Chapter 39 Neurons and Nervous Systems

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43.1 How Animals Reproduce

  • There are two patterns of reproduction.

1. Asexual–only one parent is involved and the offspring are identical to the parent.

2. Sexual–two parents are involved and the offspring are genetically unique.

A. Asexual Reproduction

1. Hydra reproduce asexually by budding; a new individual arises as an outgrowth (bud) of a parent.

2. Obelia undergoes an alternation of generations: an asexual colonial stage and a sexual medusa stage.

3. Flatworms can constrict into two halves, each half becoming a new individual.

4. Fragmentation followed by regeneration is seen among annelids, sponges, and echinoderms.

5. Parthenogenesis is found among some insects, worms, fish, lizards and some other animals; an unfertilized egg develops into a complete individual.

6. In honeybees, the queen can fertilize or not fertilize the eggs, producing diploid female workers (if fertilized) or haploid male drones (if unfertilized).

B. Sexual Reproduction

1. In sexual reproduction, the egg of one parent is fertilized by the sperm of the other; most animals are dioecious (having separate sexes).

2. Hermaphroditic organisms possess both male and female organs.

  1. Earthworms undergo cross-fertilization.

  2. Tapeworms are capable of self-fertilization.

  3. Sequential hermaphroditism, or sex reversal, involves the changing of sex; a male wrass (a reef fish) has a harem but if the male dies, the largest female becomes a male.

3. Gonads are organs specialized to produce gametes.

a. Sponges are an exception since their collar cells give rise to sperm and eggs.

b. Hydras produce only temporary gonads in the fall when sexual reproduction occurs.

c. Animals in other phyla have permanent gonads.

4. There are two types of gonads: testes produce sperm and ovaries produce eggs.

5. Eggs and sperm cells derive from germ cells that specialize early for this development.

6. Other cells in the gonads support and nourish the developing gametes or produce hormones for reproduction.

7. Accessory organs form ducts and storage areas that aid in bringing gametes together.

8. Sexually-reproducing animals have various methods to ensure that the gametes unite.

a. Aquatic animals that practice external fertilization must synchronize egg release.

b. The lunar cycle is one trigger that cues animals by tides.

c. Hundreds of thousands of palolo worms rise to the surface to release eggs during a 2–4 hour period.

9. Copulation is sexual union to facilitate the reception of sperm by a female.

a. The penis is a male copulatory organ typical of terrestrial males; it deposits sperm into the female’s vagina.

b. Aquatic animals have other types of copulatory organs or employ other strategies for delivering sperm:

1) Lobsters and crayfish have modified swimmerets.

2) Cuttlefish and octopuses use an arm.

3) Sharks have a modified pelvic fin to pass packets of sperm to the female shark.

c. Birds lack a penis or vagina; they transfer sperm from cloaca to cloaca.

C. Life History Strategies

1. Many aquatic animals use external fertilization; eggs and sperm join outside the body in the water.

2. Terrestrial animals tend to practice internal fertilization; eggs and sperm join inside the female’s body.

3. Both types of animals are usually oviparous; they deposit eggs in the external environment.

4. Insect eggs are produced in ovaries; they mature and increase in size as a result of the accumulation of yolk.

a. Yolk is stored food to be used by the developing embryo.

b. To prevent insect eggs from drying out, their eggshell has several layers of protein or wax.

c. In insects, small holes are left at one end to allow for the entry of sperm.

5. Some insects have a special organ to store sperm so the eggs can be fertilized later.

6. A larval stage is often quite different in appearance and way of life from the adult form.

a. The larva is able to seek its own food to sustain itself until it becomes an adult.

b. Metamorphosis is a major change in form that some animals undergo during development.

c. Incomplete metamorphosis lacks a pupal stage and the nymphs look more like adults.

d. Larval aquatic forms can utilize a different food source than the adults.

e. The bilaterally symmetrical sea star larvae attach to a substrate and become radially symmetrical adults.

f. The free‑swimming barnacle larvae metamorphose into sessile adults with calcareous plates.

g. The crayfish lacks a larval stage; eggs hatch into tiny juveniles with the same form as the adults.

