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Endocrine System
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GENERAL ENDOCRINE CHARACTERISTICS
1. Endocrine vs. Nervous
a. Similarities
i.
Both are communication systems from one part of the body to another.
ii.
Both are necessary for maintaining homeostasis
iii.
Both use chemical messengers
iv.
Both must have receptors to the chemical messengers
b. Differences
i.
Nervous system uses nerves that “direct wire” one part of the body to
another
ii.
The endocrine system uses the existing vascular system to deliver
messages from one part of the body to another
iii.
The nervous system uses an electrical signal to go from one part of the
body to another.
iv.
The endocrine systems places messenger molecules into the blood
stream. (Hormones)
v.
The nervous system emits chemical messengers from the axon
terminals of neurons. (Neurotransmitters)
2. Hormone = a molecule released in one part of the body but regulates the activity of
cells in other parts of the body.
3. Target cells = cells that a particular hormone affects. Target cells must have specific
receptors that exhibit “lock and key” fit to the hormone key.
4. Two kinds of glands:
a. Exocrine = those that deliver their product via a tube or duct to the outside of
the body. (The lumen of the intestine is technically outside the body.)
b. Endocrine = those glands that secrete their products in the interstitial fluid
surrounding the secretory cells.
5. Endocrine System is made up of:
a. Endocrine glands
i.
Pituitary
ii.
Thyroid
iii.
Parathyroid
iv.
Pineal
v.
Adrenal
b. And other cells that secrete hormones that aren’t classified as endocrine glands:
i.
Hypothalamus
ii.
Thymus
iii.
Pancreas
iv.
Gonads
v.
Kidneys
vi.
Stomach
vii.
Liver
viii.
Small intestine
ix.
Skin
x.
Heart adipose tissue
xi.
Placenta
c. A person who studies the endocrine system is an endocrinologist
d. The study of the endocrine system in endocrinology
6. Hormone Receptors
a. Only target cells of a particular hormone respond to that hormone
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b. The target cells contain a receptor molecule
c. When the hormone chemically bonds with the receptor it alters the action of the
cell having the receptor
d. A target cell may have between 2000 and 100,000 receptors for a particular
hormone.
i.
The cell can alter the number of receptors available to hormone binding
and alter its sensitivity to that hormone
1. If the cell removes receptors and becomes less sensitive then
the cell has down-regulated.
2. If the cell adds additional receptors and become more sensitive
then the cell has up-regulated.
ii.
Various drugs act by:
1. Binding to a receptor without activating it, thereby, blocking the
true hormone from binding and activating the cell.
2. Binding to a receptor and activating a cellular process.
3. Binding to a receptor and inhibiting a cellular process.
4. Binding to a hormone so that it no longer fits the receptor site
iii.
Cell often up-regulate or down-regulate and change their response to
drugs, making the drugs less effective over time.
1. Drug dosage may be adjusted as time goes on
2. Some drugs require a period of abstinence periodically to allow
the cell to return to normal receptor density
3. Example: Nicotine binds to nicotinic acetylcholine receptors and
stimulates the cells. A smoker initially over stimulates cells with
nicotinic acetylcholine receptors. The cells are being stimulated
by both the normal acetylcholine present and by the nicotine.
The cells respond by removing some receptors and reducing the
stimulation back to normal function. When a smoker quits
smoking the cells have been down-regulated and respond at
reduced stimulus levels because they now have only the
available acetylcholine.
7. Types of hormones
a. Most hormones are circulating hormones.
b. Other hormones are local hormones.
i.
Local hormones include paracrine hormones
1. Produced by cells that affect nearby cells
ii.
Local hormones include autocrine hormones
1. Produced by cells and the hormone stimulates itself or only
adjacent touching cells.
8. Chemical classes of hormones
a. Lipid soluble (not soluble in water like oil and vinegar salad dressing)
i.
Must be bound with a carrier globulin protein to make them water
soluble for transport in the blood.
1. It is released from the carrier before entering the cell
ii.
Can mix freely with other lipids like the phospholipid cell membrane
iii.
Can freely pass through the cell membrane
iv.
