Transcript Physiology of endocrine system

Physiology of endocrine system
Possible fates and actions of a hormone following its
secretion by an endocrine cell.
Not all paths apply to all hormones.
Pathways by which the nervous system influences hormone secretion. (a) Certain
neurons in the hypothalamus, some of which terminate in the posterior pituitary,
secrete hormones. The secretion of hypothalamic hormones from the posterior
pituitary and the effects of other hypothalamic hormones on the anterior pituitary
are described later in this chapter. (b) The autonomic nervous system controls
hormone secretion by the adrenal medulla and many other endocrine glands.
• Secretion of the anterior pituitary
hormones is largely regulated by
hormones produced by the
hypothalamus and collectively
called hypophysiotropic
hormones.
• These hormones are secreted
by neurons that originate in
diverse areas of the
hypothalamus and terminate in
the median eminence around the
capillaries that are the origins of
the hypothalamo-pituitary portal
vessels.
• The generation of action
potentials in these neurons
causes them to release their
hormones, which enter the
capillaries and are carried by the
hypothalamopituitary portal
vessels to the anterior pituitary.
There they act upon the various
anterior pituitary cells to control
their hormone secretions.
Hypophysiotropic
Hormones
Hypophysiotropic hormones
• Each hypophysiotropic hormone is named for
the anterior pituitary hormone whose secretion it
controls.
• Thus, secretion of ACTH (corticotropin) is
stimulated by corticotropin releasing hormone
(CRH), secretion of growth hormone is
stimulated by growth hormone releasing
hormone (GHRH), secretion of
thyroidstimulating hormone (thyrotropin) is
stimulated by thyrotropin releasing hormone
(TRH), and secretion of both luteinizing hormone
and follicle-stimulating hormone (the
gonadotropins) is stimulated by gonadotropin
releasing hormone (GnRH).
Neural Control of
Hypophysiotropic Hormones
• Neurons of the hypothalamus receive synaptic
input, both stimulatory and inhibitory, from
virtually all areas of the central nervous system,
and specific neural pathways influence secretion
of the individual hypophysiotropic hormones.
• A large number of neurotransmitters (for
example, the catecholamines and serotonin) are
released at the synapses on the hormonesecreting hypothalamic neurons, and this
explains why the secretion of the
hypophysiotropic hormones can be altered by
drugs that influence these neurotransmitters.
Anterior Pituitary Hormones
• The anterior pituitary secretes at least eight hormones,
but only six have well-established functions.
• All peptides, these six “classical” hormones are
folliclestimulating hormone (FSH), luteinizing
hormone (LH), growth hormone (GH), thyroidstimulating hormone (TSH, thyrotropin), prolactin, and
adrenocorticotropic hormone (ACTH, corticotropin).
Each of the last four is probably secreted by a distinct
cell type in the anterior pituitary, whereas FSH and LH,
collectively termed gonadotropic hormones (or
gonadotropins) because they stimulate the gonads, are
both secreted by the same cells.
Activation of punishment center
• Section
through an
adrenal
gland
showing
both the
medulla and
cortex, as
well as the
hormones
they
secrete.
1. Thyroid-releasing hormone (TRH)
is released from neurons in the
hypothalamus and travels in the
blood to the anterior pituitary
gland.
2. TRH stimulates the release of
thyroid-stimulating hormone (TSH)
from the anterior pituitary gland.
TSH travels in the blood to the
thyroid gland.
3. TSH stimulates the secretion of
thyroid hormones (T3 and T4) from
the thyroid gland into the blood.
4. Thyroid hormones act on tissues to
produce responses.
5. Thyroid hormones also have a
negative-feedback effect on the
hypothalamus and the anterior
pituitary to inhibit both TRH
secretion and TSH secretion. The
negative feedback helps keep
blood thyroid hormone levels
within a narrow range.
Hormonal
Regulation of T3 &
T4 Secretion
1. During the menstrual
cycle, before ovulation,
small amounts of
estrogen are secreted
from the ovary.
