Hypothalamus

Hypothalamus

Is a part of Diencephalon, made of a collection of nuclei below the thalamus.

Each nucleus is a collection of neurons that regulates specific  body function(s).

Functions:

Regulation of water  intake & loss by:

Thirst center.

Supraoptic & paraventricular nuclei that secrete Vasopressin.

Stimuli of the thirst centre & Vasopressin secretion include :

Hyperosmolality of the plasma by hypernatraemia or  hyperglycaemia.

Hypovolaemia: Decreased blood volume after haemorrhage,diarrhoea, sweating, polyuria, vomiting.

Hypotension.

AngiotensinII.

Thirst center stimulation → ↑ water intake

Vasopressin stimulates water reabsorption from the renal collecting duct → ↓ the water loss.

2.Regulation of food intake & body weight.

Feeding center located laterally & satiety center ventromedially in hypothalamus.

Feeding centre Activation → hunger felling ↑ the appetite for food intake.

Activation of satiety centre inhibits the feeding centre → Satiety feeling ↓ the appetite for food intake.

The chemical messengers acting on the hypothalamus to ↑ the appetite for food intake, include:

Neuropeptide Y.

Orexins.

melanin-concentrating hormone. MCH

Chemical meseengers acting on the hypothalamus to ↓ the appetite for food:

Leptin.

Cocaine-& Amphetamine-regulated transcript CART.

Corticotropin-releasing hormone CRH.

Catecholamines.

Melanocytes stimulating hormone MSH.

Hypotheses controlling the food intake:

lipostatic hypothesis States that:

↑ fats in the adipose tissue → release of  a chemical messenger  Leptin .

Leptin acts on the hypothalamus to ↓ Neuropeptide Y.

Ø   → Satiety feeling ↓ing  the appetite.

II. The glucostatic hypothesis States that :

Increased glucose utilization in hypothalamus activate satiety centre →  satiety feeling.

III. Gut Peptides  Hypothesis: States that:

food entering the GIT triggers release of GIT hormones which act on the brain to cause satiety. They include:

Ø  Gastrin releasing peptide GRP.

Ø  Glucagon.

Ø  Somatostatin.

Ø  Cholecystokinin CCK.

IV. Thermostatic hypothesis: States that:

Ø  ↑in body or climatic temperature ↓ the appetite for food intake.

Ø  Cold climate↑ the appetite for  food intake

This theory is important for regulation of body temperature.

The hypothalamus is a main regulatory centre in the Autonomic nervous system.

Regulation of body temperature by two centres:

Heat sensitive area in the anterior hypothalamus. Stimulation gives responses :

Ø  Satiety feeling ↓ing  appetite for food.

Ø  Dilation of skin (cutaneous) blood vessels to ↑ flow of the warm blood to skin surface.

Ø  Sweating. & apathy.

Ø  Assuming relax posture to ↑ exposed skin area

Cold sensitive area in the posterior hypothalamus, Stimulation leads to:

Ø  Hunger to increase food intake.

Ø  Cutaneous vasoconstricion.

Ø  Increased muscular activity (shivering).

Ø  Assuming tense posture to ↓ the exposed skin area

5.Neuroendocrine function:

The hypothalamus is connected by neural tract to the posterior lobe of the pituitary gland, & by vascular (portal) to the anterior lobe.

Cell bodies of supraoptic & paraventricular nuclei synthesize Vasopressin & Oxytocin that pass via hypothalamohypophysial tract to the posterior pituitary.

Hypothalamic releasing & inhibiting factors  are hypophysiotropic hormones that control the anterior pituitary.

6. Hypothalamic suprachiasmatic nucleus SCN control the cyclic phenomena (circadian rhythm) which are physiologic effects related to the day- night cycle.

7. Hypothalamus is a part sleep zone.

8. As a functional part of the limbic system. Its involved in the defense reactions (fear & rage), emotions & motivations (reward & punishment).