Anatomy and Physiology

Clitoris and Surrounding Erectile Tissue

There is a considerable density of tactile receptors in the clitoris. The anterior vaginal wall is also rich in tactile receptors. Freud (23) entertained a developmental idea about excitability to explain how "a little girl turns into a woman." He argued that from the onset of puberty, libido increases in boys; at the same time, in girls, "a fresh wave of repression" occurs that affects "clitoridal sexuality." This finite period of "anasthesia," Freud thought, was necessary to enable successful transferrence of a girl's erotogenic susceptibility to stimulation from the clitoris to the vaginal orifice. Even though his suggestion that there are also tactile receptors in the anterior vaginal wall is correct, there is no evidence that the anterior wall becomes excitable at the expense of clitoral sensitivity. Contrary to Freud's belief, there is ample evidence that women who learned to know their own sexuality through masturbation are able to transfer this knowledge (or skill) to coital stimulation with a partner (12). For a long time, ideas similar to those of Freud have been used to suppress masturbation in girls and women. Even today there are many women with a partner, who feel guilty when masturbating.

The clitoris contains two stripes of erectile tissue (corpora cavernosum) that diverge into the crura inside the labia majora. On the basis of recent anatomical studies, O'Connell et al. (24) proposed to rename these structures as bulbs of the clitoris. They found that there is erectile tissue connected to the clitoris and extending backwards, surrounding the perineal part of the urethra. However, most anatomical facts have been known for a long time (25). The clitoris' parasympathetic innervation comes from lumbosacral segments L2-S2, while its sympathetic supply is from the hypogastric superior plexus. The pudendal and hypogastric nerves serve its sensory innervation. It responds with increased blood flow and tumescence on being stimulated through sexual arousal. Nitric oxide synthase (NOS), among many other neuropeptides, has been identified in the complex network of nerves in the clitoral tissue (26).

The Anterior Vaginal Wall

When Masters and Johnson (10) published their account of the physiology of the sexual response, they opposed Freud's theory of the transition of erogeneous zones in women. According to these famous sexologists, nerve endings in the vagina are extremely sparse. Therefore, during coital stimulation the clitoris is stimulated indirectly, possibly through the movement or friction of the labia. Hite's data supported this point of view. Almost all women who reached orgasm through stimulation from coitus alone had experienced orgasm through masturbation. Many women needed additional manual stimulation to orgasm during coitus, and an even larger number was unable to orgasm during coitus at all (12).

Apparently, coitus alone is not a very effective stimulus for orgasm in women. In 1950, Grafenberg (25) provided an alternative to Masters and Johnson's explanation for the relative ineffectiveness of coitus to induce orgasm. He described an area of erectile tissue on the anterior wall of the vagina along the course of the urethra, about a third of the way in from the intro-itus and below the base of the bladder. Strong digital stimulation of this zone would activate a rapid and high level of sexual arousal which, if maintained, induced orgasm. This paper was ignored until 1982, at which time this area was renamed as the G-spot (27). According to Levin (28), however, there is no convincing scientific evidence for the presence of either a unique G-spot with its own plexus of nerve fibers or for the fluid that is often expelled when orgasm is reached from stimulation of this area being anything other than urine. Because it is difficult to see how strong stimulation of this "G-spot" would not also stimulate other erogeneous structures such as the urethra and cli-toral tissue, Levin argues that the whole area should be regarded as the "anterior wall erogeneous complex." Grafenberg pointed out that coitus in the so-called missionary position (ventral-ventral) prevents stimulation of the anterior vaginal wall and would therefore not be optimally sexually arousing for women. Instead, contact with the anterior wall is "very close, when the intercourse is performed more bestiarum or a la vache that is, a posteriori"

(25, p. 148). Thus, Grafenberg's suggestion was not that coitus itself is an ineffective sexual stimulus for women, but only coitus in the missionary position.

Sensitivity of the entire vaginal wall has been explored in several studies. Weijmar Schultz et al. (29) used an electrical stimulus for exploration under non-erotic conditions. This study confirms sensitivity of the anterior vaginal wall, even though sensitivity of this area was much lower than that of the clitoris.

Central Nervous System and Spinal Chord Pathways

Neural and spinal components of female sexual arousal anatomy have been examined in animals and spinal cord-injured (SCI) women only. There is strong evidence for the occurrence of sexual arousal and orgasm in women with SCI who have an intact S5-S5 reflex arc. Not only were genital and extragenital responses to vibrotactile stimulation similar between able-bodied and SCI subjects in a recent study of Sipski et al. (30), subjective descriptions of sensations were indistinguishable between groups. SCI subjects did take longer than able-bodied subjects to achieve orgasm. Whipple and Komisaruk (31) suggested that, on the basis of their studies in SCI women in whom cervical stimulation was applied, the vagus nerve conveys a sensory pathway from the cervix to the brain, bypassing the spinal cord, which is responsible for the preservation of sexual arousal and orgasm in these women.

There remain large gaps in our understanding of the central nervous control of female sexual function. Most of the animal work relates to receptive behavior in female rats and very little to the control of genital responses. According to McKenna (32), the autonomic and somatic innervation of the genitals is based upon spinal mechanisms, modulated by supraspinal sites. Sensory information from the genitals project to interneurons in the lower spinal cord, which possibly generate the coordinated activity of sexual responses. The spinal reflex mechanisms are under inhibitory (through serotonergic activity) and excitatory (through adrenergic activity) control from supraspinal nuclei. These nuclei are highly interconnected. Many of them also receive genital sensory information. It is likely that during sexual activity, sensory activation of supraspinal sites causes a decrease in the inhibition, and an increase in the excitation of the spinal reflexive mechanisms by the supraspinal sites. Higher order sensory and cognitive processes may modulate the activity of supraspinal nuclei controlling sexual function.

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