Chapter 39 Neurons and Nervous Systems



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Infertility is the inability of a couple to achieve pregnancy after one year of regular, unprotected intercourse.

  1. Causes of Infertility

  1. The most frequent causes of male infertility are low sperm count and abnormal sperm; this can be due to a sedentary lifestyle coupled with smoking and alcohol consumption.

  2. The major factors involved in female infertility are body weight, blocked oviducts, and endometriosis.

  1. Endometriosis is the presence of uterine tissue beyond the uterus.

  2. Blocked oviducts can be due to pelvic inflammatory disease.

  3. If no obstruction is apparent and body weight is normal, females can be given fertility drugs.

  4. Hormone treatments carry the risk of multiple pregnancy.

  1. Assisted Reproductive Technologies

  1. Artificial Insemination by Donor (AID)

  1. A sperm sample is injected into the vagina.

  2. If the husband’s sperm count is low, many samples can be combined.

  3. Artificial insemination from a donor is necessary when the husband lacks viable sperm.

  4. Intrauterine insemination can be coordinated with drugs used to stimulate the ovaries.

  5. With artificial insemination, sperm can be sorted into those that are X-bearing (producing a girl) or Y-bearing (producing a girl).

  1. In Vitro Fertilization (IVF)

  1. In IVF, conception occurs in laboratory glassware.

  2. Ultrasound machines spot maturing follicles and a laparoscope is used to harvest the eggs using a needle.

  3. When sperm and egg are combined in glassware, they can be transferred to the uterus after 2–4 days.

  4. While in glassware, the new embryos can be tested for genetic diseases.

  1. Gamete Intrafallopian Transfer (GIFT)

  1. A gamete is a sex cell, either a sperm or an egg.

  2. Due to the low success rate of IVF (15–20%), GIFT immediately places the sperm and egg in the oviduct.

  3. A variation is to fertilize the eggs in the laboratory and then place the zygotes in the oviducts.

  1. Surrogate Mothers

  1. Women can be contracted and paid to have babies; they are then surrogate mothers.

  2. The sperm and/or egg can be contributed by the contracting parents.

  1. Intracytoplasmic Sperm Injection (ICSI)

  1. A single sperm is injected into an egg.

  2. This is used when a man has severe infertility problems.

43.5 Sexually Transmitted Diseases

1. Sexually transmitted diseases (STDs) are caused by organisms ranging from viruses to arthropods.

2. Humans cannot develop lasting immunity to any STDs; therefore, prompt medical treatment should be received when exposed to an STD.

3. To prevent STDs, a condom can be used.

4. It is difficult to cure STDs caused by viruses; treatment is available for AIDS and genital herpes.

5. STDs caused by bacteria (e.g., gonorrhea, chlamydia, and syphilis) are treatable with antibiotics.

A. AIDS


  1. Acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV).

  2. HIV attacks the helper T cells that stimulate the activity of B lymphocytes to produce antibodies.

  3. After an HIV infection begins, helper T cells decline in number and a person becomes more susceptible to infections.

  4. Symptoms

  • AIDS has three stages of infection called category A, B, and C

a. The category A stage may last about a year.

1) An individual is asymptomatic but can pass on the infection.

2) Immediately after infection but before testing positive, a large number of infectious viruses are in the blood.

3) After testing positive, a person may remain well for as long as he or she can maintain sufficient helper T cells (above 500 mm3).

b. The category B stage may last six to eight years.

1) Lymph nodes swell.

2) There is weight loss, night sweats, fatigue, fever, and diarrhea.

3) Infections such as thrush and herpes reoccur.

c. The category C stage is full‑blown AIDS.

1) Nervous disorders and opportunistic diseases (e.g., an unusual type of pneumonia or skin cancer) occur.

2) Without intensive medical treatment, an AIDS patient usually dies by about 7–9 years after infection.

3) A recent combination therapy of several drugs allows AIDS patients in the United States to live longer.



  1. Transmission

a. AIDS is transmitted by sexual contact with an infected person (vaginal or rectal intercourse and oral/genital contact).

b. Needle sharing among intravenous drug users is a high‑risk behavior.

c. Transfusions of blood or clotting factors is now a rare mode of transmission and can be screened.

d. The largest increases in AIDS cases now involve heterosexual contact and intravenous drug use.

e. Women now account for 19% of all newly diagnosed cases of AIDS.


  1. Treatment

  1. Even though there is no cure for AIDS, a treatment called highly active antiretroviral therapy (HAART) is usually able to stop HIV replication.

