Human sexuality
Sexual response cycle

The sexual response cycle is a model that describes the physiological responses that occur during sexual activity. This model was created by William Masters and Virginia Johnson. According to Masters and Johnson, the human sexual response cycle consists of four phases; excitement, plateau, orgasm, and resolution, also called the EPOR model. During the excitement phase of the EPOR model, one attains the intrinsic motivation to have sex. The plateau phase is the precursor to orgasm, which may be mostly biological for men and mostly psychological for women. Orgasm is the release of tension, and the resolution period is the unaroused state before the cycle begins again.

The male sexual response cycle starts in the excitement phase; two centers in the spine are responsible for erections. Vasoconstriction in the penis begins, the heart rate increases, the scrotum thickens, the spermatic cord shortens, and the testicles become engorged with blood. In the plateau phase, the penis increases in diameter, the testicles become more engorged, and the Cowper's glands secrete pre-seminal fluid. The orgasm phase, during which rhythmic contractions occur every 0.8 seconds, consists of two phases; the emission phase, in which contractions of the vas deferens, prostate, and seminal vesicles encourage ejaculation, which is the second phase of orgasm. Ejaculation is called the expulsion phase; it cannot be reached without an orgasm. In the resolution phase, the male is now in an unaroused state consisting of a refractory (rest) period before the cycle can begin. This rest period may increase with age.

The female sexual response begins with the excitement phase, which can last from several minutes to several hours. Characteristics of this phase include increased heart and respiratory rate, and an elevation of blood pressure. Flushed skin or blotches of redness may occur on the chest and back; breasts increase slightly in size and nipples may become hardened and erect. The onset of vasocongestion results in swelling of the clitoris, labia minora, and vagina. The muscle that surrounds the vaginal opening tightens and the uterus elevates and grows in size. The vaginal walls begin to produce a lubricating liquid. The second phase, called the plateau phase, is characterized primarily by the intensification of the changes begun during the excitement phase. The plateau phase extends to the brink of orgasm, which initiates the resolution stage; the reversal of the changes begun during the excitement phase. During the orgasm stage the heart rate, blood pressure, muscle tension, and breathing rates peak. The pelvic muscle near the vagina, the anal sphincter, and the uterus contract. Muscle contractions in the vaginal area create a high level of pleasure, though all orgasms are centered in the clitoris.
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