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Physiology of sexual function

 
Sexual Anatomy
The external genitalia, referred to collectively as the vulva, consist of the mons pubis, clitoris and bulbs, labia majora, and labia The mons pubis is an area of fatty tissue overlying the pubic symphysis and covered by pubic hair. Clitoral body extends into the mons for several centimeters before bifurcating into the crura (singular ¼ crus), which run bilaterally under the inferior pubic rami. Between the crura lie the clitoral bulbs, draped over the urethra, with the bulk of the bulbar tissue lateral to the walls of vagina Usually, only the glans clitoris is visible externally, approximately 1 cm anterior to the urethral meatus, and the glans is often covered by the clitoral hood The labia majora are fatty, elongated, hair-bearing folds of tissue forming the lateral boundaries of the vulva. The medial sides of the labia majora unite with the labia minora, which are thin crease of skin circumscribing the introitus, or inlet to the vagina. The anterior sides of the labia minora split and fuse with the frenulum of the glans clitoris on its ventral aspect and fuse over the glans as the hood. The posterior aspects of the labia minora connect in the midline at the lower aspect of the introitus Gross anatomic and histologic study of the clitoris, clitoral bulbs, labia minora, and urethra show them to contain specialized vascular tissues that are sexually responsive and include two histologically well defined categories of vascular tissue. Trabeculated erectile tissue construct the clitoris and the bulbs. The erectile tissue of both the clitoris and bulbs resemble that of the male corpora cavernosa and corpus spongiosum. The trabecular nature of the erectile tissue allows for engorgement with blood and volume expansion throughout sexual arousal. In contrast to the clitoral and bulbar erectile tissue, the labia minora and glans clitoris are composed of non-erectile vascular tissue, in which the blood vessels are distributed within a fibrous matrix, with particular least amount of smooth muscle. Non-erectile, sexually reactive vascular tissue also is found surrounding the urethral lumen and within the walls of the vagina. These tissues also respond throughout sexual arousal with increases in blood flow, but the structure of the tissues does not accommodate vascular engorgement as in the erectile tissues
The urethra is a midline structure and sits toward the vertex of the introitus. Although not classically identified as a sex organ, it manifest changes in blood flow through out arousal. The distal urethra is surrounded by the erectile tissue of the clitoral bulbs. There are paraurethral glands accompanying the length of the lumen. Not so many of these glands stain positively for prostate-specific antigen  leading some to declare that this area is correspondent to the male prostate.
However, the glands are isolated and do not give the impression to have any endocrine or exocrine function in ladies.An area of the anterior wall of the vagina covering the midurethra has been described as the Grafenberg spot (or G-spot), a region that, in some ladies, is particularly sensitive to tactile stimulation. Grafenberg’s original article demonstrated that the erogenous area as arising from the urethra. Controversy still exists over the possible anatomic structure and site of the G-spot
The vagina is considered an internal genital component. It is a potential space consisting of a flattened fibromuscular tube with anterior and posterior walls collapsed onto each other, extending from the introitus to the fornices that encircle the cervix. The vaginal wall consists of three layers: (i) the stratified squamous non-keratinized epithelium and an underlying lamina propria of connective tissue; (ii) the muscular layer, composed of smooth muscle fibers disposed longitudinally and circularly; and (iii) the adventitia, a dense connective tissue that blends with the surrounding fascia. The lamina propria and connective tissue layers contain a rich supply of vascular channels. During sexual stimulation, the marked increase in fluid production in the vagina is caused by transudation across the vaginal wall, providing the lubrication needed for non-painful and non-traumatic vaginal intercourse. There are no glands in the vaginal wall 
Physiology of sexual function
Sexual desire may be defined differently by different individuals, but for clinical purposes it has been descriped as ‘the sum of forces that lean us toward and push us away from sexual behavior’ It is a complex construct and has been delineated into three main components of drive, expectations/wish and motivation Drive is the biological component of desire governed by neuroendocrine physiology and relates to the chemical mediators in the nervous system such as dopamine and hormones such as testosterone. Drive may diminish with ageing in terms of key hormones such as oestrogen, dehydroepiandrosterone (DHEA) and testosterone Drive may also be greatly impacted by common medical conditions that occur more as women age such as diabetes, obesity and metabolic syndrome that negatively affect hormones.