  1. HAART uses a combination of drugs that interfere with the life cycle of HIV.

  1. Entry inhibitors stop HIV from entering a cell.

  2. Reverse transcriptase inhibitors (e.g., AZT) interfere with the enzyme reverse transcriptase.

  3. Integrase inhibitors prevent HIV from inserting its own genetic material into that of the host.

  4. Protease inhibitors prevent protease from processing newly created polypeptides.

B. Genital Warts

1. Genital warts are caused by the human papillomaviruses (HPVs).

2. Many carriers are asymptomatic or they have minimal symptoms.

3. If visible warts are removed, they may recur.

4. HPVs are now associated with cancer of the cervix as well as tumors of the vulva, vagina, anus, and penis.

5. Some researchers believe viruses are involved in 90–95% of all cases of cancer of the cervix.

C. Genital Herpes

1. Genital herpes is caused by the herpes simplex virus.

2. Type 1causes cold sores and fever blisters; type 2 more often causes genital herpes.

3. Individuals infected with this type of virus can be asymptomatic carriers.

4. Symptoms include painful ulcers on the genitals, fever, painful urination, and swollen lymph nodes.

5. Exposure to herpes in the birth canal can cause neurological disorders and even death in a newborn; birth by cesarean section avoids this possibility.

D. Hepatitis

1. Hepatitis A is usually acquired from sewage‑contaminated drinking water but is also an STD contracted by oral/anal contact.

2. Hepatitis B is spread in the same manner as AIDS but is more infectious; a vaccine is available.

3. Hepatitis C is called post‑transfusion hepatitis, but it can be transmitted through sexual contact.

4. Hepatitis infections infect the liver and can lead to liver failure, liver cancer, and death.

E. Chlamydia

1. Chlamydia is named for the bacterium that causes it: Chlamydia trachomatis.

2. New chlamydial infections have increased faster than any other STD.

3. It also causes cervical ulcerations which increase the risk of acquiring AIDS.

4. It also causes pelvic inflammatory disease (PID).

5. If a baby is exposed at birth, inflammation of the eyes or pneumonia can result.

F. Gonorrhea



  1. Gonorrhea is caused by the bacterium Neisseria gonorrhoeae.

  2. Male diagnosis is not difficult: typical symptoms include urination pain and a thick, greenish yellow discharge.

  3. In males and females, latent infections lead to pelvic inflammatory disease (PID); the vasa deferentia or the oviducts become infected and inflamed.

  4. As these tubes heal, they may become partially blocked, resulting in sterility or infertility.

  5. If a baby is exposed at birth, an eye infection can lead to blindness; therefore all newborns are given eye drops.

  6. Gonorrhea proctitis is an infection of the anus; gonorrhea can infect the mouth, throat, tonsils, heart, and joints.

  7. Previously easily cured by antibiotics, nearly 40% of modern strains are now antibiotic resistant.

G. Syphilis

1. Syphilis is caused by the bacterium Treponema pallidum.

2. This disease has three stages typically separated by latent periods.

a. The primary stage involves the appearance of a hard chancre (ulcerated sore).

b. The second stage involves the appearance of a rash all over the body, including the palms and feet.

c. The third stage involves neurological and cardiac disorders.

1) An infected individual may become mentally retarded, blind, walk with a shuffle, or become insane.

2) Large destructive ulcers (gummas) develop on the skin or within internal organs.

3. Syphilitic bacteria can cross the placenta, causing birth defects or stillbirth.

4. Unlike the other STDs discussed, there is a blood test to diagnose syphilis.

5. Tracing sexual partners is very important in controlling syphilis.

H. Two Other Infections

1. Bacterial vaginosis is caused by Gardnerella vaginalis, Trichomonas vaginalis (a flagellated protozoa), or Candida albicans (a yeast).

2. The protozoan infection causes a frothy, foul‑smelling discharge with itching.

3. Trichomoniasis is most often transmitted through sexual intercourse.

4. The Candida yeast infection causes a white, curdy discharge with itching.

5. Candida albicans is a normally-occurring organism in the vagina; yeast infections can result from taking birth‑control pill or antibiotics.

Chapter 44 Animal Development

This chapter explores the topic of animal development. Beginning with a study of the stages of early development, the discussion moves into developmental processes, and then into human embryonic and fetal development. The chapter concludes with a discussion of the process of birth. A Health Focus box discusses “Preventing Birth Defects.”



Chapter Outline

44.1 Early Developmental Stages

1. Fertilization requires that sperm and egg interact to form a zygote.

a. A human sperm cell has three parts.

1) The head contains a haploid nucleus covered by a caplike acrosome containing enzymes, allowing the sperm to penetrate the egg.

2) A middle piece contains ATP‑producing mitochondria.

3) The tail is a flagellum that allows the sperm to swim.

2. The plasma membrane of the egg is surrounded by the zona pellucida.

a. The zona pellucida is surrounded by a few layers of adhering follicular cells, collectively called the corona radiata.

b. These cells nourished the egg when it was in a follicle of the ovary.

