Chapter 34
Endocrine Control
Albia Dugger • Miami Dade College
34.1 Hormones in the Balance
• Many synthetic chemicals we release into the environment
are endocrine disruptors that interfere with hormones
• DDT (a pesticide) and PCBs (used in electronic products,
caulking, and solvents) were banned in the 1970s
• Atrazine, a synthetic herbicide, remains in wide use in the
United States
• Biologist Tyrone Hayes showed that atrazine affects sexual
development in frogs and other aquatic organisms
Tyrone Hayes and
Frogs Exposed to Atrazine
34.2 The Vertebrate Endocrine System
• Animal cells communicate with one another by way of a
variety of short-range and long-range chemical signals
• Animal cells communicate with adjacent cells through gap
junctions and by releasing molecules that bind to receptors in
or on other cells
Mechanisms of Intercellular Signaling
• Only neurons secrete neurotransmitters, which diffuse
across the synaptic cleft to the target cell
• Many cells secrete local signaling molecules – such as
prostaglandins released by injured cells – which affect only
neighboring cells
• Animal hormones are secreted into interstitial fluid, enter the
blood, and are distributed throughout the body
Discovery of Hormones
• Hormones were first discovered by William Bayliss and Ernest
Starling, who were studying how the secretion of pancreatic
juices is regulated
• They discovered that exposure to acid causes the small
intestine to release a chemical signal into the blood that
encourages the pancreas to secrete bicarbonate into the gut
Hormones and the Endocrine System
• Internal secretions carried by the blood that influence the
activities of specific body organs are called hormones
• Endocrine glands and other structures that secrete hormones
make up an animal’s endocrine system
• Some of the major endocrine glands also have functions
unrelated to hormone secretion
Hypothalamus
Pineal gland
Parathyroid glands
Pituitary gland
Thyroid gland
Thymus gland
Adrenal glands
Pancreas
Gonads
Figure 34-2 p587
Neuroendocrine Interactions
• Nervous and endocrine systems interact – both respond to
the hypothalamus, a command center in the forebrain
• Most organs receive and respond to both nervous signals and
hormones
• Hormones affect brain development and nervous processes
such as sleep cycles, emotion, mood, and memory
• The nervous system regulates hormone secretion
Take-Home Message: How
do animal cells
communicate with one another?
• In all animals, cells release molecules that influence other
cells. Each type of signal acts on all target cells that have
receptors for it. Hormones are intercellular signaling
molecules that travel in the bloodstream.
• Most vertebrates have the same types of hormones produced
by similar structures. Collectively, hormone-secreting glands
and cells make up an endocrine system.
• Integrated interactions between the nervous system and
nearly all endocrine glands coordinate many different
functions for the body as a whole.
34.3 The Nature of Hormone Action
• For a hormone to have an effect, it must bind to protein
receptors on or inside a target cell
• Hormone action involves three steps:
1. A hormone activates a target cell receptor
2. The signal is transduced (changed into a form that affects
target cell behavior)
3. The cell makes a response
From Signal Reception to Response
Signal
Reception
Signal
Transduction
Cellular
Response
Table 34-1 p588
Intracellular Receptors
• Steroid hormones are made from cholesterol and can diffuse
across the plasma membrane
• Most steroid hormones form a hormone-receptor complex that
binds to a promoter inside the nucleus and alters the
expression of specific genes
1 A steroid hormone
molecule is moved from
blood into interstitial fluid
bathing a target cell.
2 Being lipid
soluble, the
hormone
easily
diffuses
across the
cell’s plasma
membrane.
3 The hormone
diffuses through the
cytoplasm and nuclear
envelope. It binds with
its receptor in the
nucleus.
5 The
resulting
mRNA moves
into the
cytoplasm and
is transcribed
into a protein.
gene product
receptor
hormone–
receptor
complex
4 The
hormone–
receptor
complex
triggers
transcription of a
specific
gene.
Figure 34-3a p589
Receptors at the Plasma Membrane
• Large amine, peptide and protein hormones bind to a receptor
at the plasma membrane
• Binding triggers formation of a second messenger (molecule
that relays signal into cell)
• Enzyme converts ATP to cAMP
• cAMP activates a cascading series of reactions
A peptide hormone
1 molecule,
glucagon,
diffuses from blood into
interstitial fluid bathing
the plasma membrane
of a liver cell.
unoccupied glucagon
receptor at target cell’s
plasma membrane
ATP
cyclic
AMP
+ Pi
2 Glucagon binds
with a receptor.