7. Reptiles and birds provide their eggs with plentiful yolk; there is no larval stage.

a. Complete development takes place within a shell containing extraembryonic membranes.

b. The chorion is the outermost membrane that lies next to the shell and functions in gas exchange.

c. The amnion forms a water‑filled sac around the embryo ensuring that it will not dry out.

d. A yolk sac holds yolk which nourishes the embryo.

e. The allantois holds nitrogen waste products.

f. A shelled egg frees an animal from any need to reproduce in water and also helps it live completely on land.

8. Birds tend their eggs.

a. Newly hatched birds have to be fed before they develop to where they can seek food on their own.

b. Parent bird’s reproductive behaviors involve complex hormone and neural regulation.

9. In oysters and sea horses; the eggs remain inside the body until they hatch fully-developed.

10. Garter snakes, water snakes, and pit vipers also retain eggs until they hatch and give birth to live young.

11. Mammals are viviparous, producing living young.

a. The nutrients needed for development are constantly supplied by the mother.

b. Viviparity represents the ultimate in caring for the zygote and the embryo.

c. The evolution of viviparity can be seen in the primitive mammals.

1) The exceptions are the duckbill platypus and the spiny anteater, which are egg‑laying mammals.

2) Marsupials give birth to immature offspring that finish developing within a pouch.

3) In all other mammals, development occurs in a placenta.

12. The placenta is a complex organ comprised of maternal and embryonic tissues.

a. A placenta exchanges O2, CO2, nutrients, wastes, etc., between the fetal and maternal circulations.

b. Evolution allowed embryos to exchange materials with the mother; this made the shell unnecessary.

43.2 Male Reproductive System

1. Paired testes are suspended in the scrotal sacs of the scrotum.

2. The testes begin development in the abdominal cavity but descend into the scrotal sac during development.

3. If the testes do not descend, without surgery or hormonal therapy, sterility results.

4. The lower temperature of the scrotum is vital to normal sperm production.

5. Sperm produced in the testes mature within the epididymides.

a. These are tightly coiled tubules outside of the testes in which the sperm undergo maturation.

b. The maturation time in the epididymis is required for the sperm to develop the ability to swim to the egg.

6. Once sperm have matured, they are propelled into the vasa deferentia by muscular contractions.

7. Sperm are stored in both the epididymides and the vasa deferentia.

8. When a male is sexually aroused, the sperm enter the urethra, part of which extends through the penis.

9. The penis is a cylindrical copulatory organ used to introduce spermatozoa into the female vagina.

a. Three columns of spongy, erectile tissue extend down the penile shaft.

b. During sexual arousal, nervous reflexes cause an increase in the arterial blood flow to the penis.

c. Increased blood flow fills and distends the erectile tissue, and the penis stiffens and increases in size.

d. These changes cause an erection; failure to achieve an erection is called impotency.

10. Semen (seminal fluid) is thick, whitish fluid that contains sperm and glandular secretions.

a. The seminal fluid is formed by the seminal vesicles, prostate gland, and bulbourethral glands.

b. The seminal vesicles lie at the base of the urinary bladder.

1) Each joins a vas deferens to form an ejaculatory duct that enters the urethra.

2) They secrete into the ejaculatory duct a thick fluid containing nutrients for use by the sperm.

c. The prostate gland is located just below the urinary bladder and surrounds the upper portion of the urethra.

1) It secretes a milky, slightly alkaline solution that promotes sperm motility and viability.

2) In older men, the prostate gland may become enlarged and constrict the urethra.

3) Prostate cancer is also common in older men.

d. The bulbourethral glands are located below the prostate gland and on either side of the urethra; they release mucus secretions that provide lubrication.

