Hypothalamus is part of the brain, consisting of numerous
nuclei, located above pituitary gland and connected to pituitary gland at
infundibulum. Hypothalamus links nervous and endocrine system together–it
receives neural signals from brain, sends these signals to the pituitary gland
via hypophyseal portal system (capillary) or hypothalamic hypophyseal tract
(nerves). Pituitary gland is a master gland, controlling nearly all other
endocrine glands in the body, consisting of anterior and posterior pituitary.

Hypothalamus has many regions. Region of paraventricular and
supraoptic nuclei is responsible for production of antidiuretic hormone and
oxytocin. Region of arcuate nuclei produces releasing and inhibitory hormones.

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After receiving neural signal, hypothalamus nuclei (e.g.
arcuate or preoptic nuclei) produce releasing or inhibiting hormone, sends the
hormone to anterior pituitary gland. Anterior pituitary obtains signals from
hypothalamus via pituitary portal system  (small blood vessels flowing between hypothalamus
and pituitary gland). These hormones signal pituitary gland to synthesize and
release its hormones, which go to the other endocrine glands (tropic hormones-
FSH, LH, ACTH, TSH), or direct hormones (PRL, GH), which go directly to the
body part they affect.

Paraventricular and supraoptic nuclei produce ADH and
oxytocin, which travels to posterior pituitary via nerves, and it gets stored
there. After receiving the neural signal, hypothalamus sends signal down the
nerves to posterior pituitary to release the stored hormones.

Hormone TRH (thyrotropin-releasing hormone) is made and
released, after certain neural signal, from hypothalamus to anterior pituitary.
It stimulates anterior pituitary to produce and release TSH (thyroid
stimulating hormone), which travels out of hypophyseal veins down in blood
until it reaches thyroid gland, which has specific receptor for TSH. TSH
signals thyroid gland to produce and release triodothyronine and thyroxine, which
main function is regulation of metabolism. T3 and T4 are circulating all over
body, until they reach their receptors. Some of the receptors for T3 or T4 are
located on pituitary gland and hypothalamus. When the sufficient level of
these  hormones reach pituitary and
hypothalamus receptors, it signals to hypothalamus and pituitary gland to stop
producing TRH and TSH (double check). This is negative feedback system.

Hormone oxytocin is released from posterior pituitary to
blood, after hypothalamus receives neural signal and sends the signal down the
nerves to posterior pituitary. It travels to the target organ, e.g. uterine
muscles. When the baby moves down to the birth canal, cervix and uterus get
stretched, which sends neural impulse to hypothalamus, which sends signal to posterior
pituitary to release oxytocin, which travels to uterine muscles by blood and
causes contractions. a)     
This reaction starts in eye, which sends signal
to amygdala, nervous tissue in brain, which triggers a neural response to
hypothalamus. Hypothalamus activates sympathetic nervous system, which sends
impulses to adrenal medulla to produce catecholamines: epinephrine and
norepinephrine. Release of these hormones results in short-term stress
response.  Heart rate and blood pressure
increase. Digestion stops, because blood flow to the digestive system
decreases, blood flow to muscles increases, so the supply of oxygen needed for
muscle function increases. Liver transforms glycogen to glucose, which goes to
blood and provides increase of energy for the muscles and other body parts.
Bronchioles (air ways) expand, breathing gets faster, which increases the
oxygen intake essential for the ATP conversion needed for brain (increased
alertness-better eye sight, hearing) and muscle movement and contraction. Metabolic
rate increases, so the body burns calories faster, to supply the body (mainly
muscles and brain) with more energy. At the same time, paraventricular nuclei
of hypothalamus send CRH to the anterior pituitary, which produces ACTH that travels
down to adrenal cortex and causes release of glucocorticoids and
mineralocorticoids. Cortisol is glucocorticoid, which raises blood sugar level by
triggering gluconeogenesis-conversion of proteins and fats into glucose. Immune
system is suppressed, because number of proteins involved in immune system
signaling is decreased. Mineralocorticoid called aldosterone causes kidneys to
reabsorb sodium, which leads to reabsorption of water and secrete potassium,
which leads to increased blood pressure and volume.  Cortisol is controlled by negative
feedback-cortisol travels to hypothalamus and pituitary gland, where it stops
CRH and ACTH release.


Adrenal medulla produces catecholamines-
adrenaline and noradrenaline. Function of adrenaline is the stress response
(e.g. increased heart rate, blood sugar level). Noradrenaline increases
alertness in the brain in the stress response, but it is also produced in brain
and it has neurotransmitter function. Adrenal cortex produces
mineralocorticoids (mainly aldosterone) and glucocorticoids (mainly cortisol). These
hormones are very important for the processes in our body. Aldosterone makes  collecting ducts in kidney retain sodium,
excretes potassium, which causes osmotic pressure and water reabsorption. Blood
pressure and volume increases, which leads to the release of ANP, hormone inhibiting
release of aldosterone.  In case of
dehydration or sodium deficiency, blood volume and pressure decrease, which
triggers the reaction that increases the blood pressure, one of its products
called angiotensin II signals to adrenal cortex to produce aldosterone, which
raises blood pressure even more. Hypoaldosteronism causes hypotension,
hyperaldosteronism causes hypertension, both may be fatal without treatment.
Cortisol regulates blood sugar levels, fights the inflammation, increases
sodium and water absorption and excretes potassium. Adrenal cortex damage may
result in Addison’s disease, lack of cortisol, with the symptoms as
hypotension, weight loss, skin darkening. Addisonian crisis may be fatal. Hormones
produced by adrenal cortex, cortisol and aldosterone, are very important for
healthy body functioning, although we can survive without adrenal cortex.
Without adrenal cortex, life long steroid therapy is necessary and quality of
life decreases.

 Hormones from adrenal medulla-catecholamines
producing fight or flight response are evolutionary important as a part of
selective advantage, but they are not necessary for life-we can survive without
stress response. Therefore, I agree with the statement: ‘Comparatively, it is
better to damage the adrenal medulla than adrenal cortex.’



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