Transcript Unit 12 Animal Anatomy and Physiology Part 2

UNIT 12
ANIMAL ANATOMY AND
PHYSIOLOGY
Excretory, Endocrine, Reproductive,
and Nervous Systems
(Chapters 44, 45, 46, and 48)
Excretory System (Chapter 44)
Osmoregulation = management of the body’s water content and solute composition
Elimination of Nitrogenous Waste
Ammonia =
water soluble
and very toxic
Tolerable at low
levels
Urea =
-Low toxicity
-Tolerable at higher
concentrations = store
-Requires less water to
produce it
Uric Acid =
-Relatively nontoxic
-Insoluble in water
-Excreted as a semisolid paste
-Shells of eggs impermeable to
liquids, so stored as a solid
Water Balance in Two Terrestrial Mammals
Kidney Functions of Excretory Systems
Filtration =
-Filtering through a selectively permeable membranes of transport epithelia
-The membranes retain cells as well as proteins and other large molecules
-Hydrostatic pressure forces water and small solutes, such as salts, sugars,
amino acids, and nitrogenous wastes, into the excretory system
-”Filtrate” is the filtered fluid
Reabsorption =
Epithelium reclaims valuable substances from the filtrate and returns them to body fluids
Secretion =
Other substances, such as toxins and excess ions, are extracted from body fluids and
added to the contents of the excretory tubule (glucose, certain salts, and amino acids)
Excretion =
“Elimination”
The Human Excretory System at Four Size Scales
Kidneys (2) = Bean shaped and ~10cm long
Renal Artery and Renal Vein supply blood
Receive ~20% of resting cardiac output
Blood  Kidneys  Ureters  Urinary Bladder  Urethra (sphincter muscles)
Nephron = the functional unit
of the vertebrate kidney
- ~1 million per kidney
-- Total Tubule Length = 80 km
Glomerulus =
A single long tubule
and a ball of capillaries
Bowman’s Capsule =
-Cup-shaped swelling at the end
of the tubule
-Surrounds the Glomerulus
Afferent Arteriole =
Supplies each nephron with blood
to be filtered
Pathway of Filtration and Filtrate:
Blood  Glomerulus  Bowman’s Capsule  Proximal Tubule  Loop of Henle (Descending  Ascending)  Distal Tubule  Collecting Duct 
Renal Pelvis  Ureter
-From 1,100 to 2,000 L of blood flows through a pair of human kidney’s each day
-This volume is ~275 times the total volume of blood in the body
-Nephrons process about 180 L of initial filtrate, equivalent to two or three times the body weight of an average person
-Of this, nearly all of the sugar, vitamins, and other organic nutrients, and about 99% of the water, are reabsorbed into the blood
leaving only about 1.5 L of urine to be voided
The Nephron and Collecting Duct
Filtrate:
H2O
Salts (NaCl, etc.)
HCO3H+
Urea
Glucose; Amino Acids
Some Drugs
Orange Arrows =
Active Transport
Purple Arrows =
Passive Transport
Steps Showing the Concentration of Urine by the Kidney
Step 1
Step 2
Step 3
Hormonal Control of the Kidney by Negative Feedback Circuits
Antidiuretic Hormone (ADH) =
Produced by the Hypothalamus and
stored/released in the Pituitary Gland
Osmoreceptor cells also
stimulate thirst
ADH enhances fluid retention by making the
kidneys reclaim more water
The release of ADH is triggered when
osmoreceptor cells in the hypothalamus
detect an increase in the osomolarity of
the blood
Drinking reduces the osmolarity of
the blood, which inhibits the secretion
of ADH, completing the feedback circuit
3 In the blood, renin initiates the conversion of
angiotensinogen to angiotensin II
4 Angiotensin II increases blood pressure
by causing arterioles to constrict
It also increases blood volume by:
-signaling the proximal tubules of the nephrons
to reabsorb more NaCl and water
-stimulating the adrenal glands to release
aldosterone, a hormone that makes the distal
tubules reabsorb more Na+ and water
This leads to an increase in blood volume and
pressure, completing the feedback circuit
by suppressing the release of renin
2 The JGA responds to a decrease in
blood pressure or blood volume by
releasing the enzyme renin into the
bloodstream
1 The renin-angiotensin-aldosterone system (RAAS)
centers on the juxtaglomerular apparatus (JGA)
Endocrine System (Chapter 45)
Endocrine System = all of an animal’s hormone-secreting cells
Endocrine Glands = hormone-secreting organs
Secrete hormones directly into body fluids
Neurosecretory Cells = specialized nerve cells that secrete hormones
An Example of How Feedback Regulation Maintains Homeostasis
Antagonistic (opposite) Actions
Mechanisms of Chemical Signaling
The chemical signal (hormone) binds to a receptor protein on the surface of a target cell
This triggers a signal-transduction pathway which is the series of step by which a signal on a cell’s surface is converted into a specific cellular response
A chemical signal penetrates the target cell’s plasma membrane and binds to a receptor inside the cell
When the signal is bound to an intracellular receptor, the receptor acts as a transcription factor, causing a change in gene expression
The binding of signal to a surface receptor can lead to either a change in gene expression or a change in a cytoplasmic activity
One Chemical Signal With Different Effects
Acetylcholine is a neurotransmitter
It can produce different responses in different target cells
Different response may result because the receptors are different or because signal-transduction pathways within target cells are different
Human Endocrine Glands and Hormones
Hormones can affect one or a few tissues
Sex Hormones affect most of the tissues of the body
Tropic Hormones target other endocrine glands
Hormones of the Hypothalamus and Pituitary Glands
-The pituitary gland, located at the base of the brain and surrounded by bone, consists of
the posterior pituitary (neurohypophysis) and the anterior pituitary (adrenohypophysis)
-Note that the posterior pituitary is actually an extension of the hypothalamus
The Posterior Pituitary =
-Neurosecretory cells in the hypothalamus synthesize antidiuretic hormone (ADH) and
oxytocin, which are transported down the axons to the posterior pituitary, where they
are stored
-The posterior pituitary releases them into the blood circulation
-ADH binds to target cells in the kidneys, oxytocin to target cells in the mammary glands
and uterus
The Anterior Pituitary =