Multiple models have been developed to describe a healthy sexual response
The female sexual response cycle is a sequence of body changes that occur in response to sexual stimulation, affected by many factors such as psychological, biological and environmental factors
It is a cycle of four overlapping physiological phases which varies in intensity and time spent from person to another person:Arousal phase, plateau phase, orgasmic phase and resolution phase
The Excitement phase or Arousal phase which is the first stage of the cycle, may last from minutes to hours, sexual desire and arousal co-exist and compound each other, desire is triggered by erotic stimulation such as physical or mental stimulation like fantasies, memories, sexual thoughts, kissing or viewing erotic images.
It is influenced by beliefs, traditions, sexual orientation, fatigue and illness.
 It is characterized generally with Increased muscle tension, tachycardia, tachypnea, sexual flushing (vasocongestion of the skin with formation of maculopapular erythematous rash on the chest and epigastrium and swelling of external genitalia), vaginal transudative lubrication occurs, the bartholin glands secrete few lubricant,engorged breasts and erected nipples as a myotonic response generally the excitement phase is caused by activation of the parasympathetic fibers of the autonomic nervous system which will be affected in ladies receiving anticholinergic drugs
The plateau phase is the period of arousability before orgasm, All physical changes that occurred in the excitement phase are markedly intensified. With continuous increase in blood flow to external genitalia, the lower third of the vaginal wall become dark purple in colour and more swollen decreasing 50% of its diameter which allows better friction against the penis with dilation of the upper two thirds also the clitoris withdraws slightly under the clitoral hood, become more sensitive(may be painful to touch), the bartholin glands increase lubricant discharge.
It is the peak of pleasure in ladies who never experienced orgasm
The orgasm phase(the climax) is the most euphoric sensation in the sexual response cycle, It is accomplished by rhythmic involuntary contractions of the lower perineal striated muscles that lasts only a few seconds range from 2 to 4seconds and repeat at 0.8-2 seconds intervals, It is commonly associated with Increased vaginal lubrication, also blood pressure,heart rate and respiratory rate reach their highest rates, with sudden release of sexual tension, women can have multiple orgasms quickly in a short period of time with the presence of effective sexual stimulation, orgasm is caused by stimulation of the sympathetic system. In some cases antihypertensive or antidepressant drugs affect orgasmic response.
The resolution phase is the relaxation from sexual tension and slow resumption of normal level of body functions with enhanced intimacy and increased sense of well-being.
Masters and Johnson suggested that the clitoris plays The main role in sexual satisfaction in ladies. others suggested that the cervix plays a role too depending on the fact that it is highly supplied with nerve fibers actually no scientific data support this theory.
Human female sexual arousal encompasses a mental state (emotional and cognitive) and a physical state induced by various stimuli that usually produce feelings of pleasure and initiate a desire to continue the activity often to orgasmChanges in the physical state involve increases in heart rate, blood pressure, respiration, genital blood flow and vasocongestion, vaginal lubrication, and nipple erection. However, there are conditions in which mental arousal can occur without genital physical changes and physical changes can occur without mental changes Any or all of the senses (vision, hearing, taste, smell, and touch) and fantasy can be involved in creating the arousal  However, touch is of great importance, because most intrapersonal sexual scenarios involve this sense. Many sites on the female body can evoke arousal when caressed—for example, the lips, nape and back of neck, ears, underarms, breasts and nipples pubic hairline, inside of thighs, buttocks, anus, and perineum—but the most sensitive structures lie in and around the genitalia The sites can be stimulated by different means (vibrators, dildoes, digitally, and lingually) and by PVI.
Because of this, it would be expected that the neural inputs from PVI-stimulated multiple sites would facilitate rapidly induced sexual arousal leading to orgasm more quickly than that from the stimulation of a single site, such as the clitoral shaft and glans. In fact, when questioned, women find that the latter stimulation needs less energy, creates arousal faster, and yields more intense orgasms, although these are not always the most satisfying. 

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