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ENDOCRINE SYSTEM: Chapter Goals
1. describe the structure of the pituitary gland and explain the functional
relationship between the pituitary and the hypothalamus.
2. list the hormones released by the posterior pituitary, state the origin of these
hormones, and explain how the hypothalamus regulates their release.
3. list the hormones of the anterior pituitary and explain how their secretion is
regulated by the hypothalamus.
4. describe the production and actions of the thyroid hormones and explain how
thyroid secretion is regulated.
5. describe the location of the parathyroid glands and explain the actions of
PTH and the regulation of its secretion.
6. describe the types and actions of corticosteroids and explain how the
secretions of the adrenal cortex are regulated.
7. describe the actions of epinephrine and norepinephrine, and explain how the
secretions of the adrenal medulla are regulated.
8. explain why the pancreas is both an exocrine and an endocrine gland and
describe the structure and functions of the pancreatic islets.
ENDOCRINE SYSTEM: Chapter Goals
9. describe the actions of insulin and glucagon, and explain the regulation of
their secretion.
10. list the hormones secreted by the gonads and placenta.
ENDOCRINE SYSTEM
• General Principles of Endocrinology
• Central Endocrine Glands
– Pineal Gland
– Hypothalamus
– Hypophysis cerebri (Pituitary)
• Peripheral Endocrine Glands
–
–
–
–
–
Thyroid
Parathyroids
Adrenal Glands
Pancreas
Gonads
Principles of Endocrinology
• Endocrine Glands - Glands of internal
secretion (products are emptied directly into
the circulation)
Hormones
• 1. Definition - Chemical messengers (produced in
one location and work somewhere else)
• 2. Types
• Peptides
• Amines
– Catecholamines (epinephrine, nor-epinephrine)
– Thyroxine
• Steroids
18-2
Hormones
• 3. Chemical Properties
• Hydrophilic and Lipophobic
– Peptides -- work via a second messenger, cAMP
Adenylate Cyclase
ATP ___________________> 2',3'-cAMP
– Catecholamines - via 2nd messengers
• Hydrophobic and Lipophilic
– Thyroxine - Activates genes
– Steroids -- activate genes
18-4
11.8
18-5
18-6
Central Endocrine Glands
• Pineal Gland
• Hypothalamus
• Hypophysis cerebri (Pituitary)
18-1
Pineal Gland
• Melatonin - regulates circadian rhythms (increases
in dark)
• May induce sleep
• May initiate puberty
• May inhibit ovulation/spermatogenesis
• May be antioxidant
• May slow aging
• May enhance immunity
18-9
Hypothalamic Hormones
• Generalities
• Regulators of Adenohypophysis
• Neurohypohysis - hormones produced in
hypothalamus
Generalities
• If purified and structure is known =
hormone
• otherwise = factor.
• Materials transported from hypothalamus to
hypophysis by hypothalamic-hypophyseal
portal system
Regulators of Adenohypophysis
• if final target is another endocrine gland,
production of hypothalamic hormone (hh) is
regulated by negative feedback, so only one
hh is required
• otherwise, 2 hh are required: one to cause
release and one to inhibit release of
adenohypophyseal hormone.
Hypophysis cerebri
• (= pituitary) - called the "master gland", because it
regulates other glands.
– Adenohypophysis (=anterior lobe)
– Neurohypophysis (= Posterior Lobe)
Neurohypophysis (=posterior
lobe)
• oxytocin - stimulates smooth muscle
contraction --> childbirth & milk release.
• Antidiuretic hormone (ADH)
(=vasopressin) --> increased permeability of
collecting tubules to water
18-10
20-32
Adenophpophysis
• Growth Hormone (GH) (= somatotropin) - regulates
growth.
• Prolactin (= Luteotropic hormone, LTH) - stimulates milk
production; no known role in males.
• Adrenocorticotropic hormone (ACTH) (= corticotropin) stimulates adrenal cortex.
• Thyroid Stimulating Hormone (TSH) (=Thyrotropin) Stimulates thyroid gland to secrete thyroxine
• Gonadotropins (Act on gonads)
– Follicle Stimulating Hormone (FSH)
– Luteinizing Hormone (LH)
18-13
18-11
18-14
18-15
Growth Hormone
• Regulates growth
• Overproduction in children ---> gigantism; in adults --->
acromegaly. Underproduction --> pituitary midget (perfect
proportions). Available commercially via recombinant
DNA technology.
