Chemical Coordination
Chapter 34
Hormones
 A hormone is a chemical signal that is secreted into
the circulatory system and communicates regulatory
messages within the body.
 Hormones may reach all parts of the body, but only
certain types of cells, target cells, are equipped to
respond.
Systems of Internal Communication
 Animals have two systems of internal communication
and regulation:
 The nervous system
 The endocrine system
Systems of Internal Communication
 The nervous system conveys high-speed electrical
signals along specialized cells called neurons.
 The endocrine system, made up of endocrine
glands, secretes hormones that coordinate slower but
longer-acting responses to stimuli.
Hormones
 Advantages of using chemical messengers:
 Chemical molecules can spread to all tissues through the
blood.
 Chemical signals can persist longer than electrical ones.
 Many different kinds of chemicals can act as hormones;
different hormones can target different tissues.
Glands
 Many hormones are secreted by ductless endocrine
glands.
 Obtain raw materials from and secrete hormones directly
into the bloodstream.
 Exocrine glands have ducts for discharging secretions
onto a free surface.
 Sweat glands, salivary glands, enzyme-secreting glands
in the digestive tract.
Hormones
 Hormones convey information via the bloodstream to
target cells throughout the body.
 Pheromones carry messages outside the body – to
other individuals.
Hormones
 Three major classes of molecules function as
hormones in vertebrates:
 Proteins and peptides
 Amines derived from amino acids
 Steroids
Hormones
 Signaling by any of these molecules involves three key
events:
 Reception
 Signal transduction
 Response
Hormones
 The hypothalamus regulates the neuroendocrine
system, maintaining homeostasis in the body.
 The hypothalamus can use motor nerves to send shortlived electrical messages or hormones to send chemical
messages with a longer duration.
The Chain of Command
 The hypothalamus produces seven different
“releasing” hormones that travel to the
pituitary gland.
 Each releasing hormone stimulates the pituitary
to release a corresponding hormone which
travels to an endocrine gland and causes it to
start producing a particular endocrine hormone.
Membrane-Bound Receptors
 Many hormones are too
large, or too polar, to
pass through plasma
membranes.
 Bind to transmembrane
proteins that act as
receptor sites on target
cell membranes.
 Hormone is first
messenger.
 Causes activation of a
second messenger in
the cytoplasm.
 cAMP
Nuclear Receptors
 Steroid hormones are
lipid soluble molecules
that bind to hormone
receptors in the cytoplasm
of the target cell.
 Site of activity is the
nucleus.
 Steroids are manufactured
from cholesterol.
 Estrogen, progesterone,
testosterone, cortisol.
Nuclear Receptors
 Thyroid hormones and
insect-molting
hormone (ecdysone)
also act through
nuclear receptors.
 Binds to
transmembrane
protein that uses ATP
to move it into the cell.
Control Pathways and Feedback
Loops
 A common feature
of control pathways
is a feedback loop
connecting the
response to the
initial stimulus.
 Negative feedback
regulates many
hormonal pathways
involved in
homeostasis.
Invertebrate Hormones
 Ecdysone
regulates
molting in
insects.
 Juvenile
hormone
favors the
retention of
juvenile
characteristics.
The Pituitary
 The pituitary gland is
located below the
hypothalamus.
 Nine major hormones
are produced here.
 These hormones act
primarily to influence
other endocrine
glands.
The Pituitary
 The posterior lobe of the pituitary regulates water
conservation, milk letdown, and uterine contraction in
women.
 The anterior lobe regulates the other endocrine
glands.
The Anterior Pituitary
 Thyroid stimulating hormone (TSH) – stimulates the
thyroid gland to produce thyroxine which stimulates
oxidative respiration.
 Luteinizing hormone (LH) plays an important role in
the menstrual cycle. It also stimulates the production of
testosterone in males.
The Anterior Pituitary
 Follicle-stimulating hormone (FSH) – plays
an important role in the menstrual cycle. In
males, it causes the testes to produce a
hormone that regulates sperm production.
 Adrenocorticotropic hormone (ACTH) –
stimulates the adrenal gland to produce steroid
hormones. Some regulate glucose production,
others balance sodium & potassium in the
blood.
The Anterior Pituitary
 Growth hormone (GH) – stimulates the growth of
muscle and bone.
 Prolactin – stimulates milk production.
 Melanocyte-stimulating hormone (MSH) – in reptiles
& amphibians, this hormone stimulates color change.
