The human sexual response cycle is a four-stage model of physiological responses during sexual stimulation. These phases, in order of their occurrence, are the excitement phase, plateau phase, orgasmic phase, and resolution phase. The term was coined by William H. Masters and Virginia E. Johnson in their 1966 book Human Sexual Response.
 Excitement phase
The excitement phase (also known as the arousal phase or initial excitement phase) is the first stage of the human sexual response cycle. It occurs as the result of any erotic physical or mental stimulation, such as kissing, petting, or viewing erotic images, that lead to sexual arousal. During the excitement stage, the body prepares for coitus, or sexual intercourse, in the plateau phase.
 Excitement in both sexes
Among both sexes, the excitement phase results in an increase in heart rate, an increase in breathing rate, and a rise in blood pressure. An erection of the nipples, especially upon direct stimulation, will occur in nearly all females and approximately 60% of males. Vasocongestion of the skin, commonly referred to as the sex flush, will occur in approximately 50-75% of females and 25% of males. The sex flush tends to occur more often under warmer conditions and may not appear at all under cooler temperatures.
During the female sex flush, pinkish spots develop under the breasts, then spread to the breasts, torso, face, hands, soles of the feet, and possibly over the entire body. Vasocongestion is also responsible for the darkening of the clitoris and the walls of the vagina during sexual arousal. During the male sex flush, the coloration of the skin develops less consistently than in the female, but typically starts with the epigastrium (upper abdomen), spreads across the chest, then continues to the neck, face, forehead, back, and sometimes, shoulders and forearms.
The sex flush typically disappears soon after orgasm occurs, but this may take up to two hours or so, and sometimes, intense sweating will occur simultaneously. The flush usually diminishes in reverse of which it appeared.
An increase in muscle tone (myotonia) of certain muscle groups, occurring voluntarily and involuntarily, begins during this phase among both sexes. Also, the external anal sphincter may contract randomly upon contact (or later during orgasm without contact).
Sexual excitement in both sexes can be strongly modified by subjective psychological factors. Neuro-psychoanalyst Mark Blechner calls this "the Lady Chatterley phenomenon" based on the passage in the D. H. Lawrence novel Lady Chatterley's Lover in which Lady Chatterley is first completely unexcited during intercourse and ridicules the sex act, and then, shortly thereafter, becomes intensely sexually excited.
 Excitement in males
In males, the beginning of the excitement phase is observed when the penis becomes partially erect, often after only a few seconds of erotic stimulation. The erection may be partially lost and regained repeatedly during an extended excitement phase. Both testicles become drawn upward toward the perineum, notably in circumcised males where less skin is available to accommodate the erection. Also, the scrotum can tense and thicken during the erection process.
 Excitement in females
In females, venous patterns across the breasts become more visible and the breasts very slightly increase in size, which becomes more observable if lying on the back. The labia majora become flatter, thinner, and rise upwards and outwards in nulliparous women (those who have not given birth). In parous women (those who have given birth), they may increase two or threefold in size. The labia minora may increase in size and may protrude from the labia majora, depending on the size they are in a relaxed state. The clitoral glans becomes tumescent, or swollen, like the glans of the penis and vaginal lubrication is produced by the vasocongestion of the vaginal walls. They darken in color and become smoother than normal. Also, the uterus elevates, more vertically as time passes, and the inner two-thirds of the vagina expand, usually a total of 7 to 10 cm.
 Plateau phase
The plateau phase is the period of sexual excitement prior to orgasm. The phase is characterised by an increased circulation and heart rate in both sexes, increased sexual pleasure with increased stimulation, and further increased muscle tension. Also, respiration continues at an elevated level.
During this phase, the male urethral sphincter contracts (so as to prevent urine from mixing with semen, and guard against retrograde ejaculation) and muscles at the base of the penis begin a steady rhythmic contraction. Males may start to secrete seminal fluid or pre-ejaculatory fluid and the testicles rise closer to the body.
At this stage females show a number of effects. The areola and labia further increase in size, the clitoris withdraws slightly and the Bartholin glands produce further lubrication. The tissues of the outer third of the vagina swell considerably, and the pubococcygeus muscle tightens, reducing the diameter of the opening of the vagina and creating what Masters and Johnson refer to as the orgasmic platform. For those who never achieve orgasm, this is the peak of sexual excitement. Both men and women may also begin to vocalize involuntarily at this stage.
Prolonged time in the plateau phase without progression to the orgasmic phase may result in frustration if continued for too long (see orgasm control).
 Orgasmic phase
Orgasm is the conclusion of the plateau phase of the sexual response cycle, and is experienced by both males and females. It is accompanied by quick cycles of muscle contraction in the lower pelvic muscles, which surround both the anus and the primary sexual organs. Women also experience uterine and vaginal contractions. Orgasms are often associated with other involuntary actions, including vocalizations and muscular spasms in other areas of the body, and a generally euphoric sensation. Heart rate is increased even further.
In men, orgasm is usually associated with ejaculation. Each ejection is associated with a wave of sexual pleasure, especially in the penis and loins. Other sensations may be felt strongly among the lower spine, or lower back. The first and second convulsions are usually the most intense in sensation, and produce the greatest quantity of semen. Thereafter, each contraction is associated with a diminishing volume of semen and a milder wave of pleasure.
Orgasms in females may also play a significant role in fertilization. The muscular spasms are theorized to aid in the locomotion of sperm up the vaginal walls into the uterus.
 Resolution phase
The resolution phase occurs after orgasm and allows the muscles to relax, blood pressure to drop and the body to slow down from its excited state.
Men and women may or may not experience a refractory period, and further stimulation may cause a return to the plateau stage. This allows the possibility of multiple orgasms in both sexes. However, typically men enter this refractory period and some may find continued stimulation to be painful after the orgasmic phase. Women may not have a similar refractory period and may be able to repeat the cycle almost immediately. Many women experience clitoral hypersensitivity after orgasm, which essentially mimics the effect of the males refractory period. These women may be capable of additional orgasms, but the pain involved makes the prospect undesirable.
In addition, refractory periods range from human to human, with some being immediate (no refractory) and some being as long as 12 to 24 hours.
 See also
- ^ "The Sexual Response Cycle". http://www.soc.ucsb.edu/sexinfo/article/the-sexual-response-cycle.
- ^ "Human Sexual Response Cycle". http://www.csupomona.edu/~gdbrum/bio301/sexualresponsecycle.html.
- ^ Masters & Johnson Human Sexual Response, Bantam, 1981 ISBN 978-0553204292; 1st ed. 1966
- ^ The Journal of Sexual Medicine
- ^ Blechner, M. J. (2009) 'Sex Changes: Transformations in Society and Psychoanalysis.' New York and London: Taylor & Francis.
- ^ Lawrence, D. H. (1928/2003) 'Lady Chatterley's Lover.' New York: Signet.
- ^ a b "Does size matter". TheSite.org. http://www.thesite.org/sexandrelationships/havingsex/performanceproblems/doessizematter. Retrieved 2006-08-12.
- ^ "do big penises hurt?". AskMen.com. http://www.askmen.com/love/dzimmer/17_love_answers.html. Retrieved 2006-08-14.
- ^ Rathus, Spencer A.; Nevid, Jeffrey S.; Fichner-Rathus, Lois; Herold, Edward S.; McKenzie, Sue Wicks (2005), Human sexuality in a world of diversity (Second ed.), New Jersey, USA: Pearson Education, ISBN 1-205-46013-5
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