18-16
Peripheral Endocrine Glands
• Thyroid Gland
• Parathyroid Glands
• Adrenal Glands
• Pancreas
19-1a
Thyroid Gland
• -->Thyroxine (tyrosine + 4 I; therefore low
iodine --> goiter) - Regulates BMR
• a. Hyperthyroidism --> high metabolic rate (extreme
--> exophthalmos). Treated by propylthiouracil,
thyroidectomy, or radioactive iodine.
• b. Hypothyroidism --> cretinism in children &
myxedema in adults
19-1b
19-3
19-4
11.25
11.27
Parathyroid Gland
• (4 next to thyroid) --> parathyroid hormone (=
parathormone).
• a. Regulates free Calcium metabolism (99% of Ca++ in bones,
0.5% bound to blood protein, 0.5% free.
• b. Controlled by Ca++, not pituitary
• c. Role of pH (e.g. low pH --> increased Ca++ absorption and
mobilization from bone, and decreased Ca++ excretion).
• d. Role of Blood Ca++
– i. Too much Ca++ --> depressed neural and muscle function
– ii. Too little Ca++ --> enhanced excitability of nerves and
muscles.
19-22
19-7
Adrenal Gland
• a. Adrenal Medulla --> epinephrine &
norepinephrine (A modified sympathetic
postganglionic neuron)
• b. Adrenal Cortex --> numerous steroids (3 major
groups)
• i. Mineralocorticoids (e.g. aldosterone - regulates Na+ & K+ in blood).
• ii. Glucocorticoids (e.g. cortisone - causes: gluconeogenesis,
mobilization of amino acids, mobilization of fats. Anti-inflammatory
agent released in response to stress)
• iii. Androgens - Normally have no effect
See Fig 16-18
Pancreas
• Structure
– i. Exocrine portion --> enzymes
– ii. Endocrine portion = Islets of Langerhans
• (a) Alpha-cells --> glucagon
• (b) Beta-cells --> insulin
• Function
Function (cont’d)
insulin
• Glucose < ------------------------------- >Glycogen
glucagon
• Too little insulin --> diabetes --> hyperglycemia (cells can't
get glucose, so use fats --> keto acids (acetone breath) -->
low blood pH --> high free Ca++ -->depressed nerve and
muscle function --> hyperglycemic shock.
• Too much insulin --> hypoglycemia. Since brain can only
use glucose for an energy source --> hypoglycemic shock
(insulin coma).
19-16
19-18
Gonads
• Testes --> testosterone (Starts in fetus -->
development of male sex organs & descent of
testes. At puberty --> secondary male sexual
characteristics.
• Ovaries --> estrogens (e.g. 17-beta-estradiol) &
progesterone --> secondary female sexual
characteristics. Also play a role in the reproductive
cycle (details on board).
Chapter Summary
Endocrine Glands and Hormones
I. Hormones are chemicals that are secreted into the blood by endocrine
glands.
A. The chemical classes of hormones include amines, polypeptides, glycoproteins
and steroids.
B. Nonpolar hormones, which can pass through the cell membrane of their target
cells, are called lipophilic hormones.
II. Precursors of active hormones may be classified as either prohormones or
prehormones.
A. Prohormones are relatively inactive precursor molecules made in the endocrine
cells.
B. Prehormones are the normal secretions of an endocrine gland that in order to be
active must be converted to other derivatives by target cells.
III. Hormones can interact in permissive, synergistic, or antagonistic ways.
IV. The effects of a hormone in the body depend on its concentration.
A. Abnormally high amounts of a hormone can result in atypical effects.
Chapter Summary
Mechanisms of Hormone Action
I. The lipophilic hormones (steroids and thyroid hormones) bind to nuclear
receptor proteins, which function as ligand-dependant transcription factors.
A. Some steroid hormones bind to cytoplasmic receptors, which then move into the
nucleus. Other steroids and thyroxine bind to receptors already in the nucleus.
B. Each receptor binds to both the hormone and to a region of DNA called a
hormone-response element.
C. Two units of the nuclear receptor are needed to bind to the hormone-response
element to activate a gene; as a result, the gene is transcribed (makes mRNA).
Chapter Summary
Mechanisms of Hormone Action
II. The polar hormones bind to receptors located on the outer surface of the cell
membrane. This activates enzymes that enlist second-messenger
molecules.
A. Many hormones activate adenylate cyclase when they bind to their receptors.
This enzyme produces cyclic AMP (cAMP), which activates protein kinase
enzymes within the cell cytoplasm.
B. Other hormones may activate phospholipase C when they bind to their receptors.
This leads to the release of inositol triphosphate (IP3), which stimulates the
endoplasmic reticulum to release Ca2+ into the cytoplasm, activating calmodulin.
