Biology of Sexual Dysfunction--论文代写范文精选

正常性功能是与认知过程参于的事情，一般的问题可能导致性功能障碍。性功能障碍会导致各种心理和生理因素的存在。病理生理学的性功能障碍包括性激素水平的紊乱和神经递质。下面这篇paper代写范文进行叙述。

Abstract
Sexual activity is a multifaceted activity, involving complex interactions between the nervous system, the endocrine system, the vascular system and a variety of structures that are instrumental in sexual excitement, intercourse and satisfaction. Sexual function has three components i.e., desire, arousal and orgasm. Many sexual dysfunctions can be categorized according to the phase of sexual response that is affected. In actual clinical practice however, sexual desire, arousal and orgasmic difficulties more often than not coexist, suggesting an integration of phases. Sexual dysfunction can result from a wide variety of psychological and physiological causes including derangements in the levels of sex hormones and neurotrensmitters. This review deals with the biology of different phases of sexual function as well as implications of hormones and neurotransmitters in sexual dysfunction 
Key Words: Sexual dysfunction, Hormones, Neurotransmitters  

Introduction
Sexual activity is a multifaceted activity, involving complex interactions between the nervous system the endocrine system, the vascular system and a variety of structures that are instrumental in sexual excitement, intercourse and satisfaction.(1) Sexual function is our physiological capacity to experience desire, arousal and orgasm. Normal sexual function requires the integrity of the genitalia, the reliable co-ordination of blood flow, the activation of various smooth and skeletal muscles and the stimulation of local secretions. This is linked with cognitive processes attending to the sexual meaning of what is happening. Problem anywhere in the entire sequence may lead to sexual dysfunction. Sexual dysfunction can result from a wide variety of psychological and physical causes. Pathophysiology of sexual dysfunction involves derangements in the levels of sex hormones and neurotransmitters.

Sexual Response Cycle
As defined by Kaplan, this consists of three phases- desire, arousal (excitement) and orgasm.(2) However, the division is arbitrary and it only helps to organize clinical and research oriented problems involving sexuality. In clinical practice sexual desire, arousal and orgasmic difficulties more often than not coexist, suggesting an integration of phases.

1. Desire (Libido) 
It is a complex construct involving physiologic, cognitive, behavioural, developmental and cultural components,(3) which are thought of as the broad interest in sexual experiences including thoughts, fantasies, dreams and wishes along with an interest in initiating or engaging in sexual activity and frustration due to lack of opportunity for sexual expression.(1) This stage is hypothetically a dopaminergic phenomenon mediated by mesolimbic dopaminergic "reward centre" including medial preoptic area (MPA) and anterior hypothalamus. The reward centre receives inputs from serotonergic and noradrenergic neurons and contains dopaminergic cell bodies.(4) Other contributing factors include testosterone and oxytocin, as well as personality characteristics and psychosocial context. 2. Arousal Sexual arousal is characterized by a subjective sense of sexual excitement associated with observable physiological changes such as penile tumescence in men and pelvic vasocongestion, vaginal lubrication and swelling of external genitalia in women. These responses may be accompanied in both sexes by other bodily changes like skin flushing, tachycardia etc.(5) Psychogenic penile erections are mediated by impulses descending from the cerebral cortex and limbic system to reach thoracolumbar sympathetic ganglia (inferior hypogastric plexus) and sacral parasympathetic ganglion. (6) Baseline non-erect state is maintained by tonic adrenergic stimulation of µ1 and µ2 adrenoceptors.(7) The sacral plexus via the pudendal and perineal nerves, initiates tactile reflexogenic erections. 

Hypogastric plexus mediates psychogenic penile erections. Efferent impulses from the parasympathetic vasodilator fibers in sacral plexus initiate vasodilatory mechanism for penile erection.(8) Psychological sexual arousal in women begins with increased clitoral length and diameter, and vasocongestion of the vagina, vulva, clitoris, uterus, and possibly the urethra. Pelvic nerve stimulation results in clitoral smooth muscle relaxation and arterial smooth muscle dilation. With sexual arousal, there is an increase in clitoral cavernosal artery inflow and an increase in clitoral intracavernous pressure that leads to tumescence and extrusion of the clitoris. Engorgement of the genital vascular network increases pressure inside the vaginal capillaries and results in lubrication of the epithelial surface of the vaginal wall. The neurotransmitters that mediate clitoral and arterial smooth muscle dilation remain undetermined.(9)

3. Orgasm 
Orgasm in both sexes is characterized by climax of sexual pleasure associated with rhythmic contractions of perineal muscles, cardiovascular and respiratory changes and a release of sexual tension.(3) In men, there are two physiological stages of orgasm – (i) Emission, (ii) Ejaculation Emission involves the propulsion of seminal fluid to the bulbar urethra by the contraction of the smooth muscles of the vas deferens, prostate and seminal vesicles. This process is thought to be under the control of thoracolumbar sympathetic nerves which release norepinephrine on to alpha receptors. After emission, the external sphincter is opened to allow release of fluid. In the second stage, a local, parasympathetic, sacral spinal reflex activates the striated muscles surrounding the bulbar urethra, producing ejaculation. 

