Human Sexual Response
Successful reproduction is the ultimate definition of evolutionary fitness. Because fertilization occurs within the reproductive tract of humans, intimate contact between male and female is necessary for spontaneous conception to occur. Therefore, from an evolutionary view, human sexual behavior should ultimately be directed toward the physiology of coitus, which results in the deposition of sperm within the female reproductive tract. Of course, this purely procreational approach to sexual behavior is too simplistic. Humans differ from most animals, whose mating is seasonal and determined by hormonal cycles, in being sexually receptive regardless of fertility potential. Human sexuality is defined not only by procreation, but also by recreation and pleasure. The nonreproductive aspect of human sexuality is quite plastic and subject to individual and cultural influences. What is pleasurable to one individual may not be so to another. Normative behavior in one culture may be unacceptable in another. What does seem to be common to all human sexual responses is that both physiologic and psychologic satisfaction are central and motivating.
Most sexual encounters pass through five stages. The first stage, sexual attraction or arousal, was not included in initial descriptions of the human sexual response cycle. The latter four stages were first defined by the pioneering work of Masters and Johnson. Using hundreds of observations made during heterosexual interactions and masturbation, they divided the human sexual response into excitement, plateau, orgasm and resolution phases (Fig. 15.1). Although the validity of some of the data gathered by Masters and Johnson has been subject to question, their model remains the single best description of the physiologic aspects of the human sexual response.
Phases of the sexual response
Sexual attraction or arousal is the most individualized stage of the human sexual response. In many respects, sexual attraction and arousal are closely tied to personality. They are also the most culturally deter- mined. For example, incest taboos forbidding marriage and intercourse between closely related family members are almost universal among cultures over time. In contrast, attention to women’s breasts or weight in a sexual context varies tremendously among cultures. Interestingly, two variables of attractiveness do appear to be both universal and related to reproductive success: youth and health.
The nature of erotic stimuli can also be quite varied and include mental images, smells, sounds and physical events such as touching or stroking. If self-report and measurements of pelvic blood flow are used to indicate the level of arousal, men and women seem to be equally arousable. They differ dramatically, however, in the types of things that result in arousal. Novel or unpredictable situations and explicit visual stimuli, particularly body images, appeal to men more than women. Women generally prefer images with an emotional, romantic or familiar context. An individual’s physical health and mental state contribute greatly to the threshold at which they can be aroused by a given stimulus.
During the next phase of the sexual response cycle, the physiologic excitement phase, sexual interest is stimulated by these psychologic or physiologic stimuli. This aroused state intensifies during the plateau phase. If stimulation is sufficient, orgasm or climax occurs. Orgasm is typically experienced as an explosive and pleasant release of sexual tension. Finally, during the resolution phase, sexual arousal dissipates. The physiologic changes associated with arousal and orgasm return to baseline. Although both men and women progress through the same phases of sexual response, they may differ in length and intensity in any given sexual encounter. The most notable physiologic difference between males and females is the presence of a refractory period in men. This is a part of the resolution phase following orgasm. During this period of time, sexual arousal cannot be restored and orgasm cannot occur in men. In contrast, sufficient stimulation can induce orgasm in women at any point during the resolution phase.
The basic physiologic responses of the human body to sexual stimulation are twofold. The primary reaction is vascular congestion. The secondary response is generalized muscle tension or myotonia. Reflexes activated within the spinal cord are modulated by the higher central nervous system and control each response.
Male sexual response
The human male’s first physiologic response to effective sexual stimulation is penile erection. Erection occurs during the excitement phase, with vasodilatation of the lacunar smooth muscle of the penis leading to its engorgement and hardening (Chapter 13). Only a minimal degree of sexual tension may accompany excitation and this phase of the sexual response can vary significantly in length.
Erectile stimuli may be either psychogenic or somatogenic. Psycho- genic stimuli can include imagined sensory cues or direct visual cues, including explicitly erotic images. These signals are integrated within the limbic system of the brain and transmitted via descending projections to the spinal cord. They then travel via autonomic and visceral efferent nerves to the penis. Somatogenic stimuli include touching the penis or adjacent perineum. These tactile stimuli will reflexively activate the same efferents as the spinal cord pathway. This tactile reflex is typically preserved following spinal cord transection. The erection of the excitement phase may be quite susceptible to external signals and may resolve without progression. Changes in the physical surroundings, such as sudden loud noises, can impair penile erection in the excitation phase. Erection of the penis can also occur independent of the excitation phase of sexual arousal, observable in the newborn period and during sleep, especially in pubescent boys.
