Transcript of The endocrine system is one of the body's two major communication system the nervous system being...
1The endocrine system
The endocrine system
The endocrine system
is one of the body's two major communication system the nervous
system being the other.
The endocrine system consist s of all those glands termed endocrine
glands or gland of internal secretion . The endocrine glands are
glands without excretory ducts that secrete hormones.
Hormones are chemical messengers that are carried by the blood from
endocrine glands to the cells upon which they act . the cells
influenced by a particular hormone are the target cells for that
hormone.
The endocrine system differs form most of the other organ systems
of the body in that the various glands are not anatomically
continuous however they do form a system in the functional
sense.
Classification of hormones
1-amine hormones
2-peptide hormones
The great majority of hormones are either peptides or proteins .
they range in size from small peptides having only three amino
acids to small proteins (some of which are glycoproteins.) in many
cases the initially synthesized on the ribosomes of the endocrine
cells . many peptides serve as both neurotransmitters and as
hormones . for example most of the hormone secreted by the
endocrine glands in the gastrointestinal tract (for example
cholecystokinin) are also produced by neurons in the brain where
they function as neurotransmitters
3- steroid hormones
Hormone transport in the blood
Peptide and catecholamine hormones are water –soluble .therefore
with the exception of a few peptides these hormones are transported
simply dissolved in plasma .in contrast the steroid hormones and
the thyroid hormones circulate in the blood largely bound to plasma
proteins .
Effects of peptide hormones and catecholamines
The receptors for peptide hormones and the catecholamine hormones
are located on the outer surface of the target cells plasma
membrane .when activated by hormones binding the receptors trigger
one or more of the signal transduction pathways .that is the
activated receptors directly influence :
1-ion channels that are part of the receptors .
2-Enzymne activity that is part of the receptors.
3-G proteins coupled in the plasma membrane to effector proteins
–ion channels and enzymes .
The changes in enzyme activity produce changes in the conformation
and hence the activity of various cellular proteins .in some cases
the signal transduction pathways also lead to stimulation or
inhibition of the synthesis of new proteins by the cell .
Effects of steroid hormones
The steroid hormones are lipid –soluble messengers .their receptors
are intracellular and are inactive when no messenger is bound to
them for certain lipid –soluble messengers the inactive receptors
are in the cytosol.
The messenger diffuses across the cells plasma membrane and enters
the cytosol .where the receptor is in the cytosol the messenger
combines with it there and hormones \receptor complex then moves
into the nucleus .with intra nuclear receptors the hormones
diffuses by itself into the nucleus and binds to the receptor there
.the receptor activated by the the binding of hormone to it then
functions in the nucleus as any regulatory protein that directly
influences gene transcription .the result is an increase in the
cellular concentration of the protein or its rate of secretion
.
Inputs that control hormone secretion
Hormone secretion controlled mainly by three types of inputs to
endocrine cells :
1-changes in the plasma concentration of mineral ions or organic
nutrients .there are at least 5 hormones whose secretion is
directly controlled by the plasma concentrations of specific
mineral ions or organic nutrients .in each case a major function of
the hormone is to regulate in a negative –feedback manner the
plasma concentration of the ion or nutrient controlling its
secretion .for example insulin secretion is stimulated by an
elevated plasma glucose concentration and the additional insulin
then causes by several actions the plasma glucose concentration to
decrease .
2-neurotransmitters released from neurons impinging on the
endocrine cell .the autonomic nervous system is the neural input
controlling many hormones (the adrenal medulla is stimulated by
sympathetic preganglionic fibers the secretion of insulin and
gastrointestinal hormones are stimulated and inhibited by both
sympathetic and parasympathetic inputs ) the controlled by neurons
in the brain-
3-another hormone acting on the endocrine cell .In many cases the
secretion of a particular hormone is directly controlled by the
blood concentration of another hormone .a hormone that stimulates
the secretion of another hormone is often referred to as a topic
hormone .the topic hormones usually stimulate not only secretion
but the growth of the stimulated gland as well.
