Johannes Bitzer1
(1)
Department Obstetrics and Gynecology, University Hospital Basel, Basel, Switzerland
Johannes Bitzer
Email: JBitzer@uhbs.ch
Email: Johannes.Bitzer@usb.ch
Keywords
External genitaliaPhysiology of sexual responsePeripheral and central regulation
The anatomy and physiology of the female sexual response comprises the structural and functional elements of the female external genitalia, the physiological changes occurring during the sexual response cycle, and the endocrine and neurobiological regulation of this response.
18.1 The Female External Genitalia, the Vulva
The external genitalia are composed of the mons pubis, the labia majora, the labia minora, Bartholin’s glands, the Skene’s glands, minor vestibule glands, the clitoris, the vulvar vestibule, the perineum, and the urethral and anal openings, collectively representing the vulva (Fig. 18.1) [1–3].
Fig. 18.1
The female external genitalia
The mons pubis consists of fatty tissue that covers the pubic bone. During puberty, it becomes covered with hair. The mons pubis contains oil-secreting (sebaceous) glands that release substances that are involved in sexual attraction (pheromones).
The labia majora (“big lips”) are two marked folds of skin that extend from the mons pubis downward and backward to merge with the skin of the perineum. The major function of the labia majora is protection of the softer tissues of the vulva. Unlike the inner structures of the vulva, the labia majora contain many pubic hairs that help to protect the rest of the vulva from mechanical stress and friction.
The labia minora are “small lips” that lie inside the labia majora and surround the openings to the urethra and vagina. The labia minora can be very small or up to 2 in. wide. They are covered with hairless skin and contain very little adipose tissue. Blood flowing through the many capillaries in the connective tissue layer gives the labia minora their pinkish color. During sexual stimulation, the small blood vessels become engorged with blood, causing the labia minora to swell and become more sensitive to stimulation. At their anterior end, the labia minora meet at the clitoral hood, or prepuce, where they envelope the lateral sides of the clitoris. From the clitoral hood, the labia minora extend inferiorly toward the anus, where they gradually decrease in size before merging with the skin of the perineum. Many sebaceous glands are also present in the connective tissue and extend to the surface of the labia minora via ducts.
The vulvar vestibule surrounds the opening of the vagina. It extends from Hart’s line on the medial aspect of the labia minora to the hymen. The vestibule is covered by a nonkeratinized squamous endothelium and contains many nerve fibers and the ostia of the Bartholin’s glands, Skene’s glands (the female prostate), and the minor vestibule glands—all of which are androgen-dependent mucin-secreting glands. The tissues below the skin include the vestibular bulbs, which contain erectile tissue and are part of or the internal portion of the clitoris (see below).
The Bartholin’s glands are androgen-dependent mucin-secreting glands that are responsible for lubrication during arousal. These glands are innervated by the autonomic nervous system and the ostia of the gland are at the 4 and 8 o’clock positions of the vulvar vestibule.
The Skene’s glands are androgen-dependent glands that are embryologic equivalents to the male prostate. The Skene’s glands surround the urethra and, combined with the clitoral crura, make up the anatomic suture known as the G-spot. The ostia of the Skene’s glands are adjacent to the urethral meatus [4, 5].
18.2 Clitoris
The clitoris can be subdivided into several parts: clitoral hood, glans clitoris, clitoral body, clitoral crurae (Fig. 18.2).
Fig. 18.2
The clitoris
18.2.1 Clitoral Hood
The clitoral hood projects at the front of the labial commissure, where the edges of the labia majora meet at the base of the pubic mound; it forms as part of the external folds of the labia minora and covers the glans and external shaft. There is considerable variation in how much of the glans protrudes from the hood and how much is covered by it, ranging from completely covered to fully exposed, and tissue of the labia minora also encircles the base of the glans.
18.2.2 The Glans Clitoris
The glans forms the pointed tip of the clitoris extending outward from the body and beyond the prepuce that covers the rest of the clitoris. It consists of a midline shaft lying in the medial sagittal plane about 2–4 cm long and 1–2 cm wide.
18.2.3 The Clitoral Body
The clitoral body forms a wishbone-shaped structure containing the corpora cavernosa—a pair of sponge-like aggregations of erectile tissue that contain most of the blood in the clitoris during clitoral erection. Arterial inflow includes the dorsal and clitoral cavernosal arteries, which arise from the iliohypogastric pudendal bed. The autonomic efferent motor innervation arises from the cavernosal nerve of the clitoris arising from the pelvic and hypogastric plexus.
