chap. 24. In the instances of neurologic diseases with hypersexuality, there are usually other signs of disinhibited behavior as well.
On the other hand, sexual drive may be present but penile erection impossible to attain or sustain. The most common cause of erectile dysfunction is a depressive state. Prostatectomy is another, the result of damage to the parasympathetic nerves embedded in the capsule of the gland. It occurs also in patients who suffer disease of the sacral cord segments and their afferent and efferent connections (e.g., cord tumor, myelitis, tabes, and diabetic and many other polyneuropathies), in which case nocturnal erections are absent. The parasympathetic nerves cannot then be activated to cause tumescence of the corpora cavernosa and corpus spongiosum. The phosphodiesterase inhibitors such as sildenafil (Viagra) have proved to be useful in the treatment of erectile dysfunction in some patients with sexual dysfunction of neurologic cause. During sexual stimulation, it enhances the effect of local nitric oxide on the smooth muscle of the corpus cavernosum; this results in relaxation of the smooth muscle and inflow of blood. The high rate of success of this drug in patients with spinal cord injury indicates that segmental innervation is all that is required for reflexive erection in response to tactile stimulation of the penis.