Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
14/MHS03/022 PHYSIOLOGY OF ERECTION INTRODUCTION The peripheral pharmacology of local mechanisms of penile erection is known today thanks to recent advance in the study of the regulation of erectile tissue smooth muscle tone. Smooth muscle fibers present in the corpus cavernosum and arteries destined to the penis relax in response to the release of non adrenergic non cholinergic neuromediators synthetized by postganglionic parasympathetic nerve fibers present in the cavernous nerves. Nitric oxide is the main proerectile neuromediator. Noradrenaline, released by sympathetic fibers, contracts penile smooth muscle fibers and is antierectile. Recent progress in the peripheral pharmacology of penile erection allows new perspectives in the treatment of erectile dysfunction. The spinal cord represents a major site for the neural regulation of penile erection. The latter occurs in response to stimuli from peripheral or supraspinal origin. Different neural structures in the brainstem (nucleus paragigantocellularis), pons and hypothalamus (nucleus paraventricularis) send projections to the thoracolumbar sympathetic and lumbosacral parasympathetic nuclei at the origin of proerectile peripheral pathways. Serotonin and oxytocin are candidates as neuromediators involved in the supraspinal control of penile erection. Studying the central command of penile erection allows an approach to the pathophysiology of psychogenic erectile dysfunction. Penile Erection—Role of the Parasympathetic Nerves. An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle and direction of an erection varies considerably in humans. Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system (ANS), causing nitric oxide (a vasodilator) levels to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpora spongiosum) to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood. Erection subsides when parasympathetic activity reduces to baseline. As an autonomic nervous system response, an erection may result from a variety of stimuli, including sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence. A penis which is partly, but not fully, erect is sometimes known as a semi-erection (clinically: partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft Penile erection is the first effect of male sexual stimulation, and the degree of erection is proportional to the degree of stimulation, whether psychic or physical. Erection is caused by parasympathetic impulses that pass from the sacral portion of the spinal cord through the pelvic nerves to the penis. These parasympathetic nerve fibers, in contrast to most other parasympathetic fibers, are believed to release nitric oxide and/or vasoactive intestinal peptide in addition to acetylcholine. The nitric oxide especially relaxes the arteries of the penis, as well as relaxes the trabecular meshwork of smooth muscle fibers in the erectile tissue of the corpora cavernosa and corpus spongiosum in the shaft of the penis, This erectile tissue consists of large cavernous sinusoids, which are normally relatively empty of blood but become dilated tremendously when arterial blood flows rapidly into them under pressure while the venous outflow is partially occluded. Also, the erectile bodies, especially the two corpora cavernosa, are surrounded by strong fibrous coats; therefore, high pressure within the sinusoids causes ballooning of the erectile tissue to such an extent that the penis becomes hard and elongated. This is the phenomenon of erection. MECHANISM OF ERECTION There are two major mechanisms involved in the physiology of penile erection which are; 1, Increased blood inflow 2, decreased blood outflow The firm and enlarged state of the penis is called an erection. There are four factors that lead to the erection of the male genitals; neural, endocrine, vascular and physiological. The complex interaction between these four factors causes the penis to erect. When there is a disruption in any of the functions of these four factors, erection dysfunction or impotence may occur. In order to prevent any event of disruption, it is necessary that a male knows the mechanics of erection. The physiological explanation for the mechanics of erection is simple. There are only two mechanisms that are involved in erection; increased blood inflow and decreased blood outflow. Initially, erection is triggered upon the functioning of physiological stimuli. It starts with the libido function which is directly happening in the cerebral cortex. This is the sexual urge that makes a man feel the need for sex. When the libido arises, an impulse is sent to the spinal center which is then sent to the penis nerves. The blood inflow mechanism of erection starts right after the impulse is sent to the penis nerves. When the penis nerve endings receive the impulse, arterioles dilate as the smooth muscles relax. Because of this relaxation that occurs in the muscles, the spaces in the corpora nervosa, the chambers in the penis, are filled in. When these spaces are completely filled in, the space is no longer enough to accommodate the blood flow, thus expansion follows. The blood outflow mechanism of erection starts upon copulation. Copulation is also called sexual intercourse. This takes place when the penis finally enters the vagina. This act continues until the ejaculation and orgasm is attained. Ejaculation and orgasm is reached by the continuous friction between the vaginal mucosa and penis glans. The friction is reinforced by the stimulation of psychogenic factors to discharge sperm from the sympathetic pathway to the seminal pathway. When the sperm reaches the seminal pathway, it is led to the posterior urethra by the blood flow. The brain sends impulses again to the muscles and this signifies the end of the copulation because by the time the impulses were received by the penis nerves, orgasm is reached. After orgasm and ejaculation are reached, the discharge resumes and once again, the expanded muscles go back to their normal size as they contract and the blood that comes from the sinusoidal spaces is flowed out of the muscles. When the above mechanism of erection is disrupted at anytime of the process, orgasm and ejaculation is reached. Continuous events of disruption may totally damage the nerves in the penis and this will result to impotence or