Medical Physiology, 3rd Edition

Female Sex Act

Female sexual desire—libido—is a complex phenomenon that includes physical and psychological effects, all modulated by circulating sex steroids. Libido varies during the ovarian cycle, and the frequency of female sexual activity increases around the time of ovulation. There may also be an increase in the rate of initiation of sexual activity by women around the time of ovulation. These changes may, in part, reflect the increased secretion of androgenic steroids that occurs just before and during ovulation secondary to the LH surge.

The female sex response occurs in four distinct phases

Although sexual function has a strong physiological basis, it is not possible to separate sexual response from the other emotional and contributing factors involved in sexual relationships. image N55-8  The four stages of the sex response in women are excitement or seduction, plateau, orgasm, and resolution.

N55-8

Kinsey, and Masters and Johnson

Contributed by Ed Moczydlowski

Masters and Johnson, in their now classic work Human Sexual Response, published a discussion of data obtained on the sexual cycles of 700 subjects. Our current understanding of the female sex response is based on their findings. Masters and Johnson described four stages of the sex response in women: excitement or seduction, plateau, orgasm, and resolution. A brief description of each stage is provided in the text. Masters and Johnson suggested that prolonged stimulation during the excitement phase leads to more pronounced orgasmic activity.

This work by Masters and Johnson followed the so-called Kinsey Reports by Alfred C. Kinsey and coworkers, who wrote Sexual Behavior of the Human Male (1948) and Sexual Behavior of the Human Female (1953).

References

Kinsey AC, Pomeroy WB, Martin CE. Sexual Behavior in the Human Male. WB Saunders: Philadelphia; 1948.

Kinsey AC, Pomeroy WB, Martin CE, Gebhard P. Sexual Behavior in the Human Female. WB Saunders: Philadelphia; 1953.

Masters WH, Johnson VE. Human Sexual Response. Bantam Books: New York; 1966 [paperback: Bronx, NY, Ishi Press International, 2010].

Excitement

The excitement or arousal phase of the female sex response may be initiated by a multitude of internal or external stimuli, including psychological factors, such as thoughts and emotions, and physical factors, such as sight and tactile stimuli. Table 55-2 summarizes the responses of the excitement phase, many of which reflect activity of the parasympathetic division of the autonomic nervous system (ANS). Sexual intensity rises in crescendo fashion.

TABLE 55-2

Female Sex Response Cycle

Excitement

Warmth and erotic feelings

Increased sexual tension

Deep breathing

Increased heart rate

Increased blood pressure

Generalized vasocongestion

Skin flush

Breast engorgement

Nipple erection (myotonic effect)

Engorgement of labia and clitoris

Vaginal “sweating” (transudative lubrication)

Secretions from Bartholin's glands

Uterine tenting into pelvis

Plateau

Marked vasocongestion

“Sex flush” (maculopapular rash on breasts, chest, and epigastrium)

Nipple erection

Engorgement of the labia

Engorgement of lower third of the vagina, with narrowing of diameter

Dilation of upper two thirds of vagina

Clitoral swelling and erection

Vaginal “sweating”

Uterine tenting

Orgasm

Release of tension

Generalized, rhythmic myotonic contractions

Contractions of perivaginal muscles and anal sphincter

Uterine contractions

Resolution

Return to pre-excitement state

Personal satisfaction and well-being

New excitement cycles may be initiated

Plateau

The plateau stage is the culmination of the excitement phase as it reaches its peak. It is associated with a marked degree of vasocongestion throughout the body.

Orgasm

During orgasm, the sexual tension that has built up in the entire body is released. The climax, or orgasm, is intense and includes a myotonic response throughout the body. Muscle contractions start 2 to 4 seconds after the woman begins to experience orgasm, and they repeat at 0.8-second intervals. The actual number of contractions, as well as their intensity, varies from woman to woman. Some women observed to have orgasmic contractions are not aware that they are having an orgasm. Whereas the excitement phase is under the influence of the parasympathetic division of the ANS, as is the erection phase in men, orgasm seems to be related to the sympathetic division, as is the emission phase in men (see pp. 1106–1107).

Resolution

The last phase of the female sex response is a return of the woman's physiological state to the pre-excitement level. During the resolution phase, the woman generally experiences a feeling of personal satisfaction, well-being, and relaxation of sexual desire. A new sexual excitement cycle may be initiated at any time after orgasm without the refractory phase that occurs in men.

Both the sympathetic and the parasympathetic divisions control the female sex response

Much of the response in the excitement phase results from stimulation of the parasympathetic fibers of the ANS. In some cases, anticholinergic drugs may interfere with a full response in this stage. Dilatation of blood vessels in the erectile tissues causes engorgement with blood and erection of the clitoris, as well as distention of the peri-introital tissues and subsequent narrowing of the lower third of the vagina. Parasympathetic fibers emanating from the sacral plexus (see p. 339) innervate these erectile tissues, just as in men (see pp. 1105–1106). In addition, the parasympathetic system innervates Bartholin's glands, which empty into the introitus, as well as the vaginal glands. Adequate lubrication is necessary to minimize the friction of intercourse and thus maximize the stimulation to achieve orgasm.

Both physical and psychological stimuli are important for female orgasm. Psychological stimuli are coordinated via the cerebrum, which causes the generalized tension throughout the body, as discussed above, and also modulates the autonomic response. The female orgasm is also coordinated via a spinal cord reflex that results in rhythmic contractions of the perineal muscles. The afferent pathways for this spinal cord reflex follow the pudendal nerves, which emanate via sacral segments 2 to 4 and are the primary innervation to the perineum and the female external genitalia. This spinal cord reflex is similar to that observed in men.

The female sex response facilitates sperm transport through the female reproductive tract

The spinal reflexes previously discussed may also increase uterine and cervical activity and may thus promote transport of gametes. The cervix dilates during orgasm, which facilitates sperm transport into the upper part of the reproductive tract. The release of oxytocin at the time of orgasm stimulates uterine contractility, which also facilitates gamete transport. Although 150 to 600 million sperm cells (see p. 1103) are normally deposited in the vagina during sexual intercourse, ~100,000 reach the cavity of the uterus and only 50 to 100 viable sperm reach the distal fallopian tube where fertilization occurs. Aside from the one or more sperm that will fertilize the ovum (or ova), most sperm degenerate, to be disposed of by the female genital tract. Sperm transport is accomplished by swimming movements of the sperm tail through the mucus of the cervical canal. The sperm reach the ampulla of the fallopian tubes within 5 minutes of ejaculation. Clearly, this rapid rate of transport could not be achieved by the swimming activity of the sperm alone. Therefore, uterine or tubal activity must play a major role in sperm transport.

LESION

REFLEXOGENIC ERECTION

PSYCHOGENIC ERECTION

EFFECT ON EJACULATION

Upper motor neuron

Present

Absent

Significantly impaired

Lower motor neuron

Absent

Present

Less impaired