11. The urethra also conducts urine from the bladder during urination.

A. Ejaculation

1. Ejaculation results in the expulsion of semen; this is achieved at the peak of sexual arousal.

2. The first phase of ejaculation is emission.

a. Nerve impulses from the spine trigger the epididymides and vasa deferentia to contract.

b. Subsequent motility causes the sperm to enter the ejaculatory duct; seminal vesicles, the prostate gland, and the bulbourethral glands release their secretions.

c. A small amount of secretion from the bulbourethral glands may leak from the end of penis; it functions to clean the urethra of acid but it may contain sperm.

3. The second phase of ejaculation is expulsion.

a. Rhythmical contractions at the base of the penis and within the urethral wall expel the semen in spurts.

b. Rhythmical contractions are a release from myotonia, or muscle tenseness, an important sexual response.

4. An erection lasts for a limited time and the penis generally returns to a flaccid state following ejaculation.

5. A refractory period follows during which stimulation does not bring about an erection.

6. Orgasm is the physiological and psychological sensations that occur at the climax of sexual stimulation.

B. The Testes

1. A longitudinal section shows compartments called lobules, each of which contains one to three seminiferous tubules.

a. Altogether, seminiferous tubules have a combined length of about 250 meters.

b. In a microscopic cross section, tubules show cells undergoing spermatogenesis, a process of meiosis.

c. The sustentacular (Sertoli) cells support, nourish, and regulate spermatogenic cells.

2. Mature sperm (spermatozoa) have three parts.

a. The sperm head contains a nucleus covered by an acrosome.

1) An acrosome is a caplike covering over the anterior end of nucleus; it stores enzymes to penetrate the egg.

2) A human egg is surrounded by several layers of cells and thick membrane; the enzymes allow the sperm to penetrate.

b. The middle piece contains mitochondria wrapped around microtubules of the flagellum; the mitochondria provide the energy for movement.

c. The tail also contains microtubules as components of a flagellum; its movement propels sperm.

3. The ejaculate of a normal human male contains several hundred million sperm.

4. Fewer than 100 ever reach the vicinity of an egg; and only one sperm normally enters an egg.

C. Hormonal Regulation in Males

1. The hypothalamus has ultimate control of the testes’ sexual function through secreting of gonadotropic‑releasing hormone (GnRH) that stimulates the pituitary to produce gonadotropic hormones.

2. There are two gonadotropic hormones: follicle‑stimulating hormone (FSH) and luteinizing hormone (LH) found in both males and females.

3. In males, FSH stimulates spermatogenesis in the seminiferous tubules.

4. In males, luteinizing hormone (LH) is also called interstitial cell‑stimulating hormone (ICSH); it stimulates testosterone secretion by interstitial cells of the testes.

5. The seminiferous tubules also release the hormone inhibin.

6. The hypothalamus‑pituitary‑testis system are involved in a negative feedback relationship that maintain a fairly constant production of sperm and testosterone.

7. Functions of Testosterone

  1. Testosterone is the main sex hormone in males.

  2. Testosterone is essential for the development of male secondary sex characteristics that develop at puberty, and for the maturation of sperm.

  1. It causes the tallness, longer legs and broader shoulders of males.

  2. Testosterone causes the larynx and vocal cords to enlarge, thus causing a deeper voice.

  3. It is responsible for greater muscle strength of males; some athletes take supplemental anabolic steroids (that are natural or synthetic testosterone).

  4. Testosterone causes males to develop hair on the face, chest, and back.

  5. Testosterone is also involved in triggering baldness if baldness genes are present.

43.3 Female Reproductive System

1. The female reproductive system includes: ovaries, oviducts, uterus, and vagina.

2. The ovaries produce a secondary oocyte each month; the ovaries are located in the pelvic cavity.

3. The oviducts (uterine tubes, fallopian tubes) extend from the ovaries to the uterus.

a. The oviducts are not attached to the ovaries.

b. Fingerlike projections called fimbriae sweep over the ovaries and waft in the egg when it erupts.

c. This is the normal site for fertilization; the embryo is slowly moved by ciliary movement toward the uterus.