4-Chemical and physical factors in the lumen of the
gastrointestinal tract .this applies only to the hormones secreted
by the gastrointestinal tract
Control system involving the hypothalamus and pituitary
pituitary gland comprising the anterior pituitary and posterior
pituitary is connected to the hypothalamus by a stalk containing
nerve axons and blood vessels
The axons whose cell bodies are in the hypothalamus terminate in
the posterior pituitary and release oxytocin or vasopressin.
The anterior pituitary secretes growth hormone (GH) ,
thyroid-stimulating hormone (TSH) , adrenocorticotropic hormone
(ACTH) , prolactin ,and two gonado- tropic hormones- follicle-
stimulating hormone (LH).
Secretion of the anterior pituitary hormones is controlled mainly
by hypophysiotropic hormones secreted into capillaries in the
median eminence of the hypothalamus and reaching the anterior
pituitary via the portal vessels connecting the hypothalamus and
anterior pituitary.
2-Growth hormone releasing hormone (GHRH) stimulates secretion of
growth hormone (GH).
3-Thyrotropin releasing hormone (TRH) stimulates secretion of
thyroid –stimulating hormone (thyrotropin) .
4-Gonadotropin releasing hormone (GnRH) stimulate secretion of both
luteinizing hormone and follicle – stimulating hormone (the
gonadotropins) .
5-Somatostatin (SS) inhibits secretion of growth hormone .
6-Prolactin- inhibiting hormone (PIH) inhibits secretion of
prolactin
7-Prolactin releasing hormone (PRH) stimulates secretion of
prolactin
Neural control of Hypophysiotropic hormones
CRH- ACTH – cortisol sequence . The stress input to the
hypothalamus is via neural pathways ,cortisol exerts a negative –
feedback control . over the system by acting on (1) the
hypothalamus to inhibit CRH secretion and (2) the anterior
pituitary to reduce responsiveness to CRH Hormonal feedback control
of the hypothalamus and anterior pituitary .
A prominent feature of each of the hormonal sequences initiated by
a hypophysiotropic hormone is negative feedback exerted upon the
hypothalamo-pituitary system by one or more of the hormones in its
sequence . for example , in the CRH – ACTH – cortisol sequence ,
the final hormone , cortisol , acts upon the hypothalamus to reduce
secretion of CRH by causing a decrease in the frequency of action
potentials in the neurons secreting CRH . In addition , cortisol
acts directly on the anterior pituitary to reduce the response of
the ACTH- secreting cells to CRH . Thus , by a double barreled
action , cortisol exerts a negative – feedback control over its own
secretion. Such a system is effective in damping hormonal response,
that is , in limiting the extremes of hormone secretory
rates.
Another adaptive function of these negative – feedback mechanisms
is that they maintain the plasma concentration of the final hormone
in a sequence relatively constant whenever a disease- induced
primary change occurs in the secretion or metabolism of that
hormone.
Hormones not in a particular sequence can also influence secretion
of the hypothalamic and anterior pituitary hormones in that
sequence. For example, estrogen markedly enhances the secretion of
prolactin by the anterior pituitary , even though estrogen
secretion is not controlled by prolactin.
Hypothalamus -1
Hypothalamus -1
The hypothalamus lies below the thalamus , it contains different
cell groups and pathways that form the master command center for
neural and endocrine coordination . the pituitary gland is
connected to the hypothalamus by a stalk containing nerve axons and
blood vessels.
The axons, whose cell bodies are in the hypothalamus terminate in
the posterior pituitary and release oxytocin or vasopressin.
Between the hypothalamus and anterior pituitary gland there is an
unusual blood flow directly from the hypothalamus to the anterior
pituitary.
Hypothalamus secretes hormones , which are collectively termed
hypophysiotropic hormones (hypothalamic releasing hormones). The
major function of these hormones is control of secretion of
hormones is control of secretion of hormones by the anterior
pituitary.
