Hypothalamus is part of the brain, consisting of numerousnuclei, located above pituitary gland and connected to pituitary gland atinfundibulum. Hypothalamus links nervous and endocrine system together–itreceives neural signals from brain, sends these signals to the pituitary glandvia hypophyseal portal system (capillary) or hypothalamic hypophyseal tract(nerves). Pituitary gland is a master gland, controlling nearly all otherendocrine glands in the body, consisting of anterior and posterior pituitary. Hypothalamus has many regions. Region of paraventricular andsupraoptic nuclei is responsible for production of antidiuretic hormone andoxytocin. Region of arcuate nuclei produces releasing and inhibitory hormones.
After receiving neural signal, hypothalamus nuclei (e.g.arcuate or preoptic nuclei) produce releasing or inhibiting hormone, sends thehormone to anterior pituitary gland. Anterior pituitary obtains signals fromhypothalamus via pituitary portal system (small blood vessels flowing between hypothalamusand pituitary gland). These hormones signal pituitary gland to synthesize andrelease its hormones, which go to the other endocrine glands (tropic hormones-FSH, LH, ACTH, TSH), or direct hormones (PRL, GH), which go directly to thebody part they affect. Paraventricular and supraoptic nuclei produce ADH andoxytocin, which travels to posterior pituitary via nerves, and it gets storedthere. After receiving the neural signal, hypothalamus sends signal down thenerves to posterior pituitary to release the stored hormones. Hormone TRH (thyrotropin-releasing hormone) is made andreleased, after certain neural signal, from hypothalamus to anterior pituitary.
It stimulates anterior pituitary to produce and release TSH (thyroidstimulating hormone), which travels out of hypophyseal veins down in blooduntil it reaches thyroid gland, which has specific receptor for TSH. TSHsignals thyroid gland to produce and release triodothyronine and thyroxine, whichmain function is regulation of metabolism. T3 and T4 are circulating all overbody, until they reach their receptors. Some of the receptors for T3 or T4 arelocated on pituitary gland and hypothalamus. When the sufficient level ofthese hormones reach pituitary andhypothalamus receptors, it signals to hypothalamus and pituitary gland to stopproducing TRH and TSH (double check). This is negative feedback system.
Hormone oxytocin is released from posterior pituitary toblood, after hypothalamus receives neural signal and sends the signal down thenerves to posterior pituitary. It travels to the target organ, e.g. uterinemuscles. When the baby moves down to the birth canal, cervix and uterus getstretched, which sends neural impulse to hypothalamus, which sends signal to posteriorpituitary to release oxytocin, which travels to uterine muscles by blood andcauses contractions. a) This reaction starts in eye, which sends signalto amygdala, nervous tissue in brain, which triggers a neural response tohypothalamus. Hypothalamus activates sympathetic nervous system, which sendsimpulses to adrenal medulla to produce catecholamines: epinephrine andnorepinephrine.
Release of these hormones results in short-term stressresponse. Heart rate and blood pressureincrease. Digestion stops, because blood flow to the digestive systemdecreases, blood flow to muscles increases, so the supply of oxygen needed formuscle function increases. Liver transforms glycogen to glucose, which goes toblood and provides increase of energy for the muscles and other body parts.Bronchioles (air ways) expand, breathing gets faster, which increases theoxygen intake essential for the ATP conversion needed for brain (increasedalertness-better eye sight, hearing) and muscle movement and contraction. Metabolicrate increases, so the body burns calories faster, to supply the body (mainlymuscles and brain) with more energy. At the same time, paraventricular nucleiof hypothalamus send CRH to the anterior pituitary, which produces ACTH that travelsdown to adrenal cortex and causes release of glucocorticoids andmineralocorticoids.
Cortisol is glucocorticoid, which raises blood sugar level bytriggering gluconeogenesis-conversion of proteins and fats into glucose. Immunesystem is suppressed, because number of proteins involved in immune systemsignaling is decreased. Mineralocorticoid called aldosterone causes kidneys toreabsorb sodium, which leads to reabsorption of water and secrete potassium,which leads to increased blood pressure and volume. Cortisol is controlled by negativefeedback-cortisol travels to hypothalamus and pituitary gland, where it stopsCRH and ACTH release. b) Adrenal medulla produces catecholamines-adrenaline and noradrenaline. Function of adrenaline is the stress response(e.
g. increased heart rate, blood sugar level). Noradrenaline increasesalertness in the brain in the stress response, but it is also produced in brainand it has neurotransmitter function. Adrenal cortex producesmineralocorticoids (mainly aldosterone) and glucocorticoids (mainly cortisol). Thesehormones 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. Bloodpressure and volume increases, which leads to the release of ANP, hormone inhibitingrelease of aldosterone.
In case ofdehydration or sodium deficiency, blood volume and pressure decrease, whichtriggers the reaction that increases the blood pressure, one of its productscalled angiotensin II signals to adrenal cortex to produce aldosterone, whichraises blood pressure even more. Hypoaldosteronism causes hypotension,hyperaldosteronism causes hypertension, both may be fatal without treatment.Cortisol regulates blood sugar levels, fights the inflammation, increasessodium and water absorption and excretes potassium.
Adrenal cortex damage mayresult in Addison’s disease, lack of cortisol, with the symptoms ashypotension, weight loss, skin darkening. Addisonian crisis may be fatal. Hormonesproduced by adrenal cortex, cortisol and aldosterone, are very important forhealthy body functioning, although we can survive without adrenal cortex.Without adrenal cortex, life long steroid therapy is necessary and quality oflife decreases.
Hormones from adrenal medulla-catecholaminesproducing fight or flight response are evolutionary important as a part ofselective advantage, but they are not necessary for life-we can survive withoutstress response. Therefore, I agree with the statement: ‘Comparatively, it isbetter to damage the adrenal medulla than adrenal cortex.’