-The release of anterior pituitary hormones is controlled by the
hypothalamus
-Neurosecretory cells in the hypothalamus secrete releasing
hormones and inhibiting hormones into a capillary network
located above the stalk of the pituitary
-The hormones travel through short portal vessels into a
secondary capillary network within the anterior pituitary
-In response to specific releasing hormones, endocrine cells in
the anterior pituitary secrete certain hormones into the
circulation
Two Thyroid Hormones
-Structurally identical, except that T3 has three iodine atoms and T4 has four,
these hormones regulate metabolism in most body cells
-The thyroid secretes mainly T4, most of which is converted to T3 by an enzyme
in target cells
-T3 binds more avidly than T4 to receptors in the target cells
Feedback Control Loops Regulating the
Secretion of Thyroid Hormones T3 and T4
-The hypothalamus secretes TRH (TSH-releasing hormone), which stimulates the anterior
pituitary to secrete TSH (thyroid-stimulating hormone)
-When TSH binds to specific receptors in the thyroid gland, a signal-transduction pathway
involving cAMP as a second messenger triggers the synthesis and release of the thyroid
hormones T3 and T4
-The system is balanced by negative feedback loops
-In the two main ones (red arrows), high levels of T3 and T4 in the blood inhibit TSH
secretion by the anterior pituitary and TRH secretion by the hypothalamus
Hormonal Control of Calcium Homeostasis in Mammals
-A negative feedback system involving two antagonistic hormones, calcitonin and parathyroid hormone (PTH), maintains the concentration of calcium in blood very close to 10 mg/100mL
-A rise in blood Ca2+ induces the thyroid gland to secrete calcitonin, which lowers the Ca 2+ concentration by increasing bone deposition and reducing reabsorption in the kidneys
-By interfering with PTH, the calcitonin also helps reduce Ca2+ uptake by the intestines, not shown
-These effects are reversed by PTH, which is secreted from the parathyroid glands when the concentration of blood Ca 2+ falls below the set point
-Blood calcium levels begin to increase as target cells in the bones and kidneys respond to PTH
-In addition to stimulating Ca2+ uptake in these organs directly, PTH also acts indirectly by helping activate vitamin D in the kidneys
-The active form of vitamin D then stimulates the intestines to increase Ca2+ uptake from food
-But blood Ca2+ will rise only so far before the thyroid counters by secreting more calcitonin
Glucose Homeostasis Maintained by Insulin and Glucagon
-A rise in blood glucose above the set point (about 90 mg/100mL in humans) stimulates the pancreas to secrete insulin, which triggers its target cells to
take up the excess glucose from the blood
-Once the excess is removed and blood glucose concentration dips below the set point, the pancreas responds by secreting glucagon, which acts on
the liver to raise the blood glucose level
The Synthesis of Catecholamine Hormones
Epinephrine (also known as adrenaline)
Norepinephrine (also known as noradrenaline)
Catecholamines = compounds synthesized from the amino acid tyrosine
-These hormones are secreted in response to positive and negative stress –
everything from extreme pleasure to increased cold to life-threatening danger
-Their release into the blood gives the body a rapid bioenergetic boost,
increasing the basal metabolic rate (BMR) and having dramatic effects on
several targets
Increasing the availability of energy sources:
-They both increase the rate of glycogen breakdown in the liver and skeletal
muscles and glucose release into the blood by liver cells
-They also stimulate the release of fatty acids from fat cells, which may be used
by cells for energy
Effects on the cardiovascular and respiratory systems:
-They increase both the rate and stroke volume of the heartbeat and dilate the
bronchioles in the lungs, effects that increase the rate of oxygen delivery to body cells
-Doctors prescribe epinephrine as a heart stimulant and to open breathing tubes
during asthma attacks
-They also cause smooth muscles of some blood vessels to contract and muscles
of other vessels to relax, with an overall effect of shunting blood away from the
skin, digestive organs, and kidneys while increasing the blood supply to the
heart, brain, and skeletal muscles
Cells in the adrenal medulla synthesize the catecholamines
norepinephrine and epinephrine from the amino acid tyrosine
Steroid Hormones from the Adrenal Cortex and Gonads
-Cortisol and Aldosterone, both made in the adrenal cortex, are structurally similar to the sex hormones testosterone (an androgen), estradiol (as estrogen),
and progesterone (a progestin)
-The precursor for the synthesis of all steroid hormones is cholesterol
-Most of the androgens (male hormones) that circulate in the blood are made in the testes
-Most of the estrogens and progestins (female hormones) are produces by the ovaries; however, small amounts of these sex hormones are also made
by the adrenal cortex
Stress and the Adrenal Gland
Stressful stimuli cause the hypothalamus to activate the adrenal medulla via nerve impulses and the adrenal cortex via hormonal signals
The adrenal medulla mediates short-term responses to stress by secreting the catecholamine hormones epinephrine and norepinephrine
The adrenal cortex controls more prolonged responses by secreting steroid hormones
Corticosteroids =
A family of steroids from the
adrenal cortex
Short-term Stress Response
Effects of Epinephrine and Norepinephrine:
1. Glycogen broken down to glucose; increased blood glucose
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
5. Change in blood flow patterns, leading to increased alertness
and decreased digestive and kidney activity
Long-term Stress Response
Effects of Mineralocorticoids (like Aldosterone):
1. Retention of sodium ions and water by kidneys
2. Increased blood volume and blood pressure
Effects of Glucocorticoids (like Cortisol):
1. Proteins and fats broken down and converted to glucose, leading to
increased blood glucose
2. Immune system may be suppressed
Reproduction (Chapter 46)
Sexual Behavior in Parthenogenetic Lizards (C. uniparens)
-The desert-grassland whiptail lizard (Cnemidophorus uniparens) is an all-female species.
