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
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.
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.
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.
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
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.
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.
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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