The penis introduces sperm into the vagina. There are two types of tissue: 2 masses of corpora cavernosa penis which are rich in blood vessels & the urethra-containing corpus spongiform penis. The release of nitric oxide causes the relaxation of smooth muscle that results in vasodilation; as blood enters the penis stiffens which blocks the veins allowing the release of the blood. During stimulation, large quantities of blood enter causing the tissue to swell. The penis ranges in size from 4.7 to 9.2 inches. Smaller penises have a greater enlargement upon arousal (Jones, 1991).

The male sexual response involves diverse regions of the nervous system. Initially, the parasympathetic division of the ANS is required for arousal and sympathetic stimulation (through stress, anxiety, fear) will inhibit erection. During emergencies, blood is directed towards muscles and away from the reproductive system. During ejaculation however, the sympathetic division is engaged and, through NE release, it mediates emission (by the release of the components of the prostate, seminal vesicle, and vas deferens), contraction of the urethral sphincter, and the propulsion of semen into the penile urethra. In addition to the components of the ANS, somatic innervation through the pudendal nerve causes the rhythmic contractions of bulbocavernosus and other muscles which expel the ejaculate (Levin, 2007).
The pudendal nerve carries stimuli from the external genitalia to the gray matter of the lumbar and sacral segments and from there to the brain through spinothalamic and spinoreticular pathways (Levin, 2007). An erection is primarily a spinal cord reflex. A man with a spinal cord injury above this center can still have an erection although he can't feel it (Jones, 1991).

Impotence is most commonly caused by the ability to maintain an erection; may be caused by depression, anxiety (most cases are psychological), drugs & illnesses that affect ANS, or plaques blocking arteries. Temporary periods of impotence are common in normal individuals.

The distal end composes glans penis which is covered at birth by the foreskin. The foreskin is removed in circumcision. Some feel this lowers susceptibility to infection and disease (one of the reasons is that preputial glands on skin & neck of penis & foreskin secrete a waxy material known as smegma which can be used as a food source for bacteria). Others contend that the removal of this sensitive area deprives men of much of the pleasure they would otherwise experience in intercourse.

Skeletal muscles at base of penis contract to expel ejaculate. Ejaculation causes the internal urethral sphincter to contract (so that urine doesn't enter semen and semen doesn't enter the bladder) and results in powerful muscle contractions of the 2 cavernosus muscles.

Temperature receptors are located in the scrotum. When the scrotum cools, the dartos contracts causing the scrotum to wrinkle and the testes to ascend. The cremaster contracts in arousal, fear, and anxiety (Jones, 1991). During stimulation, the scrotal skin becomes congested and narrows. The cremaster muscle contracts in a reflex which elevates the testes (Jones, 1991).

During intercourse, reflexes stimulate peristalsis of the testes, epididymis, vas deferens, seminal vesicles, prostate gland.


The vagus nerves offer an alternate route through which women with spinal cord injury can perceive vaginal stimulation and reach orgasm, utilizing the same brain regions identified in normal women (such as the medial amygdala, hypothalamus, frontal and parietal cortex, and the cerebellum) (Komisaruk, 2004).


Free nerve endings are located in the wall of the outer third of the vagina which are sensitive to stimulation. The majority of the vagina has very little nervous supply; most female stimulation from friction against walls of exterior vaginal opening.In the absence of sexual arousal, the vagina receives little blood supply. During arousal, vaginal blood supply increases, blood is retained in the vaginal wall (congestion), and fluid leaks through intercellular channels into the vaginal lumen. In the lumen, the fluid mixes with proteins released by the cervix to produce a lubricating fluid. Vaginal congestion occurs in the anterior third of the vagina (Mah, 2001). The vaginal epithelia maintains sodium which allows it to reabsorb fluid quickly (Jones, 1991; Levin, 2007).

The vagina measures and average of 4 inches but this increases during arousal and can even be increased voluntarily. (Jones, 1991). There are two layers of smooth muscle in the vagina: the inner longitudinal layer contracts to make it shorter and wider while the outer circular layer can constrict the vagina (Levin, 2007). As arousal continues, the uterus pulls away from the vaginal wall (vaginal "tenting") and the vagina expands at its distal end (Levin, 2007).

After childbirth, vaginal wall may relax and provide less stimulation during intercourse. Exercises and different sexual positions may help (Jones, 1991).

Anatomical reference to the clitoris could disappear for the period of a century. When "rediscovered", some confused it with pseudohermaphroditism and advocated its removal. While its removal in ancient history and in a variety of cultures was justified on religious grounds, Western medicine advocated its removal as a treatment for disorders such as insanity, epilepsy, and hysteria (O'Connell, 2005).
The clitoris possesses paired bulbs and erectile corpora. As in the penis, nitric oxide causes the vasodilation of the smooth muscle which increases blood flow during arousal. The glans clitoris is the only portion which is visible externally. (O'Connell, 2005). While manual and oral stimulation of the clitoris stimulates the pudendal nerve, intercourse stimulates the pudendal, pelvic, hypogastric, and vagal nerves. The pelvic nerves seem to mediate some of the effects of oxytocin (Brody, 2006b).
In most women, clitoral stimulation is the trigger for orgasm but vaginal pressure and rhythm and even cervical stimulation can also trigger orgasm (Cutler, 2000).Women report that there is no distinguishable difference between clitoral and vaginal orgasms. In surveys, the majority of women feel that clitoral stimulation is more important than vaginal stimulation although about 10% feel the opposite. Many women reach orgasm more regularly during masturbation than through intercourse without clitoral stimulation and some rarely achieve orgasm through intercourse (Mah, 2001).

Durint stimulation, the clitoris becomes wider and the vaginal walls darken. In plateau phase, clitoris retracts and is coverd by its hood (Jones, 1991).

Surveys indicate that the Grafenberg or "G" spot in upper anterior vagina is rated as second to the clitoris as a source of stimulation for orgasm. The G spot is not a separate structure and may be associated with the external urethral sphincter (O'Connell, 2005; Bianchi-Demicheli, 2007).
The benefits of sexual intercourse (as opposed to other types of sexual activity) include improved cardiovascular function, weight loss, (Brody, 2006a). Improvements in cardiovascular and psychological function occur in response to penile-vaginal intercourse moreso than with other sexual activities (Brody, 2006b). Increased sexual activity is associated with a greater longevity (Brody, 2006b). Self image is higher in the morning after intercourse (Van Anders, 2007).


During intercourse, the uterus becomes larger because of increase blood flow. The labia minora become larger and the breasts can increase their size by 25% (Jones, 1991). Uterine contractions become rhythmic, perhaps caused by oxytocin (Jones, 1991).

Female ejaculate from lesser vestibular glands near urethra; homolog of prostate (Jones, 1991). It is uncertain whether the fluid released by the urethra at high levels of arousal, often called 'female ejaculation', is composed of urine, secretions of paraurethral glands, or a mix of both. Paraurethral glands may be homologs of prostatic tissue in females. (Levin, 2007; Mah, 2001).