Under the surface of the skin, two legs of erectile tissue known as the clitoral crura fan out to support the exterior structures of the clitoris and attach to the underlying tissues. Associated are the urethral sponge, perineal sponge, a network of nerves and blood vessels, and the suspensory ligament of the clitoris, muscles, and the pelvic floor.
Innervation of the clitoris is mainly supplied by the dorsal nerve, a branch of the paired pudendal nerve (left and right), which carries sensory and motor signals to the perineum of both women and men. There may be additional innervation of the clitoris from the genitofemoral nerve as well as the ilioinguinal nerve. Other structures involved in the autonomic innervation of the clitoris are cavernous nerves originating from the vaginal nervous plexus and traveling at the 5 and 7 o’clock positions along the urethra. The genitofemoral nerve supplies the labia majora with sensory fibers. The mons and labia majora contain sensory fibers from the ilioinguinal nerve.
18.2.4 Vestibular or Clitoral Bulbs
The vestibular bulbs , also known as the clitoral bulbs , are aggregations of erectile tissue that are an internal part of the clitoris. They can also be found throughout the vestibule—next to the clitoral body, clitoral crura, urethra, urethral sponge, and vagina. Thousands of touch and pressure-sensitive nerve endings are found throughout the clitoris. Nerve endings in the clitoral body and glans are sensitive to direct external touch and pressure, while the nerve endings of the crus are sensitive to stimulation from within the vagina.
18.3 The Vagina
The vagina links the vulvar structures to the inner genitalia (Fig. 18.3).
Fig. 18.3
The vagina relative to other components of the female reproductive tract
The vagina has three layers: the internal mucosal layer, the intermediate muscularis layer, and the external adventitial layer.
18.3.1 The Internal Mucosal Layer
The vaginal mucosa has folds, or rugae, and is composed of the epithelium and the lamina propria. The vaginal epithelium is nonkeratinized stratified squamous epithelium. There are no glands, so there is no mucin secretion. The lamina propria of the vaginal mucosa contains many elastic fibers as well as a dense network of blood and lymphatic vessels and nerves. Transudate from these blood vessels, combined with cervical mucus, provides lubrication during sexual arousal and intercourse.
18.3.2 The Muscularis
The vaginal muscularis layer consists of autonomically innervated smooth muscle fibers arranged into outer longitudinal and inner circular layers.
18.3.3 The Adventitia
The adventitia is rich in collagen and elastin, provides structural support to the vagina, and allows for expansion of the vagina during intercourse and childbirth. Surrounding the adventitia are three sets of powerful pelvic striated muscles: (a) the ischiocavernosus and bulbocavernosus, which are the most superficial layer, (b) the transverse perinei providing intermediate support, and (c) the levator ani forming the deeper layer of the pelvic diaphragm across the anterior of the pelvis. The largest medial portion is the pubococcygeus and puborectalis.
18.3.4 The Grafenberg Spot: G-Spot
In 1950, the German gynecologist Ernst Gräfenberg first, published his work on “the role of the urethra in female orgasms” and hypothesized that an erogenous zone might be located on the anterior wall of the vagina, along the course of the urethra (Fig. 18.3) [6]. Stimulation of this area would lead to discharge of fluid from the Skene’s gland ostia, which some have consequently considered female ejaculation. There is one case report in which, during orgasm, this area was being “pressed downwards against the finger like a small cystocele” protruding into the vaginal canal. In reference to this anecdotal observation, Addiego [7] described this area as the “G-spot” in 1981, and ever since, especially after publication of the book The G-Spot and Other Recent Discoveries About Human Sexualityby three American sexologists a year later [8, 9], public interest has grown around this topic. Taking into account the controversy about the separate existence of this anatomical site, the G-spot may at this moment be considered an excitable area along the entire length of the urethra running along the anterior vaginal wall.
18.3.5 Halban’s Fascia
Halban’s fascia is the space , between the trigone of the bladder and the anterior part of the vaginal wall. It is filled with mesenchymal lamina, a fibroelastic sheet composed of collagen, elastic and muscular fibers with a rich blood supply, and a nerve supply containing Krause bodies or pseudo-corpuscular nerve endings. On stimulation, this space becomes vasocongested and creates an erotic pleasurable response.
