Sexual response is an extremely individual process. People vary in their physical, mental, and emotional reactions to sexual stimulation. However, almost all people experience certain basic physiological changes that happen, and those fit with some general patters about what happens when one is sexually aroused.
Before embarking on our exploration of the sexual response cycle, there are a few points to remember. First, these stages describe the response to any form of sexual stimulation; they are not limited to penile-vaginal intercourse. The full sexual response cycle can be experienced during masturbation, manual stimulation by one's partner, oral sex, and fantasy.
Additionally, it can be difficult to avoid using these stages as a kind of personal checklist when observing our own sexual responses. However, excess use of these stages in evaluating our personal responses can lead to "spectatoring," where you seem to be a third person in the bedroom, watching and commenting on your own sexual responses. This can detract from the pleasure both you and your partner might achieve, so beware of excess spectatoring.
A final note before continuing: the simplification of the sexual response into a small number of stages does not do justice to the richness and beauty of the individual variability in sexual responses. Everyone is different, and while the responses are biologically somewhat predictable, there is extreme variation in each person's subjective experiences.
Masters and Johnson's Four-Phase Model
The sexual responses of men and women have many similarities. Two basic physiological responses to sexual stimulation are vasocongestion and myotonia, and they occur in both women and men. Vasocongestion occurs when body tissues fill up with blood and swell in size. Vasocongestion is responsible for the erection of the penis in the male, and it causes women's breasts to increase in size and their vaginas to lubricate when sexually excited. It can affect other parts of the body too, such as the labia, testicles, clitoris, and nipples. The other response, myotonia, is the increased muscle tension that happens during sexual arousal. This includes both flexing (which is voluntary) and contractions (which are involuntary). The most obvious examples of this are the muscle contractions that occur during both male and female orgasm, but myotonia also causes facial grimaces and twitches in the hands and feet.
The excitement phase is the first phase of the sexual response cycle. Like all phases of the sexual response, it varies from person to person and situation to situation: It can last anywhere from less than a minute to over several hours. It includes myotonia and increased heart rate and blood pressure. Also, many sexual areas can become engorged with blood, including the clitoris, breast, penis, and testes. The sexual organs often gain a deeper color in this phase as well. A sex flush, which is a pink or red rash on the chest or breasts, can occur in both men and women, though is more common in women. During the excitement phase, a man's penis becomes erect, though the erection is not necessarily completely hard-it can vary between unaroused, partially aroused, and fully aroused states. The testes also elevate and engorge with blood. In women, the clitoral shaft gets bigger, the labia majora separate, and the labia minora enlarge while often becoming darker. Also, some women produce considerable amounts of lubrication at this point, though others only produce a small amount.
In the plateau phase, sexual tension continues to grow as a precursor to orgasm. This phase can be very brief, from two seconds to a few minutes. Many people find that extending the length of the plateau period can lead to more intense orgasms. This phase does not have a clear starting point in which a person obviously shifts from excitement to plateau: In the plateau phase, everything that happened in the excitement phase continues and it becomes more prominent. Heart and breathing rates keep rising, muscle tension increases, and sex flushes and genital coloration becomes more noticeable. During the plateau phase, the outer one-third of the vagina becomes especially engorged with blood, creating a structure called "the orgasmic platform."
Note that the word "plateau" is usually used to describe a leveling-off, where there are no real changes. In our stages, however, plateau is not a static, boring place. There are very powerful surges of sexual tension or pleasure that occur in this stage; for example, both men and women often experience faster heart and breathing rates.
Orgasm is the shortest phase of the sexual response cycle, typically lasting only several seconds. Women can have a slightly longer orgasm than men. While men almost always experience orgasm after the plateau phase, many women can experience the plateau stage without reaching orgasm. Usually men experience orgasm and ejaculation in conjunction with each other, but ejaculation does not always occur at the time of orgasm. Before ejaculation can occur, the seminal fluids gather in the ejaculatory ducts and upper urethra. This produces a feeling that orgasm is inevitable. Then the semen is expelled out of the penis at the time of orgasm. When a woman orgasms, the uterus and orgasmic platform contract in rhythmic waves of muscular movement.
It is interesting to note that orgasms do not seem to differ by gender; that is, men and women feel quite similar things during orgasm. In one study, college students provided descriptions of orgasm. Researchers compared the descriptions using a standard psychological rating scale, and there were no distinguishable differences between men's and women's descriptions. Both males and females tended to describe orgasm with words such as "waves of pleasure in my body," corresponding to the rhythmic muscle contractions that occur during orgasm. In an earlier study, 70 expert judges could not reliably differentiate between the reports of orgasms in men and women.
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In this phase, the body returns to its original, nonexcited state. Some of the changes occur rapidly, whereas others take more time. The resolution phase begins immediately after orgasm if there is no additional stimulation.
The refractory period is the one significant difference in the sexual response cycle of males and females. The refractory period occurs during the resolution phase. It is a time when a man cannot reach excitement, plateau, or orgasm through any kind of sexual stimulation. This period can last from a few minutes to days, depending on age and frequency of sexual activity, among other things. Women do not experience a refractory period, and they are capable of reaching orgasm from anywhere during resolution. Women have the potential to have multiple orgasms, but a woman may not want to have a second or third orgasm. In order to determine the woman's feelings on this matter, good communication between partners is extremely important.
Kaplan's Three-Stage Model
Helen Singer Kaplan described three stages of sexual response: desire, excitement, and orgasm. The most notable aspect of her model is that she includes sexual desire as a crucial stage. Most other writers do not discuss changes that are separate from genital changes.
A note about desire
Not all sexual activity is prefaced by sexual desire. For example, a couple may be trying to have a baby, so even though they may not have sexual desire on one day, they may still engage in sexual activity. Many times people will respond to their partner's sexual approaches even if desire was not present to begin with.
It sometimes seems that women experience sexual desire less than men. In many cases, men initiate sex so often that women do not notice their rising sexual desire since the man has expressed his.
Paraphrased from Crooks and Baur (2002), Our Sexuality, pages 158-168; and from McAnulty and Burnette (2001), Exploring Human Sexuality, pages 108-110.