sexual response cycle

n. The cycle of arousal (or excitement), plateau, orgasm, and resolution which is characteristic of sexual response in both men and women. The arousal phase is the first stage, which can result from any mental or physical stimulation. It is characterized by bodily preparation for intercourse. In both sexes the arousal phase results in an increase in heart rate, breathing rate, and blood pressure. The nipples become erect in almost all women and the majority of men. Vasocongestion, or a reddening of the skin of the breasts, upper chest, and lower neck, occurs in most women and some men and typically lasts throughout the cycle. An increase in muscle tone occurs both voluntarily and involuntarily among both sexes. The external anal sphincter may also contract upon contact. It has been observed that the degree of the sex flush can predict the intensity of orgasm to follow. The sex flush typically disappears soon after orgasm occurs. In males, arousal phase includes erection of the penis, which may be lost and regained repeatedly. The testicles draw in and the scrotum can tense and thicken. In females the breasts very slightly increase in size, the labia tend to swell and spread, and the clitoris swells as the penis does. Vaginal lubrication is produced by the vasocongestion of the vaginal walls, the uterus elevates, and the inner two-thirds of the vagina expand. The plateau phase is the period of sexual excitement prior to orgasm, during which most sexual activity occurs. During this phase, the male urinary bladder closes and prevents urine from mixing with semen, muscles at the base of the penis begin steady rhythmic contraction, and seminal fluid begins to drip from the head of the penis. At this stage in females the areola and labia further increase in size, the clitoris withdraws slightly, and further lubrication is produced. The tissues of the outer third of the vagina swell considerably, and the muscles of the vagina wall tighten, reducing the diameter of the opening of the vagina. Orgasm is the conclusion of the plateau phase and consists of waves of pleasure along with short cycles of muscle contraction and relaxation in the lower pelvic muscles for both sexes, and women also have uterine and vaginal contractions. In men orgasm usually includes ejaculation of seminal fluid and the cessation of sexual desire. The resolution phase occurs after orgasm and is characterized by a deep sense of relaxation and return of all the physical symptoms of arousal to normal. Some individuals of both sexes experience a refractory period in which sexual excitement does not occur, and some find sexual stimulation temporarily aversive, although this is more common in males and women are often capable of multiple consecutive orgasms while only a minority of males can do so. The refractory period is variable and can last as long as 24 hours but is usually much shorter with appropriate stimulation.