Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Neural Basis of Instinctive Behavior and Emotions
Sexual Behavior

Sexual function is a complex phenomenon regulated by multiple divisions of The Nervous system. The sexual act itself is driven by a series of Reflexes integrated in the Spinal Cord and lower Brainstem centers, whereas the behavioral components accompanying the act, sexual desire, and the coordinated sequence of male and FEMALE SEXUAL RESPONSES culminating in Pregnancy are largely regulated by the Limbic System and Hypothalamus. Learning plays an important role in shaping behavioral adaptive responses, particularly in primates; however, in nonprimate mammals, courtship and successful mating can occur without prior experience. Consequently, the primary responses are innate and undoubtedly characteristic of all mammals. In humans, however, sexual Functions are heavily encephalized and subject to social and psychological influences. This is why the physiological mechanisms of sexual behavior are first examined in experimental animals and subsequently compared with corresponding responses in humans.

Relationship with Endocrine Function

In nonprimate mammals, removal of the Gonads results in a reduction or loss of sexual activity in both males and females, although this process occurs very slowly in males of certain species. Injection of gonadal Hormones restores sexual activity in castrated animals. The most prominent effect is observed after the administration of testosterone to males and estrogen to females. Large doses of testosterone or other androgens in castrated females induce typical male sexual behavior patterns, whereas large doses of estrogens in males induce typical mating responses characteristic of females. It remains unclear why the administration of opposite-Sex Hormones elicits responses appropriate to the sex of the experimental animal.

Hormonal Effects in Humans

Removal of the Ovaries in an adult woman does not necessarily lead to a decrease in libido (in terms of sexual interest and drive) or the capacity for sexual life. Postmenopausal women continue to engage in sexual activity, often with the same frequency as before. This phenomenon is presumably due to the secretion of Steroid Hormones by the adrenal cortex (which are converted into circulating estrogens) or the significant encephalization of human sexual function and its relative independence from instinctive and hormonal control. The administration of sex hormones enhances sexual interest and drive in humans. For example, testosterone increases libido in men, as does estrogen when used in the Treatment of prostate carcinoma. In this case, sexual behavior is stimulated, but its pattern is not altered. For instance, the administration of testosterone to homosexual individuals increases their homosexual drive rather than converting it into a heterosexual one.

Nervous Regulation in Males

Extirpation of the neocortex in male animals predominantly inhibits sexual behavior. Partial removal of the cortex also leads to such inhibition, and the degree of this effect does not depend on accompanying motor impairments. This phenomenon is more pronounced following damage to the frontal lobes. On the other hand, in cats and monkeys, bilateral damage to the limbic lobe in the region of the pyriform cortex above the amygdaloid nuclei (Fig. 15-3) results in a marked enhancement of sexual activity. These animals mount not only adult females but also sexually immature individuals, as well as other males, attempting copulation with animals of other species and even inanimate objects. This behavior is driven by the presence of testosterone and is not altered by an increase in its secretion.

The hypothalamus also participates in The regulation of sexual behavior in males. Stimulation of areas along the medial Forebrain bundle and adjacent hypothalamic regions induces penile erection and prominent accompanying emotional responses in monkeys. Administration of testosterone into the hypothalamus of castrated rats restores characteristic sexual behavior. Lesions in corresponding areas of the anterior hypothalamus in rats result in a loss of sexual interest.

Although it is difficult to determine the extent to which data obtained from experiments on male animals with lesions surrounding the amygdala can be extrapolated to humans, there are reports of hypersexuality in men with bilateral damage to the amygdaloid nuclei or adjacent areas.

Female Sexual Behavior

Sexual activity in male mammals is more or less constant, whereas in many species the sexual activity of females is cyclical. Most of the time, females avoid males, rejecting their sexual advances. Periodically, however, a sudden change occurs in their behavior, and they solicit mating. These short periods of estrus are so characteristic that the sexual cycles of mammalian species lacking menses have likewise been termed estrous cycles.

Changes in female sexual behavior are driven by rising levels of circulating Blood estrogen. Some animals, particularly rabbits and ferrets, enter estrus and remain in it until pregnancy occurs.

