Sexual Response Cycle
From Encyclopedia of Sex and Sexuality
By the time they reach adulthood, almost all persons are aware that their bodies go through a series of changes when they become sexually aroused and proceed to masturbation or sexual intercourse. The most obvious changes for the males are erections and ejaculation, and for females increased vaginal lubrication and enlargement of the clitoris and the external fold of the vagina. A systematic study of this sexual response cycle was first carried out by Dr. William Masters and Dr. Virginia Johnson, who studied over 10,000 response cycles in 694 men and women during the late 1950s and early 1960s. Masters and Johnson described the sexual response cycle as having four stages, or phases, that blend into one another: excitement, plateau, orgasm, and resolution. As they reported in their scholarly publications, however, there is a wide variety of individual variation in each of these stages with respect to their duration and intensity. Furthermore, for any given individual, the perception of the intensity of the excitement and response to stimulation may vary with the type of stimulation: oral, manual, penile, or vibrator (see Masters and Johnson).
Helen Singer Kaplan modified the Masters and Johnson sexual response cycle based on her work treating sexual dysfunction problems in men and women. Dr. Kaplan added a prior stage of sexual desire (its absence is a common sexual dysfunction treated by Kaplan and other sex therapists), while eliminating the resolution phase that she believes to be an absence of sexual response rather than part of the cycle itself. In addition, Kaplan eliminated the plateau phase as defined by Masters and Johnson because she believed that it is essentially a continuation of the excitement phase and because it is of little value in sex therapy, due to the virtual impossibility of a patient distinguishing it from the excitement phase. Therefore, her model has only three phases: sexual desire, excitement, and orgasm (see Kaplan, Dr. Helen Singer).
 Sexual Desire Phase
(Kaplan model only.) Sexual desire, sometimes termed libido, is the interest or sexual energy that precedes physical or psychological stimulation. Kaplan described the physiology of sexual desire as originating in the “sex circuits of the brain—mediated by testosterone.” She pointed out that while all the mechanisms involved in sexual desire are not known, “adequate levels of testosterone and a proper balance of the neurotransmitters serotonin and dopamine and the catecholamines are necessary for the normal functioning of the brain’s sex circuits, both in male and females.” This inner sexual feeling needs no erotic stimulation and is the basic sensation that may initiate sexual activity and upon which sexual stimulation then builds. When sexual desire is low or absent, sexual functioning may be inadequate, unsatisfying, or absent.
 Excitement Phase
(Kaplan and Masters and Johnson.) This stage is marked by vasocongestion (in this instance, a swelling of the genitalia due to more blood coming into the tissues than can be quickly drained away) leading to an erection of the penis in males, and for females a swelling of the clitoris and vaginal lips, increased vaginal lubrication, an increase in breast size, and erection of the nipples. These are usually precipitated by physical, visual, or psychological stimulation by one’s self or a partner. In many men and women a “sex flush” appears on the upper abdomen and may spread to the chest. Arm and leg muscles, among others, begin to tense and there is an increase in heartbeat, breathing rate, and blood pressure.
 Plateau Phase
(Masters and Johnson only.) This is a continuation of the excitement phase, with tensions building in all the processes cited above until an orgasm is triggered. For the male, an indication of reaching this phase is the presence of two or three “love drops,” called Cowper’s fluid, at the tip of the penis, composed of the preejaculatory fluid (these drops may contain sperm, thereby making withdrawal before ejaculation an ineffective form of contraception). The testes of the male are enlarged and are pulled closer to the body. (Kaplan incorporates these aspects of the plateau phase in her description of the excitement phase.)
 Orgasm Phase
(Kaplan and Masters and Johnson.) For both men and women there are strong contractions in the penis and vagina at 0.8–second intervals, with the male ejaculation occurring in two stages: the feeling of inevitability (see moment of inevitability) and the actual ejaculation of semen. (Some authors have written about female ejaculation, but there is no scientific evidence that it exists.) Muscular contractions and spasms, including contortions of facial expression, appear for both sexes as does an increase in the respiratory rate to about 40 breaths per minute, in heart beat to as high as 180 beats per minute, and a further increase in blood pressure.
 Resolution Phase
(Masters and Johnson only.) In this last stage of the sexual response cycle, the body slowly returns to the conditions that existed before the onset of the excitement phase. However, the rate of return appears to be much faster for males than for females. In addition, Masters and Johnson claim that males have a refractory period almost immediately after ejaculation, during which the body does not respond to further sexual stimulation for lengths of time varying from a few minutes (usually only in adolescents) to several hours or days depending upon one’s age and physical condition. For the female, however, the length of the resolution phase may be considerably longer.