The hypophysiotropic hormones are :
.1 Corticotropic releasing hormone (CRH) , which controls the
secretion of ACTH (STIMULATION).
2. Thyrotropin releasing hormone (TRH) , Which controls the
secretion of TSH (STIMULATION) .
3. Growth hormone releasing hormone (GHRH) , Which controls the
secretion of GH (inhibition) .
4. Somatostatin (SS) , which controls the secretion of GH
(inhibition) .
5. Gonadotropin releasing hormone (GnRH) ,which controls secretion
of LH and Fsh (stimulation)
6. Dopamine (DA,also called prolactin – inhibiting hormone , PIH),
which controls secretion of prolactin (inhibition).
II-PITUITARY GLAND (Hypophysis)
II-PITUITARY GLAND (Hypophysis)
The pituitary gland lies in a pocket (the sella turcica) of the
sphenoid bone at the base of the brain , just below hypothalamus .
The pituitary gland is composed of two adjacent lobes:the anterior
pituitary (adenohypophysis) which has blood vessel connection with
hypothalamus , and the posterior pituitary (neurohypophysis),which
has neural connection with hypothalamus.
1-Posterior pituitary hormones
The hormones are not synthesized in the posterior pituitary itself
but in the hypothalamus , and by the axons , which terminate in
posterior pituitary release them . the hormones are :
2-Anterior pituitary hormones
a-Growth hormone (GH, Somatotropin ) is the major stimulus of
postnatal growth : induces precursor cells to differentiate and
secrete insulin –like growth factor I(IGF-I) , WHICH Stimulates
cell division .GH stimulates protein synthesis.
b-Thyroid-stimulating hormone (TSH.thyrotoropin) .which stimulates
secretion of T3; T4(thyroid gland) .
c-adrenocorticotropic hormone (ACTH. Corticotrophin) which
stimulates secretion of adrenal cortex.
d- Prolaction ,which stimulates breast growth and milk production .
Prolaction many be permissive for certain reproductive functions in
male .
e-Gonadotropic hormones :Follicle-stimulating hormone(FSH),and
Luteinzing hormone(LH), which controls of gonads (gamete production
and sex hormone secretion)
f-Beta lipotropin ,which has unknown function.
g-Beta endorphin,which has unknown function.
III-The pineal gland
III-The pineal gland
The pineal gland is an outgrowth from the roof of the diencephalons
of the brain. It is a rudimentary organ. Pineal gland secretes the
melatonin (candidate hormone). The exact functions of melatonin in
hormons are unknown, but this hormone probably plays an important
role in the setting of the bodys circadian rhythms.
IV-The thyroid gland
IV-The thyroid gland
The thyroid gland is located in the lower pasrt of the neck wrapped
around the front of the trachea.It composed of many spherical
structures called follicles, each consisting of a single layer of
epithelial cell surrounding an extracellular central space filled
with a glycoprotein colloid called amine hormones- thyroxine (T4)
and triiododothyronine (T3), parafollicular cells, which are
located between follicles, secrete a third hormone- a peptide
called calcitonin, this hormone does not contain iodine and not
included in the term (thyroid hormone). Iodine is an essential
element that functions as a component of T4 and T3. T4 is secreted
in much larger ammounts than is T3. However, a variety of tissues,
particularly the liver and kidneys, convert most of this T4 into
T3. This is an important point because T3 is a much more active
hormone than T4. Indeed, it is likely that T4 has little or no
action unless it is converted into T3.
Major function of thyroid hormones:
Virtually every tissue in the body is afficted by the thyroid
hormone. This effect include :
1-Regulation of netabolic rate . The Thare the single most
important determinat of basal metabolic rate. TH increase the
oxygen consumption and heat production of most body tissues
,anotable exception being the brain . This ability to increase BMR
is termed a calorigenic effect.