-These reptiles reproduce by parthenogenesis; eggs undergo a chromosome doubling after meiosis and develop into lizards
without being fertilized. However, ovulation is enhanced by courtship and mating rituals that imitate the behavior of closely related
species that reproduce sexually
Reproductive Anatomy of a
Parasitic Flatworm
Insect Reproductive Anatomy
Most insects have separate sexes with complex reproductive systems. In the male, sperm
develop in a pair of testes and are conveyed along a coiled duct to two seminal vesicles, where
they are stored. During mating, sperm are ejaculated into the female reproductive system. In the
female, eggs develop in a pair of ovaries and are conveyed through ducts to the vagina, where
fertilization occurs. In many species the female reproductive system includes a spermatheca, a
sac in which sperm may be stored for a year or more
Most flatworms (phylum Platyhelminthes) are hermaphroditic, with complex male and female
reproductive systems. Both systems open to the outside through the genital pore. Sperm
produced in the testes pass through a pair of ducts (vasa efferentia) into a single sperm duct
(vas deferens) and are stored in the seminal vesicle. During copulation, sperm are ejaculated
into the female system (usually of another individual) and then move through the uterus to the
seminal receptacle. (In some flatworms, sperm are injected into the body tissues through the
body wall and then migrate to the female reproductive tract.) Eggs from the ovary pass into the
oviduct, where they are fertilized by sperm from the seminal receptacle and coated with yolk and
tough shell material secreted by the yolk glands. From the oviduct, the fertilized, shelled eggs
pass into the uterus from which they are shed through the genital pore. Usually only a small
fraction of the eggs will develop into the next generation of adult worms
Reproductive Anatomy of
the Human Male
In most mammalian species, including humans, the male’s external
reproductive organs are the scrotum and penis. The internal reproductive
organs consist of gonads that produce gametes (sperm cells) and hormones,
accessory glands that secrete products essential to sperm movement, and
ducts that carry the sperm and glandular secretions
Path of Sperm:
Testes  Epididymis  Vas Deferens  Ejaculatory Duct  Urethra  Exit Penis
Accessory Glands:
Seminal Vesicle
Prostate
Bulbourethral Gland
Reproductive Anatomy of
the Human Female
The female’s external reproductive structures are the
clitoris and two sets of labia surrounding the clitoris
and vaginal opening. The internal reproductive organs
consist of a pair of gonads and a system of ducts and
chambers to conduct the gametes and house the
embryo and fetus
The female gonads, the ovaries, lie in the abdominal cavity, flanking, and
attached by a mesentery to, the uterus. Each ovary is enclosed in a tough
protective capsule and contains many follicles. A follicle consists of one egg
cell surrounded by one or more layers of follicle cells, which nourish and
protect the developing egg cell. All of the 400,000 follicles a woman will ever
have are formed before her birth. Of these, only several hundred will release
egg cells during the woman’s reproductive years. Starting at puberty and
continuing until menopause, usually one follicle matures and releases its egg
cell during each menstrual cycle. The cells of the follicle also produce the
primary female sex hormones, the estrogens. The egg cell is expelled from
the follicle in the process of ovulation. The remaining follicular tissue then
grows within the ovary to form a solid mass called the corpus luteum. The
corpus luteum secretes additional estrogens and progesterone, the hormone
that maintains the uterine lining during pregnancy. If the egg cell is not
fertilized, the corpus luteum disintegrates, and a new follicle matures during
the next cycle
Ovulation
Spermatogenesis
Structure of a Human Sperm Cell
The structure of a sperm cell fits its function. In most species, a head
containing the haploid nucleus is tipped with a special body, the
acrosome, which contains enzymes that help the sperm penetrate the egg.