3 Cyclic AMP
Binding activates an
activates another
enzyme that catalyzes enzyme in the cell.
the formation of
cyclic AMP from ATP
inside the cell.
4 The enzyme activated by
cyclic AMP activates another
enzyme, which in turn
activates another kind that
catalyzes the breakdown of
glycogen to its glucose
monomers.
5
The enzyme
activated by
cyclic AMP
also inhibits
glycogen
synthesis.
Figure 34-3b p589
Receptor Function and Diversity
• Only cells with appropriate and functional receptor proteins
can respond to a hormone
• Gene mutations that alter receptor structure can prevent or
change cell response to a hormone
• Examples:
• Androgen insensitivity syndrome
• Variations in ADH receptors
Take-Home Message: How do hormones exert
their effects on target cells?
• Hormones exert their effects by binding to protein receptors,
either inside a cell or at the plasma membrane.
• Steroid hormones often enter a cell and act by altering the
expression of specific genes.
• Peptide and protein hormones usually bind to a receptor at
the plasma membrane. They trigger formation of a second
messenger, a molecule that relays a signal into the cell.
• Variations in receptor structure affect how a cell responds to a
hormone.
ANIMATION: Hormones and target cell
receptors
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34.4 The Hypothalamus and Pituitary Gland
• The hypothalamus is the main center for control of the
internal environment – it connects structurally and functionally
with the pituitary gland
• The pituitary gland has two parts:
• The posterior lobe secretes hormones made in the
hypothalamus
• The anterior lobe makes its own hormones
• The hypothalamus signals the pituitary by way of secretory
neurons that make hormones
The Hypothalamus and Pituitary Gland
hypothalamus
anterior
lobe of
pituitary
posterior
lobe of
pituitary
Posterior Pituitary Function
• Secretory neurons of the hypothalamus make two hormones
that move through axons into the posterior pituitary, which
releases them
• Antidiuretic hormone (ADH) affects certain kidney cells
• Oxytocin (OT) triggers muscle contractions during childbirth
Cell bodies of
secretory neurons
in hypothalamus
synthesize ADH or
oxytocin.
1
The ADH or
oxytocin moves
downward inside the
axons of the secretory
neurons and
accumulates in the
axon terminals.
2
Action potentials
trigger the release of
these hormones,
which enter blood
capillaries in the
posterior lobe of the
pituitary.
3
Blood
vessels carry
hormones to
the general
circulation.
4
Figure 34-5 p591
Anterior Pituitary Function
• The anterior pituitary produces hormones of its own:
• Adrenocorticotropic hormone (ACTH)
• Thyroid-stimulating hormone (TSH)
• Follicle stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Prolactin (PRL)
• Growth hormone (GH)
Control of Anterior Pituitary
• Hormones from the hypothalamus control the release of
anterior pituitary hormones
• Releasing hormones encourage secretion of hormones by
target cells
• Inhibiting hormones reduce secretion of hormones by target
cells
• Releasing and inhibiting hormones are secreted into the stalk
that connects the hypothalamus to the pituitary
Cell bodies of secretory
neurons in hypothalamus
synthesize inhibitors or
releasers that are secreted
into the stalk that connects
to the pituitary.
1
The inhibitors or
releasers picked up by
capillaries in the stalk
get carried in blood to
the anterior pituitary.
2
When encouraged
by a releaser,
anterior pituitary
cells secrete
hormone that enters
blood vessels that
lead into the general
circulation.
4
The inhibitors or
releasers diffuse
out of capillaries in
the anterior
pituitary and bind
to their target cells.
3
Figure 34-6 p591
Table 34-2 p590
Feedback Controls of Hormone Secretion
• Positive feedback mechanisms
• Response increases the intensity of the stimulus
• Example: Oxytocin and childbirth contractions
• Negative feedback mechanisms
• Response decreases the stimulus
Take-Home Message: How do the
hypothalamus and pituitary gland interact?
• Some secretory neurons of the hypothalamus make
hormones (ADH, OT) that move through axons into the
posterior pituitary, which releases them.