Has receptors inside the cell
v.
Steroid hormones derived from cholesterol are lipid hormones.
1. Sex hormones
2. Corticotropic hormones
vi.
Other Lipid hormones
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1. Thyroid hormones made from the amino acid tyrosine which has
a lipid soluble benzene ring.
2. Nitric Oxide gas hormone
b. Water soluble
i.
Can not freely pass through the hydrophobic cell membrane
ii.
Has receptor on the cell membrane.
1. Transmembrane protein receptor
2. Glycoprotein receptors
iii.
May be further classified as
1. Amine hormones made from amino acids with the carboxyl
group (COO) removed.
a. Epinephrine
b. Norepinephrine Catecholamines
c. Dopamine
d. Histamine
e. Serotonin
f. Melatonin
2. Peptide Hormones
a. Hormones that are a short string of amino acids
connected by peptide bonds. (Depending on the book
either < 100 or this book < 50)
3. Protein Hormones Longer chain of peptide bonded amino acids.
a. Protein hormones with an attached carbohydrate group
are called glycoprotein hormones.
4. Eicosanoid Hormones
a. Twenty carbon chain of fatty acid produced from
arachidonic acid which is made from a membrane
phospholipid.
b. Two types of eicosanoid hormones
i. Prostaglandins – first discovered in the prostate,
later found throughout the body
ii. Leukotrienes – immune system communicators
9. Hormone Action
a. The same hormone may do different things in different places.
i.
Example: Insulin Removes glucose from the blood by:
1. Increasing facilitated transport into cells.
2. Increasing triglyceride production in adiposities (making fat)
3. Increasing glycogen production in hepatic cells (liver storage of
energy)
b. Lipid Hormone action
i.
Lipid hormones are released from their carrier
ii.
Hormone diffuses from the blood into the interstitial space
iii.
Lipid hormones diffuse through the phospholipid cell membrane to find
their receptor in the cytoplasm or inside the nucleus.
iv.
It turns on or off specific genes
c. Water soluble hormone action
i.
Hormone diffuse from the blood into the interstitial space
ii.
Hormone binds to a receptor on the cell membrane
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The hormone is called the first messenger because it activates a 2nd
messenger system.
Cyclic AMP 2nd messenger system
iv.
The first messenger activates a G protein within the phosphate heads of
the plasma membrane.
v.
A portion of the G protein (α,β,or ) travels in the membrane between
the phosphate heads and activates Adenylate cyclase
vi.
Adenylate cyclase changes ATP into cAMP
vii.
cAMP activates protein kinase
viii.
The protein kinase causes an ATP to phosphorylate a protein
1. The protein is usually an enzyme
2. Phosphorylation may activate the enzyme
3. Phosphorylation may inactive the enzyme
ix.
The phosphorylated protein alters the cellular function by turning on or
off a specific metabolic pathway in the cell
x.
Phosphordiesterase inactivates the cAMP
Calcium 2nd messenger system
xi.
The first messenger activates a G protein
xii.
A portion of the G protein opens a Ca2+ channel in the membrane
xiii.
The Ca2+ binds and activates calmodulin
xiv.
Calmodulin activates a protein kinase which phosphorylates an enzyme
xv.
The enzyme turns on or off a metabolic pathway in the cell
There are other 2nd messenger systems, but these two are principal 2nd messenger
systems.
d. Amplification of hormone effects.
i.
Hormones are difficult to detect and quantify in the blood because they
exist in minute quantities.
ii.
Their effect is amplified by the target cell after binding to the receptor
1. Example:
a. One hormone molecule binds to a receptor
b. The receptor activates 100 G proteins
c. Each G protein activates an adenylate cyclase
d. Each adenylate cyclase molecule can activate 1,000 cAMP
making 100,000 activated cAMP’s from one hormone
molecule.
e. Each cAMP can activate 1,000 phosphate kinase
molecules making one million activated phosphate kinase
molecules from a single hormone molecule.
e. Interaction between hormones
i.
