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Lecture 20
Coordination and Control
In Animals - 2
Coordination and Control Systems
• The nervous system coordinates rapid and precise
responses to stimuli using action potentials.
• The endocrine system maintains homeostasis and
long-term control using chemical signals.
• The endocrine system works in parallel with the
nervous system to control growth and maturation
along with homeostasis.
The Endocrine System
• Is a collection of glands that secrete chemical
messages we call hormones.
• These signals are passed through the blood to
arrive at a target organ, which has cells possessing
the appropriate receptor.
• Exocrine glands (not part of the endocrine system)
secrete products that are passed outside the body.
Sweat glands, salivary glands, and digestive
glands are examples of exocrine glands.
Function of Endocrine System
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controls numerous body processes
growth and development
metabolism
reproduction
homeostasis
Structure of The Endocrine System
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2.
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6.
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Comprises a number of separate organs and tissues
scattered throughout the body.
These include:
The pituitary (hypophysis) attached to the underside of the
brain.
The thyroid, situated in the neck close to the larynx.
The parathyroids, associated with the thyroid but
comprising separate structures.
The adrenals, situated on top of each kidney.
The endocrine portion of the pancreas.
The ovaries or testes (The endocrine role of the ovaries and
testes will be dealt with later in the course, in association
with the reproductive system.
Produce hormones.
Hormones
• PROTEINS (eg. insulin).
• PEPTIDE (at least 3 amino acids eg. oxytocin).
most abundant, cell surface receptors.
• AMINO ACID DERIVATIVES (eg. thyroxine).
• AMINE (eg adrenaline) cell surface or intracellular
receptors modified amino acid residues.
• STEROID (eg. cortisol) intracellular receptors.
Steroids
• Are lipids derived from
cholesterol.
• Testosterone is the male sex
hormone.
• Estradiol, similar in structure
to testosterone, is responsible
for many female sex
characteristics.
• Specific protein carrier
molecules.
• Slow acting - genomic
actions.
Glucocorticoids
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•
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The glucocorticoids get their name from their effect of
raising the level of blood sugar (glucose). One way they do
this is by stimulating gluconeogenesis in the liver: the
conversion of fat and protein into intermediate metabolites
that are ultimately converted into glucose.
The most abundant glucocorticoid is cortisol (also called
hydrocortisone).
Cortisol and the other glucocorticoids also have a potent antiinflammatory effect on the body. They depress the immune
response, especially cell-mediated immune responses.
Widely used in therapy:
1. to reduce the inflammatory destruction of rheumatoid
arthritis and other autoimmune diseases.
2. to prevent the rejection of transplanted organs.
3. to control asthma.
Mineralocorticoids
• The mineralocorticoids get their name from their effect on
mineral metabolism.
• The most important of them is the steroid aldosterone.
• Aldosterone acts on the kidney promoting the reabsorption
of sodium ions (Na+) into the blood. Water follows the salt
and this helps maintain normal blood pressure.
• Aldosterone also :
- acts on sweat glands to reduce the loss of sodium in
perspiration.
- acts on taste cells to increase the sensitivity of the taste
buds to sources of sodium.
• The secretion of aldosterone is stimulated by:
- a drop in the level of sodium ions in the blood.
• - a rise in the level of potassium ions in the blood.
Androgens
• The adrenal cortex secretes precursors to
androgens such as testosterone.
• In sexually-mature males, this source is so much
lower than that of the testes that it is probably of
little physiological significance. However,
excessive production of adrenal androgens can
cause premature puberty in young boys.
• In females, the adrenal cortex is a major source of
androgens. Their hypersecretion may cause some
masculinization in adult females, producing a
masculine pattern of body hair and cessation of
menstruation.
Synthesis of Sex Hormones
Peptide Hormones
• Water soluble,
• Stored in vesicles,
• Fast acting,
• Short half-life
• They are secreted by the pituitary,
parathyroid, heart, stomach, liver, and
kidneys
Amine Hormones
• Derivatives of single amino acid residues, primarily
TYROSINE
• Secreted by thyroid and adrenal medulla
• Can behave either like peptides or steroids
• Key characteristics:
- Carrier molecules required
- Surface and intracellular receptors
- Fast and slow acting
Blood Levels of Hormones
• The concentration of a hormone in the blood
stream will depend on rate of secretion vs rate of
removal.