3. Fertilization involves the following steps.

a. Several sperm penetrate the corona radiata and several sperm attempt to penetrate the zona pellucida.

b. One sperm enters the egg and their nuclei eventually fuse.

c. After the sperm head binds tightly to the zona pellucida, the acrosome enzymes digest and form a pathway for the sperm through the zona pellucida.

d. The head, middle piece and usually the tail enters the egg.



  1. Prevention of polyspermy depends on changes in the egg plasma membrane when the sperm touches the egg and depolarizes the egg plasma membrane; this is called Fast block.”

  2. Vesicles in the egg called cortical granules secrete enzymes that turn the zona pellucida, forming an impenetrable fertilization membrane; this is called “Slow Block.”

f. As soon as plasma membranes of the sperm and egg fuse, the zona pellucida lifts away from the surface of the egg, forming a moat that prevents entrance of any other sperm.

g. The diploid zygote forms when a nuclear envelope surrounds the sperm and egg chromosomes.



  1. Embryonic Development

  1. Cellular Stages of Development

  1. Development is all of the changes that occur during the life cycle of an organism.

  2. An organism is an embryo during the first stages of development.

  3. After fertilization, a zygote undergoes cleavage, cell division without growth.

  4. DNA replication and mitosis occur repeatedly, and the cells get smaller with each division.

  5. In the lancelet, the cell divisions are equal in the resulting morula.

  6. A cavity called the blastocoel develops forming a hollow ball called the blastula.

  1. Tissue Stages of Development

  1. The tissue stages of development are early gastrula and late gastrula.

  2. The early gastrula stage begins with the invagination of certain cells into the blastocoel to form two of the three primary germ layers.

  3. The outer layer of cells becomes ectoderm; ectoderm gives rise to the epidermis of the skin, the epithelial lining of the mouth and rectum, and the nervous system.

  4. The inner layer of cells becomes the endoderm that gives rise to the epithelial lining of the digestive tract and the respiratory tract, associated glands of the digestive and respiratory system, and the lining of the urinary bladder; a pore created by invagination is the blastopore.

  5. The late gastrula has, in addition to ectoderm and endoderm, a middle layer of cells called the mesoderm.

1) The outpocketings grow and fuse, forming a two layered mesoderm.

2) The space between them is the coelom that contains the body organs.

3) The mesoderm gives rise to the skeleton, the dermis of the skin, the skeletal system, the muscular system, the excretory system, the reproductive system (including most epithelial linings), and the outer layers of respiratory and digestive systems.


  1. These germ layers then develop into those future organs.

  1. Organ Stages of Development

  1. The newly formed mesoderm cells along the main axis coalesce to form a dorsal notochord; it persists in lancelets but is replaced in frogs, chicks, and humans by the vertebral column.

  2. The nervous system develops from the midline ectoderm located just above the notochord.

  1. At first, the cells on the dorsal surface of the embryo thicken, forming the neural plate.

  2. Then neural folds develop on either side of a neural groove which becomes the neural tube when the folds fuse.

  3. At this point the embryo is called a neurula.

  4. Later, the anterior end of the neural tube develops into the brain; the rest becomes the spinal cord.

  1. Midline mesoderm cells that did not contribute to the formation of the notochord now become two longitudinal masses of tissue.

  1. The two tissue masses become blocked off into somites.

  2. The somites give rise to segmental muscles in all chordates; in vertebrates the somites also form the vertebral bones.

44.2 Developmental Processes

1. Cellular differentiation occurs when cells become specialized in their structure and function.

2. Morphogenesis produces a change in the shape and form of a body part; this includes both early cell movement and later pattern formation.

A. Cellular Differentiation


  1. Each body cells contain a full set of chromosomes; therefore differentiation is not due to parceled out genes.

  2. Cells in the adult body are totipotent; each contains all of the instructions to form any specialized cell.

  3. Since only muscle cells produce myosin, only red blood cells produce hemoglobin, and only skin cells produce keratin, there must be differential gene expression.

  4. Two mechanisms–cytoplasmic segregation and induction–seem especially important.

  5. Cytoplasmic Segregation

  1. Differentiation begins long before we can recognize specialized cell types.

  2. Eggs contain substances called maternal determinants that influence the course of development.

  3. Cytoplasmic segregation parcels out the maternal determinants as mitosis occurs and determines how the various cells of morula develop.

  4. Early experiments showed the cytoplasm of a frog egg is not uniform in content.

  5. After the first cleavage of a frog embryo, only a daughter cell that receives a portion of the gray crescent develops into a complete embryo.