C. The membrane receptors for insulin and various growth factors are tyrosine
kinase enzymes that are activated by binding to the hormone. Once activated,
the receptor kinase phosphorylates signaling molecules in the cytoplasm that can
have many effects.
Chapter Summary
Pituitary Gland
I. The pituitary gland secretes eight hormones.
A. The anterior pituitary secretes growth hormone, thyroid-stimulating hormone,
adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone,
and prolactin.
B. The posterior pituitary releases antidiuretic hormone (also called vasopressin)
and oxytocin, both of which are produced in the hypothalamus and transported to
the posterior pituitary by the hypothalamo-hypophyseal nerve tract.
II. The release of posterior pituitary hormones is controlled by neuroendocrine
reflexes.
III. Secretions of the anterior pituitary are controlled by hypothalamic hormones
that stimulate or inhibit these secretions.
A. Hypothalamic hormones include TRH, CRH, GnRH, PIH, somatostatin, and a
growth hormone-releasing-hormone.
B. These hormones are carried to the anterior pituitary by the hypothalamohypophyseal portal system.
Chapter Summary
Pituitary Gland
IV. Secretions of the anterior pituitary are also regulated by the feedback
(usually negative feedback) exerted by target gland hormones.
V. Higher brain centers, acting through the hypothalamus, can influence
pituitary secretion.
Chapter Summary
Adrenal Glands
I. The adrenal cortex secretes mineralocorticoids (mainly aldosterone),
glucocorticoids (mainly cortisol), and sex steroids (primarily weak
androgens).
A. The glucocorticoids help to regulate energy balance. They also can
inhibit inflammation and suppress immune function.
B. The pituitary-adrenal axis is stimulated by stress as a part of the general
adaptation syndrome.
II. The adrenal medulla secretes epinephrine and lesser amounts of
norepinephrine. These hormones complement the action of the sympathetic
nervous system.
Chapter Summary
Thyroid and Parathyroid Glands
I. The thyroid follicles secrete tetraiodothyronine (T4, or thyroxine) and lesser
amounts of triiodothyronine (T3).
A. These hormones are formed within the colloid of the thyroid follicles.
B. The parafollicular cells of the thyroid secrete the hormone calcitonin,
which may act to lower blood calcium levels.
II. The parathyroids are small structures embedded within the thyroid gland.
They secrete a parathyroid hormone (PTH) which promotes a rise in blood
calcium levels.
Chapter Summary
Pancreas and Other Endocrine Glands
I. Beta cells in the islets secrete insulin and alpha cells secrete glucagon.
A. Insulin lowers blood glucose and stimulates the production of glycogen,
fat, and protein.
B. Glucagon raises blood glucose by stimulating the breakdown of liver
glycogen. It also promotes lipolysis and the formation of ketone bodies.
C. The secretion of insulin is stimulated by a rise in blood glucose following
meals. The secretion of glucagon is stimulated by a fall in blood glucose
during periods of fasting.
II. The pineal gland, located on the roof of the third ventricle of the brain,
secretes melatonin.
A. Melatonin secretion is regulated by the suprachiasmatic nucleus of the
hypothalamus, which is the major center for the control of circadian
rhythms.
B. Melatonin secretion is the highest at night, and this hormone has a
sleep-promoting effect. In many species, it also has an antigonadotropic
effect and may play a role in timing the onset of puberty in humans,
Chapter Summary
Pancreas and Other Endocrine Glands
III. The thymus is the site of T cell lymphocyte production and
secretes a number of hormones that may play a role in
regulating the immune system.
IV. The gastrointestinal tract secretes a number of hormones that
help regulate digestive functions.
V. The gonads secrete sex steroid hormones.
A. Leydig cells in the interstitial tissue of the testes secrete testosterone
and other androgens.
B. Granulosa cells of the ovarian follicles secrete estrogen.
C. The corpus luteum of the ovaries secretes progesterone, as well as
estrogen.
VI. The placenta secretes estrogen, progesterone, and a variety of polypeptide
hormones that have actions similar to some anterior pituitary hormones.
Chapter Summary
Autocrine and Paracrine Regulation
I. Autocrine regulators are produced and act within the same tissue of an organ,
whereas paracrine regulators are produced within one tissue and regulate a
different tissue of the same organ. Both types are local regulators, they do
not travel in the blood..
II. Prostaglandins are special, twenty-carbon-long fatty acids produced by many
different organs. They usually have regulatory functions within the organ in
which they are produced.