The Posterior Pituitary
 Antidiuretic hormone
(ADH) regulates the
kidney’s retention of water.
 Oxytocin initiates uterine
contraction during
childbirth and milk release
in mothers.
 These hormones are
actually synthesized in the
hypothalamus and stored
in the posterior pituitary.
Biological Clocks
 The pineal gland is
located in the brain of
most vertebrates.
 Evolved from a light
sensitive “third eye”.
 Primitive fish & some
reptiles still have a third
eye.
Biological Clocks
 In other vertebrates it functions as an
endocrine gland secreting melatonin.
 Melatonin controls color change in amphibians
& reptiles.
 Release of melatonin is controlled by light/dark
cycles.
 The primary functions of melatonin appear to
be related to biological rhythms associated with
reproduction.
 Circadian rhythms – 24 hours long.
The Thyroid
 The thyroid gland, located
in the neck, produces:
 Thyroxine – increases
metabolic rate and promotes
growth.
 Two iodine-containing
hormones, triiodothyronine
(T3) and thyroxine (T4).
 Calcitonin – stimulates
calcium uptake by bones.
The Thyroid
 The hypothalamus and
anterior pituitary
control the secretion of
thyroid hormones
through two negative
feedback loops.
The Thyroid
 The thyroid
hormones play
crucial roles in
stimulating
metabolism and
influencing
development
and maturation.
The Parathyroids
 The parathyroid glands are
four small glands attached to
the thyroid.
 The hormone they produce
is parathyroid hormone
(PTH) which regulates the
level of calcium in the blood.
 Essential that calcium is
kept within narrow limits
for muscle contraction,
including the heart.
Calcium Homeostasis
 Two antagonistic
hormones,
parathyroid
hormone (PTH) and
calcitonin, play the
major role in calcium
(Ca2+) homeostasis in
mammals.
Calcium Homeostasis
 Calcitonin, secreted by the thyroid gland,
stimulates Ca2+ deposition in the bones and
secretion by the kidneys, thus lowering blood
Ca2+ levels.
 PTH, secreted by the parathyroid glands, has
the opposite effects on the bones and kidneys,
and raises Ca2+ levels.
 Also has an indirect effect, stimulating the kidneys to
activate vitamin D, which promotes intestinal uptake
of Ca2+ from food.
The Adrenals
 Mammals have an adrenal gland above each kidney.
 Adrenal medulla is the inner core which produces
adrenaline (epinephrine) and norepinephrine.
 Adrenal cortex is the outer shell that produces the
steroid hormones cortisol and aldosterone.
Adrenal Medulla
 The adrenal medulla releases adrenalin (epinephrine)
and norepinephrine in times of stress.
 Identical to the effects of the sympathetic nervous
system, but longer lasting.
 Accelerated heartbeat, increased blood pressure, higher
levels of blood sugar and increased blood flow to heart and
lungs.
Adrenal Cortex
 The adrenal cortex produces the steroid hormone
cortisol (hydrocortisone).
 Reduces inflammation.
 Synthetic derivatives such as prednisone are used as antiinflammatory agents.
 Stimulates carbohydrate metabolism.
Adrenal Cortex
 The adrenal cortex also produces aldosterone.
 Aldosterone acts in the kidney to promote the uptake of
sodium & other salts from the urine.
 These salts are important in nerve conduction.
 Aldosterone and PTH are the only two hormones
essential for survival.
The Pancreas
 The pancreas is located
behind the stomach and is
connected to the small
intestine by a small tube.
 It secretes digestive
enzymes into the digestive
tract (exocrine function).
 Endocrine function –
production of insulin and
glucagon.
Glucose Homeostasis
 The islets of
Langerhans in the
pancreas secrete
insulin and glucagon.
 Insulin removes
glucose from the
blood.
 Glucagon returns
glucose to the blood.
Diabetes
 Diabetes mellitus, perhaps the best-known endocrine
disorder, is caused by a deficiency of insulin or a
decreased response to insulin in target tissues.
 Marked by elevated blood glucose levels.
Diabetes
 Type I diabetes mellitus (insulin-dependent
diabetes) is an autoimmune disorder in which
the immune system destroys the beta cells of
the pancreas.
 Type II diabetes mellitus (non-insulindependent diabetes) is characterized either by
a deficiency of insulin or, more commonly, by
reduced responsiveness of target cells due to
some change in insulin receptors.