The rhythmic contraction of these muscles propels the semen outward. It is unclear to what degree the occurrence and intensity of orgasm are shaped by central neurochemical and cognitive processes.(5) Smooth muscle controlled emission and striated muscle determined ejaculation involve separate nerve inputs and can be pharmacologically distinguished. In women less is known about the neural mechanisms of orgasm and the process is assumed to be largely similar to that in the male. Women are capable of experiencing multiple orgasms in rapid succession, without the more prolonged refractory periods. Mechanisms that make this possible are not clear, although it is hypothesized that women are less sensitive to the post-orgasmic secretion of prolactin that is hypothesized to produce a refractory period in men and women.(10)

Some women ejaculate fluid from the urethra with orgasm. Chemical analysis of this fluid confirms that while it may be mixed with small amounts of urine, it is predominantly a secretion from the paraurethral glands, thought to be a vestigial equivalent of the prostate. But this goes unrecognized when it occurs in a retrograde direction.(11)

Testosterone Testosterone has been shown to restore nocturnal penile tumescence responses in hypogonadal men, in whom this is impaired.(12) A recent study showed testosterone increased sexual arousal and enjoyment among hypogonadal and normal men, and had a positive effect on mood only among men with abnormally low testosterone levels.(13) In normal adult males, there exists wide individual variability in circulating testosterone levels that do not seem to be linked in any meaningful, way with individual differences in levels of drive or sexual behavior.(14) 

It is believed that the level of testosterone required for sexual interest and activity in adult males is lower than normal males’ circulating levels of testosterone. Among males with normal testosterone levels, testosterone has not been shown to facilitate erection.(15) In an earlier study of women who had undergone bilateral oophorectomy and adrenalectomy, removal of the ovaries decreased sexual desire to a certain extent, but removal of the adrenal glands had an even more deleterious effect on desire.(16) Studies of surgically menopausal women generally indicate that desire drops from presurgery level and may be restored with exogenous administration of supraphysiological levels of testosterone with or without estradiol.(17)

Estrogens 
Most research suggests that estrogens have little direct influence on sexual desire in either males or females. In men, relatively high levels of exogenous estrogen have been somewhat effective in inhibiting sexual desire among sex offenders and men who experience uncontrollable sexual urges.(18,19) In women, some early studies have claimed that estrogen (especially estradiol) is important for normal sexual desire, but most current researches agree that estrogens play only a minimal role in female sexual desire.(20,21) Estrogen deficiency, as occurs with menopause, causes a decrease in genital vasocongestion, lubrication and atrophy of the vaginal epithelium. Such changes not only impair the physiological sexual arousal response in women and may cause dyspareunia (painful intercourse), but can adversely influence the psychological experience of sexual arousal. These changes could be expected to indirectly impair sexual desire.(22)

Progesterone 
Early studies have revealed a decrease in sexual desire in men receiving intramuscular injections of progesterone. (23,24) No controlled studies have been conducted on the relationship between progesterone treatment and sexual desire in men. Certain oral contraceptives that increase progesterone levels throughout the menstrual cycle in female have been associated with decreased sexual interest and desire.(25,26) It is generally agreed on, however, that progesterone treatment does not have a substantial influence on the sexual desire of either premenopausal or postmenopausal women.(17,27,28)

Prolactin 
Men and women with abnormally high levels of prolactin report a decrease in sexual interest that is restored with bromocriptine treatment.(29) It is unclear whether the reversal of sexual symptoms secondary to bromocriptine treatment is attributable to the lowering of serum prolactin levels, to the correction of hypothalamic dopaminergic dysregulation, or to an interaction between these two mechanisms.(30) Prolactin's effect on other aspects of human sexual behaviour remains equivocal. Erectile dysfunction has been described in men with abnormally high levels of prolactin, (31) but has also been described in men with unusually low levels of prolactin, (32) suggesting more than a simple inhibitory role of prolactin on erectile ability. In women, abnormally high levels of prolactin have been associated with amenorrhoea, infertility and decreased sexual ability.(33) Oxytocin Circulating levels of oxytocin increase during sexual arousal and orgasm in both men and women.(34) 

Using a continuous blood sampling technique and electromyography, it was reported to have a positive correlation between oxytocin levels and the intensity, but not the duration of orgasmic contraction in males and females.(35) Most of what we know about the influence of oxytocin on sexual behaviour however is based on animal studies. Cortisol Hypercortisolism (Cushing syndrome) can produce a constellation of symptoms including depression, insomnia and decreased libido in males and females. Cortisol levels were higher in men with psychogenic erectile dysfunction who demonstrated a poor response to intracavernosal injection of a smooth muscle relaxant.(36)(paper代写)

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