During the plateau phase, a minor involuntary increase in vasocongestion occurs and penile erection increases slightly. The size of the testes likewise increases and the scrotum and testes are drawn toward the perineum. There is a measurable rise in heart rate and systolic blood pressure. Just prior to ejaculation, a warm red rash may develop over the upper abdomen, trunk, neck and face. There is a diffuse and near maximal increase in muscular tension throughout the body. Emission immediately precedes ejaculation. During emission, muscular contractions are induced within the prostate gland, vas deferens and seminal vesicles and seminal plasma and spermatozoa are expelled into the posterior urethra. This process is mediated by sympathetic output traveling through the hypogastric plexus and can be abolished by α-adrenergic blockade. Once the plateau phase is reached, detumescence without ejaculation and orgasm is rare in healthy individuals.
During orgasm, somatic changes in the cardiovascular system are at their maximum, as is generalized muscle tension. Hyperventilation and vocalizations are common. Contraction of the smooth muscles of the urethra and the striated muscles of the bulbocavernosus and ischiocavernosus muscles expels the semen from the prostatic urethra. The pelvic floor and rectal sphincter may contract rhythmically. Ejaculation of the semen from the penis marks the height of orgasm. It is typically accompanied by release of sexual tension and an intense sense of pleasure.
Penile detumescence during the resolution phase of the male sexual response cycle occurs in two distinct stages. The primary stage of penile involution occurs very rapidly. The penis reduces in size from full erection to about 50% larger than its flaccid, unstimulated size. The penis is totally refractory to stimulation during this first stage. Secondary stage involution is a more extended process that returns the penis to its normal unstimulated size. The penis is only relatively refractory to stimulation during this stage. The penis progressively regains responsiveness. The excitement or plateau phase of the sexual cycle may be voluntarily extended by the male in an effort to delay ejaculation until his sexual partner is satisfied. This may be accompanied by a prolongation of the primary stage of detumescence after ejaculation.
Female sexual response
During the excitement phase of the sexual response cycle, somatogenic and psychogenic stimuli arouse the female through neural pathways similar to those described for the male. The clitoral response to arousal is less predictable than is that of its homolog, the penis. Tactile stimulation of the female perineum or the glans clitoris can elicit vasocongestion, engorgement of the body of the clitoris and erection, but only in some women. The response of the vagina during the excitement phase is much more predictable and consistent than that of the clitoris. Vaginal lubrication begins 10–30 s after receipt of arousing stimuli and continues progressively through orgasm. The more prolonged the excitement and plateau phases, the greater the production of vaginal lubrication. The upper two-thirds of the vagina also expand and lengthen during the excitement phase. This elevates the uterus into the false pelvis, repositions the cervix above the vaginal floor and “tents” the midvaginal plane. These changes result in an increase in the circumference of the vaginal diameter, largely at the level of the cervix. Finally, the labia minora become markedly engorged with blood during the excitement phase. The engorged labia minora displace the labia majora upward and outward away from the vaginal introitus. This increase in the diameter of the labia minora adds at least 1 cm to the functional length of the vagina.
During the plateau phase, the most striking change in the female genitalia is the florid coloration of the labia minora accompanying vascular congestion. This beet red appearance is the single most con- sistent physical marker for sexual arousal in the female. The clitoris retracts behind a tissue hood formed by the labia. The respiratory rate, heart rate and blood pressure all increase late in the plateau phase; the magnitude of these changes are not as marked in women as in men. Generalized myotonia may be present, including spastic contractions of the striated muscles of the hands and feet. The latter are referred to as carpopedal spasms.
During heterosexual coitus, penetration of the penis into the vagina can heighten a woman’s sexual arousal by indirectly stimulating the retracted clitoris. This occurs because of traction on the engorged labia minora whose fused anterior segment forms the clitoral hood. The glans of the clitoris, however, is extremely sensitive in the aroused state. For this reason, direct and prolonged contact may be irritating. Similar to the male, orgasm in the female involves rhythmic con- tractions of the muscles of the reproductive organs followed by physical release from the vasocongestive and myotonic tensions developed during arousal. Typically, orgasmic contractions begin in the lower third of the vagina and evolve to encompass the entire vagina and uterus. A sex flush, which can also include diffuse fine perspiration, may develop over the woman’s entire body. The resolution phase of the sexual response cycle of women involves decongestion of the labia, detumescence of the clitoris if it has occurred, and relaxation of the vagina.
There are four major physiologic differences between male and female orgasms. First, emission and ejaculation do not occur in the female. Second, if sexual stimulation occurs before a woman drops below plateau phase levels of arousal, the female is capable of rapidly successive orgasms. Third, females more commonly reach the plateau phase, remain there for brief or prolonged periods of time but then return to the unstimulated state without orgasm than their male counterparts. Finally, the female orgasm may last for a relatively long period compared with that of the male.