Behind the head, the sperm cell contains large numbers of mitochondria
(or a single large one, in some species) that provide ATP for movement of
the tail, which is a flagellum. Mammalian sperm shape varies from species
to species, with the head a slender comma shape, an oval form (as in the
human sperm), or nearly spherical
These drawings correlate the meiotic stages in sperm development (left) with the structure of
seminiferous tubules at the microscopic level. Primordial germ cells of the embryonic testes
differentiate into spermatogonia, the stem cells that give rise to sperm. As spermatogonia
differentiate into spermatocytes and then to spermatids, meiosis reduces the chromosome
number from diploid (2n = 46 in humans) to haploid (n = 23). The developing cells are gradually
pushed from a location near the outer wall of the seminiferous tubule toward the lumen (central
opening) and make their way to the epididymis, where they become motile. This process, from
spermatogonia to motile sperm, takes 65 to 75 days in the human male. In mature males, about
3 million spermatogonia start the process each day
Oogenesis
Oogenesis is the
development of ova-mature, unfertilized egg
cells, This diagram
shows the process and
its location, the ovary. In
the developing female
embryo, oogonia, the
stem cells that give rise
to ova, multiply and then
begin meiosis, but the
process stops at
prophase I. The cells at
this stage, called
primary oocytes, remain
quiescent within small
follicles until puberty,
when they are
reactivated by
hormones. Beginning at
puberty, FSH (folliclestimulating hormone)
periodically stimulates a
follicle to grow and
induces its primary
oocyte to complete
meiosis I and start
meiosis II. Meiosis then
stops again; the
secondary oocyte,
released during
ovulation, does not
continue meiosis II right
away. In humans,
penetration of the egg
cell by the sperm
triggers the completion
of meiosis, and only
then is oogenesis
actually complete
Development Stages of an Ovarian Follicle
This cutaway of an ovary illustrates the development stages of an ovarian follicle that accompany
oogenesis. In response to FSH, several follicles start to grow, but usually only one matures.
For convenience, the stages are presented as a cycle (arrows), although they occur at different
times and are never actually present simultaneously within the ovary. In a real ovary, a follicle
stays in one place as it goes through the sequential stages
The production of ova begins with differentiation of the primordial germ cells in the female
embryo into oogonia, which in tern develop into primary oocytes. By birth, a female’s lifetime
supply of primary oocytes is present in her ovaries. Each primary oocyte is arrested at prophase
of meiosis I. Starting at puberty, a single primary oocyte completes meiosis I each month,
developing into a secondary oocyte. The meiotic divisions in oogenesis involve unequal cytokinesis,
with the smaller cells becoming polar bodies (the first polar body may or may not divide again).
The secondary oocyte completes meiosis II only if a sperm cell enters it. After meiosis II, the
haploid nuclei of the sperm and the mature ovum fuse – fertilization
Hormonal Control of the Testes
GnRH from the hypothalamus
regulates FSH and LH release
from the anterior pituitary
FSH acts on the seminiferous
tubules to increase spermatogenesis
LH stimulates Leydig cells to make androgens,
which in turn stimulate sperm production
The anterior pituitary secretes two gonadotropic hormones with different effects on the testes, luteinizing hormone (LH) and folliclestimulating hormone (FSH). LH and FSH are themselves both regulated by gonadotropin-releasing hormone (GnRH) from the
hypothalamus. LH, FSH, and GnRH concentrations in the blood are regulated by negative feedback by androgens. GnRH is also
controlled by negative feedback from LH and FSH (not shown). In human males, these feedback loops keep the hormones at relatively
constant levels, but in many other mammalian species, seasonal cycles in hormone concentration regulate breeding patterns
The Reproductive Cycle of the Human Female
Hormones coordinate the ovarian and menstrual cycles, preparing the uterine lining (endometrium) for implantation of an embryo even before ovulation. (a) Changes in LH and FSH levels. (b)
Changes in the levels of estrogens and progesterone. (c) The ovarian cycle consists of a follicular phase, during which follicles grow and secrete increasing amounts of estrogens; ovulation; and
a luteal phase, during which the corpus luteum secretes estrogens and progesterone. Length of the follicular phase varies; the luteal phase usually lasts 13 to 15 days. (d) The menstrual cycle
consists of a menstrual flow phase, a proliferative phase, and a secretory phase. Menstruation, the shedding of the endometrium, occurs during the menstrual flow phase. The first day of flow
marks day 1 of the menstrual cycle. During the proliferative phase, estrogens from the growing follicle stimulate the endometrium to thicken and become increasingly vascularized. During the
secretory phase, the endometrium continues to thicken, its arteries enlarge, and endometrial glands grow. These endometrial changes require estrogens and progesterone, secreted by the
corpus luteum after ovulation. Thus, the secretory phase of the menstrual cycle parallels the luteal phase of the ovarian cycle. Disintegration of the corpus luteum at the end of the luteal phase
reduces the amount of estrogens and progesterone available to the endometrium, so it is shed. In the event of pregnancy, additional mechanisms maintain high levels of estrogens and
progesterone, preventing loss of the endometrium
Formation of the Zygote and Early Postfertilization Events
1. Ovulation releases a secondary oocyte, which enters the oviduct
2. Fertilization. Entry of a sperm causes the oocyte to complete meiosis and become an ovum. Fertilization occurs when the nuclei of the ovum and sperm
fuse, producing a zygote.
3. Cleavage (cell division) begins in the oviduct as the embryo is moved toward the uterus by peristalsis and the movements of cilia
4. Cleavage continues. By the time the embryo reaches the uterus, cleavage has transformed the embryo into a ball of cells (blastula). It floats in the uterus
for several days, nourished by endometrial secretions
5. The blastocyst implants in the endometrium about 7 days after conception
Placental Circulation
From the fourth week of development until birth, the placenta, a combination of maternal and embryonic tissues, transports nutrients, respiratory
gases, and wastes between the embryo or fetus and the mother. Maternal blood enters the placenta in arteries, flows through blood pools in the
endometrium, and leaves via veins. Embryonic or fetal blood, which remains in vessels, enters the placenta through arteries and passes
through capillaries in fingerlike chorionic villi, where oxygen and nutrients are acquired. As indicated in the drawing, the fetal (or embryonic)
capillaries and villi project into the maternal portion of the placenta. Fetal blood leaves the placenta through veins leading back to the fetus.