2-Control of growth .TH are essential for normal growth because
they are required for both the synthesis of growth hormone and the
growth-promoting effects of the hormone. Accordingly , infants and
children with hypothyroidism (deficient thyroid function) manifest
retarded growth due to slowed bone growth .
This effect on brain development must be distinguished from other
stimulatory effects TH exerts on the nervous system throughout life
not just during infancy . Ahypothyroid person exhibits sluggishness
and poor mental function and these effects are completely
reversible at any time with administration of TH .conversely a
person with hyperthyroidism (excessive secretion of TH) is jittery
and hyperactive.
Major functions of calcitonin:
1-Control of plasma calcium .It has long been thought that
calcitonin helps to regulate plasma calcium but present evidence
indicates that it plays a minor role in this regard.
V-Parathyroid glands
V-Parathyroid glands
The Parathyroid glands are located on the posterior surface of the
thyroid gland. There are four Parathyroid glands .They secrete a
hormone called parathyroid hormone .Parathyroid hormone production
is controlled by the extracellular calcium concentration acting
directly on the secretory cells ( via a plasma –membrane calcium
receptor ). Decreased plasma calcium concentration stimulates
parathyroid hormone secretion and an increased plasma calcium
concentration does just the opposite.
Parathyroid hormone exerts multiple action that increase
extracellular calcium concentration:
1-It directly increases the resorption of bond by osteoclasts which
results in the movement of calcium ( and phosphate ) from bone into
extracellular fluid.
2-It directly stimulates the activation of vitamin D and this
latter hormone then increases intestinal absorption of calcium
.Thus the effect of parathyroid hormone on the intestinal tract is
an indirect one.
3- It directly increases renal tubular calcium reabsorption thus
decreasing urinary calcium excretion .
In addition parathyroid hormone directly reduces the tubluar
reabsorption of phosphate thus raising its uninary excretion .This
keeps plasma phosphate from increasing at a time when parathyroid
hormone is stimultaneously causing increased release of both
calcium and phosphate from bone .
VI-THE ADRENAL (SUPRARENAL ) GLANDS
VI-THE ADRENAL (SUPRARENAL ) GLANDS
There are two adrenal glands one on the top of of each kidney .Each
adrenal gland constitutes two distinct endocrine glands an inner
adrenal medulla which secretes amine hormones and a surrounding
adrenal cortex which secretes steroid hormones .
A drenal medulla is really a modified sumpathetic ganglion whose
cell bodies do not have axons but instead release their secretions
into the blood thereby fulfilling a criterion for an endocrine
gland . The adrenal medulla secretes mainly two amine hormones
epinephrine (E) and norepinephrine (NE) .In humans the adrenal
medulla secretes approximately four times more epinephrine than
norepinephrine .
epinephrine and norepinephrine exert actions similar to those the
sympathetic nerves .These effects are :
1-Increased hepatic and muscle glycogenolysis ( provides a quick
source of glucose).
2-Increased breakdown of adipose tissue triacylglycerol (provides a
supply of glycerol for gluconeogenesis and of fatty acids for
oxidation).
3-Decreased fatigue of skeletal muscle.
4-Increased cardiac output secondary to increased cardiac
contractility and heart rate.
5-Shunting of blood from viscera to skeletal muscles by means of
vasoconstriction in the former beds and vasodilation in the
latter.
6-Increased ventilation .
7-Increased coagulability of blood.
The adrenal medulla also secretes small amounts of dopamine and
several substances.
Hormones of the adrenal cortex
The five hormones normally secreted in physiologically significant
amounts by the adrenal cortex are :
1-ALDOSTERONE (secreted by zona glomerulosa ) is known as a
mineralocoticoid because its effects are on salt (mineral ) balance
mainly on the kidneys handlind of sodium potassium and hydrogen
ions.
2-CRTISOL and CORTYCOSTERONE (secreted by zona fasiculata and zona
reticulais) are called glucocorticoids because they have important
effects on the metabolism of glucose and other organic nutrients
.Cortisol is by far the more important of two glucocorticoids in
human
The main functions of the cortisol are :
1-Effect on organic metabolism :increased plasma concentrations of
amin acids glucose and free fattu acids.
