The Sexual Response Cycle: Detailed Explanation of Physiological Changes in the Excitation and Plateau Phases
Sexual response cycle
One of Masters and Johnson's key discoveries was revealing the fundamental patterns of male and female sexual responses. They divided the human sexual response into four phases: excitement, plateau, orgasm, and resolution. They found that male and female responses share similar, clear patterns, but also have their own unique characteristics.
The sexual response cycle is simply an artificial division into four stages. This is an effective structural model provided to describe the physiological process of sexual response in detail. Although they may appear to have independent parts, they are actually a coherent, inseparable, and complete dynamic process.
(1) Excitation period
This refers to the arousal of sexual desire and the entry of the body into a state of sexual tension. For both men and women, sexual stimulation, whether physical or psychological, can cause sexual arousal. The duration of sexual arousal varies and is influenced by many factors, including psychological state, emotional state, and the effectiveness of the stimulation. The longer the arousal period, the longer the resolution period.
The main difference between men and women during the sexual arousal phase is that men can generally achieve sexual arousal quickly and easily, desiring immediate intercourse from the outset. Women's sexual arousal, however, is greatly influenced by socio-psychological factors; they tend to crave caresses, hugs, and sweet words. Once women better understand and treat themselves appropriately, they can also quickly enter the sexual arousal phase.
For an individual, if the stimulus maintains sufficient intensity, the intensity of the response will rapidly increase. In this case, the excitement phase will be accelerated or shortened. If the stimulus causes physiological or psychological discomfort, or even interrupts it, the excitement phase may be greatly prolonged, or even aborted. As a result, only the excitement phase and the resolution phase exist in the entire sexual response cycle.
The excitement phase marks the beginning of sexual arousal, and vasocongestion is the fundamental physiological process of this phase. In men, the most obvious response to sexual arousal is penile erection. Erection occurs due to the filling of the corpora cavernosa and corpus spongiosum with blood. Erection usually occurs rapidly within seconds of sexual stimulation, but certain factors, including age, alcohol consumption, and physical condition, can alter the rate of arousal.
The most obvious response in women during arousal is vaginal lubrication. While this response may seem different from men's, the physiological process is actually the same: vasocongestion. Masters and Johnson discovered that vaginal lubrication is caused by vasocongestion in the tissues surrounding the vagina, allowing fluid to seep into the vagina through the semi-permeable vaginal wall epithelium. Although this response is not as rapid as in men, it is still quite quick, generally starting 10-30 seconds after sexual stimulation. While vaginal lubrication is the initial sign of female sexual arousal, its appearance serves as a good indication to men that the woman is ready for intercourse. Many other changes occur before a woman reaches orgasm, such as clitoral swelling, nipple erection, separation of the labia majora, swelling and protrusion of the labia minora, and vaginal dilation. Clitoral swelling is due to vasocongestion in the cavernous sinuses, which is very similar to the physiological response of penile erection in men. Nipple erection is the result of contraction of the smooth muscle fibers around the nipple. Nipple erection quickly becomes less noticeable because the swelling and congestion of the areola around the nipple become more prominent, obscuring the nipple's erection. During the later stages of arousal, the breasts may swell slightly due to increased blood flow. Many men also experience nipple erection during sexual arousal.
During the excitement phase, both men and women may develop a maculopapular rash called "sexual erythema," which is more common in women. It usually starts on the upper abdomen and then spreads rapidly to the chest.
Other physiological changes that occur during the excitement phase in both men and women include increased voluntary muscle tension, increased heart rate, and elevated blood pressure.
(2) Plateau period
The plateau phase, also known as the plateau or climax phase, refers to a short, stable period of sexual tension following the excitement phase and preceding the orgasm, lasting from about half a minute to a few minutes. Men with premature ejaculation experience an extremely short plateau phase. The plateau phase is essentially the period of thrusting during intercourse. For women, a very short plateau phase indicates a very intense orgasm. Compared to the excitement phase, the plateau phase is not marked by sudden physiological changes; rather, the physiological response continues and intensifies on the basis of excitement.
During the plateau phase, vasocongestion reaches its peak. The penis is fully erect, and the testicles are 50% larger than when not aroused. The scrotum and testicles rise further, becoming closer to the body. The bulbourethral glands may secrete a few drops of fluid from the urethral opening of the penis; although this is not ejaculated semen, it may contain motile sperm.
The most significant change in a woman during the plateau phase is the formation of the "orgasmic plateau." This refers to the significant thickening of the outer third of the vaginal wall due to congestion and swelling during the plateau phase, narrowing the vaginal canal and thus significantly increasing the gripping force on the penis. Another change is the elevation of the clitoris, which actually retracts towards the pubic symphysis. Other changes include further swelling of the breasts and an increase in uterine volume. Finally, the labia minora change color; in multiparous women, the labia minora change from dark red to dark purplish-red, while in nulliparous women, they change from pink to bright red. This change indicates that orgasm is approaching. If appropriate stimulation continues and other conditions are favorable, the woman will experience orgasm soon after the change in labial color.
Myotonia is another important form of sexual physiological response that occurs after sexual tension increases to a certain level. As a general rule, clinically significant myotonia is limited to the plateau phase. Myotonia initially manifests as hypersensitivity and extensibility of the muscles, followed by regular, repetitive contractions and strong spasmodic contractions of the involved muscle groups. These responses are primarily involuntary, but may also include voluntary movements.
Both men and women may experience a further increase in respiratory rate, leading to shortness of breath; heart rate may reach 100–175 beats/min, without palpitations being felt under the cover of sexual arousal; blood pressure may further increase, with systolic blood pressure increasing by 2.67–10.7 kPa (20–80 mmHg) and diastolic blood pressure increasing by 1.33–5.33 kPa (10–40 mmHg). Nervous system arousal also reaches a higher level.