• The rate of secretion, which may be divided into:
I) the rate of synthesis, dependent on chemical
precursors from diet or from other cells in the
body. Some hormones are stored, others undergo
continuous turnover.
II) the rate of release of preformed hormone into
the blood, dependent on the stimulus for release.
Blood Levels of Hormones
• The rate of removal, which may be divided into:
I) the rate of inactivation, dependent on the
rate at which the hormone is used up at its site of
action or rendered inactive during passage through
the liver or kidney.
II) the rate of excretion, dependent upon renal
handling of either the inactivated hormone or its
metabolic breakdown products (often used as a
basis for "drug" testing).
Mechanism of Action of Hormones
• Proteins, peptides and amines are not lipid soluble, so they
cannot cross the plasma membranes of cells.
• Hormones classified as these types of compound have their
receptor on the outside of the cell membrane.
• Combination of the hormone with its membrane receptor
causes the production within the cell of a second messenger
compound which then exerts the hormone's actions within the
cell.
• The second messenger activates, accelerates or inhibits preprogrammed activities within the cell. These activities usually
involve changing enzyme action, altering protein synthesis
(through messenger RNA) or opening or closing membrane
channels.
E.g cyclic adenosine monophosphate (cAMP), cyclic
guanosine monophosphate (cGMP), inositol triphosphate
(IP3) and calcium ions.
.
Action of Secondary Messengers
Mechanism of Hormone Action
• The second mechanism involves steroid hormones,
which pass through the plasma membrane and act in a
two step process.
• Steroid hormones bind, once inside the cell, to the
nuclear membrane receptors, producing an activated
hormone-receptor complex.
• The activated hormone-receptor complex binds to
DNA and activates specific genes, increasing
production of proteins.
Action of Steroids
Control of Hormone Release
•
There are three ways in which the release of a hormone
from an endocrine gland may be controlled.
1.
2.
3.
Substrate control. This sets up a simple control loop in which
the substrate is controlling release of the hormone, which by its
action(s) is altering the level of the substrate. E.g calcitonin
(substrate is calcium), aldosterone (substrate is potassium) and
insulin (substrate is glucose).
Nervous control. Some hormones are released directly in
response to nervous stimuli. Endocrine glands which are most
influenced by nerve inputs include the pituitary, where the
nervous control arises in the hypothalamus, and the adrenal
medulla, where the nervous control arises from the sympathetic
nervous system.
Trophic hormones. In some cases, the endocrine gland is itself a
target organ for another hormone. Hormones of this type are
termed trophic hormones, and they are all released from the
anterior pituitary gland (adenohypophysis). Endocrine glands
controlled principally by trophic hormones include the thyroid
gland and the adrenal cortex.
The Adrenal Glands
• Each kidney has an adrenal gland located above it.
• The adrenal gland is divided into an inner medulla and an outer cortex.
• The medulla synthesizes amine hormones, the cortex secretes steroid
hormones.
• The adrenal medulla consists of modified neurons that secrete two
hormones: epinephrine and norepinephrine.
• Stimulation of the cortex by the sympathetic nervous system causes
release of hormones into the blood to initiate the "fight or flight"
response.
• The adrenal cortex produces several steroid hormones in three classes:
mineralocorticoids, glucocorticoids, and sex hormones.
• Mineralocorticoids maintain electrolyte balance.
• Glucocorticoids produce a long-term, slow response to stress by raising
blood glucose levels through the breakdown of fats and proteins; they
also suppress the immune response and inhibit the inflammatory
response.
The Adrenal Medulla
• The adrenal medulla consists of masses of neurons. Instead of
releasing their neurotransmitters at a synapse, these neurons
release them into the blood.
• The adrenal medulla releases:
- adrenaline (also called epinephrine) and
- noradrenaline (also called norepinephrine)
• Both are derived from the amino acid tyrosine.
