Biology 102
Fall 2001
R. Brundage
Lecture 2: Part III
Endocrine Control
I.The Endocrine System
A.Hormones and Other Signaling Molecules
1.Signaling molecules are hormones and secretions that can bind to target
cells and elicit in them a response.
2.There are four main types of signaling molecules:
a.Hormones are secreted from endocrine sources and some
neurons, and are then transported by the blood to remote targets.
b.Neurotransmitters are secreted from neurons and act on
immediately adjacent target cells for a short time.
c.Local signaling molecules are secreted from cells of many
different tissues; they act locally and are swiftly degraded.
d.Pheromones, which are secreted by exocrine glands, have
targets outside the body; they integrate social activities between
animals.
B.Discovery of Hormones and Their Sources
1.In the early 1900s, Bayliss and Starling first demonstrated that a hormone
(later named secretin) released into the blood triggers secretion of
pancreatic juices.
2.Starling coined the word hormone for internal secretions released into
the bloodstream that influence the activities of other tissues and organs.
3.The sources of hormones may be collectively called the "endocrine
system," which shows intimate connections with the nervous system.
II.Signaling Mechanisms
A.The Nature of Hormonal Action
1.Different hormones activate different cellular response mechanisms.
2.Not all cells have receptors for all hormones; the cells that respond are
selected by means of the type of receptor they possess.
B.Characteristics of Steroid Hormones
1.Steroid hormones, assembled from cholesterol, are lipid-soluble and
therefore cross plasma membranes readily.
2.Steroids stimulate or inhibit protein (especially enzyme) synthesis by
switching certain genes on or off.
a.They easily diffuse through the lipid bilayer of the plasma
membrane, bind to chromosomal proteins in the nucleus, and
then activate transcription.
b.Testosterone is the male hormone with receptors throughout the
body; however, in testicular feminization syndrome, none of the
target cells respond correctly, so the XY individual develops
female characteristics.
C.Characteristics of Peptide Hormones
1.Peptide hormones include peptides, polypeptides, and glycoproteins.
2.All peptide hormones issue signals at a receptor located on the target cell’s
membrane.
a.First, the receptor binds to the peptide hormone because it cannot
pass into the cell as steroid hormones do.
b.The hormone-receptor complex stimulates the production of
cyclic AMP, a "second messenger" which amplifies the signal by
activating numerous enzymes.
III.The Hypothalamus and Pituitary Gland
A.The hypothalamus and pituitary gland work jointly as the neural-endocrine control
center.
1.The hypothalamus is a portion of the brain that monitors internal
conditions and emotional states.
2.The pituitary is a pea-sized gland connected to the hypothalamus by a
stalk.
a.The posterior lobe of the pituitary consists of nervous tissue and
releases two neurohormones made in the hypothalamus.
b.The anterior lobe consists of glandular tissue and secretes six
hormones and controls the release of others.
c.An intermediate lobe (not in humans) produces secretions that
induce color changes in fur color of vertebrates.
B.Posterior Lobe Secretions
1.The axons of neuron cell bodies in the hypothalamus extend down into
the posterior lobe of the pituitary.
2.Two hormones are released into the capillary bed:
a.Antidiuretic hormone (ADH) acts on the walls of kidney tubules
to control the body’s water and solute levels.
b.Oxytocin triggers uterine muscle contractions to expel the fetus
and acts on mammary glands to release milk.
C.Anterior Lobe Secretions
1.The anterior lobe releases six hormones that stimulate ("tropic") other
endocrine glands:
a.Corticotropin (ACTH) stimulates the adrenal cortex.
b.Thyrotropin (TSH) stimulates the thyroid gland.
c.Follicle-stimulating hormone (FSH) stimulates egg formation in
females and sperm formation in males.
d.Luteinizing hormone (LH) also acts on the ovary to release an
egg and on the testes to release sperm.
e.Prolactin acts on the mammary glands to sustain milk
production.
f.Somatotropin (STH), or growth hormone (GH), acts on body
cells in general to promote growth.
2.The hypothalamus produces releasing and inhibiting hormones that target
the anterior pituitary.
IV.Examples of Abnormal Pituitary Output
A.The body does not produce large quantities of each hormone.
B.But experience has shown that the amounts, no matter how tiny, are critical to
normal body functioning.
1.In childhood, too little STH can cause pituitary dwarfism, while too much
causes gigantism.
2.Oversecretion of STH in adulthood causes a thickening of skin and bones
called acromegaly.
V.Sources and Effects of Other Hormones
A.When considering the actions of hormones, three points are relevant:
1.Hormones interact to oppose (antagonistic), add to (synergistic), or prime
(permissive interaction) target cells for another hormone’s effects.