2-Antigrowth effects : inhibits bond growth stimulates protein
catabolism responsible for the retarded growth that occuas with
illness .
3-Effects during stress:
a-Effect on organic metabolism :stimulation of protein catabolism
stimulation of liver uptake of amino acids and their conversion to
glucose (gluconeogenesis) inhibition of glucose uptake and
oxidation by many body cells (insulin antagonism ) but not by the
brain and stimulation of triacylglycerol catabolism in adipose
tissue with release of glycerol and fatty acids into the blood
.
b-Enhanced vascular reactivity that is increased ability to
maintain vasoconstriction in response to norepinephrine and other
stimuli .
c-Unidentified protective effects against the damaging influences
of stress .
d-Inhibitione of inflammation and specific
immune response.
4-Regulation of the immune system .
3- ANDROGENS ( secreted by zona fasiculata and zona reticularis)
consist of dehydroepiandrosterone and androstenedione. Androgens
also includes the male sex hormone testosterone produced bu the
testes . All androgens have actions similar to those of
testosterone .
Because the adrenal androgens are much less potent than
testosterone they are of little physiological significance in the
male they do however play roles in the female .
Effect of testosterone (androgens ) in the male :
1-Required for initation and maintenance of spermatogenesis.
2-Decreases GNRH secretion via an action on the hypothalamus.
3-Inhibits LH secretion via an action on the anterior
pituitary.
4-Induces differentiation of male accessory reproductive organs and
maintains
their function .
5- Induces male secondary sex characteristic opposes action of
estrogen on breast growth .
6-Stimulates protein anabolism bone growth and cessation of bone
growth.
7-Required for sex drive and may enhance aggressive behavior.
Androgens in women:
Testosterone is found in low concentration in blood of normal
women. Other androgens are also found in significant concentrations
in the blood of women. All these androgens play several important
roles in the female, including stimulation of the growth of pubic
hair , axillary hair , and possibly skeletal muscle. In several
diseases states, the female adrenals may secrete abnormally large
quantities of androgens , which produce virilism.
VII-Thymus:
VII-Thymus:
This gland is situated in the anterior mediastinum of the thorax.
It is relatively large at birth and continues to increase in size
until puberty, when it gradually diminishes. Thymus is primary
lymphoid organ.
It secretes Thymopoietin hormone, which control T-lymphocyte
function.
VIII- The pancreas:
VIII- The pancreas:
The pancreas is an elongated gland located behind the stomach , has
both endocrine and exocrine functions. Hormones are secreted by the
islets of Langerhans , clusters of endocrine cells in the pancreas
. There are several distinct types of islet cells, each of which
secretesa different hormone. These hormones are:
1 – Insulin, which secreted by beta cells , Insulin's many actions
are often divided into two broad categories :
(A) – metabolic effects on carbohydrate, lipid, and protein
synthesis, and (B) – growth-promoting effects on DNA synthesis,
cell division , and cell differentiation. The major controlling
factor for insulin secretion is the plasma glucose concentration ,
which affects the beta cells directly.
2- Glucagon, is the peptide hormone produced by the alpha cells of
the pancreatic islets . The major physiological effects of
glucagons are all in the liver and are opposed to those of insulin
: (a) increased glycogen breakdown , (b) increased gluconeogenesis
, and ( c) synthesis of ketones.
3- Somatostatin , secreted by delta cells has unknown
function.
4- Pancreatic polypeptide has unknown function.