Materials are exchanged by diffusion, active transport, and selective absorption between the fetal capillary bed and the maternal blood pools
Human Fetal Development
(a) 5 weeks. Limb buds, eyes, the heart, the liver, and rudiments of all other organs have started to develop in the embryo, which is only about 1 cm long.
(b) 14 weeks. Growth and development of the offspring, now called a fetus, continue during the second trimester. This fetus is about 6 cm long.
(c) 20 weeks. By the end of the second trimester (at 24 weeks), the fetus grows to about 30 cm in length
Hormonal Induction of Labor
The Three Stages of Labor
Birth, or parturition, occurs
through a series of strong,
rhythmic uterine contractions,
commonly known as labor
(FIGURE 46.20). The first stage is
the opening up and thinning of the
cervix, ending with complete
dilation. The second stage is
expulsion, or delivery, of the baby.
Continuous strong contractions
force the fetus down and out of
the uterus and vagina. The
umbilical cord is cut and clamped
at this time. The final stage of
labor is delivery of the placenta,
which normally follows the baby
A complex interplay of hormones (estrogens and oxytocin) and local regulators
(prostaglandins) induces and regulates labor (FIGURE 46.19). Estrogens, which
reach their highest level in the mother’s blood during the last weeks of pregnancy,
trigger the formation of oxytocin receptors on the uterus. Oxytocin, produced by the
fetus and the mother’s posterior pituitary, stimulates powerful contractions by the
smooth muscles of the uterus. Oxytocin also stimulates the placenta to secrete
prostaglandins, which enhance the contractions. In turn, the physical and emotional
stresses associated with the contractions stimulate the release of more oxytocin and
prostaglandins, a positive feedback system that underlies the three stages of labor
Mechanisms of Some Contraceptive Methods
Red arrows indicate
where these methods,
devices, or products
interfere with the flow
of events from the
production of sperm
and egg (secondary
oocyte) to the birth of a
baby
Except for complete abstinence from sexual intercourse, the methods
that prevent the release of gametes are the most effective means of
birth control. Chemical contraceptives--birth control pills--have
pregnancy rates of less than 1% , and sterilization is nearly 100%
effective. The most commonly used birth control pills are combinations
of synthetic estrogens and a synthetic progestin (progesteronelike
hormone). These two hormones act by negative feedback to stop the
release of GnRH by the hypothalamus, and of FSH (an estrogen
effect) and LH (a progestin effect) by the pituitary. By blocking LH
release, the progestin prevents ovulation. As a backup mechanism,
the estrogen inhibits FSH secretion so that no follicles develop.
Combination birth control pills can be prescribed in high doses as
morning after pills (MAP). Taken within 3 days of unprotected
intercourse, they prevent fertilization or implantation, with an
effectiveness of about 75%
A second type of birth control pill, called the minipill, contains only
progestin. The minipill prevents fertilization mainly by altering a
woman’s cervical mucus so that it blocks sperm from entering the
uterus. In 1990, the FDA approved a progestin-only capsule
(Norplant) that is implanted under the skin. Steadily releasing a tiny
amount of progestin into the blood, the implant produces effective
birth control for about 5 years. A product called Depo-Provera is a
synthetic progestin that is injected every 3 months
Of all contraceptives for sexually active
individuals, latex condoms are the only
ones that offer some protection against
sexually transmitted diseases,
including AIDS. This protection is,
however, not absolute
Sterilization is the permanent prevention of gamete release. Tubal
ligation in women usually involves cauterizing or tying off (ligating) a
section of the oviducts to prevent eggs from traveling into the uterus.
Vasectomy in men is the cutting of each vas deferens to prevent
sperm from entering the urethra. Both male and female sterilization
are relatively safe and free from harmful effects. Both are also difficult
to reverse, so the procedures should be considered permanent
Recent scientific and technological advances have made it possible to deal with problems of reproduction in striking ways. For example, it is now possible to diagnose many genetic
diseases and congenital (present at birth) disorders while the fetus is in the uterus. Amniocentesis and chorionic villus sampling are invasive techniques in which amniotic fluid or fetal cells
are obtained for genetic analysis. Noninvasive procedures usually use high-frequency sound waves, or ultrasound imaging, to detect fetal condition. An alternate new technique relies on
the fact that a few fetal blood cells leak across the placenta into the mother’s bloodstream. A blood sample from the mother yields enough fetal cells that can be identified with specific
antibodies (which bind to proteins on the surface of fetal cells) and then tested for genetic disorders.
Ultrasound Imaging
This color-enhanced image shows a fetus in the uterus at about 18 weeks. The image is
produced on a computer screen when high-frequency sounds from an ultra-sound scanner held
against a pregnant woman’s abdomen bounce off the fetus
The Nervous System (Chapter 48)
Overview of a Vertebrate Nervous System
In general, a nervous system has three overlapping functions: sensory input, integration, and motor output Sensory receptors, such as the light-detecting cells in the eyes, collect
information about the physical world outside the body as well as processes inside the organism; this sensory input is then conveyed to integration centers. Integration is the process by
which the input is interpreted and associated with appropriate responses of the body. The circular arrow in FIGURE 48.1 indicates that integration is a continuous background activity.
