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Male Sexual Behavior And The Male Sex Act
Testosterone is required for normal sexual behavior. Testosterone enters certain cells within the brain, especially within the hypo-thalamus, and influences their functions. The blood levels of testosterone remain relatively constant throughout the lifetime of a male, from puberty until about 40 years of age. Thereafter, the levels slowly decline to approximately 20% of this value by 80 years of age, causing a slow decrease in sex drive and fertility.
The male sex act is a complex series of reflexes that result in erection of the penis, secretion of mucus into the urethra, emission, and ejaculation. Emission (̄e -mish′ ̆u n) is the movement of sperm cells, mucus, prostatic secretions, and seminal vesicle secretions into the prostatic, membranous, and spongy urethra. Ejaculation (̄e -jak′ ̄u -l̄a ′ sh̆u n) is the forceful expulsion of the secretions that have accumulated in the urethra to the exterior.
Sensations, normally interpreted as pleasurable, occur during the male sex act and result in an intense sensation called an orgasm (̄o r′ gazm), or climax. In males, orgasm is closely associated with ejaculation, although they are separate functions and do not always occur simultaneously. A phase called resolution occurs after ejaculation. During resolution, the penis becomes flaccid, an overall feeling of satisfaction exists, and the male is unable to achieve erection and a second ejaculation.
Sensory action potentials from the genitals are carried to the sacral region of the spinal cord, where reflexes that result in the male sex act are integrated. Action potentials also travel from the spinal cord to the cerebrum to produce conscious sexual sensations.
Rhythmic massage of the penis, especially the glans, and sur-rounding tissues, such as the scrotal, anal, and pubic regions, pro-vide important sources of sensory action potentials. Engorgement of the prostate gland and seminal vesicles with secretions or irritation of the urethra, urinary bladder, ducta deferentia, and testes can also cause sexual sensations.
Psychological stimuli, such as sight, sound, odor, or thoughts, have a major effect on male sexual reflexes. Ejaculation while sleep-ing (nocturnal emission) is a relatively common event in young males and is thought to be triggered by psychological stimuli associ-ated with dreaming.
Erection is the first major component of the male sex act.Parasympathetic action potentials from the sacral region of the spinal cord cause the arteries that supply blood to the erectile tissues to dilate. Blood then fills small venous sinuses called sinusoids in the erectile tissue and compresses the veins, which reduces blood flow from the penis. The increased blood pressure in the sinusoids causes the erectile tissue to become inflated and rigid. Parasympathetic action potentials also cause the mucous glands within the urethra and the bulbourethral glands to secrete mucus.
Failure to achieve erections, or erectile dysfunction (ED), sometimes called impotence, can be a major source of frustration. The inability to achieve erections can be due to reduced testoster-one secretion resulting from hypothalamic, pituitary, or testicularcomplications. In other cases, ED can be due to defective stimula-tion of the erectile tissue by nerve fibers or reduced response of the blood vessels to neural stimulation. Some men can achieve erections by taking oral medications, such as sildenafil (Viagra), tadalafil (Cialis), or verdenafil (Livitra), or by having specific drugs injected into the base of the penis. These drugs increase blood flow into the erectile tissue of the penis, resulting in erection for many minutes.
Before ejaculation, the ductus deferens begins to contract rhythmically, propelling sperm cells and testicular fluid from the epididymis through the ductus deferens. Contractions of the ductus deferens, seminal vesicles, and ejaculatory ducts cause the sperm cells, testicular secretions, and seminal fluid to move into the urethra, where they mix with prostatic secretions released by contraction of the prostate.
Emission is stimulated by sympathetic action potentials that originate in the lumbar region of the spinal cord. Action potentials cause the reproductive ducts to contract and stimulate the seminal vesicles and the prostate gland to release secretions. Consequently, semen accumulates in the urethra.
Ejaculation results from the contraction of smooth muscle in the wall of the urethra and skeletal muscles surrounding the base of the penis. Just before ejaculation, action potentials are sent to the skeletal muscles that surround the base of the penis. Rhythmic contractions are produced that force the semen out of the urethra, resulting in ejaculation. In addition, muscle tension increases throughout the body.
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