2.Many hormones are linked to the neural-endocrine control center by
negative feedback mechanisms.
3.The response of target cells depends on the number of their receptors and
the concentration of the hormone.
4.Environmental cues may be important mediators of hormonal action.
B.[Consult Table 37.3 for hormone source, secretion(s), main targets, and primary
actions.]
VI.Feedback Control of Hormonal Secretions
A.A shift in the amount of hormone in the blood causes a feedback mechanism to
operate.
1.With negative feedback, an increase or decrease in the concentration of a
hormone triggers events that inhibit further secretion.
2.With positive feedback, an increase in the concentration of hormone
triggers events that stimulate further secretion.
B.Negative Feedback From the Adrenal Cortex
1.One adrenal gland is located on top of each kidney.
2.Among the secretions of the outer portion are the glucocorticoids such as
cortisol, which helps control blood glucose levels.
a.Cortisol secretion is an example of a negative feedback loop.
b.When blood levels of glucose fall (as in hypoglycemia), the
hypothalamus releases CRH ——> anterior pituitary ——> ACTH
——> adrenal cortex ——> cortisol, which prevents muscle cells
from withdrawing glucose from the blood.
c.When the body is stressed, as in painful injury, the nervous
system provides an override mechanism in which the levels of
cortisol remain high to promote healing.
C.Local Feedback in the Adrenal Medulla
1.The inner medulla portion secretes epinephrine and norepinephrine
under direction from sympathetic nerves from the hypothalamus.
2.Its secretions mobilize the body during times of excitement or stress
("fight-or-flight" response).
D.Skewed Feedback From the Thyroid
1.The human thyroid gland lies at the base of the neck in front of the
trachea.
2.Its hormones, thyroxine and triiodothyronine, influence metabolic
rates, growth, and development.
a.These two hormones contain critical amounts of iodine.
b.If the blood levels of iodine are too low, the pituitary responds
with too much TSH causing the thyroid gland to enlarge
abnormally in what we call a goiter.
3.Hypothyroidism in adults results in lethargy and weight gain.
4.Hyperthyroidism increases heart rate and blood pressure and causes
weight loss.
E.Feedback Control of the Gonads
1.The ovaries and testes—gonads—produce not only gametes but also
hormones governing reproduction via feedback loops.
2.Sex hormones also influence secondary sexual traits.
VII.Responses to Local Chemical Changes
A.Secretions From Parathyroid Glands
1.Four parathyroid glands are embedded in the thyroid gland and respond
to the changing levels of calcium in the blood.
a.A drop in calcium level causes parathyroid hormone (PTH)
levels to rise, resulting in removal of calcium from bone and an
activation of vitamin D (to help in calcium absorption from the
gut).
b.When calcium levels rise, the PTH levels are reduced.
2.Calcitonin from the thyroid gland acts antagonistically to PTH and
promotes deposition of calcium in bones [this information is from Table
37.3].
B.Effects of Local Signal Molecules
1.Prostaglandins are produced in tissues throughout the body; their actions
remain localized near the site of secretion.
2.Epidermal growth factor (EGF) influences the growth of many cell types.
3.Nerve growth factor (NFGF) promotes growth and survival of neurons
in the developing embryo.
C.Secretions From Pancreatic Islets
1.The pancreas is dual function gland; its exocrine function is to secrete
digestive enzymes.
2.Certain cells within the pancreas have an endocrine function:
a.Alpha cells secrete glucagon, which causes glycogen stored in
the liver to be converted to glucose, raising its levels in the
blood.
b.Beta cells secrete insulin, which stimulates the uptake of
glucose by liver, muscle, and adipose to reduce glucose levels in
the blood, especially after a meal.
c.Delta cells secrete somatostatin, which can inhibit the secretion
of glucagon and insulin.
3.Diabetes mellitus is a disease resulting from imbalances of insulin; its
effects include weight loss, ketone production, water-solute problems,
and possible death.
a.In type 1 diabetes, insulin is no longer produced because the beta
cells have been destroyed by an autoimmune response; treatment
is by insulin injection.
b.In type 2 diabetes, the insulin levels are near normal but the target
cells cannot respond to the hormone; controlling diet is an
effective treatment.
VIII.Hormonal Responses to Environmental Cues
A.Daylength and the Pineal Gland
1.The pineal gland is a photosensitive organ located in the brain.
2.In the absence of light, melatonin is secreted; thus in winter, high levels
of the hormone are instrumental in suppressing reproductive activity in
hibernating animals.
3.Decreased melatonin secretion in humans might help trigger the onset of
puberty.
B.Comparative Look at a Few Invertebrates
1.Molting in arthropods is controlled by hormones.
2.Ecdysone promotes molting in insects; juvenile hormone retards the same
process.