CHEMICAL SIGNALING
WITHIN THE ANIMAL BODY
CHAPTER 30
HORMONES
• A hormone is a chemical
signal produced in the
body.
Hypothalamus
Pineal
gland
Pituitary
gland
Parathyroid glands
(attached to the thyroid)
• It typically acts at a site distant
from where it was produced.
• Most hormones are
produced in glands that are
completely enclosed in
tissue.
• These glands are called
endocrine glands.
Thyroid
gland
Thymus
Adrenal
glands
Pancreas
Ovaries
(in females)
Testes
(in males)
HORMONES
• There are three big advantages to using
chemical hormones as messengers rather
than speedy electrical signals (like nerve
signals).
• Chemical molecules can spread to all tissues via
the blood.
• Chemical signals can persist much longer than
electrical ones.
• Many different kinds of chemicals can act as
hormones.
HORMONES
• The glands that produce hormones are
generally controlled by the nervous system.
• The endocrine system and the motor nervous
system are the two main routes the CNS uses to
issue commands to the organs of the body.
• The two are so closely linked that they are often
considered a single system—neuroendocrine
system.
• The hypothalamus is the main switchboard of the
neuroendocrine system.
HORMONES
• The CNS regulates the body’s hormones
through a chain of command.
• For example, the hypothalamus controls the
pituitary gland with thyrotropic-releasing
hormone (TRH).
• This causes the pituitary to release or thyroidstimulating hormone (TSH).
• TSH then causes the thyroid gland to release
thyroid hormones.
• The hypothalamus also secretes inhibiting
hormones that keep the pituitary from secreting
specific hormones.
HORMONES
• Hormones are effective messengers within
the body because a particular hormone
can influence a specific target cell.
• Cells that the body has targeted to respond to a
particular hormone have receptor proteins
shaped to fit that hormone and no other.
HORMONES
• Hormones secreted by endocrine glands
belong to four different chemical
categories:
• polypeptides
• glycoproteins
• amines
• steroids
HORMONES
•
The path of communication taken by a
hormonal signal is a series of simple steps:
1. Issuing the command - the hypothalamus controls
2.
3.
4.
the release of many hormones.
Transporting the signal - most are transported
throughout the body by the bloodstream.
Hitting the target - the hormone binds to a receptor
on the target cell.
Having an effect - when the hormone binds to the
receptor protein, the protein changes shape and triggers
a change in cell activity.
KEY BIOLOGICAL PROCESS: HORMONAL
COMMUNICATION
Dehydration
Blood volume and
pressure drops.
Osmotic concentration
in the blood increases.
3
Osmoreceptors
1
4
Hypothalamus
ADH
Antidiuretic
hormone (ADH)
2
4
Bloodstream
1
Reduced urine
volume causes
increased water
retention.
3
Increased
vasoconstriction
leads to higher
blood pressure.
Posterior
pituitary
Generally, a part of
the neuroendocrine
system receives
sensory information
and issues a
command in the form
of a chemical
messenger (hormone).
ADH
2
The hormone is
transported to target
cells via the
bloodstream.
3
The hormone
reaches the target
cells and binds to the
cell receptors.
4
The hormonerecept or complex
triggers changes in the
target cells.
HOW HORMONES TARGET CELLS
• The steroid hormones are recognized by
protein receptors located in the cytoplasm
or nucleus of the target cell.
• Steroids are manufactured from cholesterol.
• Steroid hormones can pass across the lipid bilayer
of the cell plasma membrane.
HOW HORMONES TARGET CELLS
• The complex of a steroid hormone and its
receptor inside the target cell bind to DNA
in the nucleus.
• This activates the transcription of a specific gene
and a protein is subsequently synthesized.
HOW STEROID HORMONES WORK
Tissue fluid
Blood plasma
Target cell
E
Steroid
hormone
Plasma membrane
1
Cytoplasm
E
Transport
protein
1
Estrogen (E) is a lipid soluble steroid
hormone and thus readily passes through the
plasma membrane of cells lining the uterus.
E
2
Inside the cell, estrogen binds to a specific receptor
protein associated with the DNA in the nucleus.
4
Receptor
protein
3
The estrogen-receptor complex activates the
transcription of genes.
Steroid hormonereceptor complex
Protein
synthesis
4
2
Protein synthesis is induced. In this case,
the protein produced is a receptor for
another steroid protein, progesterone.
