Sexual response refers to the set of physiological and emotional changes that lead to and follow orgasm. Different researchers have constructed various models. Usually, these models include 3, 4 or 5 distinct phases, with the exact components of each phase differing across models.
William Masters and Virginia Johnson, prominent sex researchers and therapists, suggested that there are four identifiable phases in the sex response cycle: excitement, plateau, orgasm, and resolution. Using various instruments designed to monitor changes in heart rate and muscle tension, Masters and Johnson were able specify the bodily changes that characterize each of these phases.
1. The First Phase of Sexual Response: Excitement
Excitement can last for just a few minutes or extend for several hours. Characteristics of this phase include: an increasing level of muscle tension, a quickened heart rate, flushed skin (or some blotches of redness may occur on the chest and back), hardened or erect nipples, and the onset of vasocongestion, resulting in swelling of the woman's clitoris and labia minora and erection of the man's penis. Other changes also occur. In the woman, the vaginal walls begin to produce a lubricating liquid, her uterus elevates and grows in size, and her breasts become larger. At the same time, the woman's vagina swells and the muscle that surrounds the vaginal opening, called the pubococygeal muscle, grows tighter. These changes prepare the woman's body for orgasm and were called the "orgasmic platform" by Masters and Johnson. Additional changes in men include elevation and swelling of the testicles, tightening of the scrotal sac, and secretion of a lubricating liquid by the Cowper's glands.
2. The Second Phase of Sexual Response: Plateau
This phase is characterized primarily by the intensification of all of the changes begun during the excitement phase. During this period, the woman's clitoris may become so sensitive that it is painful to the touch. The plateau phase extends to the brink of orgasm, which initiates the reversal of all of the changes begun during the excitement phase. During the second phase, called the expulsion phase, the urinary bladder closes to block the possibility of urine mixing with the semen. At this point, muscles at the base of the penis begin a steady rhythmic contraction that finally expels the semen from the urethral opening at the head of the penis.
3. The Third Phase of Sexual Response: Orgasm
The peak of sexual excitement is reached during the third phase. Involuntary muscle contractions, heightened blood pressure and heart rate, rapid intake of oxygen, sphincter muscle contraction, spasms of the carpopedel muscles in the feet, and sudden forceful release of sexual tension characterize the orgasmic phase. For men, orgasm generally climaxes in the ejaculation of semen, which contains millions of sperm. Ejaculation consists of two steps. During the first phase, called the emission phase, seminal fluid builds up in the urethral bulb of the prostate gland. As the fluid accumulates, the male senses he is about to ejaculate. This is often experienced as inevitable and uncontrollable. For women, orgasm also consists of rhythmic muscle contractions, in this case of the uterus, at about the same pace as in men. Tightening of the woman's muscles puts pressure on the man's penis and assists in male orgasm. For both sexes, barring the presence of some form of sexual dysfunction, orgasm is an intensely pleasurable experience. Indeed, some see it as the most pleasurable experience possible.
4. The Final Phase of Sexual Response: Resolution
In this phase, the resolution, the body returns to normal levels of heart rate, blood pressure, breathing, and muscle contraction. Swelled and erect body parts return to normal and skin flushing disappears. The resolution phase is marked by a general sense of well being and enhanced intimacy and possibly by fatigue as well. Many women are capable of a rapid return to the orgasmic phase with minimal stimulation and may experience continued orgasms for up to an hour. Males, especially as they age, experience a refractory period of varying duration after orgasm. During this period, men cannot achieve orgasm, although partial or full erection may sometimes be maintained. The duration of the refractory period can vary from just a few minutes to several days and there is great variability in the length of the refractory period both within and between men.