18.3.6 Innervation of the Vagina
Autonomic efferent innervation to the, upper two thirds of the vagina is through the uterovaginal plexus, which contains both sympathetic and parasympathetic fibers. Sympathetic efferent fibers from the lumbar splanchnic nerves travel first through the superior hypogastric plexus, then through the bilateral hypogastric nerves to reach the inferior hypogastric plexuses, and finally the uterovaginal plexus. Parasympathetic efferent input to the uterovaginal plexus is from the pelvic splanchnic nerves. Nerves from the uterovaginal plexus travel within the uterosacral and cardinal ligaments to supply the proximal two thirds of the vagina. Autonomic efferent innervation to the lower vagina is carried through the pudendal nerve (S2, 3, 4), which reaches the perineum through Alcock’s canal.
Autonomic afferent fibers from the upper vagina travel through the pelvic splanchnic nerves to sacral spinal cord segments, whereas autonomic afferents from the lower vagina leave the sacral spinal cord through the pudendal nerve. Somatic sensation exists primarily in the distal one third of the vagina and is also carried by the pudendal nerve to the sacral spinal cord [3].
18.3.7 The Physiology of the Female Sexual Response Cycle
By observing sex workers under laboratory , conditions, Masters and Johnson were able to describe visible changes in the genital organs during sexual encounters, which led them to define a linear model of the human sexual response, applicable to both genders. Their model still serves as a basic description of physical changes during sex and has served as the blueprint from which further knowledge about the anatomy and physiology of the female sexual response has been accumulated [10, 11].
They differentiate four phases with typical characteristics: (1) excitement, (2) plateau, (3) orgasmic, and (4) resolution [12].
18.3.7.1 Excitement Phase
The excitement phase (also known as the arousal or initial excitement phase) results in an increase in heart and breathing rates and a rise in , blood pressure. Vasocongestion of the skin, commonly referred to as the sex flush, will occur in approximately 50–75 % of females. During this sex flush, pinkish spots develop under the breasts and then spread to the breasts, torso, face, hands, soles of the feet, and possibly over the entire body. Vasocongestion is also responsible for the darkening of the clitoris and the walls of the vagina during sexual arousal (Fig. 18.4a, b). The sex flush typically disappears soon after orgasm occurs, but this may take up to 2 h and sometimes results in simultaneous intense sweating. An increase in muscle tone (myotonia) of the levator ani muscles occurring voluntarily and involuntarily begins during this phase. Also, the external anal sphincter may contract upon contact (or later during orgasm without contact).
Fig. 18.4
(a, b) Physical changes during the excitement phase of the sexual cycle (Modified from Clayton AH. Sexual function and dysfunction in women. Psychiatric Clinics of North America 2003;26:673–682 [13])
In females, the excitement phase can last from several minutes to several hours. The onset of vasocongestion is due to an increase in vaginal blood flow and can be measured by plethysmography, which provides information about the basal vaginal pulse amplitude. Vasocongestion during the excitement phase results in swelling of the woman’s clitoris, labia minora, and vagina. On sexual arousal, the blood supply to the vaginal epithelium is rapidly increased by parasympathetic inputs from S2 to S4, and at the same time, the venous drainage is probably reduced, resulting in vasocongestion.
The increased blood flow to the genitals is activated by the VIPergic innervation of the large vessels supplying the epithelium and possibly aided by calcitonin gene regulating peptide (CGRP) enhanced permeability of the capillary tufts. This allows serum to leak out of blood vessels through the vaginal mucosa, and this transudate acts as a lubricant during intercourse. Neuropeptide Y (NPY), a known vasoconstrictor, may be involved in constricting the venous drainage. There appears to be very little nitric oxide synthase (NOS) in the blood vessels of the premenopausal vagina and none in the postmenopausal. During the excitement phase, the androgen-dependent glands of the vulvar vestibule (Bartholin’s, Skene’s) secret mucin to act as a lubricant at the introitus. In addition to the vascular changes during the excitement phase, the pubococcygeus muscle surrounding the vaginal opening tightens and the uterus elevates and increases in size. Meanwhile, the breasts increase slightly in size and nipples become hardened and erect.
18.3.7.2 Plateau Phase
The plateau stage in females represents a continuation , of the changes that occur during the excitement phase. The glans clitoris becomes extremely sensitive and withdraws slightly under the clitoral hood, and the vestibular glands produce further lubrication (Fig. 18.5a, b). The tissues of the outer third of the vagina swell, and the pubococcygeus muscle tightens, reducing the diameter of the vaginal opening [12].