In these species, ovulation is triggered by a neuroendocrine reflex. Genital stimulation or other sensory stimulation during mating leads to the release of LH by the Pituitary Gland, resulting in the rupture of the ovarian follicle. In many other species, such as monkeys (and particularly anthropoid apes), spontaneous ovulation occurs regularly at fixed intervals, and estrous periods coincide with it. In captivity, these species mate infrequently, but in the wild, females permit males to approach them during each ovulation. In women, sexual activity is manifested throughout the entire Menstrual cycle, though detailed studies have shown that, as in other primates, it peaks around the time of ovulation.

Pheromones

Substances secreted by animal organisms that induce hormonal, behavioral, and other changes in other animals of the same species are called pheromones. The best-known Examples are sex attractants in certain insect species, as well as olfactory pheromones acting on the vomeronasal organ in rodents (see Chapter 10). In monkeys, male sexual drive is stronger when they are permitted access to a female during her ovulation period than at other times of her cycle. The signal transmitted from the female to the male during this time is the odor of specific Fatty acids in the vaginal secretions. An increase in the concentration of these fatty acids in vaginal secretions during midcycle has also been detected in women. There is reason to believe that human females produce olfactory pheromones. For example, close female friends or women living together tend to synchronize their menstrual cycles; the axillary odor of women can, as already noted, modify the menstrual cycle. In addition, infants show a preference for the breast and axillary pad of their own mother rather than those of any other woman.

Nervous Regulation in Females

Extirpation of the neocortex and limbic cortex in female animals leads to a loss of The ability to actively seek out and solicit males during estrus (loss of the proceptivity response). Damage to the area surrounding the amygdaloid Nucleus does not lead to hypersexuality, unlike in males. However, selective lesions of the anterior hypothalamic area result in the loss of all behavioral responses characteristic of estrus (see Fig. 15-3) without affecting the pituitary-ovarian cycle (see Chapter 23).

The administration of a small amount of estrogen into the anterior hypothalamus induces estrus in ovariectomized rats (see Fig. 23-33), whereas its administration into other Brain regions or outside the brain fails to produce this effect. Apparently, certain structures within the hypothalamus are sensitive to the action of circulating estrogen, and their stimulation triggers estrus-typical behavior.

Effects of Sex Hormones Administered During Early Developmental Stages on Adult Responses

Steroid sex hormones administered during intrauterine development or early postnatal life cause marked disruptions in animal sexual behavior upon reaching maturity. Female rats injected with a relatively small single dose of androgen up to the fifth day of life lost their normal estrous periods upon reaching maturity. They failed to exhibit the cyclical release of pituitary gonadotropin characteristic of adult females; this process acquired the tonic, uniform pattern typical of adult males, meaning their brains were "masculinized" following a single, brief exposure to androgen. Such animals also displayed manifestations of male sexual behavior. Conversely, in male rats castrated immediately after birth, the administration of Ovarian Hormones induced female-type cyclical gonadotropin secretion and the appearance of female sexual behavior characteristics. This effect was not observed in intact males. Thus, The formation of the "female hypothalamus" during early Selection/3.html">Stages of development is driven more by the absence of androgens than by the action of Female Sex Hormones.

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Fig. 15-3. Hypothalamic areas in sheep whose lesions block estrous behavior without disrupting ovarian cycles. PHI - posterior hypothalamic area; MB - mamillary body; OC - optic chiasm; Hypo - hypophysis (after M.T. Clegg and W.F. Ganong)

Newborn rats are exceptionally underdeveloped. In other species born in a more mature state, the changes induced by postnatal androgen administration are not observed. However, in these animals, sexual abnormalities occur when they are exposed to androgen during intrauterine development (see Chapter 23). Female monkeys exposed to androgen during intrauterine development did not lose their "female" pattern of gonadotropin secretion, but they did exhibit abnormal sexual behavior upon reaching maturity.

No effect of androgen administered during human female intrauterine development has been found on the cyclical characteristics of gonadotropin secretion when these individuals reach maturity (see Chapter 23). However, some masculinizing effect on sexual behavior has indeed been documented.

Maternal Behavior

Lesions to the cingulate and retrosplenial Regions of the limbic cortex deprive animals of characteristic maternal behavior. It is likely that hormones, apart from prolactin, which is secreted in large quantities during pregnancy and Lactation, do not significantly affect this behavior.

In female mice, the knockout of the fosB Gene, one of the four immediate-early fos genes (see Chapter 1), results in the loss of postpartum acceptance and nurturing of newborns. Neglected newborn mice perish, whereas those cared for—even by an adoptive yet normal nursing mother—grow and develop successfully. Consequently, genetic factors are essential for the regulation of maternal behavior.