Modern Biology I
BIOL103
ENDOCRİNE SYSTEM AND HORMONES
Hormonal Control
ENDOCRİNE SYSTEM
What is a hormone?
• Hormone is a chemical signal secreted into the circulatory system
and communicates regulatory messages within the body.
• Hormones may reach all parts of the body.
• But only certain types of cells (target cells) are equipped to
respond. (WHY??)
There are two systems which act individually
and together to regulate animal’s physiology
NERVOUS SYSTEM
Communication between
and within neurons via
Neurotransmitters
ENDOCRINE SYSTEM
Constituted by hormone-secreting cells and
glands
Secrets hormones that coordinate slower but
longer-acting responses to stimuli
Hormonal Control
In multicellular complex organisms, giving appropriate response towards
internal and external stimuli.
Coordination and control of working of cell, tissue, organ and systems.
Protection of homeostasis is provided by nervous system and endocrine system.
These two systems are called as supervisory systems.
Control of Endocrine system is provided by special organic substances which are
called hormones.
Endocrine System
It helps coordination of many psychological reactions.
Endocrine glands secrete hormones into blood circulation.
Hormones have a special role on nervous system and general organisation of our
body.
Hormones activate some behaviours (For exp.; Sexual Behaviours).
The interaction between hormones and behavior is bidirectional: hormones can influence behavior, and
behavior can sometimes influence hormone concentrations.
General Features of Hormones
Every living being produces its own hormones. But when hormones are taken from outside, it is
also effective.
Hormones are organic molecules. (They can be Polypeptide, amino acid, steroid or purine form)
Polypeptide-structured hormones are effected by digestive enzymes.
Hormones are effective even little quantities. (like vitamins)
Hormones can cause abnormalities when it is secreted little or more.
Hormones are not effect its glands. It effects its target structures. For example: FSH is secreted
in pituitary gland but target structure is ovary.
Some hormones have general effects. Whole body is a target structure. For exp: STH, Thyroxine,
STH etc.
Cont.
Some hormones have local effect. İt is effective in a special tissue or system. For exp: Secretin,
FSH,ACTH etc.
When hormone finishes its mission, it is eliminated by cell metabolism or liver.
Some hormones have different effects on different target structures. For exp: while oestrogen
shows primary effect on reproductive system, it shows secondary effect on skeletal muscle and
skin-based structures.
Secretion of hormones is controlled under nervous system and pituitary gland.
Different hormones have different effect on the same target structure.
Hormones which are secreted by nervous system are called neurohormones.
Compared to nerve effect, Hormonal effect happens too slow but its effect takes long.
Nerve control on endocrine system is linked to internal and external stimuli.
Hormone composition
• Three major classes are:
• Proteins and peptides (soluble) (GH, ADH, Glucagon, Insulin)
• Amines derived from amino acids (soluble) (Thyroxin, Adrenalin, Noradrenalin)
• Steroids (insoluble) (Cortisol and Aldosterone)
• Signalling by any of these molecules involved three key events:
• Reception
• Signal transduction
• Response
Hormone-Receptor interactions
• Definition: a protein that binds a ligand with high affinity and low capacity. This
binding must be saturable.
• A tissue becomes a target for a hormone by expressing a specific receptor for
it. Hormones circulate in the blood stream but only cells with receptors for it are
targets for its action.
Types of receptors
 Receptors for the water soluble hormones are found on the surface of the
target cell, on the plasma membrane.
 These types of receptors are coupled to various second messenger systems which
mediate the action of the hormone in the target cell.
 Receptors for the lipid soluble hormones reside in the nucleus (and sometimes
the cytoplasm) of the target cell.
 Because these hormones can diffuse through the lipid bilayer of the plasma
membrane, their receptors are located on the interior of the target cell
Negative Feedback System in Hormones
The MAJOR human endocrine glands
Human Endocrine glands and their hormones
Cntd.
The hypothalamus links the nervous
and endocrine systems by way of the
pituitary gland. Its function is to
secrete releasing hormones and
inhibiting hormones that stimulate or
inhibit (like their names imply)
production of hormones in the
anterior pituitary.
Tropic hormones function by
stimulating the endocrine gland
to facilitate the release of
another hormone. For example,
the thyroid-stimulating hormone
stimulates the release of
thyroxin from the thyroid gland.
On the other hand, non-tropic
hormones act straight on the
target organ for the specified
function.
Non-tropic hormones are
hormones that directly stimulate
target cells to induce effects.
(T3)
(T4)
GONADS
Gonads secrete oestrogen and androgen hormones in
ovary and testes.
•LH and FSH in women stimulate the ovaries to
produce estrogens and progesterone. Depending on
the phase of the menstrual cycle, those hormones act
back on the hypothalamus and pituitary gland in
either a stimulatory or inhibitory manner.
◦ FSH helps manage the menstrual cycle and stimulates
the ovaries to produce eggs.
◦ LH: triggers the release of an egg from the ovary.
◦ Progesterone prepares the endometrium for the potential of
pregnancy after ovulation.
◦ Estrogen helps stimulate the growth of the egg follicle.
•In men, LH stimulates the testes to release
testosterone, which feeds back on the hypothalamus
and pituitary.
Gonads