4. The uterus is a hollow, thick‑walled muscular organ the size and shape of an inverted pear.

a. An embryo completes development by embedding itself in uterine lining, the endometrium.

b. The narrow end of the uterus is the cervix.

c. A small opening at the cervix of the uterus leads to the vaginal canal.

5. The vagina is a tube at a 45o angle with the small of the back.

a. Its mucosal lining lies in folds and it can extend, as necessary in childbirth.

b. It receives the penis during copulation and also serves as the birth canal.

6. The external genitalia of women are known collectively as the vulva.

  1. The mons pubis, labia minora, and labia majora are to the side of the vaginal and urethral openings.

  2. At the front juncture of the labia minora is the clitoris.

  1. This is homologous to the penis in males.

  2. The clitoris has a short shaft of erectile tissue and is capped by a pea‑shaped glans.

  3. It contains many sensory receptors that allow it to function as a sexually sensitive organ.

  1. Orgasm involves the release of neuromuscular tension in the muscles of the genital area, vagina, and uterus.

A. The Ovaries

  1. The ovaries alternate in producing one oocyte each month.

  2. The ovaries produce both the egg (ovum) and the female sex hormones, estrogens and progesterone, during the ovarian cycle.

  3. The Ovarian Cycle

  1. In a longitudinal section, an ovary shows many cellular follicles, each containing an oocyte (egg).

  2. As a follicle matures during the ovarian cycle, it develops from a primary follicle to a secondary follicle to a vesicular (Graafian) follicle.

  3. As oogenesis is occurring, a secondary follicle contains a secondary oocyte pushed to one side of fluid‑filled cavity.

  4. The vesicular follicle fills with fluid until the follicle wall balloons out on the surface and bursts, releasing a secondary oocyte surrounded by a zona pellucida and follicular cells.

  5. Ovulation is the rupture of the vesicular follicle with the discharge of the secondary oocyte into the oviduct.

  6. The secondary oocyte completes a second meiotic cell division when fertilization occurs.

  7. Meanwhile, the follicle develops into the corpus luteum; if pregnancy does not occur, the corpus luteum begins to degenerate in 10 days.

4. Phases of the Ovarian Cycle

  1. The ovarian cycle is under the control of gonadotropic hormones: follicle‑stimulating hormone (FSH) and luteinizing hormone (LH).

  2. The gonadotropic hormones are not present constantly but are secreted at different rates during the cycle.

  3. During the follicular phase, FSH promotes the development of a follicle that secretes estrogens.

  4. As the estrogen level in the blood rises, it exerts feedback control over the anterior pituitary secretion of FSH; the follicular phase comes to an end.

  5. Estrogen levels in the blood rise, causing the hypothalamus to secret more GnRH; this causes a surge in LH secretion.

  6. The LH surge then triggers ovulation.

  7. The luteal phase is the second half of the ovarian cycle following ovulation.

  1. LH promotes the development of the corpus luteum, which secretes large amounts of progesterone.

  2. As the blood level of progesterone rises, negative feedback to the anterior pituitary’s secretion of LH causes the corpus luteum to degenerate.

  3. As the luteal phase ends, menstruation occurs.

B. The Uterine Cycle

  1. Estrogens and progesterone affect the endometrium of the uterus to cause a cycle of events known as the uterine cycle.

  2. An average 28-day uterine cycle is divided into four sections.

a. During days 1–5, low levels of estrogen and progesterone in the body cause menstruation.

b. During days 6–13, an increased production of estrogens by an ovarian follicle causes the endometrium to thicken and become vascular and glandular (proliferative phase).

c. Ovulation usually occurs on day 14 of the 28‑day cycle.

d. Days 15–28 see increased production of progesterone by the corpus luteum that causes the endometrium to double in thickness; uterine glands mature, producing a thick mucoid secretion; this is the secretory phase.