• Other hypothalamic neurons produce releasers and inhibitors
that are carried by the blood into the anterior pituitary. These
hormones regulate the secretion of anterior pituitary
hormones (ACTH, TSH, LH, FSH, PRL, and GH).
34.5 Growth Hormone
Function and Disorders
• Growth hormone (GH) encourages production of cartilage and
bone and increases muscle mass
• Excessive GH secretion in childhood results in gigantism
• Excessive GH secretion in adulthood results in acromegaly
• A deficiency of GH during childhood can cause dwarfism
Gigantism
Acromegaly
Dwarfism
Take-Home Message: What are the effects of
too much or too little growth hormone?
• Excessive growth hormone causes faster-than-normal bone
growth. When the excess occurs during childhood, the result
is gigantism. In adults, the result is acromegaly.
• A deficiency of GH during childhood can cause dwarfism.
34.6 Sources and Effects
of Other Vertebrate Hormones
• In addition to the hypothalamus and pituitary gland, endocrine
glands and endocrine cells secrete hormones
• The gut, kidneys, and heart are among the organs that are
not glands, but include hormone-secreting cells
Table 34-3 p593
Multiple Hormone Receptors
• Most cells have receptors for multiple hormones, and the
effect of one hormone can be enhanced or opposed by
another one
• Example: Skeletal muscle hormone receptors
• Glucagon, insulin, cortisol, epinephrine, estrogen
testosterone, growth hormone, somatostatin, thyroid
hormone and others
Take-Home Message: What are the sources
and effects of vertebrate hormones?
• In addition to the pituitary gland and hypothalamus, endocrine
glands and endocrine cells secrete hormones.
• The gut, kidneys, and heart are among the organs that are
not considered glands, but do include cells that secrete
hormones.
• Most cells have receptors for multiple hormones, and the
effect of one hormone can be enhanced or opposed by that of
another.
34.7 Thyroid and Parathyroid Glands
• The thyroid gland, located at the base of the neck, regulates
metabolic rate
• The adjacent parathyroids regulate calcium levels
Human Thyroid and Parathyroid Glands
epiglottis
thyroid
cartilage
(Adam’s
apple)
pharynx
Thyroid
Gland
Parathyroid
Glands
trachea
(windpipe)
anterior
posterior
Thyroid Function and Disorders
• Thyroid gland
• Secretes iodine-containing thyroid hormones and
calcitonin
• Regulated by a negative feedback loop
• Hypothyroidism
• Low levels of thyroid hormone; caused by iodine
deficiency or thyroid disease; causes goiter
• Graves’ disease
• Excess thyroid hormone production
RESPONSE
STIMULUS
Blood level of
thyroid hormone falls
below a set point.
+
Hypothalamus
TRH
Anterior Pituitary
TSH
Rise of thyroid
hormone level
in blood inhibits
the secretion of
TRH and TSH.
Thyroid Gland
Thyroid hormone is secreted.
Stepped Art
Figure 34-9 p594
Goiter
Thyroid Disruptors
• When a chemical pollutant called perchlorate is present,
fewer iodide ions enter thyroid cells, and thyroid hormone
production declines
• Most municipal water treatment plants cannot remove
perchlorate from water
The Parathyroid Glands
• Parathyroid glands
• Release parathyroid hormone (PTH) in response to low
blood calcium levels
• Targets bone cells and kidney cells
• Stimulates conversion of vitamin D to calcitriol
Parathyroid Diseases
• Inadequate vitamin D is the most common cause of the
nutritional disorder known as rickets
• Tumors and other conditions that cause excessive PTH
secretion weaken bone and increase risk of kidney stones
• Disorders that reduce PTH output lower blood calcium and
can cause deadly seizures and muscle contractions
Rickets
Take-Home Message: What are the functions of
thyroid and parathyroid glands?
• The thyroid gland has roles in regulation of metabolism and in
development. Iodine is required to make thyroid hormone.
• The parathyroid glands are the main regulators of blood
calcium level.