2. Estrogen stimulates the
release of gonadotropinreleasing hormone
(GnRH) from the
hypothalamus and
luteinizing hormone (LH)
from the anterior
pituitary.
3. GnRH also stimulates the
release of LH from the
anterior pituitary.
4. LH causes the release of
additional estrogen from
the ovary. The GnRH
and LH levels in the
blood increase because
of this positive-feedback
effect.
Positive feedback
regulation of estrogen
1. During the menstrual
cycle, after ovulation,
the ovary begins to
secrete progesterone
in response to LH.
2. Progesterone inhibits
the release of GnRH
from the hypothalamus
and LH from the
anterior pituitary.
3. Decreased GnRH
release from the
hypothalamus reduces
LH secretion from the
anterior pituitary.
GnRH and LH levels in
the blood decrease
because of this
negativefeedback
effect.
Negative feedback
regulation of estrogen
Gonadal production of steroids. Only the ovaries have high
concentrations of the enzymes (aromatase) required to
produce the estrogens estrone and estradiol.
Aldosterone action
Notion about hormones
• The endocrine system is composed of glands that
secrete chemical signals into the circulatory system.
• The secretory products of endocrine glands are called
hormones (hoЇrmoЇnz), a term derived from the Greek
word hormon, meaning to set into motion.
• Traditionally, a hormone is defined as a chemical signal,
or ligand, that (1) is produced in minute amounts by a
collection of cells; (2) is secreted into the interstitial
spaces; (3) enters the circulatory system, where it is
transported some distance; and (4) acts on specific
tissues called target tissues at another site in the body
to influence the activity of those tissues in a specific
fashion. All hormones exhibit most components of this
definition, but some components don’t apply to every
hormone.
• Hormone molecules
diffuse from the blood
through the walls of the
capillaries into the
interstitial spaces. Once
within the interstitial
spaces, they diffuse to
the target cells. As the
concentration of free
hormone molecules
increases in the blood,
more molecules diffuse
from the capillary to the
target cells.
• As the concentration of
free hormone molecules
decreases in the blood,
fewer diffuse from the
capillary to the target
cells.
Hormone
Concentrations at
the Target Cell
• An equilibrium
exists between
free hormone
molecules and
hormone
molecules bound
to plasma
proteins.
• The free hormone
molecules can
diffuse from the
capillaries to the
interstitial spaces.
Effect of Changes in
Plasma Protein
Concentration on the
Concentration
of Free Hormone
Effect of decrease in plasma
protein concentration
• A decrease in plasma protein
concentration reduces the
number of hormone molecules
bound to plasma proteins.
• This increases the rate at
which free hormone molecules
diffuse from the capillaries.
More importantly, hormones
that diffuse from capillaries are
eliminated from the blood by
the kidney and liver.
• The rapid loss of hormone
from the circulatory system
reduces the hormone
concentration in the body and
fewer hormone molecules are
available to bind to receptors.
Response of Target Cells to Hormones
•
•
(a) Down-regulation occurs when the number of receptors for a hormone decreases within
target cells. For example, gonadotropin-releasing hormone (GnRH) released from the
hypothalamus binds to GnRH receptors in the anterior pituitary. GnRH bound to its receptors
causes down-regulation of the GnRH receptors so that eventually the target cells become less
sensitive to the GnRH.
(b) Up-regulation occurs when some stimulus causes the number of receptors for a hormone to
increase within a target cell. For example, FSH acts on cells of the ovary to up-regulate the
number of receptors for LH. Thus the ovary becomes more sensitive to the effect of LH.
The Cascade Effect
The combination of a hormone with a membrane-bound receptor activates several G
proteins. The G proteins, in turn, activate adenylyl cyclase enzymes, which
cause the synthesis of a large number of cAMP molecules. The cAMP molecules, in
turn, activate many protein kinase enzymes, which produce a rapid and
amplified response.
The endocrine system