For the most part, integration is carried out in the central nervous system (CNS), which consists of the brain and spinal cord in vertebrates. Motor output is the conduction of signals
from the integration center, the CNS, to effector cells, the muscle cells or gland cells that actually carry out the body’s responses to stimuli. The signals are conducted by nerves,
ropelike bundles of extensions of neurons tightly wrapped in connective tissue. The nerves that communicate motor and sensory signals between the central nervous system and the
rest of the body are collectively called the peripheral nervous system (PNS). From receptor to effector, information is communicated along a pathway of neurons by a combination of
electrical and chemical signals. In this chapter we concentrate on communication within the nervous system. Chapter 49 connects the nervous system to its inputs and outputs by
discussing sensory receptors and the physiology of movement
Structure of a Vertebrate Neuron
The structural and functional unit of the nervous system is the neuron, or nerve cell. A neuron has a cell body, which contains the
nucleus and other organelles, and it has fiberlike processes (extensions) of two general types. Dendrites (from the Greek dendron ,
tree) are short, highly branched processes that receive incoming messages from other cells and carry this information as an electrical
signal toward the cell body. Axons, usually much longer than dendrites, convey outgoing messages from the neuron to other cells.
Some axons, such as the ones connecting your spinal cord to your foot, can be over a meter long. The conical region of the axon
where it joins the cell body is called the axon hillock; this region plays an essential role in the transmission and integration of nerve
signals. Many axons are enclosed by an insulating layer called the myelin sheath
The “Knee-Jerk” Reflex
For simplicity, only one neuron
of each type is shown here, but
actually many neurons are
involved in the knee-jerk reflex
1.
2.
3.
4.
5.
6.
The knee-jerk, or patellar, reflex is caused by tapping the tendon connected to the quadriceps muscle
Sensory (stretch) receptors detect a sudden stretch in the quadriceps (extensor) muscle I the thigh
Sensory neurons convey the information to neurons in the spinal cord
In the spinal cord, the information travels via synapses between sensory neurons and motor neurons
Motor neurons serving quadriceps. The motor neurons convey signals to the quadriceps muscle to contract, jerking the lower leg forward
Interneurons. Only two kinds of neurons (sensory and motor) mediated the actual reflex action, but the sensory neurons from the quadriceps also
communicate with interneurons in the spinal cord
7. In turn, the interneurons inhibit certain motor neurons. this inhibition prevents the flexors from contracting, which would resist the action of the quadriceps
Schwann Cells
In the peripheral nervous system, supporting cells called Schwann cells wrap many axons with an insulating myelin sheath.
Gaps between successive Schwann cells are called nodes of Ranvier
Schwann cells (in the PNS) are glia that form insulating myelin sheaths around the axons of many neurons. Neurons become myelinated in a
developing nervous system when Schwann cells or oligodendrocytes grow around axons such that their plasma membranes form concentric layers,
somewhat like a jelly roll. The membranes are mostly lipid, which is a poor conductor of electrical currents. Thus, the myelin sheaths provide electrical
insulation of the axon, analogous to the insulation that covers copper electrical wires. We will see later in this chapter that the myelin sheath also
increases the speed of propagation of nerve impulses. In the degenerative disease known as multiple sclerosis, myelin sheaths gradually deteriorate,
resulting in a progressive loss of coordination due to the disruption of nerve impulse transmission. Clearly, supporting cells are indispensable partners
of neurons in a working nervous system
The Basis of Membrane Potential
(a) Shown here are the approximate concentrations for a mammalian cell (in millimoles per liter, abbreviated mM) of potassium, [K +]; sodium [Na+];
chloride, [Cl-]; and anions that remain inside the cell, [A-]. K+ diffuses out of the cell down its concentration gradient, but the A- anions cannot follow, so
the interior of the cell develops a net negative charge. (b) There is a steady diffusion of K+ out of the cell and steady diffusion of Na+ into the cell; the
thickness of the arrows indicates the relative permeability of the membrane to K+ and Na+ (the permeability mainly reflects the number of ion-specific
channels). Over time, diffusion would cause the ionic gradients shown in part (a) to dissipate. Dissipation is prevented by the sodium-potassium pump,
which uses ATP to actively transport Na+ out of the cell and K+ into the cell
Measuring Membrane Potentials
(a) Microelectrodes inside and outside the cell measure the voltage
(membrane potential) across a cell’s plasma membrane.
