Physiology of the Male Reproductive System

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Chapter: Anatomy and Physiology for Health Professionals: Reproductive System

The physiology of the male reproductive system involves primary phases of sexual response. Physiology Function of the Male Reproductive System - Male Sexual Response: Erection, Ejaculation ; Spermatogenesis ; Hormonal Regulation: Effects of Testosterone

Physiology of the Male Reproductive System

Physiology of the Male Reproductive System

The physiology of the male reproductive system involves primary phases of sexual response. These include erec-tion of the penis for penetration of the female vagina and ejaculation, which allows semen and the sperm it contains to be propelled into the vagina.

 

Male Sexual Response

When sexual stimulation occurs, parasympathetic nerve impulses from the sacral area of the spinal cord release nitric oxide, which dilates the arteries leading into the penis. Arterial pressure in the erectile tissue compresses the veins to reduce blood flow away from the penis.

Erection

The erectile tissues expand with blood and the penis swells and elongates to produce an erection. Before sexual arousal, arterioles that supply the erectile tissue are constricted and the penis is flaccid. Sexual arousal triggers the parasympathetic reflex that causes nitric oxide to be released locally, relaxing the smooth mus-cle in the walls of the penile blood vessels. The arteri-oles dilate and the erectile bodies become filled with blood. As the corpora cavernosa expand, their drain-age veins become compressed. The engorgement of the penis is maintained because outward blood flow cannot occur. The corpus spongiosum only slightly expands in comparison and functions to keep the ure-thra open when ejaculation occurs. When erect, the penis should not bend excessively. This is prevented by the way the collagen fibers surrounding the penis are arranged in longitudinal and circular fashion.

Ejaculation

Male orgasm is accompanied by emission and ejaculation­, which is the propulsion of semen from the duct system. A typical ejaculation releases approx-imately 300 million sperm. The movement of sperm cells from the testes and secretions of the prostate gland and seminal vesicles into the urethra is known as emission. In the urethra, all these components mix to form semen. Emission occurs as a result of spinal sympathetic nerve impulses that stimulate peristaltic contractions in the testicular ducts, epididymides, ductus deferentia, and ejaculatory ducts. The sympa-thetic nerves involved are mostly at the level of L1 and L2. Other sympathetic impulses simultaneously cause rhythmic contractions of the seminal vesicles and prostate gland.

The urethra fills with semen as sensory impulses pass into the sacral portion of the spinal cord. The bladder sphincter muscle constricts to prevent urine expulsion or semen reflux into the bladder. Contrac-tion of the reproductive ducts and accessory glands helps to fill the urethra with semen. Somatic motor impulses are then transmitted to certain skeletal mus-cles, causing a reflex in which the penile erectile col-umns contract rhythmically because of the actions of the bulbospongiosus muscles. This increases pres-sure inside the erectile tissues, helping to force semen through the urethra to outside the body, which is the process of ejaculation. Semen is propelled at a speed of nearly 11 miles per hour! Physiological and psycholog-ical release, known as an orgasm, is the culmination of sexual stimulation. Intense pleasure is experienced, and systemic changes include elevated blood pressure, rapid heartbeat, and generalized muscle contraction.

Fluid from the bulbourethral glands is expelled first during emission and ejaculation, followed by fluid from the prostate gland, passage of sperm cells, and finally, fluid from the seminal vesicles. After ejac-ulation, the arteries of the erectile tissue immediately constrict. Smooth muscles in the vascular spaces con-tract partially, and veins of the penis carry away excess blood, gradually returning the penis to its flaccid state. TABLE 25-1 summarizes the functions of the male reproductive organs.


After orgasm, a period of resolution quickly occurs, in which muscles relax. The internal puden-dal arteries and penile arterioles are constricted by activity of the sympathetic nerve fibers. Blood flow is reduced into the penis. Small muscles are activated, squeezing the cavernous bodies to force blood from the penis into the general circulation. The penis ­eventually becomes flaccid again. There is a latent, refractory period after ejaculation in which the male is unable to achieve another orgasm. This period length-ens because of aging.

1. Explain the structures of the male reproductive duct system.

2. Describe the functions of the prostate gland.

3. Explain the processes involved in erection and ejaculation.

 

Spermatogenesis

Spermatogenesis is the process by which sperm cells or spermatozoa are formed. Spermatogenic cells form sperm cells and line the seminiferous tubules. Interstitial cells lie in spaces between the seminifer-ous tubules. Male sex hormones are produced and secreted by these interstitial cells.

