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Edapt Endocrine System Notes! ALREADY RATED A+
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Edapt Endocrine System Notes! ALREADY RATED A+
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ENDOCRINE SYSTEM NOTESENDOCRINE SYSTEM NOTES
The purpose of the endocrine system is to maintain the body’s homeostasis using hormones. Hormones are signaling
molecules. Although a wide variety of hormones function within the body, they share certain general characteristics:
Hormones have specific rates and rhythms of secretion. Three basic secretion patterns are: (1) circadian or diurnal
patterns, (2) pulsatile and cyclic patterns, and (3) patterns that depend on levels of circulating substrates (e.g., calcium,
sodium, potassium, or the hormones themselves).
Hormones operate within feedback systems, either positive or negative, to maintain an optimal internal environment.
Hormones affect only cells with specific receptors and then act on those cells to initiate specific cell functions or
activities.
When an endocrine cell receives a stimulus or command, this stimulates the endocrine cell to secrete hormones into the
blood stream. The hormones will then target and bind onto a specific receptor on a target cell. This will cause the target
cell to initiate a response as shown in the diagram below:
Signaling Hormones
It is important to understand the signaling aspect of hormones. First, there are three types of signaling hormones,
steroid, peptide and amine. The table below provides a description of the signaling hormones and their properties.
Class Description Properties Examples
Lipids derived from Lipophilic – can cross
Testosterone
Steroid cholesterol membrane
Undergoes constitutive Estrogen /
secretion Progesterone
Short polypeptide Hydrophilic – cannot cross
Insulin
Peptide chains membrane
Undergoes regulatory
Glucagon
secretion
Hydrophilic – cannot cross
Derived from aromatic membrane
Thyroxine
amino acid Undergoes regulatory
Amine secretion
Note that peptide and amine hormones are hydrophilic (water-
soluble). This means that they are easily dissolved in fluid and
do not have to bind to a protein in order to circulate.
Characteristically, they also have a short half-life of just seconds
to minutes as they are catabolized by circulating enzymes.
Insulin, for example is a peptide hormone. Shortly after its
release, it is catabolized by insulinase enzymes within 3-5
minutes.
,ENDOCRINE SYSTEM NOTES
Lipid-soluble hormones, in contrast, are transported bound to a protein. Because they are bound, they can remain in
the blood for hours to days. It is very important to note here that when a hormone is bound to a protein, it cannot exert
its effects. Only free circulating hormones can initiate responses inside of a target cell. This will be revisited as we delve
into the diseases of the endocrine system. Upon arrival
to the cell membrane, the protein-bound hormone must
disengage from the protein in order to diffuse into the
cell where its effects can be exerted.
Hormone Regulation
Two important concepts to understand before
reviewing endocrine system disorders is how hormone
release is regulated and once arriving to the target cell,
how it enters the cell to exert its effect. Let’s begin with
hormone regulation.
Hormone regulation:
1. Hormone release is regulated by chemical
factors (e.g. blood glucose), endocrine factors
(e.g. a hormone from one gland controls
another gland) and neural control.
2. Hormone release is regulated by feedback
systems. These are responsible for monitoring
and controlling the cellular environment. You
are likely familiar with the negative and positive
feedback system. The negative feedback system
will activate when there is a change in
endocrine, chemical or neural response. It will
decrease the synthesis and secretion of a
hormone. In contrast, positive feedback systems
result when the endocrine, chemical or neural
response increases the synthesis and secretion
of a hormone. This is illustrated in the diagram below.
The feedback loop processes are simple. Positive feedback results when the hormone is needed to exert its effects on
the target cell. The hormone is produced and secreted. When there is less need for the hormone, negative feedback
results that stops the production and release of the hormone. The positive and negative feedback system is illustrated in
an example below in response to a low blood glucose level.