A target cell’s response to a hormone depends on:
1. Concentration of the hormone
2. Number of specific receptors available to the hormone
3. Interaction by other hormones.
a. Permissive effect – When the hormone action depends on
simultaneous or recent exposure to a 2nd hormone.
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b. Synergistic effect – when the combination of two
hormones produces an effect greater than the sum of the
two hormones acting alone.
c. Antagonistic effect – when one hormone opposes the
action of another.
f. Control of hormone secretion – Hormone secretion is regulated by:
i.
Signals from the nervous system can stimulate or inhibit hormone
release
ii.
Chemical changes in the blood varies the amount of hormone released
1. Feedback systems
a. Positive feedback – when the release of a hormone
causes even more of the same hormone to be released.
Example – Oxytocin
b. Negative feedback – when the release of a hormone
causes a decrease in the production of the hormone as
the desired level is reached.
2. Chemical shifts cause by external factors such as food intake,
exercise, etc.
iii.
Other hormones can stimulate or inhibit the release of a hormone
SPECIFIC ENDOCRINE GLANDS
1. The master gland
a. Many glands secrete their hormone in response to hormones released from the
pituitary gland. For years the pituitary gland was called the “master gland”
b. The pituitary only secretes its controlling hormones when stimulated by the
hypothalamus. Therefore, the pituitary is not actually considered the master
gland by most endocrinologists.
c. The hypothalamus portion of the brain integrates neural and endocrine system
responses.
2. Hypothalamus – lower portion of the diencephalon part of the brain
a. Contains nuclei with cell bodies that are influenced by other brain signals
b. Contains sensors that sense, body temperature, blood pH, blood osmolarity, etc.
c. Communicates with the pituitary with neural signals and by hormones it
produces.
3. Pituitary – sits in the sella turcica of the sphenoid bone
a. Attached to the hypothalamus by the pituitary stalk = infundibulum
i.
The infundibulum contains the pars tuberalis – the tube portion
extending up from the anterior pituitary, and
ii.
The axons between the hypothalamus and the posterior pituitary.
iii.
The hypophyseal portal system is a specialized circulatory system
between the hypothalamus and the anterior pituitary
1. A capillary bed (primary plexus) in the hypothalamus absorbs
hormones from the hypothalamus.
2. The capillary bed combines into a vein along the infundibulum
3. The vein fans back out into a capillary bed (secondary plexus)
among the glandular cells of the anterior pituitary delivering the
hypothalamic hormones to the anterior pituitary.
b. The pituitary consists of
i.
Posterior pituitary
1. contains axon terminals whose synaptic vesicles contain the
posterior hormones
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ii.
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2. Instead of synapsing on another neuron, muscle or gland as
most axon terminals the synaptic vesicles exocytosis their
contents into posterior pituitary interstitial space to be absorbed
into capillaries and carried to all parts of the body by the blood
stream.
3. The posterior pituitary is therefore, made of neural tissue type.
4. The posterior pituitary is also called the Neurohypophysis.
5. The terminal vesicle products are actually produced in cell
bodies located in nuclei of the hypothalamus. The cellular
products are transported by axonal transport to the terminal
vesicles just as in any other neuron.
6. The posterior pituitary only secretes two hormones.
a. Oxytocin – this hormone causes uterine contraction for
giving birth (parturition), and causes the release of milk
from the breasts of lactating mothers.
b. ADH (Antidiuretic hormone) this hormone causes the
kidneys to conserve water in the body.
i. The hypothalamus contains sensors that sense the
direction of push of osmotic pressure.
1. When the blood becomes concentrated water
wants to flow by osmosis from the
hypothalamus into the blood. This is called
high blood osmotic pressure.
2. When the blood becomes dilute water wants
to flow by osmosis from the blood into the
hypothalamus. This creates and osmotic
pressure in the opposite direction and is
called low osmotic pressure of the blood.
ii. ADH is also called vasopressin. It can raise the
blood pressure by increasing the volume of the
blood. The more volume you force into a closed
system the higher the pressure in that system
The anterior Pituitary = Adenohypophysis Glandular epithelial tissue
1. Derived from the same embryonic tissue as the epithelial lining
of the roof of the mouth.
