Adrenal Medulla

Adrenal medulla

Inner part of adrenal gland

Neuroendocrine organ → controlled by NS but release hormones into blood

Works alongside sympathetic nervous system (fight or flight response)

Structure of adrenal gland

Cortex - steroid hormones (cortisol and aldosterone)

Medulla - catecholamines (epinephrine, norepinephrine)

Adrenal medulla sympathetic

Giant sympathetic ganglion → instead of sending signals through nerves, releases hormones in blood

Preganglionic sympathetic neurons from spinal cord goes straight to medulla

These neurons release ACh → stimulates chromaffin cells (which act as postganglionic neurons)

Chromaffin cells release epinephrine and norepinephrine into bloodstream

Regulation of sympathetic adrenal medulla

Hypothalamus is the mean control center

Sends signals to activate SNS

This also stimulates the pituitary gland which releases ACh → stimulates cortisol from the adrenal cortex → prolonged the effects of epinephrine

Catecholamines

Play a key role in body’s stress response

Derived from amino acid tyrosine

Hydrophilic (hydro-soluble), circulate freely in blood

Like epinephrine, norepinephrine and dopamine

Catecholamine - epinephrine

Produced in adrenal medulla

Involved in fight or flight

Increases heart rate, blood pressure, blood sugar and dilates airways

Catecholamine - norepinephrine

Produced in renal medulla

Acts more like a neurotransmitter than a hormone

Causes vasoconstriction and increases blood pressure

Catecholamines - dopamine

Precursor to norepinephrine

Functions mainly as a neurotransmitter in brain

Also plays a role in motor control and hormone regulation

Catecholamine synthesis in chromaffin cells

Cytoplasm - Tyrosine hydroxylase adds a -OH (hydroxyl group) to tyrosine → L-DOPA

Cytoplasm - DOPA decarboxylase removes CO2 (carboxyl) group → dopamine

Secretory vesicles - Dopamine β-hydroxylase adds -OH group to carbon chain → norepinephrine

Adrenal medulla - PNMT adds methyl (CH3) group to norepinephrine → epinephrine

Trigger of catecholamine secretion

Stressor activates SNS

ACh from preganglionic fibers causes depolarization of chromaffin cells

Depolarization opens voltage-gated calcium channels, Ca2+ rushes in

This triggers exocytosis of hormone-containing granules

Alpha (α) receptors

Mostly cause constriction and help raise blood pressure

α1 - in blood vessels, causes vasoconstriction, sensitive to norepinephrine

α2 - nerve endings and pancreas, inhibits further release of norepinephrine (and insulin)

Beta (β) receptors

Mostly cause relaxation, stimulation or energy release

β1 - heart, increase heart rate and contractions, sensitive to epinephrine and NE

β2 - in lungs (bronchodilation), skeletal muscle (vasodilation) and liver (glycogenolysis - release of sugar), mostly epinephrine

β3 - adipose tissue, breaks down fat for energy, epinephrine

Carbohydrate metabolism

Liver - ↑ glycogenolysis (breakdown glycogen), ↑ gluconeogenesis (produce glucose)

Pancreas - ↑ glucagon (raise blood sugar), ↓ insulin (reduce glucose storage)

Muscles - ↑ glycogen breakdown for quick energy

OVERALL - ↑ glycemia (blood glucose for energy during stress)

Lipid metabolism

Adipose tissue - ↑ lipolysis (releases free fatty acids)

Liver - uses FFAs via β-oxidation to generate energy