A&P II CH.16 1-5 THE ENDOCRINE SYSTEM

Two main classes of hormones

Amino acid–based hormones, Steroids

Steroids

Synthesized from cholesterol ( lipid soluble, can pass through cell membrane)

Amino acid–based hormones

Amino acid derivatives and proteins (cannot pass through cell membrane without a receptor)

Water-soluble hormones (all amino acid–based
hormones except thyroid hormone)

Act on plasma membrane receptors
 Act via second messengers
 Cannot enter cell

has to bind to receptors for that hormone

Lipid-soluble hormones (steroid and thyroid
hormones)

Act on intracellular receptors that directly activate
genes.

Can enter cell

Endocrine glands are stimulated to
synthesize and release hormones in
response to one of three stimul

– Humoral stimuli
– Neural stimuli
– Hormonal stimuli

Humoral stimuli

Changing blood
levels of ions and
nutrients directly
stimulate secretion of
hormones

Neural stimuli

Nerve fibers stimulate
hormone release. Sympathetic nervous
system fibers stimulate
adrenal medulla to
secrete catecholamines

Hormonal stimuli

Hormones stimulate other endocrine
organs to release their hormones
 Hypothalamic–pituitary–target
endocrine organ feedback loop
–Hormones from final target organs
inhibit release of anterior pituitary
hormones

When the pancreas releases insulin in direct response to
blood glucose, this is an example of __________
stimulation.

humoral

Target cells ________
to which hormone binds

must have specific receptors

Target cell activation depends on three factors:

Blood levels of hormone
2. Relative number of receptors on/in target cell
3. Affinity (strength) of binding between receptor and hormone

Up-regulation

target cells form more
receptors in response to low hormone
levels

Permissiveness

one hormone cannot exert its effects without another
hormone being present

Synergism

more than one hormone produces same effects on
target cell, causing amplification

Antagonism

one or more hormones oppose(s) action of another
hormone. Example: insulin and glucagon

Endocrine
system acts with __________ To
coordinate activity of body cells

nervous system

Endocrine system controls

Reproduction
– Growth and development
– Maintenance of electrolyte, water, and
nutrient balance of blood
– Regulation of cellular metabolism and
energy balance
– Mobilization of body defenses

Endocrine glands

secrete hormones into the
interstitial fluid. These
hormones then enter the blood.

– Produce hormones
– Lack ducts

Chemical messengers of endocrine system

Hormones, Autocrines, Paracrines

Hormones

long-distance chemical
signals; travel in blood or lymph

Autocrines

chemicals that exert effects
on same cells that secrete them

Paracrines

affect cells other than those
that secrete them

Major endocrine organs of the body

Pineal gland, pituitary gland, thyroid gland, parathyroid gland, thymus, adrenal glands

example of humoral stimuli

rise in blood sugar

example of neural stimuli

adrenal glands initiating flight or fight response by releasing epinephrine or norepinephrine

example of hormonal stimuli

the hypothalamus producing hormones that stimulate the pituitary gland

Hypothalamic–pituitary–target
endocrine organ feedback
loop

Hormones from final target
organs inhibit release of
anterior pituitary hormones

Down-regulation

prolonged exposure to
high concentrations of hormones
decrease the number of receptors
(desensitizes)

Antagonism Example

insulin and glucagon ;

Glucagon breaks down glycogen
into glucose subunits which are
released into the blood stream
(glucose blood level rises)
▪ Insulin promotes glucose uptake
into cells (glucose blood level
decreases)

permissiveness example

the thyroid hormone increasing the number of receptors for epinephrine at its target cells ( for reproduction system )

Synergism example

epinephrine stimulating the breakdown of glycogen stores in the liver, thus glucose being released.

cyclic AMP (second messenger mechanism of water soluble hormones) step 1

Hormone (1st messenger)
binds receptor

cyclic AMP (second messenger mechanism of water soluble hormones) step 2

Receptor
activates G
protein (Gs)

cyclic AMP (second messenger mechanism of water soluble hormones) step 3

G protein
activates
adenylate
cyclase

cyclic AMP (second messenger mechanism of water soluble hormones) step 4

Adenylate
cyclase converts
ATP to cAMP
(2nd messenger

cyclic AMP (second messenger mechanism of water soluble hormones) step 5

cAMP activates
protein kinases

Direct Gene Activation
Mechanism of Lipid-Soluble
Hormones (steroids) step 1

The steroid
hormone diffuses
through the
plasma membrane
and binds an
intracellular
receptor

Direct Gene Activation
Mechanism of Lipid-Soluble
Hormones (steroids) step 2

The receptor-
hormone complex
enters the nucleus

Direct Gene Activation
Mechanism of Lipid-Soluble
Hormones (steroids) step 3

The receptor-
hormone complex
binds a specific
DNA region

Direct Gene Activation
Mechanism of Lipid-Soluble
Hormones (steroids) step 4

Binding initiates
transcription of the
gene to mRNA

Direct Gene Activation
Mechanism of Lipid-Soluble
Hormones (steroids) step 5

The mRNA
directs protein
synthesis.