The epithelium that makes up the seminiferous tubules contains supporting sustentacular or Sertoli cells and spermatogenic cells. These cells provide a framework that nourishes and regulates sperm cells, which are continually produced beginning at puberty (usually around age 14). Sperm cells collect in the lumen of each tubule, pass to the epididymis, and then mature. Each mature sperm cell is about 0.06 mm (60 μm) in length and appears like a tiny tadpole. It has a flattened “head,” a cylinder-shaped “body,” and a long “tail” (FIGURE 25-4). Approximately, 400 million sperm are produced daily by a healthy male.


The head of a sperm cell has a nucleus and com-pacted chromatin containing its 23 chromosomes. The acrosome is a small protrusion that contains enzymes needed to help it to penetrate an egg cell during fer-tilization. The midpiece or body of the sperm cell has a filamentous core and spiraled mitochondria. The tail or flagellum consists of microtubules in an exten-sion of the cell membrane. The tail moves via adenos-ine triphosphate from the mitochondria, propelling the sperm cell through its containing fluid. Normal development of spermatozoa in the testes requires temperatures that are about 1.1°C or 2°F lower than temperatures found elsewhere in the body.

A mature spermatozoan lacks an endoplasmic reticulum, a Golgi apparatus, lysosomes, peroxisomes, and many other intracellular structures. The loss of these organelles reduces the cell’s size and mass. Sper-matozoa are basically mobile carriers for the enclosed chromosomes and can be slowed down by an extra weight. Because the sperm cell lacks glycogen and other energy reserves, it must absorb fructose and other nutrients from the surrounding fluid.

In a male embryo, spermatogenic cells are undif-ferentiated. Also called spermatogonia, they contain 46 chromosomes. During embryonic development, spermatogonia undergo mitosis, creating two daugh-ter cells. One of these is a new “type A” spermatogo-nium that maintains supplies of undifferentiated cells, whereas the other is a “type B” spermatogonium that enlarges to become a primary spermatocyte.

During puberty, primary spermatocytes reproduce via meiosis, a type of cell division that includes first and second meiotic divisions (FIGURE 25-5). It is different from mitosis, which is the process by which most body cells divide. Meiosis I is the first division, which sepa-rates chromosome pairs that are homologous, meaning “gene for gene.” This does not mean they are identical since genes may vary because of hereditary factors. Meiosis is also called reductional division because 46 chromosomes are reduced to 23. Each homologous chromosome is replicated before meiosis I occurs, so it consists of two complete DNA strands called chroma-tids. These attach at areas called centromeres and carry all the genetic information associated with that specific chromosome. Each of the four daughter cells produced have half as many chromosomes as a typical diploid body cell. In meiosis, corresponding maternal and paternal chromosomes unite during synapsis.


Meiosis II causes one member of each homolo-gous pair via a condition called haploid to separate its chromatids. This produces other haploid cells with one set of chromosomes, but with the chromosomes no longer in the replicated form. Meiosis II causes each of the chromatids to become an independent chromo-some. Each primary spermatocyte divides into two secondary spermatocytes; these divide again to form two spermatids, which mature. For each primary sper-matocyte that undergoes meiosis, four sperm cells with 23 chromosomes in each of their nuclei are formed. A matched set of four chromatids is known as a tetrad. Meiosis II is also known as equational division because the number of chromosomes is not changed.

The final part of spermatogenesis is called ­spermiogenesis, in which each spermatid matures into a single sperm or spermatozoon. The Sertoli cells are also called nurse cells because they assist in the steps of spermatogenesis. A summary of these steps is: (1) maintenance of the blood–testis bar-rier, (2) support of mitosis and meiosis, (3) support of spermiogenesis, (4) secretion of inhibin, (5) secre-tion of androgen-­binding protein, and (6) secretion of müllerian-inhibiting factor. Müllerian-inhibiting factor is a hormone­ that causes regression of the parameso-nephric ducts in the fetus, which eventually form the uterine tubes and uterus in females. When there is not enough of this hormone, the testes fail to descend into the scrotum.

1. Summarize the events of spermatogenesis.

2. List the major structural and functional regions of sperm.

3. Define the term meiosis and contrast it with mitosis.

 

Hormonal Regulation

Male reproductive functions are controlled by hor-mones from the hypothalamus, anterior pituitary gland, and testes. The hormones begin and maintain sperm cell production, overseeing development and maintenance of secondary sex characteristics. Before puberty, the male body cannot reproduce and its sper-matogenic cells are undifferentiated. The hypothala-mus controls the changes during puberty that make a male’s body able to reproduce.