• Release of adrenaline and noradrenaline is triggered by nervous
stimulation in response to physical or mental stress. The
hormones bind to adrenergic receptors - transmembrane
proteins in the plasma membrane of many cell types.
Adrenaline
• Some of the effects are:
a) increase in the rate and strength of the heartbeat resulting in
increased blood pressure,
b)blood shunted from the skin and viscera to the skeletal muscles,
coronary arteries, liver, and brain, rise in blood sugar, increased
metabolic rate,
c) bronchi dilate
d) pupils dilate,
e) hair stands on end ("gooseflesh" in humans),
f) clotting time of the blood is reduced, increased ACTH secretion
from the anterior lobe of the pituitary.
• All of these effects prepare the body to take immediate and vigorous
action.
Adrenal Gland
The Thyroid Gland
• The thyroid gland is located in the neck. Follicles
in the thyroid secrete thyroglobulin, a storage form
of thyroid hormone. Thyroid stimulating hormone
(TSH) from the anterior pituitary causes
conversion of thyroglobulin into thyroid hormones
T4 and T3. Almost all body cells are targets of
thyroid hormones.
• Thyroid hormone increases the overall metabolic
rate, regulates growth and development as well as
the onset of sexual maturity. Calcitonin is also
secreted by large cells in the thyroid; it plays a
role in regulation of calcium.
The Pancreas
• The pancreas contains exocrine cells that secrete digestive
enzymes into the small intestine and clusters of endocrine
cells (the pancreatic islets).
• The islets secrete the hormones insulin and glucagon,
which regulate blood glucose levels.
• After a meal, blood glucose levels rise, prompting the
release of insulin, which causes cells to take up glucose,
and liver and skeletal muscle cells to form the
carbohydrate glycogen.
• As glucose levels in the blood fall, further insulin
production is inhibited.
• Glucagon causes the breakdown of glycogen into glucose,
which in turn is released into the blood to maintain glucose
levels within a homeostatic range. Glucagon production is
stimulated when blood glucose levels fall, and inhibited
when they rise.
Diabetes
• Results from inadequate levels of insulin.
• Type I diabetes is characterized by inadequate levels of
insulin secretion.
• Type II usually develops in adults. Loss of response of
targets to insulin rather than lack of insulin causes this type
of diabetes.
• Diabetes causes impairment in the functioning of the eyes,
circulatory system, nervous system, and failure of the
kidneys.
• Diabetes is the second leading cause of blindness in the
US. Treatments involve daily injections of insulin,
monitoring of blood glucose levels and a controlled diet.
The Pituitary Gland
• Often called the master gland.
• Is located in a small bony cavity at the base of
the brain.
• A stalk links the pituitary to the hypothalamus,
which controls release of pituitary hormones.
• The pituitary gland has two lobes: the anterior
and posterior lobes. The anterior pituitary is
glandular.
Anterior Lobe
• Growth hormone is a peptide anterior pituitary hormone
essential for growth.
• Cells under the action of GH increase in size (hypertrophy)
and number (hyperplasia). GH also causes increase in bone
length and thickness by deposition of cartilage at the ends of
bones.
• Thyroid-stimulating hormone (TSH) travels to the thyroid
where it promotes production of thyroid hormones, which in
turn regulate metabolic rates and body temperatures.
• Gonadotropins and prolactin are also secreted by the anterior
pituitary. Gonadotropins (which include follicle-stimulating
hormone, FSH, and luteinizing hormone, LH) affect the
gonads by stimulating gamete formation and production of
sex hormones. Prolactin is secreted near the end of pregnancy
and prepares the breasts for milk production. .
Posterior Lobe
• The posterior lobe of the pituitary releases two hormones.
• Antidiuretic Hormone (ADH), a peptide of 9 amino
acids. It is also known as vasopressin.
• ADH acts on the collecting ducts of the kidney to facilitate
the reabsorption of water into the blood.
• This it acts to reduce the volume of urine formed (giving it
its name of antidiuretic hormone).
• Oxytocin is a peptide of 9 amino acids.
• Its principal actions are, stimulating contractions of the
uterus at the time of birth, and stimulating release of milk
when the baby begins to suckle.