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Second level
Third level
Fourth level Fifth level
Physiology DepartmentMedical School, University of Sumatera Utara
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Second level
Third level
Fourth level Fifth level
Body function controlled by endocrine and
nervous system
Both maintain the internal environment in a
state of homeostasis
It enable to alter in response to changes in
the external environment
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Second level
Third level
Fourth level Fifth level
Endocrine
Endocrine system does this by means of
glands that secrete chemicals or hormones.
Endocrine refers to a gland that releases the
chemicals directly into the celluler
compartment and thence into blood stream
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Hormones :
DefinitionClassification
By origin
By site of action
By structure
Mechanisms of action
Lipid soluble hormones (cytoplasmic receptors)
Lipid insoluble hormones (cell-surface receptors)Endocrine systems :
Control of secretion
Feedback
Long-loop, short-loop
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Hormone - a substance secreted by specialized cells,
released in to the bloodstream, causing a response in
target cells elsewhere in the body.
Response is mediated by receptors that are specific to
the hormone.
Definition
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Functions of Hormone
Controlled behaviour and sexual function
Controlled salt and water balance
Maintain blood glucose Reproductive function
Involve and influence metabolic system
Lactation
Body respons to stress
Growth function
etc
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Endocrine glands
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Classif icat ion
Hormones can be classified by several properties
1. Classi f ic at ion by s ite of act ion .
Autocr ine secretion - substance released by cell that
affects the secreting cell itself
(e.g. norepinephrine is released by a neurosecretory cell in
the adrenal medulla, and norepinephrine itself inhibitsfurther release by that cell - this is also an example of
direct negative feedback)
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Paracrine secretion - substance released by cell that affects
neighboring cells. Not released into bloodstream(e.g. histamine released at site of injury to constrict blood
vessel walls and stop bleeding)
Endocr ine secretion - substance released by cell intobloodstream that affects distant cells.
(e.g. testosterone is secreted by Leydig cells in testis, makes
hair grow on your back)
Though hormones may also have autocrine or paracrine
actions, we're mainly concerned with endocrine actions
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Exocr ine secretion - substance released by cell into a duct
that leads to epithelial surface
(onto skin or into gut). Action doesntdepend on receptors intarget tissue. (e.g. sweat, saliva)
Endocrine and exocrine secretions are glandular
secret ions; they come from specialized secretory cells thatare clumped together to form a gland. Endocrine glands are
sometimes called ductless glands.
Note that a secretion may have several si tes o f act ion
simul taneously.Example of norepinephrine above - autocrine action causes
negative feedback on secretion. Simultaneously, endocrine
action causes respiration rate to , peripheral blood vessels
to contract, etc.
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Circulating & Local Hormones
Circulating
hormones act on distant targets
travel in blood
Local hormones act on neighboring
cells or same cell
that secreted them
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2. Classi f ic at ion by orig in.
Another set of terms, related to those just discussed, is
commonly used to classify secretions, based both on
or ig in and site of action
Neurohormones - endocrine, source = nerve
Glandu lar ho rmones - endocrine, source = gland
Local Hormones - paracrine (source may not be gland)
Pheromones - exocrine
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Chemical signaling by hormones: (a) Endocrine hormones are produced by cells of
endocrine glands and transported by the blood to target cells. (b) Some hormones are
released by neurosecretory cells, which function in both the nervous and endocrine
systems. (c) A few hormones are secreted over short distances through tissue fluid.
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3. Classi f icat ion by molecular struc ture.
Amines- small molecules derived from amino acids. Thecatecholamines are epinephrine and norepinephrine,
secreted by the adrenal medulla. Lipid insoluble
Prostaglandins - fatty acids. Lipid soluble.
Steroids - cyclic hydrocarbons derived from cholesterol. Lipid
soluble
Peptides and p roteins - large, complex, structure can differamong species due to amino acid substitutions. Lipid
insoluble.
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Steroids are fat soluble, so they bind to receptors in the
cytoplasm, either in the cytosol or directly in the nucleus.
When the steroid binds to the receptor, i t dis inh ibi ts a
DNA binding si te on the receptor.
The steroid-receptor complex then moves into the nucleus
and activates or suppresses specific genes. By altering
gene expression, steroids produce long-lasting effects.
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Action of Lipid-Soluble Hormones
Hormone diffuses
through phospholipid
bilayer & into cell
Binds to receptor
turning on/off specificgenes
Gene copies are
formed & direct
synthesis of newproteins
New protein alters
cells activity
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Action of Lipid-
insoluble Hormones
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The mechanism of peptide hormones(1) Peptide hormones are produced by an endocrine gland and carried by the blood. (2)
Receptors on the plasma membrane of a target cell bind with the hormone. The hormone
itself does not enter the cell. (3) An enzyme in the plasma membrane changes ATP to cyclic
AMP (cAMP). The cAMP molecule is the secondmessenger,carrying the hormonessignal
through the cell. (4) Enzymes are activated, triggering a series of reactions. (5) These
reactions bring about the changes controlled by the hormone.
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Effects o f hormones:
mechanisms of hormone act ion on target t issue
What does a hormone do when it arrives somewhere
via the blood?
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1. First level controlling action: target/non-target.
Hormones have their effects only on target t issues. Cells in
target tissue have receptors, molecules that bind
specifically to that hormone. In non-target tissue, cells have
no receptors and are not affected by the hormone no matter
how much of it is present.
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Hormone Receptors
Hormones only affect target cellswith specificmembrane proteins called receptors
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2. Second level controlling action: receptor densi ty.
Target cells vary in the number of receptors they hold. A cell
with many receptors is more strongly affected than one withfew receptors, when exposed to the same amount of
hormone.
Receptor density for a given tissue changes through time.
Down-regulat ion is a decrease in receptor density.
Up-regulat ion is an increase in receptor density.
Down and up-regulation commonly respond to the amount of
hormone in circulation over the long-term, as a damping
mechanism. If blood concentration of cortisol (a stresshormone from the cortex of the adrenal gland) is high for a
long time, receptor densities will decrease to modulate the
response.
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Mechanisms of Activation
Hormones can alter the rateof cellular activity
They do not alter the activity of the cell
Cellular activity is controlled by: changes in PM permeability
synthesis of regulatory molecules
enzyme activity/de-activity
induction of secretion
stimulation of mitosiscell division
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Hormonal Activation
Once bound to the cell will hormone affect the
cell by one of two mechanisms
Amino Acid based hormones and a second
messenger system, the second messenger thenbrings about the desired affect
Steroids diffuse into the cell and bind to
receptors this then activates protein synthesis
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Time Scale of Activation
The level of a given hormone in the blood
reflects the balance between release and
removal
The persistence of a given hormone in the bloodis called the Half Life
Unlike the nervous system which has a rapid
response which is short lived the endocrine
system takes longer to have an effect but the
effect last longer
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Hormone Release
Negative feed back loops
Endocrine gland stimulation
Humoral - Ca+levels and PTH/calcitonin
- Glucose and Insulin Neural - Fight or Flight
Hormonal - release is triggered by other hormones
Nervous system regulation
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Feedback loops
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Feedback loops
NEGATIVE FEEDBACK
Positive Feedback:
Hormones stimulate each
other, product of target cell
increases production ofinitial hormone which
causes even more
secretion of the target cell
product
Tend to be rareandshortlived because they
can quickly get out of
control!
Negative feedback:
Short- and long-loops.
Product inhibits further
production of self and of
stimulating hormone.
Can operate on multiple
levels.
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Negative
feed
backcontrol
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