(b) Apparatus for measuring membrane potentials
Graded Potentials and the Action Potential in a Neuron
A change in membrane potential is a localized electrical event at the point of stimulation. Let us consider what happens in a region of a dendrite that is stimulated by a neurotransmitter. The
specific effect of this stimulus on membrane polarization depends on the type of chemically-gated ion channel that is opened. FIGURE a and b show two types of local responses. Some stimuli
trigger a hyperpolarization, an increase in the voltage across the membrane. One of the ways a stimulus can produce a hyperpolarization is by opening a potassium channel, which increases K+
outflow and causes the inside of the cell to become more negative. In contrast, a depolarization is a reduction in the voltage across the membrane. One of the ways this can occur is by a
stimulus opening a sodium channel; the increased inflow of Na+ makes the inside of the cell less negative. These voltage changes are called graded potentials because the magnitude of
change (either hyperpolarization or depolarization) depends on the strength of the stimulus: A larger stimulus will open more channels, producing a larger change in permeability and thus a larger
change in the membrane potential
Environmental changes can alter the cell’s membrane potential. (a) One way a neuron can be hyperpolarized is by stimuli that open potassium
channels. (b) A neuron can be depolarized by stimuli that open sodium channels. (c) A depolarizing stimulus of sufficient strength will change the
membrane potential to a critical level called the threshold potential. This triggers an action potential, or nerve impulse. Unlike a graded potential, an
action potential is an all-or-none event; the size of the action potential is not affected by the strength of the stimulus that triggered it
The Role of Voltage-Gated Ion Channels in the Action Potential
Activation gates of the sodium
channels are open, but the
potassium channels remain
closed. Sodium ions rush into
the cell, and the interior of the
cell becomes more positive
Inactivation gates close sodium channels,
and potassium channels open. Potassium
ions leave the cell, and the loss of positive
charge causes the inside of the cell to
become more negative than the outside
A stimulus opens some Na+
channels. If the Na+ influx
achieves threshold potential,
then additional Na+ gates
open, triggering an action
potential
Both the sodium and potassium channels are closed,
and the membrane’s resting potential is maintained
Both gates of the sodium channels are closed, but potassium channels remain
open because their relatively slow gates have not had time to respond to the
repolarization of the membrane. Within another millisecond or two, the resting
state is resorted, and the system is ready to respond to another stimulus
Propagation of the Action Potential
1.
2.
3.
An action potential is generated as sodium ions flow inward across the membrane at one location
The depolarization of the first action potential has spread to the neighboring region of the membrane, depolarizing it and
initiating a second action potential. At the site of the first action potential, the membrane is repolarizing the K + flows outward
A third action potential follows in sequence, with repolarization in tis wake. In this way, local currents of ions across the
plasma membrane give rise to a nerve impulse that is propagated along the axon
The three parts of this figure show the changes that occur in a portion of an axon at three
successive times as a nerve signal passes from left to right. At each point along the axon, the
voltage-gated channels go through the sequence described in FIGURE 48.9, reproducing the
sequence of voltage changes associated with the action potential. (The "status" of the
membrane is color-coded to FIGURE 48.9.)
Saltatory Conduction
In a myelinated axon, the ion current during an action potential at one node of Ranvier spreads
along the interior of the axon to the next node (blue arrows), triggering an action potential there.
The action potential thus jumps from node to node as it propagates along the axon (red arrows)
A Chemical Synapse
When an action potential depolarizes the membrane of the synaptic terminal, it
1 triggers an influx of Ca2+ that 2 causes synaptic vesicles to fuse with the
membrane of the presynaptic neuron. 3 The vesicles release neurotransmitter
molecules into the synaptic cleft. These molecules diffuse across the cleft and
bind to the receptors of ion channels embedded in the postsynaptic
membrane. 4 The binding of neurotransmitter molecules to their specific
receptors opens specific ion channels--Na+ channels, in the synapse
illustrated here, causing a Na+ influx that depolarizes the postsynaptic
membrane. 5 The neurotransmitter molecules are quickly degraded by
enzymes or are taken up by another neuron, closing the ion channels and
terminating the synaptic response
Integration of Multiple Synaptic Inputs
(a) Each neuron, especially in the central nervous system, is on the receiving end of thousands of synapses, some excitatory (green) and others
inhibitory (red). At any instant an action potential may be generated at the axon hillock if the combined effect of ion currents induced by excitatory
and inhibitory synapses depolarizes the membrane to the threshold potential. Synapses close to the axon hillock generally have a stronger effect
on the membrane potential than other synapses. (b). This micrograph reveals numerous synaptic terminals of presynaptic neurons that
communicate with a single postsynaptic cell (SEM).
Neurotransmitters
Dozens of different substances, many of them small, nitrogen containing organic molecules, are known to function as neurotransmitters, and researchers expect to find many more. Table
48.1 summarizes the major known neurotransmitters. Notice that a particular neurotransmitter can trigger different responses in postsynaptic cells. This versatility depends on the receptors
present on different postsynaptic cells and on the receptor’s mode of action. Many neurotransmitters bind with receptors that have a direct effect on ion channel proteins, altering the
membrane permeability of the postsynaptic cell. This type of synaptic communication can take only a few milliseconds, serving the rapid and precise transfer of information at a single
synapse. Other neurotransmitters take much longer (up to several minutes) because they communicate via complex signal-transduction pathways in the postsynaptic cell. In some cases
neurotransmitters in the brain--such as those regulating mood, attention, and arousal--remain active long enough after their release to diffuse to many synapses and modulate their activity
Diversity in Nervous Systems
The Nervous System of a Vertebrate
The components of the central nervous system (brain and spinal cord) develop from the dorsal, hollow nerve cord, a
hallmark of chordates. Cranial nerves (originating in the brain), spinal nerves (originating in the spinal cord), and ganglia
outside the central nervous system make up the peripheral nervous system
Functional Hierarchy of the Peripheral Nervous System
Structurally, the vertebrate PNS
consists of paired cranial and spinal
nerves and associated ganglia. The
cranial nerves originate in the brain
and innervate organs of the head
and upper body. The spinal nerves
originate in the spinal cord and
innervate the entire body. Mammals
have 12 pairs of cranial nerves and
31 pairs of spinal nerves. Most of the
cranial nerves and all of the spinal
nerves contain both sensory and
motor neurons; a few of the cranial
nerves are sensory only (the
olfactory and optic nerves, for
example).