34.8 Pancreatic Hormones
• The pancreas lies behind the stomach
• Its exocrine cells secrete digestive enzymes into the small
intestine
• Its endocrine cells group in clusters called pancreatic islets
• Each islet contains three types of cells: alpha cells, beta cells,
and delta cells
Location of the Pancreas
stomach
pancreas
small
intestine
ANIMATED FIGURE: Hormones and
glucose metabolism
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Insulin and Glucagon
• Two pancreatic hormones with opposing effects work together
to regulate blood sugar levels
• Insulin
• Increases cell uptake and storage of glucose
• Secreted by beta cells in response to high blood glucose
• Glucagon
• Increases breakdown of glycogen to glucose
• Secreted by alpha cells in response to low blood glucose
6 Stimulus
1 Stimulus
Increase in blood glucose
Decrease in blood glucose
PANCREAS
2
glucagon
LIVER
3
7
8
insulin
glucagon
insulin
MUSCLE FAT CELLS
4 Body cells, especially in muscle
and adipose tissue, take up and use
more glucose.
Cells in skeletal muscle and liver
store glucose in the form of glycogen.
5 Response
Decrease in blood glucose
9 Cells in liver break
down glycogen faster.
The released glucose
monomers enter
blood.
10 Response
Increase in blood glucose
Figure 34-12b p596
Diabetes
• Diabetes mellitus is a metabolic disorder in which cells do not
take up glucose properly
• The resulting high blood sugar (hyperglycemia) disrupts
normal metabolism
• Cells have to break down proteins and fats, yielding harmful
waste products
• High blood sugar causes some cells to overdose on glucose
and produce other harmful substances
Table 34-4 p597
Two Types of Diabetes
• Type 1 diabetes (juvenile-onset diabetes)
• Autoimmune disease that destroys insulin-producing cells
• Requires insulin injections
• Type 2 diabetes (adult onset diabetes)
• Target cells do not respond to insulin
• Usually managed by diet, exercise, and oral medications
An Insulin Pump
Take-Home Message: How do pancreatic
hormones maintain blood glucose levels?
• Insulin helps cells take up and store more glucose; it lowers
the blood level of glucose.
• Glucagon triggers breakdown of glycogen; it raises blood
glucose.
• Diabetes is a metabolic disorder in which the body does not
make insulin or the body does not respond to it. As a result,
cells do not take up sugar as they should, causing
complications throughout the body.
34.9 The Adrenal Glands
• There are two adrenal glands, one above each kidney
• The outer layer is the adrenal cortex; the inner portion is the
adrenal medulla
• The two regions are controlled by different mechanisms, and
secrete different substances
The Adrenal Cortex
• The adrenal cortex secretes steroid hormones and small
amounts of sex hormones
• The steroid hormone cortisol affects metabolism and the
stress response
• A negative feedback loop regulates cortisol level in blood
STIMULUS
Blood level
of cortisol
declines.
adrenal
cortex
RESPONSE
Hypothalamus
1
CRH
4
Anterior Pituitary
2
ACTH
adrenal
medulla
Adrenal Cortex
Rise of cortisol level in
the blood inhibits the
secretion of CRH and
ACTH.
3
Cortisol secretion
increases and has the
following effects:
kidney
Figure 34-14 p598
The Adrenal Medulla
• The adrenal medulla contains specialized neurons of the
sympathetic division that release epinephrine and
norepinephrine, which stimulate the fight-flight response
Stress, Elevated Cortisol, and Health
• Acute stress triggers increased secretion of cortisol,
epinephrine, and norepinephrine
• Chronic stress chronic stress interferes with growth, the
immune system, sexual function, cardiovascular function, and
damages cells central to memory and learning
• Hypercortisolism (Cushing’s syndrome) can be triggered by
adrenal gland tumor, oversecretion of ACTH by the anterior
pituitary, or chronic use of the drug cortisone
Cortisol Deficiency
• Hypocortisolism (Addison’s disease) results from adrenal
gland damage
• Blood pressure and blood sugar fall
• Symptoms include fatigue, weakness, depression, weight
loss, darkening of skin
• If cortisol levels get too low, blood sugar and blood pressure
fall to life-threatening levels
Take-Home Message: What are the functions of
adrenal hormones?
• The adrenal cortex’s main secretions are aldosterone, which
affects urine concentration, and cortisol, which affects
metabolism and stress responses.
• The adrenal medulla releases epinephrine and
norepinephrine, which prepare the body for danger.
• Cortisol secretion is governed by a feedback loop to the
hypothalamus and pituitary, but stress breaks that loop and
allows the level of cortisol to rise.
• Long-term cortisol elevation harms health. Insufficient cortisol
can be fatal.
