Endocrine System
What is endocrine system
Nervous system regulates body functions by means of nerve impulses
and integration of information by the spinal cord and brain.
The other regulating system of the body is the endocrine system,
which consists of endocrine glands that secrete chemicals called
hormones.
Endocrine glands are ductless; that is, they do not have ducts to take
their secretions to specific sites.
Instead, hormones are secreted directly into capillaries and circulate in
the blood throughout the body.
Each hormone then exerts very specific effects on certain organs,
called target organs or target tissues.
In general, the endocrine system and its hormones help regulate
growth, the use of foods to produce energy, resistance to stress, the pH
of body fluids and fluid balance, and reproduction.
Location of Endocrine glands
Hypothalamus - The hypothalamus links our endocrine and nervous systems
together. The hypothalamus drives the endocrine system.
Pituitary gland - The pituitary gland receives signals from the hypothalamus.
This gland has two lobes, the posterior and anterior lobes. The posterior lobe
secretes hormones that are made by the hypothalamus. The anterior lobe
produces its own hormones, several of which act on other endocrine glands.
Thyroid gland - The thyroid gland is critical to the healthy development and
maturation of vertebrates and regulates metabolism.
Adrenal glands - The adrenal gland is made up of two glands: the cortex and
medulla. These glands produce hormones in response to stress and regulate
blood pressure, glucose metabolism, and the body's salt and water balance.
Pancreas - The pancreas is responsible for producing glucagon and insulin. Both
hormones help regulate the concentration of glucose (sugar) in the blood.
Gonads - The male reproductive gonads, or testes, and female reproductive
gonads, or ovaries, produce steroids that affect growth and development and
also regulate reproductive cycles and behaviors. The major categories of
gonadal steroids are androgens, estrogens, and progestins, all of which are
found in both males and females but at different levels.
Hormone
Hormones are small molecules or proteins that are
produced in one tissue, released into the
bloodstream, and carried to other tissues, where
they act through specific receptors to bring about
changes in cellular activities.
Hormones serve to coordinate the metabolic
activities of several tissues or organs.
Virtually every process in a complex organism is
regulated by one or more hormones: maintenance
of blood pressure, blood volume, and electrolyte
balance; embryogenesis; sexual differentiation,
development, and reproduction; hunger, eating
behavior, digestion.
Chemistry of Hormones
With respect to their chemical structure, hormones
may be classified into three groups:
• Amines
• Proteins
• Steroids.
Amines—these simple hormones are structural
variations of the amino acid tyrosine. This group
includes thyroxine from the thyroid gland and
epinephrine and norepinephrine from the adrenal
medulla.
Proteins—these hormones are chains of amino
acids. Insulin from the pancreas, growth hormone
from the anterior pituitary gland, and calcitonin
from the thyroid gland are all proteins. Short chains
of amino acids may be called peptides.
Antidiuretic hormone and oxytocin, synthesized by
the hypothalamus, are peptide hormones.
Chemistry of Hormones
Protein Hormones are water soluble. So They can
easily enter into the circulatory system easily, where
they are carried to their target tissue.
Steroids—cholesterol is the precursor for the
steroid hormones, which include cortisol and
aldosterone from the adrenal cortex, estrogen and
progesterone from the ovaries, and testosterone
from the testes.
Protein hormone synthesis
Steroid hormone synthesis
Steroid hormones are very similar to cholesterol or
synthesized from cholesterol.
They are lipid soluble and consist of three
cyclohexyl rings and one cyclopentyl ring combined
a single structure.
There is little storage of steroid hormones.
Cholesterol esters in cytoplasm vacuoles rapidly
mobilized to synthesis steroid hormones after
stimulus. Cholesterol are come from plasma of
steroid producing cell.
Because of high lipid solubility after synthesis
steroid hormones diffuse the cell membrane, then
insterstitial fluid and finally in blood.
Example of steroid
hormones
Amine hormone synthesis
Amine hormones are derived from tyrosine.
They are formed by the actions of enzymes in the
cytoplasmic compartments of glandular cells. After
synthesis they are stored in the gland.
Example: thyroid hormones are synthesized from
tyrosine in response to enzyme actions in the
compartment of glandular cells. After
synthesis ,they stored in thyroid gland and
incorporated into macromolecules of the protein
thyroglobulin.
Transport of hormone in Blood
Water soluble hormones(peptide and amine)are dissolved
in plasma and transported from their site of synthesis to
target tissues where they diffuse from capillaries to
interstitial fluid &ultimately in target tissues.
Steroid and thyroid hormones circulate in the blood mainly
bound to plasma protein. Around 10% is existed free in
the plasma. Mainly protein bound hormones act as
reservoirs, they restore the plasma concentration when
lost from circulation.
Regulation of hormone secretion
Hormones are secreted by endocrine glands when there is a need
for them, that is, for their effects on target organs.
The cells of endocrine glands respond to changes in the blood or
perhaps to other hormones in the blood.
This information ,they use to increase or decrease secretion of
their own hormones.
When a hormone brings about its effects, the stimulus is reversed,
and secretion of the hormone decreases until the stimulus
reoccurs.
In any hormonal negative feedback mechanism, information
about the effects of the hormone is “fed back” to the gland, which
then decreases its secretion of the hormone. This is why the
mechanism is called “negative”: The effects of the hormone
reverse the stimulus and decrease the secretion of the hormone.
The secretion of many other hormones is regulated in a similar
way.
Hormone receptors and
their location
Hormone receptors: A hormone receptor is a
molecule or complex of molecules in or on a cell
that binds its hormone with great selectivity and in
so doing is changed in such a manner that a
characteristic response or group of responses is
initiated.
The location for different types of hormone
receptors are generally following:
1.In or on the surface of cell membrane:
the membrane receptors are specific mostly for the
protein, peptide and catecholamine hormones.
Hormone receptors and
their location
2. In the cell cytoplasm: the primary receptors for
the different steroid hormones are found mainly in
the cytoplasm.
3.In the cell nucleus: The receptors for the thyroid
hormones are found in the nucleus and are believed
to be located in direct association with one or more
of the chromosomes.
Mechanism of action of hormone
The first step of hormone’s action is to bind specific
receptors.
When hormone combine with its receptor, this action
usually initiates a cascade of reactions in the cell
Then each stage becoming more powerfully activated
so that small concentrations of the hormone can have
large effect.
Largely hormones exert its effect to its target tissues by
forming hormone receptor complex. Receptors that are
associated with hormone activation are:
1. Ion channel-Linked receptors
2. G-protein linked hormone receptors
3. Enzyme-Linked hormone receptors
G-protein linked hormone
receptors
Many hormones activate receptors that indirectly regulate the
activity of target proteins (e.g., enzymes or ion channels) by coupling
with groups of cell membrane proteins called heterotrimeric GTP-
binding proteins (G proteins).
GPCRs have seven transmembrane segments that loop in and out of
the cell membrane.
G proteins have three (i.e., trimeric) parts—the α, β, and γ subunits.
When the ligand (hormone) binds to the extracellular part of the
receptor, a conformational change occurs in the receptor that
activate the G proteins and induces intracellular signals that either
1. open or close cell membrane ion channels or
2. change the activity of an enzyme in the cytoplasm of the cell.
3. Activate gene transcription.
G-protein linked hormone
receptors