Because most nerves contain
a diversity of neurons that
play different roles, it is
convenient to divide the PNS
into a hierarchy of
components that differ in
function. The sensory
division of the PNS is made
up of the sensory, or afferent
(incoming), neurons that
convey information to the
CNS from sensory receptors
that monitor the external and
internal environment. The
motor division is composed
of the motor, or efferent
(outgoing), neurons that
convey signals from the CNS
to effector cells. The motor
division is divided, in turn, into
two functional divisions, called
the somatic and autonomic
nervous systems
The somatic nervous system
carries signals to skeletal
muscles, mainly in response to
external stimuli. The somatic
nervous system is often
considered voluntary because
it is subject to conscious
control, but a substantial
proportion of skeletal muscle
movement is actually
determined by reflexes
mediated by the spinal cord or
lower brain
The autonomic nervous system consists of two divisions that act on our body organs with opposing effects. Activation of the sympathetic
division correlates with arousal and energy generation: the heart beats faster, the liver converts glycogen to glucose, bronchi of the lungs
dilate and support increased gas exchange, digestion is inhibited, and secretion of adrenaline from the adrenal medulla is stimulated
Activity of the parasympathetic division causes approximately the mirror image of this: a calming and a return to emphasis on selfmaintenance functions. For example, activity of parasympathetic nerves decreases heart rate and energy storage and also enhances
digestion. When sympathetic and parasympathetic nerves innervate the same organ, they often (but not always) have antagonistic
(opposite) effects
The autonomic nervous
system conveys signals that
regulate the internal
environment by controlling
smooth and cardiac muscles
and the organs of the
gastrointestinal,
cardiovascular, excretory, and
endocrine systems. This
control is generally
involuntary
The Main Roles of the Parasympathetic and Sympathetic Nerves
in Regulating Internal Body Functions
The parasympathetic nerves of the autonomic
nervous system originate from the lower brain
and the sacral region of the spinal cord.
Sympathetic nerves emerge from the middle
(thoracic and lumbar) regions of the spinal
cord. Most autonomic pathways consist of a
chain of two neurons. The synapse between
the two neurons is within a ganglion in the
PNS. In each pair, the neuron that conveys
signals from the CNS to the ganglion is called
the preganglionic neuron. Preganglionic
neurons release acetylcholine at the synapse.
The neuron that conveys signals from the
ganglion to the target organ is called the
postganglionic neuron. Notice in this diagram
that several sympathetic pathways include a
synapse in prominent sympathetic ganglia
near the spinal cord. Other ganglia are less
prominent; those of the parasympathetic
neurons reside close to or within the target
organs. Most sympathetic axons release the
neurotransmitter norepinephrine at their target
organs. Parasympathetic neurons release
acetylcholine
Embryonic Development of the Brain
The Main Parts of the Human Brain
Brain Stem:
Medulla Oblongata, Pons, and Midbrain
Function in homeostasis, coordination of movement,
and conduction of information to higher brain centers
Midbrain =
-Contains centers for the receipt and integration of several
types of sensory information
-Serves as a projection center, sending coded sensory
information along neurons to specific regions of the forebrain
-Aids in auditory and visual systems (automatic responses)
Medulla Oblongata =
-Controls several visceral (automatic, homeostatic) functions
including breathing, heart and blood vessel activity,
swallowing, vomiting, and digestion
-Descending axons carrying movement instructions cross from one side
of the CNS to the other as they pass through the medulla and hence the
left side of the brain controls the right side of body movements
Cerebellum =
Pons =
Participates is some of the functions of the medulla
to including regulating breathing
-Functions in coordination and error-checking during motor, perceptual,
and cognitive performances
-Provides automatic coordination of movements and balance
(hand-eye coordination)
Thalamus =
-The main input center for sensory information going to the cerebrum
-The main output center for motor information leaving the cerebrum
-Sorts all sensory information and sends it to the appropriate higher
brain centers for further interpretation and integration
-Receives input from the cerebrum and other parts of the brain that
regulate emotion and arousal
Hypothalamus =
-Important homeostatic regulation regions
-Source of two sets of hormones – posterior pituitary
hormones and releasing hormones that act on the
anterior pituitary
-Contains the body’s thermostat
-Contains centers for regulating hunger, thirst, and many
other basic survival mechanisms
-Plays a role in sexual and mating behaviors, the
fight-or-flight response, and pleasure
Cerebrum =
-The most sophisticated center of homeostatic control and integration
-Divided into the right and left cerebral hemispheres
Corpus callosum =
Cerebral Hemispheres =
-A thick band of fibers (white matter) that communicates between the
White matter – tracts of axons
right and left cerebral hemispheres
Gray matter – nuclei, or clusters of neuron cell bodies
Gray matter functions as the cerebral cortex
Basal Nuclei – deep within the white matter that function as important centers for planning and learning movement sequences
Damage to this region can cause Parkinson’s disease and Huntington’s disease
Neocortex =
An additional outer layer of cortex, unique to mammals, consisting of six sheets of neurons running tangential to the brain surface
Houses greater cognitive abilities and more sophisticated behaviors
Humans – less than 5 mm thick and has a surface area of about 0.5 m2, due to convolutions, which accounts for about 80% of the total brain mass
Nonhuman primates and cetaceans (whales, porpoises, dolphins) also have exceptionally large and complex neocortices (dolphin’s second to humans)