5
DNA
Nucleus
5
3
mRNA
Progesterone
receptor
Later, when progesterone enters the cell, it binds to
the receptor and stimulates the cell to produce enzymes
that help prepare the uterus to nourish an embryo in the
event of a pregnancy.
HOW HORMONES TARGET CELLS
• The receptors for peptide hormones are
embedded in the plasma membrane.
• The binding of the hormone to the receptor
triggers changes in the cytoplasmic end of the
receptor protein.
• Using second messengers, this change is
amplified and causes changes in the cell
• Second messengers activate enzymes.
• one of the most common is cyclic AMP
(cAMP).
HOW PEPTIDE HORMONES WORK
1
The peptide hormone
binds with its membrane
receptor.
2
The hormone-receptor
combination triggers a
series of biochemical
reactions that produces
the second messenger.
Peptide
hormone
3
The second messenger
triggers a series of
reactions that leads to
altered cell functions.
P
P
1
Receptor
2
Production
of second
messenger
3
Alteration of
cell activity
HOW HORMONES TARGET CELLS
• A single hormone binding to a receptor in
the plasma membrane can result in the
formation of many second messengers in
the cytoplasm.
• Cyclic AMP is made from ATP by an enzyme that
removes two phosphate units.
• Each second messenger can activate many
molecules of a certain enzyme.
KEY BIOLOGICAL PROCESS: SECOND
MESSENGERS
Hormone
(first messenger)
1
After a peptide hormone binds to its receptor, the
hormone-receptor complex activates adenylyl cyclase.
Receptor
2
1
Adenylyl cyclase converts ATP into cyclic AMP
(cAMP), and cAMP acts as a second messenger
that activates enzymes called protein kinases.
3
Adenylyl
cyclase
ATP
Protein kinase
(inactive)
cAMP
2
(Second
messenger)
Protein kinase
(active)
3
Protein kinases catalyze a wide variety of
actions, depending on the nature of the first
messenger. Because of the presence of a
second messenger, the effect on the cell is
greatly amplified.
Altered cell function(regulates enzymes,
synthesizes proteins, secretes molecules)
THE HYPOTHALAMUS AND THE
PITUITARY
• The pituitary gland is located beneath the
hypothalamus and is the location where
nine hormones are produced.
• These hormones act principally to influence other
endocrine glands.
• The pituitary consists of two lobes:
• Posterior pituitary regulates water
conservation and, in women, milk
letdown and uterine contraction.
• Anterior pituitary regulates other
endocrine glands.
THE HYPOTHALAMUS AND THE
PITUITARY
• The hypothalamus and the posterior pituitary
are connected by a tract of neurons.
• Hormones are produced by cell bodies in the
hypothalamus and transported to the posterior
pituitary.
• Antidiuretic hormone (ADH) regulates the
kidney’s retention of water.
• Oxytocin initiates uterine contractions during
childbirth and milk release in mothers.
THE HYPOTHALAMUS AND THE
PITUITARY
• The anterior pituitary is a complete gland
that produces the hormones that it secretes.
• Thyroid-stimulating hormone (TSH) stimulates the
thyroid gland to produce thyroxine, which in turn
stimulates oxidative respiration.
• Adrenocorticotropic hormone (ACTH) stimulates
the adrenal gland to produce hormones.
• Growth hormone (GH) simulates the growth of
muscle and bone throughout the body.
THE HYPOTHALAMUS AND THE
PITUITARY
• Follicle-stimulating hormone (FSH)
• In females, it triggers the maturation of egg cells
and stimulates the release of estrogen.
• In males, it regulates sperm development.
• Luteinizing hormone (LH)
• In females, it triggers ovulation of a mature egg.
• In males, it stimulates the gonads to produce
testosterone.
THE HYPOTHALAMUS AND THE
PITUITARY
• Prolactin (PRL) stimulates the breasts to
produce milk.
• Melanocyte-stimulating hormone (MSH)
stimulates, in reptiles and amphibians, color
changes in the epidermis.
• Its function in humans is poorly understood.
FIGURE 30.4 THE ROLE OF THE PITUITARY
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Hypothalamus
Thyroid-stimulating hormone
(TSH)
Thyroid gland
Anterior
pituitary
Posterior
pituitary
Kidney
tubules
Muscles of
mammary
glands and
uterus
Adrenal
cortex
Gonadotropic hormones:
Follicle-stimulating
hormone (FSH) and
luteinizing hormone (LH)
Bone
and muscle
Ovary
Testis
Mammary glands
in mammals
THE HYPOTHALAMUS AND THE
PITUITARY
Cell body
• The hypothalamus
controls production
and secretion of the
anterior pituitary
hormones by means
Portal venules
of a family of special
hormones.