Fig. 18.5
(a, b) Physical changes during the plateau phase of the sexual cycle
18.3.7.3 Orgasmic Phase
Orgasm is the conclusion of the plateau phase, , and it is accompanied by quick cycles of lower pelvic muscle contraction (Fig. 18.6a, b). Between arousal and orgasm, there is an increase in vaginal luminal pressure. The smooth muscle layers contain a great variety of classical and peptidergic transmitters including serotonin (5HT), norepinephrine, acetylcholine, dopamine, vasoactive intestinal peptide (VIP), NPY, gastrin-releasing peptide (GRP), thyrotropin-releasing hormone (TRH), CGRP, somatostatin, substance P, oxytocin, cholecystokinin (CCK), and relaxin, but the exact function of each neurotransmitter in sexual response is unknown.
Fig. 18.6
(a, b) Physical changes during the orgasmic phase of the sexual cycle
A series of pelvic, clonic, striated muscle contractions occur at approximately 0.8 s intervals which gradually get longer and the contractions weaker. They can last for 5–60 s. These contractions are concomitant with the subjective feeling of orgasm. Voluntary contractions of the pelvic striated muscles do not give a feeling of intense pleasure but are often used to enhance arousal. During sexual arousal and up to orgasm, individual uterine contractions may occur, while during orgasm a series of uterine contractions occurs, mediated by the sympathetic nervous system via the hypogastric nerve. Orgasms are often associated with other involuntary actions, including vocalizations and muscular spasms in other areas of the body, and a generally euphoric sensation. Heart rate is increased even further during this phase of sexual arousal.
18.3.7.4 Resolution Phase
The resolution phase occurs after orgasm and allows the muscles to relax, blood pressure to drop, and , the body to slow down from its excited state. The refractory period, which is part of the resolution phase, is the period during which a man is unable to orgasm again. While less common, women can also experience a refractory period, although according to Masters and Johnson, women have the ability to orgasm again very quickly as long as they have effective stimulation. They are, as a result, able to have multiple orgasms in a relatively short period of time. Though generally reported that women do not experience a refractory period and thus can experience an additional orgasm, or multiple orgasms, soon after the first, for some women, the clitoris is very sensitive after climax, making additional stimulation initially painful. After the initial orgasm, subsequent orgasms for women may also be stronger or more pleasurable as the stimulation accumulates.
This model of the anatomy and physiology of the female sexual response is helpful in understanding and studying the different processes as if they were separate entities, facilitating observation and analysis. The model, however, has two major limitations:
(a)
(b)
The first broadening of the Masters and Johnson model was the introduction of sexual desire as the first phase of the female sexual response by Helen Singer Kaplan [14]. Desire has since become one of the most enigmatic and interesting phenomena in sexuality because of the inability to directly observe it. Other models have tried to describe the motivational and behavioral characteristics of the sexual response cycle, taking into account the nonlinearity of the female sexual response, pointing to a more circular model of positive and negative feedback and interaction between stimuli and motivational states [15]. One of the latest models has been constructed around the three categories: wanting, liking, and satiety [16].
18.3.7.5 The Endocrine and Neurobiological Regulation of the Female Sexual Response
Animal and human studies have contributed , to our knowledge, which is best summarized in what has been called the Dual Control System .
The Dual Control System identifies the human sexual response as a dynamic result of the interaction between excitatory processes and inhibitory processes (Sexual Wanting and Sexual Inhibition). This Dual System response is present and active in the regulation of the peripheral response (see above) and also functional in the central regulation.
Regulation of the Peripheral Response
The above-described peripheral response is regulated, and modified according to the setting resulting from interactions between:
(a)
(b)
(c)
(d)
All these systems interact with each other and upregulate or downregulate the peripheral response (Fig. 18.7) [18, 19, [22]].
Fig. 18.7
Regulatory factors affecting sexual function (Modified from Clayton AH. Sexual function and dysfunction in women. Psychiatric Clinics of North America 2003;26:673–682 [13]).
18.4 Regulatory Factors Affecting Sexual Function
18.4.1 Estrogens
Estradiol, and to a minor degree estriol, plays an important role in maintaining the ability of the vulvovaginal unit to respond to sexual stimuli and in enhancing the response through the following actions:
(a)
(b)
(c)
(d)
18.4.2 Testosterone
The role of testosterone in the peripheral sexual response is less well investigated. It seems, however, that testosterone is an important cofactor for the estrogen action and may increase vaginal blood flow. In addition, the Bartholin’s glands, Skene’s glands, and minor vestibular glands are testosterone-dependent, mucin-secreting glands that are essential for lubrication of the introitus. Lastly, testosterone may also lead to relaxation of the smooth muscle in the muscularis layer.
18.4.3 Progesterone
The role of progesterone in the sexual response has not yet been elucidated. It is assumed that progesterone exerts a protective effect on neurons, which would be nonspecific and not directly related to the peripheral sexual response.