1) The endometrium is now prepared to receive a developing embryo.

  1. If no pregnancy occurs, the progesterone and estrogen levels decline and the corpus luteum degenerates.

  2. With low levels of progesterone, the uterine lining also begins to degenerate.

  1. Menstruation is the periodic shedding of tissue and blood from the endometrium; this lining disintegrates and the blood vessels rupture.

  1. A flow of blood and tissues passes out through the vagina.

  2. The enzyme fibrinolysin prevents the blood from clotting.

  3. The first menstrual period, menarche, typically occurs between the ages of 11 and 12.

  4. If menarche does not occur by age 16, amenorrhea exists.

  1. Primary amenorrhea is usually caused by nonfunctional ovaries or developmental abnormalities.

  2. Secondary amenorrhea may be caused by weight loss or excessive exercise.

  1. Menopause, when menstruation ceases because the ovaries are no longer functioning, usually occurs between the ages of 45 and 55.

  1. Menopause is not complete until menstruation is absent for a year.

C. Fertilization and Pregnancy

1. If fertilization occurs, the embryo begins development as it travels down the oviduct to the uterus.

2. The embryo becomes embedded in the endometrium several days following fertilization.

3. The placenta develops from both maternal and embryonic tissues.

a. The placenta functions to exchange gases and nutrients between the fetal and maternal circulation.

b. There is normally no mixing of the blood between the maternal and fetal circulations.

4. Initially, the placenta produces human chorionic gonadotropin (HCG) which maintains the corpus luteum.

5. The corpus luteum is maintained by the HCG until the placenta produces its own progesterone and estrogens.

6. The progesterone and estrogens have two effects at this stage.

a. They shut down the anterior pituitary so that no new follicles mature.

b. They maintain the lining of the uterus so the corpus luteum is not needed.

7. There is no menstruation during pregnancy.

D. Estrogen and Progesterone

1. Estrogens maintain the normal development of the related organs and are responsible for the secondary sex characteristics of females.

2. There is less body and facial hair, and more fat beneath the skin provides a more rounded appearance.

3. The pelvic girdle enlarges and the pelvic cavity is larger; therefore, women have wider hips.

4. Both estrogen and progesterone are required for breast development.

E.. The Female Breast

1. The female breast contains 15–24 lobules, each with a mammary duct.

2. The mammary duct begins at the nipple and divides into numerous ducts which end in alveoli (blind sacs).

3. The hormone prolactin is needed for lactation (milk production) to begin.

  1. Production of prolactin is suppressed by the feedback inhibition that estrogens and progesterone have on the anterior pituitary during pregnancy; therefore, it takes a couple of days after delivery of a baby for milk production to begin.

  2. The breasts produce a watery, yellowish white fluid (colostrum) similar to milk but containing more protein and less fat, and it is rich in IgA antibodies providing some immunity to a newborn.

  3. Breast cancer is the most common form of cancer in females; women should have regular breast checks and mammograms when recommended.

43.4 Control of Reproduction

A. Birth Control Methods

  • The most reliable method of birth control is abstinence; it has the advantage of preventing transmission of a sexually transmitted disease.

  1. Contraceptive Injections

  1. Contraceptive injections are now being developed—possibilities include vaccination against HCG or sperm.

  1. Morning‑After Pills

  1. These regimens either prevent fertilization or stop a fertilized egg from implanting.

  2. Preven is a kit of four synthetic progesterone pills; the medication makes it difficult for the embryo to implant in the endometrium; it is considered 85% effective.

  3. Mifepristone, also known as RU‑486, causes the loss of an implanted embryo.

1) It blocks the progesterone receptors of the endometrial cells.

2) Without functioning receptors for progesterone, the uterine lining sloughs off carrying the embryo with it.

3) Taken in conjunction with a prostaglandin to induce uterine contractions, it is 95% effective.

B. Infertility

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