• Neurons in the
hypothalamus secrete
both releasing and
inhibiting hormones. Hypophyseal
portal system
Axons to
primary
capillaries
Hormones
Primary
capillaries
Pituitary stalk
Posterior pituitary
Anterior
pituitary
THE HYPOTHALAMUS AND THE
PITUITARY
• Negative feedback
(feedback inhibition) often
controls the release of
hormones from the
hypothalamus and anterior
pituitary.
• When enough of the target
hormone has been
produced, the hormone
then feeds back to the
hypothalamus and anterior
pituitary and inhibits the
release of stimulating
hormones.
Inhibition–
Hypothalamus
Releasing hormones
(TRH, CRH, GnRH)
Anterior pituitary
Inhibition–
Tropic hormones
(TSH, ACTH, FSH, LH)
Target glands
(Thyroid, adrenal cortex, gonads)
Hormones
THE PANCREAS
• The pancreas has both exocrine
and endocrine functions.
• It secretes digestive enzymes and
hormones.
• The hormones, produced in the
islets of Langerhans, are insulin
and glucagon.
• Insulin promotes the uptake of
glucose and the
accumulation of glycogen in
the liver and triglycerides in fat
cells.
• Glucagon causes liver cells to
release stored glucose and to
break down triglycerides.
After a meal
Between meals
Blood glucose
Blood glucose
Pancreas
Pancreatic islets
Pancreatic islets
Insulin secretion
Insulin secretion
Glucagon secretion
Glucagon secretion
Cellular uptake
of glucose
Release of stored glucose,
break down of fat
THE PANCREAS
• Diabetes mellitus is a serious disorder in
which affected individuals are unable to
take up glucose from the blood.
• There are two kind of diabetes mellitus:
• Type I is a hereditary autoimmune disease in
which the islets of Langerhans are attacked,
resulting in abnormally low insulin secretion.
• Type II is when cells don’t respond to insulin,
sometimes due to a reduction in the number of
insulin receptors in the target tissue.
THE THYROID, PARATHYROID,
AND ADRENAL GLANDS
• The thyroid gland makes several hormones.
• Thyroxine increases metabolic rate and promotes
growth.
• Calcitonin inhibits the release of calcium from
bones and promotes the uptake of calcium by
bones.
THE THYROID, PARATHYROID,
AND ADRENAL GLANDS
• The parathyroid glands are four small glands
attached to the thyroid.
• These glands produce parathyroid hormone
(PTH), a hormone that is absolutely essential for
survival because it regulates calcium levels in the
blood.
• Calcium ions are necessary for muscle
contractions, such as those of the heart.
• PTH stimulates the release of calcium from
bone.
• Calcitonin (released from the thyroid gland) has
the opposite effect.
MAINTENANCE OF PROPER CALCIUM
LEVELS IN THE BLOOD
Ca++
Ca++
Ca++
LOW CALCIUM LEVEL
STIMULATES PTH
SECRETION
HIGH CALCIUM LEVEL
STIMULATES CALCITONIN
SECRETION
Inactive
osteoblast
Ca++
Ca++
Ca++
Ca++
Ca++
Ca++
PTH stimulates
osteoclast
(breaking down
bone matrix)
Bone
matrix
Osteocyte
(in lacuna)
(a)
Calcitonin
stimulates
active
osteoblast
(increasing
bone
matrix)
(b)
Bone
matrix
THE THYROID, PARATHYROID,
AND ADRENAL GLANDS
• The adrenal glands are located just above
the kidney and each is comprised of two
parts.
• The medulla is the inner core and produces
epinephrine and norepinephrine.
• The cortex is the outer region and produces the
steroid hormones cortisol and aldosterone.
THE THYROID, PARATHYROID,
AND ADRENAL GLANDS
• The adrenal medulla releases epinephrine
(adrenaline) and norepinephrine in times of
stress.
• These hormones act as emergency signals that
stimulate rapid deployment of body fuel.
• The adrenal cortex produces
• Cortisol, which acts to maintain nutritional wellbeing.
• It is also released in times of stress but can become
a chronic problem if stress continues.
• Aldosterone, which affects water reabsorption in the
kidney and affects both blood volume and blood
pressure.