18.4.4 Receptors in the Vagina
It is important to note that effects of steroids depend on the presence of steroid receptors in the vagina. These receptors can be up- and downregulated and can thus modify the response to hormones. Receptors for estrogen, testosterone, and progesterone have been found in the vaginal tissues. Their differential action during the menstrual cycle and during different phases of the reproductive life of a woman remains to be investigated.
18.5 Amino Acids, Neuropeptides, and Tissue Activators
Besides the action on their receptors, sex steroids act via the autonomic nervous system, inducing either neuronal changes and/or the secretion of amino acids and neuropeptides that can be found in various vaginal tissues. VIP is an important vasodilator together with NOS, while other neuropeptides have a more vasoconstrictive action (NPY, norepinephrine, CGRP) [18]. Local prostaglandin production relaxes small blood vessels and increases blood flow.
18.6 Autonomic Nervous System
Activation of the autonomic nervous system is part of the preparation of the body for sexual activity. During this preparation, sympathetic nervous system transmitters such as epinephrine play an important role by increasing heart rate and breathing intensity, but the parasympathetic nervous system also acts via vasodilation of vessels in the genital organs. Activation of the sympathetic system facilitates genital blood flow not alone but in the context of appropriate sexual stimulation.
While norepinephrine exerts a stimulatory effect, serotonin and 5HT have an inhibitory effect on sensation and vasocongestion.
18.6.1 Central Nervous Regulation
The peripheral response regulation of the sexual response is best understood in the context of excitatory and inhibitory pathways [16, 19, 20].
18.6.1.1 Excitatory Pathways and Processes
Sexual stimuli can be principally perceived via two pathways. The first pathway is fast and transmits sexual signals (visual, tactile, auditory) to parts of limbic system including the ventral striatum (which includes the nucleus accumbens), the amygdala, the anterior cingulate cortex (ACC), and the orbitofrontal cortex (OFC). The second pathway is slower and closer to consciousness, relying on the prefrontal cortex and linking to the hippocampus as part of the explicit memory system. The regulation of these excitatory pathways is affected by gonadal steroids and neurotransmitters (Fig. 18.8) [21].
Fig. 18.8
Regulation of excitation and inhibition in the context of the sexual response—the “Sexual Tipping Point”
18.6.1.2 Gonadal Steroids
It has been shown that estrogens have central excitatory effects. While no specific centers are involved, estrogens appear to provide a priming effect by genomic and non-genomic effects on different parts of the mesolimbic system. The effect can best be described as facilitating arousability and responsiveness.
Testosterone has been shown in animal and human studies to have an “initiating” or “sex wanting” effect. Although the importance of testosterone for healthy women’s desire and arousability is still controversial, there is now evidence that lack of testosterone in specific clinical conditions such as after bilateral ovariectomy or in the setting of premature ovarian failure increases the risk for female sexual dysfunction and that replacement therapy has a therapeutic effect. This includes also women around and after natural menopause.
18.6.1.3 Neurotransmitters
Studies in animals have shown that sexual desire is stimulated by excitatory neurochemical mechanisms in the brain involving the following regions and neurotransmitters [22]:
(a)
(b)
(c)
18.7 Inhibitory Pathways and Processes
The inhibitory pathway, like the excitatory pathway, also involves brain structures, hormones, and neurotransmitters.
18.7.1 Brain Structures
The brain structures involved in sexual inhibition seem to include hyperactivity in the prefrontal cortex, increased activity in ventral prefrontal and dorsal parietal areas, and decreased activity in the middle cingulate cortex [22].
18.7.2 Hormones
Whereas gonadal steroids serve the purpose of reproduction and thus act as “turn on” substances, lowering of these hormones by other neuroendocrine mechanisms and hormones can have an inhibitory effect on sexual desire. Prolactin appears to be an inhibitory signal; whether this is an indirect effect via suppression of ovarian steroids, or a direct effect on receptors in the brain is not yet clear.
18.7.3 Neurotransmitters
The main inhibitory neurotransmitters are serotonin and to a certain extent opioids, which mediate the state of receptor saturation. Endogenous opioids represent important biological molecules involved in female sexual response and enforce a strong reward signal followed by a prolonged period of relaxation.
In summary, the anatomy and physiology of the female sexual response is characterized by a complex interaction of peripheral genital organs and neuroendocrine and neurobiological centers of processing sexual stimuli and directing responses along neuroendocrine, neurovascular, and neuromotor pathways which are still under investigation.
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