Endocrine Physiology
Endocrinology
:
•
Is the biological science concerned with the
synthesis
,
secretion
and
action
of
hormones.
•
The function of endocrine system
is to
regulate
metabolism, fluid status, growth, maturation and
sexual development.
•
The endocrine and nervous system work
Types of Hormones
:
• Hormone molecules fall into 3 general chemical classes:
1. The amines, e.g.: thyroid hormones (TH) and catecholamine, originally from amino acid Tyrosine.
2. Proteins and peptides: most of the hormones of this type e.g.: Growth Hormone (GH) and Insulin.
3. Steroids in which cholesterol is the common precursor in this
Endocrine Physiology
Synthesis & secretion of hormones
1. Polypeptide & PTN hormones are stored in
secretary vesicles until needed.
2. Steroid hormones are usually synthesized
from cholesterol & are not stored.
Hormone secretion, transport & clearance
from blood
•
There action different (few seconds – months) for full
effect
Epinephrine Thyroxine, GH
•
The conc. In the blood are very small (1 pg- a few
Endocrine Physiology
Regulation of hormone secretion
1. Feed back principal
2. Neural control
•
Negative Feedback (most common)
•
Positive Feedback (rare)
Endocrine Physiology
Negative Feedback mechanisms
• Negative feedback (-ve FB) prevent over activity of hormone
systems
• The controlled variable is often the degree of activity of the
target tissue.
• Therefore, only when the activity of the target tissue rises to
an appropriate level FB signals to the endocrine gland
Endocrine Physiology
e.g
TRH TSH TH
• TRH: Thyroid releasing hormone • TSH: Thyroid stimulatory hormone
Positive Feedback
•
It occur when biological action of the
hormone causes additional secretion of the
hormone.
•
Few e.g.
–
Surge of
Luteinizing hormone (LH)
that occur as a
Endocrine Physiology
Cyclical variation in hormone release
(chronotropic control)
•
Seasonal change
•
Various stages of development & aging
•
The diurnal (daily) cycle
•
Sleep
– e.g. GH is markedly increased during the early period of
Transport of hormone in the blood
•
1.
water soluble hormone (peptides &
catecholamines)
diffuse in the plasma, when
reaching the target tissue, they diffuse out of the
capillaries, into the ISF & ultimately to the target
cells.
•
2.
Steroid & thyroid hormone
, in contrast circulate in
Endocrine Physiology
Clearance of hormones from blood
•
2 factors can increase or decrease the conc. Of
hormone in the blood:
–
Rate of secretion
–
Rate of removal from blood (metabolic clearance
Hormones are cleared from plasma in several
ways
:
– Metabolic destruction by the tissue – Binding with the tissue
Endocrine Physiology
Mechanism of action of hormones
1. The first step is to bind to specific receptors at the target cell
2. When it combines to its receptor, this usually initiates a
cascade of reactions in the cell, with each stages becoming more powerfully activated.
3. The generated signal (2nd messenger) that causes a
quantitative change in intracellular processes by altering enzymatic activity or its concentration.
– If all the 3 steps are intact then a particular hormone effectively
The locations of different types of receptors
:
•
in or on the surface of the
cell membrane
» PTN, peptide & catecholamines
•
In the cell
cytoplasm
» Steroid hormones
•
In the cell
nucleus
Endocrine Physiology
(1) open or close cell membrane ion channels,
(2) change the activity of an enzyme in the cytoplasm of the cell, or (3) activate gene transcription.
Endocrine Physiology
Endocrine Physiology
Measurement of Hormone concentration in
the blood
because of the minute conc. of most h in the bl., it was very difficult to measure these conc. by the usual chemical means.
• The principle method of hormone measurement is
1. radioimmunoassay
The method makes use of the reaction between a hormone and an induced antibody
.
2
.
Enzyme-linked immunosorbent assay (ELISA): this type become widely used in clinical lab., bec
:.
1. radioactive isotopes not needed 2. cost-effective and accurate
3 .
The Pituitary Gland:
Anatomy
• The pituitary gland is enclosed
in the Sella turcica, bridged
over by the diaphragma Sellae;
• with the sphenoid air sinuses
below and
• the optic chiasma above, • the cavernous sinuses are
lateral to pituitary fossa and
Endocrine Physiology
The infundibular stalk connects the hypothalamus to pituitary gland carry blood from median eminence of the hypothalamus to the anterior lobe and nerve fibers to the posterior lobe. The
The Hypothalmic Releasing and Inhibiting Peptide Hormones
:
•
The Hypothalamus is a region of the brain, the size of a cherry that controls the sympathetic nervous system
.
•
All of the hypothalmic releasing and inhibiting hormones act on cell surface seven-transmembrane receptors, G-proteins
.
•
Gs, Gi (Effector Protein-Adenylate Cyclase)
•
Endocrine Physiology
Hypothalamus controls pituitary secretion
Special neurons in the hypothalamus
synthesize and secrete the HRH & HIH, that
control secretion of the ant. Pit H. these
neurons originate in various parts of the
hypothalamus and send their nerve fibers to the
median eminence
The hormones secreted by the hypothalamus &
control pituitary are
:
• GHRH :
• GHIH (somatostatin) : • CRH :
• TRH : • GnRH : • Dopamine
These H are immediately absorbed into the hypothalamic-hypophysial portal system and carried directly to the sinuses
Growth Hormone -releasing hormone
Growth Hormone-inhibitory hormone
Corticotropin-releasing hormone Thyrotropin-releasing hormone
Endocrine Physiology
The hypothalamus is a collecting center for
information concerning the internal well-being of the
body
,
• and much of this information is used to control secretions of the many globally important pit. H
• Olfactory stimuli denoting pleasant or unpleasant smells transmit strong signal components into the hypothalamus.
Anterior and posterior pituitary hormone
Neurohypophysis
(posterior)
Adenohypophysis