Video: Adrenaline and the Body
34.10 The Gonads
• Gonads are the primary reproductive organs that produce
gametes (eggs and sperm)
• Vertebrate gonads produce sex hormones, steroid hormones
that control sexual development and reproduction
• Testes produce testosterone
• Ovaries produce estrogens and progesterone
Human Gonads
testis
ovary
Control of Sex Hormone Secretion
• The hypothalamus and anterior pituitary control secretion of
sex hormones by gonads
• The hypothalamus produces gonadotropin-releasing hormone
(GnRH), which causes the anterior pituitary to secrete folliclestimulating hormone (FSH) and luteinizing hormone (LH)
• FSH and LH are “gonadotropins” that cause gonads to
produce and secrete sex hormones
Control of Sex Hormone Secretion
Hypothalamus
GnRH
Anterior Pituitary
FSH, LH
Gonads
Sex hormones
Sex Hormones and Puberty
• Sex hormone production increases during puberty, the period
of development when reproductive organs mature and
secondary sexual traits appear
• Secondary sexual traits are traits that differ between the
sexes, but do not have a direct role in reproduction
Take-Home Message:
What are sex hormones?
• Sex hormones are steroid hormones that influence the
development of sexual traits and reproduction. Production of
sex hormones increases at puberty.
• In both sexes, gonads secrete sex hormones in response to
anterior pituitary hormones (FSH and LH), which are in turn
released in response to a hypothalamic releasing hormone.
• A male’s testes secrete mainly testosterone, and a female’s
ovaries secrete mainly estrogens and progesterone.
34.11 The Pineal Gland
• The pineal gland secretes the hormone melatonin during
conditions of low light or darkness
• In nonmammalian vertebrates, the pineal gland is close to the
surface and has photoreceptors that detect light directly
• In humans, the pineal gland is deep inside the brain and
receives information about light indirectly
The Human Pineal Gland
pineal gland
Circadian Rhythms
• Melatonin secretion follows a circadian rhythm – it varies
cyclically over an approximately 24-hour interval
• Melatonin in turn influences other circadian rhythms such as
body temperature and sleep
• Routinely working night shifts or having poor sleep habits
disrupts melatonin secretion and raises the risk of cancer
Seasonal Effects
• Some people who live at high latitudes have seasonal
affective disorder (SAD), which arises when a person’s
rhythm of melatonin secretion gets out of sync with clock time
• In birds, melatonin regulates the onset of seasonal behavior
such as singing and other courtship behaviors
Take-Home Message: What is the role of
melatonin secreted by the pineal gland?
• Melatonin secreted by the pineal gland during periods of
darkness encourages sleepiness and helps set the body’s
internal clock.
• Activities that disrupt nighttime melatonin secretion raise the
risk of cancer.
• In many animals, seasonal differences in melatonin secretion
give rise to seasonal difference in behavior.
34.12 The Thymus
• The thymus lies beneath the sternum and secretes peptide
hormones that enhance wound healing and immune function
• Thymulin is a peptide hormone secreted by epithelial cells of
the thymus – it encourages maturation of white blood cells
called T cells
• Thymulin requires zinc as a cofactor
• Thymus activity decreases with age
Take-Home Message:
What is the role of the thymus?
• The thymus produces peptides that enhance immune function
and wound healing.
• The thymus is most active in childhood. During adulthood, its
secretory tissue is largely replaced by fat.
34.13 Invertebrate Hormones
• Genes for hormone receptors and enzymes involved in
hormone synthesis evolved over time
• We can trace the evolutionary roots of the vertebrate
endocrine system in invertebrates
• Cnidarians (e.g. sea anemones) and mollusks (e.g. sea slugs)
have receptors that resemble those that bind vertebrate
hormones
Hormones and Molting
• Some hormones are unique to invertebrates
• Example: ecdysone, a steroid hormone that controls molting
in arthropods
• Hormone-secreting neurons in the brain respond to
signals such as light and temperature
• Mechanisms differ in crustaceans and insects
Control of Molting in Crabs
Take-Home Message: What types of hormones
do invertebrates produce?
• Some invertebrate hormones are homologous to those in
vertebrates. Cnidarians and annelids have receptors that
resemble those that bind vertebrate hormones.
• Invertebrates also make hormones with no vertebrate
counterpart. Hormones that control molting in arthropods are
an example.