erectile dysfunction PHYSIOLOGY OF COITUS(male sexual act) Neuronal Stimulus for Performance of the Male Sexual Act The most important source of sensory nerve signals for initiating the male sexual act is the glans penis.The glans contains an especially sensitive sensory endorgan system that transmits into the central nervous system that special modality of sensation called sexual sensation.The slippery massaging action of intercourse on the glans stimulates the sensory end-organs, and the abnormal infertile sperm, compared with a normal sperm on the right. sexual signals in turn pass through the pudendal nerve, then through the sacral plexus into the sacral portion of the spinal cord, and finally up the cord to undefined areas of the brain. Impulses may also enter the spinal cord from areas adjacent to the penis to aid in stimulating the sexual act. For instance, stimulation of the anal epithelium, the scrotum, and perineal structures in generalcan send signals into the cord that add to the sexual sensation. Sexual sensations can even originate in internal structures, such as in areas of the urethra, bladder, prostate, seminal vesicles, testes, and vas deferens. Indeed, one of the causes of “sexual drive” is filling of the sexual organs with secretions. Mild infection and inflammation of these sexual organs sometimes cause almost continual sexual desire, and some “aphrodisiac” drugs, such as cantharidin, increase sexual desire by irritating the bladder and urethral mucosa, inducing inflammation and vascular congestion. Psychic Element of Male Sexual Stimulation. Appropriate psychic stimuli can greatly enhance the ability of a person to perform the sexual act. Simply thinking sexual thoughts or even dreaming that the act of intercourse is being performed can initiate the male act, culminating in ejaculation. Indeed, nocturnal emissions during dreams occur in many males during some stages of sexual life, especially during the teens. Integration of the Male Sexual Act in the Spinal Cord. Although psychic factors usually play an important part in the male sexual act and can initiate or inhibit it, brain function is probably not necessary for its performance because appropriate genital stimulation can cause ejaculation in some animals and occasionally in humans after their spinal cords have been cut above the lumbar region. The male sexual act results from inherent reflex mechanisms integrated in the sacral and lumbar spinal cord, and these mechanisms can be initiated by either psychic stimulation from the brain or actual sexual stimulation from the sex organs, but usually it is a combination of both. Stages of the Male Sexual Act Penile Erection—Role of the Parasympathetic Nerves. Penile erection is the first effect of male sexual stimulation, and the degree of erection is proportional to the degree of stimulation, whether psychic or physical.Erection is caused by parasympathetic impulses that pass from the sacral portion of the spinal cord through the pelvic nerves to the penis. These parasympathetic nerve fibers, in contrast to most other parasympatheticfibers, are believed to release nitric oxide and/orvasoactive intestinal peptide in addition to acetylcholine. The nitric oxide especially relaxes the arteries of the penis, as well as relaxes the trabecular meshwork of smooth muscle fibers in the erectile tissue of the corpora cavernosa and corpus spongiosum in the shaft of the penis. This erectile tissue consists of large cavernous sinusoids, which are normally relatively empty of blood but become dilated tremendously when arterial blood flows rapidly into them under pressure while the venous outflow is partially occluded. Also, the erectile bodies, especially the two corpora cavernosa, are surrounded by strong fibrous coats; therefore, highpressure within the sinusoids causes ballooning of the erectile tissue to such an extent that the penis becomes hard and elongated. This is the phenomenon of erection. Lubrication, a Parasympathetic Function. During sexual stimulation, the parasympathetic impulses, in addition to promoting erection, cause the urethral glands and the bulbourethral glands to secrete mucus.This mucus flows through the urethra during intercourse to aid in the lubrication during coitus. However, most of the lubrication of coitus is provided by the female sexual organs rather than by the male. Without satisfactory lubrication, the male sexual act is seldom successful because unlubricated intercourse causes grating, painful sensations that inhibit rather than excite sexual sensations. Emission and Ejaculation—Function of the Sympathetic Nerves. Emission and ejaculation are the culmination of the male sexual act. When the sexual stimulus becomes extremely intense, the reflex centers of the spinal cord begin to emit sympathetic impulses that leave the cord at T-12 to L2 and pass to the genital organs through the hypogastric and pelvic sympathetic nerve plexuses to initiate emission, the forerunner of ejaculation. Emission begins with contraction of the vas deferens and the ampulla to cause expulsion of sperm into the internal urethra. Then, contractions of the muscular coat of the prostate gland followed by contraction of the seminal vesicles expel prostatic and seminal fluid also into the urethra, forcing the sperm forward. All these fluids mix in the internal urethra with mucus already secreted by the bulbourethral glands to form the semen. The process to this point is emission. The filling of the internal urethra with semen elicits sensory signals that are transmitted through the pudendal nerves to the sacral regions of the cord, giving the feeling of sudden fullness in the internal Deep penile fascia. REFERENCE Wikipedia the free encyclopedia, Anderson RA, Baird DT: Male contraception. Endocr Rev 23:735, 2002. Barry MJ, Roehrborn CG: Benign prostatic hyperplasia. BMJ 323:1042, 2001. Brennan J, Capel B: One tissue, two fates: molecular genetic events that underlie testis versus ovary development. Nat Rev Genet 5:509, 2004. Cajochen C, Krauchi K, Wirz-Justice A: Role of melatonin in the regulation of human circadian rhythms and sleep. J Neuroendocrinol 15:432, 2003. Cheng CY, Mruk DD: Cell junction dynamics in the testis: Sertoli-germ cell interactions and male contraceptive development. Physiol Rev 82:825, 2002. Cooke HJ, Saunders PT: Mouse models of male infertility