The hypothalamus secretes gonadotropin-­ releasing hormone (GnRH), and the anterior pituitary secretes the gonadotropins known as luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Also known as interstitial cell-stimulating hormone, LH promotes development of testicular ­interstitial cells that secrete male sex hormones. FSH stimulates seminiferous tubule cells to respond to the male sex hormone testosterone. These supporting cells cause spermatogenic cells to undergo spermatogenesis, cre-ating sperm cells (FIGURE 25-6). Another hormone, inhibin, is secreted, keeping the anterior pituitary gland from oversecreting FSH via negative feedback. The hypothalamic–pituitary–gonadal axis comprises a sequence of regulatory events that govern male reproductive function (TABLE 25-2).


Androgens are the male sex hormones. They are mostly produced by the testicular interstitial cells, although the adrenal cortex synthesizes small amounts of them. Testosterone is the most import-ant androgen, loosely attaching to plasma proteins for secretion and transport via the blood. Secretion begins during fetal development, continues for sev-eral weeks after birth, and almost stops completely during childhood. Between ages 13 and 15, it restarts, producing testosterone at a rapid rate and making the body reproductively functional. This period is known as puberty . Secretion continues after puberty throughout the life of males.

Testosterone enlarges the testes and accessory reproductive organs and develops the male secondary sex characteristics:

Increased body hair on the face, chest, armpits, and pubic region

Sometimes, decreased hair growth on the scalp

Enlargement of the larynx and thickening of vocal folds, which lower the pitch of the voice

Thickening of the skin

Increased muscular growth, broadening of shoulders, and narrowing of waist

Thickening and strengthening of the bones

Testosterone also increases cellular metabolism and red blood cell production. Males usually have more red blood cells in a microliter of blood than females do because of the actions of testosterone. It also affects the brain, stimulating sexual activity.

The more testosterone received by the intersti-tial cells, the greater the speed at which the male secondary sex characteristics develop. Testosterone output is regulated by a negative feedback system in the hypothalamus. More testosterone in the blood inhibits the hypothalamus, decreasing GnRH secre-tion from the anterior pituitary. As LH secretion also falls, testosterone­ release from the interstitial cells decreases. Decreasing blood testosterone causes the hypothalamus to stimulate the anterior pituitary to release LH. Then, the interstitial cells release more testosterone and the blood testosterone levels increase again. A period in a man’s life known as the male climacteric marks a decrease in testosterone level and a decline in sexual function.

The amount of testosterone and sperm produced reflects a balance among GnRH, FSH, and LH. GnRH indirectly stimulates the testes through its effect on FSH and LH release. Both FSH and LH directly stimulate the testes. Testosterone and inhibin exert negative­ feedback controls on the hypothalamus and anterior pituitary.

In puberty, this balance is achieved over about three years. Then, produced testosterone and sperm remain in basically constant amounts throughout life. When GnRH and gonadotropins are absent, the tes-tes atrophy. Sperm and testosterone production then stops. Near puberty, higher levels of testosterone are needed to suppress GnRH release from the hypothala-mus. More GnRH release causes more testosterone to be secreted by the testes. However, the hypothalamic inhibition threshold continually rises until the adult levels of hormone interaction are developed.

Effects of Testosterone

Testosterone, like all other steroid hormones, is syn-thesized from cholesterol. It works by activating cer-tain genes, resulting in increased protein synthesis in target cells. This may require testosterone to be trans-formed into other steroid hormones in some target cells. In the prostate, it is converted to dihydrotestoster-one and in certain brain neurons to estradiol. Through-out puberty, testosterone also has a variety of anabolic effects in the body. It causes accessory reproductive organs to mature. Without it, all accessory organs atrophy, erection and ejaculation are impaired, and semen volume decreases greatly. The results are impo-tence and sterility. However, testosterone replacement therapy is very successful.

The male secondary sex characteristics occur in the nonreproductive organs because of the effects of testosterone and other androgens. These are development of hair in the facial, axillary, and pubic regions; increased hair growth elsewhere on the body; a deepening of the voice due to larynx enlarge-ment; skin thickening; increased oil production that may result in acne; increased bone density and size; and increased skeletal muscle mass. In males, testosterone increases the basal metabolic rate and changes behavior. Male libido is based on the effects of testosterone.

1. Summarize the events of spermatogenesis.

2. List the major structural and functional regions of sperm.

3. Define the term meiosis and contrast it with mitosis.





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