2. Secretes 7 hormones.
3. Control of secretion of anterior pituitary is through the action of
6 hypothalamic hormones.
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Hypothalamus: (Releasing Hormones)
Crazy
CRH
Corticotropin
Releasing Hormone
Sisters
Give
Somatostatin
GHIH Growth
Grief
GHRH
Hormone Inhibiting
Hormone
GnRH
Growth Hormone
Releasing Hormone
To
People
Thyrotropin
Releasing Hormone
Prolactin
Releasing Hormone
TRH
Gonadotropin
Releasing Hormone
PRH
Pituitary: (Stimulating Hormones)
Most
Good
Melanocyte
Stimulating
Hormone
human Growth
Hormone
MSH
hGH
People
Always
Love
Their
Families
Prolactin
(Dopamine)
Adrenal
Corticotropic
Hormone
Lutenizing
Hormone
Thyroid
Stimulating
Hormone
Follicle
Stimulating
Hormone
PRL
ACTH
LH
TSH
FSH
Body:
Brain & Skin
All Cells
Mammary
Adrenal Cortex
Gonads
Thyroid
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THYROID GLAND
13.In the neck
14.Two lobes
15.Connected with a strip of gland called the isthmus
16.Cell types
a. Thyroid Follicle Cells
i. Secrete T3 (Triiodothryonine) and T4 (Thyroxine)
b. Parafollicular cells (C cells)
i. Secrete calcitonin
1. Decreases level of Ca in the blood
2. Inhibits osteoclasts
3. Increases uptake of Ca and P into bone
4. Miacalcin – a calcitonin extract from salmon used to treat
osteoporosis b/c it is 10 X strength of human calcitonin.
PARATHYROID GLANDS
17. Located on the back of the thyroid gland
18. Most people have 4
19. Secrete parathyroid hormone
a. Major regulator of the levels of Ca2+ and Mg2+ in the blood
b. Increases the number and activity of osteoclasts
c. Decreases HPO42- in the blood
d. Converts Vitamin D to Calcitriol
i. Calcitriol increases rate of absorption of Ca2+ by the intestines
ADRENAL GLANDS
20. Located on top of each kidney like a little hat
21. Actually 2 separate glandular functions in each gland
a. Medulla – modified peripheral ganglion surrounded by the adrenal cortex
i. Secretes mostly epinephrine but ~ 20% Norepinephrine, also called
adrenaline and noradrenaline
ii. Also secretes a small amount of dopamine, also a catecholamine
iii. Responds to the autonomic sympathetic nervous stimulus
iv. Enhances the fight-or-flight response of the body
b. Cortex Glandular epithelial tissue
i. 3 regions
1. Zona Glomerulosa – secretes mineralocorticoids
a. Principal mineralocorticoid is aldosterone
i. Aldosterone regulates the Na+ and K+ balance
ii. Promotes excretion of H+ to reduce acidity
iii. The more salt in the blood the more water comes
in by osmosis, therefore more blood volume and
more blood pressure.
iv. If blood pressure drops:
1. Kidneys release renin
2. Renin converts angiotensinogen produced by
the liver into angiotensin I
3. ACE (angiotensin converting enzyme mostly from the lungs) converts angiotensin
I to angiotensin II.
4. Angiotensin II stimulates
a. Adrenal cortex to release Aldosterone
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b. Aldosterone increases Na+ in the blood
which increase osmotic pressure and
increases blood volume, which
increases blood pressure.
5. Angiotensin II also increases blood pressure
while waiting for more blood volume by
constricting arterioles
2. Zona Fasciculata – Secretes glucocorticoids including cortisone
a. Cortisone is a steroid that
i. inhibits inflation
ii. inhibits the immune system
iii. helps resist stress
iv. stimulates lipolysis
v. Gluconeogenesis
vi. Protein breakdown – muscle atrophy
3. Zona Reticularis – secretes weak androgens DHEA
a. Increases sex drive
b. Can be converted into estrogens
c. Males have some estrogen from the adrenal cortex
d. Females have some testosterone from the adrenal cortex
PANCREAS
22.
(More to Come)