Because glucose is a necessary source of energy for the body, it will attempt to restore a normal blood glucose level in
order to maintain homeostasis. Therefore, the endocrine system will attempt to increase blood glucose levels by
stimulating an endocrine cell. In this case, it is the alpha cell of the pancreas. The alpha cell will then secrete a hormone,
glucagon into the blood stream. Glucagon will travel through the blood stream to the liver which is the target cell. When
glucagon binds to a receptor on the target cell (liver cell), it will stimulate the liver to break down glycogen to secrete
glucose in the blood. The response of the liver cell is the creation of more glucose in the blood. As the blood glucose is
increased, a negative feedback signal is sent to the pancreas to stop the secretion of glucagon.
Cellular Communication
After a hormone arrives to the target cell, it must be able to enter the cell in order to exert its
effects. Before going any further with our discussion of hormones, it is important to consider
cellular biology in terms of cellular communication, signal transduction and membrane transport.
Under this week’s readings, there is a recommended chapter from your textbook that will
,ENDOCRINE SYSTEM NOTES
provide a review of these mechanisms. You are encouraged to review these in solidifying
understanding of cellular communication. These mechanisms are addressed briefly here.
First, the hormone must communicate with the target cell. Cells, in general, communicate
through many kinds of signal molecules and may occur in three ways:
1. Receptors are displayed on the plasma membrane of the cell. Think of the receptors as
signaling molecules that brings the hormone to where it specifically needs to be.
2. Affects receptor proteins inside the target cell where the signal molecule must enter the
cell to bind with them.
3. Forms protein channels (gap junctions) that coordinate the activities of adjacent cells (this
is nothing more than a way for cells to communicate with one another).
It is important to mention here the role of first and second messengers in cellular
communication. First, messengers are extracellular signaling messengers that bind to the
membrane receptors to either open or close specific membrane channels to regulate the
movement of ions in or out of the cell. Channels can be opened by the binding of an ion or
molecule to a specific membrane receptor that is closely associated with the channel (e.g. G
proteins located inside the cell transmits signals from outside the cell to inside the cell). Locate
the G protein in the diagram below and note its proximity to the cell membrane.
Signals are then transferred to an intracellular
messenger (second messenger) which then
triggers biochemical events within the cell. This is
where signal transduction comes into play.
Signal Transduction
Signal transduction involves communication
between the outside of the cell and the
inside of the cell. Remember that the
hormone that has traveled to the specific
target cell wants to get inside of the cell. To
help, extracellular chemical messengers
(first messengers) are available to convey
signals or instructions from outside of the
cell to the interior cell. Signal transduction
pathways help with this process by allowing
cells to respond to external signals. Signals
pass between the cells when a certain
type of molecule is produced by one cell
(the signaling cell) and received by
another cell (the target cell) by way of a
receptor protein that recognizes and
responds to that specific signaling
molecule. The diagram below provides a
general view of the signal transduction
pathway.
Water-Soluble and Lipid-Soluble
Hormones
Let’s one more time turn our attention to
water-soluble and lipid-soluble
hormones. Water soluble hormones do not
, ENDOCRINE SYSTEM NOTES
need to be bound to a protein in order to circulate to the target cell. It circulates freely and binds
to its specific receptor on the cell membrane, enters the cell and then exerts its effects. Lipid-
soluble hormones do circulate bound to a protein. When it arrives to the cell membrane, it must
disengage from the protein in order to cross the cell membrane. Note that in the diagram below,
this hormone binds to the receptor once inside the cell. In order to get there, some type of
energy is necessary to move it across.
Origination of Hormones
In this section the origination
of hormones will be
reviewed. First, there are
many hormones in the body
that perform different
functions and have different
responses. The responses
occur in the endocrine glands
located throughout the body.
The diagram below depicts
the types of endocrine glands
and their location in the
body.
Hypothalamus
The hypothalamus is
responsible for releasing
hormones to anterior and
posterior pituitary.
Hypothalamic releasing
hormones are summarized
below. It is also responsible
for producing the regulatory
hormones which will be
discussed shortly.
The hypothalamus also
produces antidiuretic
hormones (ADH) and
oxytocin. Once produced,
they are passed on to the
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