  6. Hans Spemann (Nobel Prize in 1935) found particular chemical signals within the gray crescent turn on the genes that control development.

  1. Induction and Frog Experiments

  1. As development proceeds, differentiation involves signals from neighboring cells.

  2. Induction is the ability of one tissue to influence the development of another tissue.

  3. Cell migration occurs during gastrulation; one set of cells can influence the migratory path of another set.

  4. Spemann showed that the dorsal lip of a blastopore (primary organizer) was necessary for development.

  1. The cells closest to the primary organizer become endoderm, those farthest away become ectoderm, and the intermediate cells became mesoderm.

  2. A molecular concentration gradient likely acts as a signal to induce germ layer differentiation.

  1. Spemann and Hilde Mangold worked on the dorsal side of the embryo where the notochord and the nervous system develop.

  1. The presumptive notochord tissue induces the formation of the nervous system when placed beneath belly ectoderm.

  2. Warren Lewis found that a developing lens induces the optic vesicle to form the optic cup in a frog embryo.

  1. Induction in Caenorhabditis elegans

a. Caenorhabditis elegans is a transparent worm, 1mm long and easily raised in Petri dishes or liquid media.

b. It is hermaphroditic and self-fertile; therefore induced recessive mutations appear in the next generation.

c. Its entire genome has been sequenced.

d. Worm development takes only three days and an adult worm contains only 959 cells.

e. The destiny of each cell can be followed in specialization and fate maps drawn.

f. The anchor cell receives the most inducers to become the inner vulva; neighboring cells receive less and become the outer vulva.

g. Work with C. elegans shows that induction requires transcriptional regulation of genes in a particular sequence.

B. Morphogenesis



  • Morphogenesis in Drosophila melanogaster (the fruit fly)

  1. The first event in successful development is the establishment of the anterior/posterior axes.

  2. In Drosophila eggs, there is a greater concentration of bicoid protein at one end.

1) Bicoid means “two-tailed”; a maternal mutation causes the egg to lack a head and it has two tails.

2) By cloning the bicoid gene and using it as a probe, mRNA was found in a gradient from anterior to posterior.

3) Proteins that influence morphogenesis are morphogens.

4) The bicoid gradient switches on the expression of segmentation genes; a gradient has a range of effects.



  1. The Segmentation Pattern

1) The bicoid gradient begins a cascade of segmentation genes.

2) Christiane Nusslein-Vollard and Eric Wieschaus won a Nobel Prize for discovering segmentation genes in Drosophila.

3) By exposing flies to mutagens and then mapping mutated genes, they located segmentation abnormalities.

4) The first genes activated are gap genes; if mutated, they cause missing blocks of segments.

5) The pair-rule genes ensure 14 segments; a mutation reduces this to half.

6) Segment-polarity genes cause each segment to have an anterior and posterior half.

7) Morphogen gradients turn on genes because they are transcription factors that regulate which genes are active in which parts of the embryo in what order.


  1. Homeotic Genes

1) Homeotic genes control pattern formation in animal morphogenesis.

2) In normal fruit fly development, homeotic genes are activated after the segmentation genes.

3) In the 1940s, Edward Lewis discovered homeotic genes that controlled which segment would bear antenna, legs, wings.

4) Homeotic genes have been found in many organisms; they all contain the same sequence of nucleic acids called a homeobox.

5) Because homeotic genes contain a homeobox in mammals, they are called Hox genes.

6) Each homeobox has a homeodomain, a sequence of sixty amino acids.

7) A homeodomain protein produced by one homeotic gene binds to and turns on the next homeotic gene, and this orderly process determines the overall pattern of the embryo.

8) Homeoboxes are derived from an original nucleic acid sequence that has been conserved because of its importance in regulation of animal development.



  1. Apoptosis

  1. Apotosis (programmed cell death) is important in morphogenesis.

  2. When a cell-death signal is received, an inhibiting protein becomes inactive, allowing a cell-death cascade to proceed.

44.3 Humans Embryonic and Fetal Development

  • Development covers events from conception (fertilization followed by implantation) to birth (parturition).

1. The time of birth is calculated by adding 280 days to the start of last menstruation.

2. Only about 5% of babies arrive on the forecasted date due to so many variables.



  • Human development is divided into embryonic and fetal development.

1. The embryonic period, during months 1 and 2 of pregnancy, is when the major organs are formed.

  1. Fetal development is during months 3–9, during which organ systems are refined.

  • Development can also be divided into trimesters.

  1. First trimester: embryonic and early fetal development occur.

  2. Second trimester: development of organs and organ systems; the fetus is distinctly human at the end of the second trimester.

  3. Third trimester: the fetus grows rapidly and the organ systems become functional.

  • Extraembryonic membranes

1. Evolution of
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