IX- Ovaries:
IX- Ovaries:
The ovaries are almond-sized organs in the upper pelvic cavity ,
one on each side of the uterus .The ovary serves a dual purpose:
(1) oogenenesis , the production of the gametes-the ova, and (2)
secretion of the female steroid hormones. The hormones secreted by
ovaries are:
1 – Estrogen is secreted during the follicular phase mainly by the
granulose cells ,
Following ovulation , it is secreted by the corpus luteum . Effects
of estrogen are:
a – Stimulates growth of ovary and follicles.
b- Stimulates growth of smooth muscle and proliferation of
epithelial linings of reproductive tact . In addition:
-Uterine tubes: increases contractions and ciliary activity.
-Uterus: increases myometrial contractions and responsiveness to
oxytocin.
-Vagina: increases layering of epithelial cells.
-Stimulates external genitalia growth .
- Stimulates female body configuration development; narrow
shoulders , broad hips , female fat distribution ( deposition on
hips , abdomen, and breast).
-Stimulates a more-fluid sebaceous gland secretion( this antiance
effect opposes the acne-producing effects of androgens)
-Stimulates development of femalepubic hair pattern ( growth , as
opposed to pattern, of pubic and axillary hair is androgen
-Stimulates bone growth and ultimate cessation of bone growth (
closure of ebibhyseal osplates );protects against osteoporosis;
does not have an anabolic effects on skeletal muscle .
-Vascular effects( deficiency products '' hot flashes'').
-Has feedback effects on hypothalamus and anterior pituitary.
-Stimulates fluid retention.
-Stimulates prolactin secretion but inhibits prolactin's
milk-inducting action on the breast.
-Protects against atherosclerosis, at least in part by effect on
plasma cholesterol.
2- Progesterone is secreted in very small amounts by the granulose
and theca cells before ovulation, but its major source is the
corpus luteum . Effecte of progesterone are:
a – Converts the estrogen-primed endometeium to an actively
secreting tissue suitable for implantation of an embryo.
b- Induces thick, sticky cervical mucus.
c-Decreases contraction of uterine tubes and myometrium.
d-Decreases proliferation of vaginal epithelial cells.
e- Stimulates breast growth , particularly glandular tissue.
f-Inhibits milk-inducting effects on prolactin.
g-Has feedback effects on hypothalamus and anterior
pituritary.
h- Probably increases body temperature.
3- Inhibin, a peptide hormone , is secreted by both the granuloses
cells and corpus luteum.
4- Relaxin and Activin , which have not clear function.
X- Placenta:
X- Placenta:
Is a combination of interlocking fetal and maternal tissues that
serves as the organ of exghange between mother and fetus for the
remainder of pregnancy.
Throughout pregnancy, plasma concentration of estrogen and
progesterone remain high
The placenta converts the androgens mainly into estriol, the major
estrogen of pregnancy
The trophoblast cells of the placenta produce inhibin , and
placental loctogen.
Gastrointestinal tract:
The hormones that control the gastrointestinal system are secreted
mainly by endocrine cells scattered throughout the epithelium of
the stomach and small intestine .One surface of each endocrine cell
is exposed to the lumen of the GIT.
The major effects of each are:
1 – Secretin is released from the small intestine in response to
increased luminal acidity.
2- Chlecystokinin is released from the small intestine in response
to the products of fat and protein digestion, and stimulates
pancreatic enzyme secretion.
3- Gastrin is released from the antrum of stomach in response to
increased amount of amino acids and peptides in stomach, and to
parasympathetic nerves.
4- Glucose-dependent insulinotropic peptide is released from small
intestine in response to glucose and fat in small intestine . GIP
stimulates insulin secretion.
XII-Testes:
XII-Testes:
The testes are the primary reproductive organs in the male.
The testes serve dual functions :
( 1) gametogenesis, and (2) secretion of steroid hormones (sex male
hormones). The Leydig cells ( interstitial cells), which lie in
small connective tissue spaces between the tubules , are the cells
that secrete testosterone .
The following hormones are released from test:
1 – Testosterone belongs to a group of steroid hormones that have
similar masculinizing actions and are collectively called
androgens. In the male, only the testes secrete significant amounts
of testosterone.
2- Inhibin is a protein hormone, released from sertoli cells.