ejaculation

ejaculation of semen is the final outcome of a complex series of reflexes induced by sexual arousal and stimulation. The reflexes involve both the sympathetic and the parasympathetic branches of the autonomic nervous system, as well as spinal motor nerves and descending nerves from the brain. Three phases of the ejaculatory response can be defined — erection, emission, and ejaculation itself.

Erection is caused by changes in blood flow to the penis that can be induced by tactile stimulation of the genital region, particularly the glans penis, which has a high density of tactile-pressure receptors. This sensory information is relayed by sensory nerves to the lower spinal cord which, via parasympathetic nerves, causes dilation of arterioles in the penis. As a result blood inflow to the spongy sinuses increases dramatically and they become engorged. Because these erectile regions are surrounded by a strong fibrous coat, the penis becomes enlarged and rigid. Also, as the erectile tissue expands the venous outflow from the penis is compressed, so while inflow of blood increases, through flow does not, and erection results. Simultaneously, parasympathetic nerves stimulate the bulbo-urethral glands to produce a mucoid substance to aid lubrication. Erection can also occur in the absence of any tactile stimulation, when thoughts, visual cues, or emotions stimulate descending nerve pathways from the brain. These, in turn, activate the same nerves as those reflexly stimulated by sensory stimulation of the genital region.

During the second phase of the response (emission), contractions of the smooth muscle in the walls of the vasa deferentia, and the ejaculatory duct formed by their junction, push sperm into the upper part of the urethra. At the same time the seminal vesicles and prostate gland contract and release seminal fluid into the urethra. (See urogenital system)

In the third phase, ejaculation proper, which occurs at orgasm, the semen (sperm plus seminal fluid) is expelled from the posterior urethra by contractions of the muscles which surround it. During coitus ejaculation is also associated with involuntary rhythmic thrusting of the pelvis. The role of the sympathetic branch of the autonomic nervous system at this point is to contract the sphincter round the neck of the bladder, so that ejaculation cannot backfire in that direction.

A feeling of intense pleasure usually accompanies ejaculation, and the whole event is known as orgasm. After this there is the resolution phase in which all the physiological changes which have occurred are reversed and a man becomes refractory to any further sexual stimulation for a while. This period can last from a few minutes to several hours.

Any interference with the relevant spinal reflexes can cause impotence and other sexual dysfunction, although libido will be unaffected. The central nervous system plays an important role in regulating the sexual response, not only in normal individuals but also in cases of disordered sexual function. Whilst sexual arousal can be stimulated or enhanced by visual or other inputs of a sexual nature in the absence of tactile genital stimulation, conversely the sexual response may be suppressed by the central nervous system, either consciously or subconsciously. This can lead to impotence, loss of sexual interest, premature ejaculation, ejaculatory failure, or a loss of the usual generalized accompaniments of orgasm. These are all common defects of this complex reflex response and may have a psychogenic basis in some patients. They are often amenable to behavioural therapy.

Saffron Whitehead

See also coitus; orgasm; penis; semen.