This post is about the Functions (physiology) of Hypothalamus so, let’s start with introduction.
INTRODUCTION
Hypothalamus is a diencephalic structure.
It is situated just below thalamus in ventral part of diencephalon.
It is formed by groups of nuclei, in walls & floor of the third ventricle.
It extends from optic chiasma to mamillary body.
FUNCTIONS
Hypothalamus is very important part of the brain because it is concerned with the homeostasis of body.
It regulates many vital functions of body like Endocrine functions, visceral functions, metabolic activities, hunger, thirst, sleep, wakefulness, emotion, sexual functions, etc.
So, let’s learn about them in brief.
1.SECRETION OF POSTERIOR PITUITARY HORMONES
- Hypothalamus is the site of secretion for the posterior pituitary hormones.
- Antidiuretic hormone (ADH) & oxytocin are secreted by supraoptic & Paraventricular.
2.CONTROL OF ANTERIOR PITUITARY
- It controls the secretions of Anterior pituitary gland by secreting releasing hormones and inhibitory hormones. It secretes 7 hormones.
I. Growth hormone-releasing hormone (GHRH)
ii. Growth hormone-releasing polypeptide (GHRP)
iii. Somatostatin
iv. Thyrotropin-releasing hormone (TRH)
v. Corticotropin-releasing hormone (CRH)
vi. Gonadotropin-releasing hormone (GnRH)
vii. Prolactin-inhibiting hormone (PIH). - These hormones are secreted from different areas of hypothalamus and transported to anterior pituitary by hypothalamohypophyseal portal blood vessels.
3. CONTROL OF ADRENAL CORTEX
- Anterior pituitary regulates adrenal cortex by secreting adrenocorticotropic hormone (ACTH).
- ACTH secretion is regulated by CRH (Corticotropin-releasing hormone), which is secreted by hypothalamus.
4.CONTROL OF ADRENAL MEDULLA
- Dorsomedial and posterior hypothalamic nuclei are excited by emotional stimuli.
- They in turn, send impulses to adrenal medulla & cause release of catecholamines, which are essential to cope up with emotional stress.
5. REGULATION OF AUTONOMIC NERVOUS SYSTEM
- Sympathetic division of ANS is regulated by posterior & lateral nuclei of hypothalamus.
- Parasympathetic division of ANS is controlled by anterior group of nuclei.
- The effects of cerebral cortex on ANS are executed through hypothalamus.
6.REGULATION OF HEART RATE
- Hypothalamus regulates sexual functions by secreting gonadotropin releasing hormones.
- Arcuate and posterior hypothalamic nuclei are involved in regulation of sexual functions.
7. REGULATION OF BLOOD PRESSURE
- Regulates the blood pressure by acting on vasomotor centre.
- Stimulation of posterior & lateral hypothalamic nuclei increases arterial BP.
- Stimulation of preoptic area decreases blood pressure.
8.REGULATION OF BODY TEMPERATURE
- Body temperature is regulated by hypothalamus.
- Under normal physiological conditions body temperature sets at 37°C.
- TWO centres are there for the regulation of body temperature:
- Heat loss centre: Present in anterior hypothalamus.
- Heat gain centre Present in posterior hypothalamus.
9. REGULATION OF HUNGER AND FOOD INTAKE
Food intake is regulated by two centres present in hypothalamus:
Feeding Centre
- It is situated in the lateral hypothalamic nucleus.
- Normally, feeding centre is always active.
- That means, it has always tendency to urge food intake.
Satiety Centre
- It is situated in the ventromedial hypothalamic nucleus.
- Stimulation of it results in total loss of appetite & cessation of food intake.
- It plays an important role in regulation of food intake by temporary inhibition of feeding centre after food intake.
10.REGULATION OF WATER BALANCE
Hypothalamus regulates water content of the body by 2 mechanisms:
1.Thirst Mechanism
Thirst centre is in lateral nucleus of hypothalamus
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Where osmoreceptors are present
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When Cerebral Fluid volume decreases, osmolality of ECF is increased
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If osmolarity increases by 1% to 2%, osmoreceptors are stimulated
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Osmoreceptors in turn activate thirst centre and thirst sensation is initiated.
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Now, person feels thirsty & drinks water
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Water intake increases the ECF volume & decreases osmolality.
2. ADH Mechanism
- Simultaneously, when volume of ECF decreases with increased osmolality,
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Supraoptic nucleus is stimulated & ADH is released.
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ADH causes retention of water by reabsorption in renal tubules.
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It increases ECF volume and brings osmolality back to normal level.
- On the contrast, when ECF volume is increased,
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Supraoptic nucleus is not stimulated and ADH is not secreted.
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In absence of ADH, more amount of water is excreted through urine and volume of ECF is brought back to normal.
11. REGULATION OF SLEEP AND WAKEFULNESS
- Mamillary body in posterior hypothalamus is wakefulness centre.
- Stimulation of it causes wakefulness and its injury leads to sleep.
- Stimulation of anterior hypothalamus also leads to sleep.
12. ROLE IN BEHAVIOR AND EMOTIONAL CHANGES
Hypothalamus has 2 centres for behavioural & emotional changes.
They are:
- Reward centre (satisfaction)
- Punishment centre (aversion)
Reward Centre: Stimulation of these areas causes sense of pleasant or satisfaction.
Punishment Centre: Stimulation of these areas causes sense of pain, fear, defence, escape reactions and other features of punishment.
Rage: Rage is a violent and aggressive emotional expression with extreme anger. It can be Developed by stimulating punishment centres.
13.REGULATION OF SEXUAL FUNCTION
- Hypothalamus regulates sexual functions by secreting gonadotropin releasing hormones.
- Arcuate and posterior hypothalamic nuclei are involved in regulation of sexual functions.
14. ROLE IN RESPONSE TO SMELL
- Posterior hypothalamus, hippocampus and brainstem nuclei are responsible for autonomic responses of body to olfactory stimuli.
- Responses include feeding activities and emotional responses like fear, excitement and pleasure.
15. ROLE IN CIRCADIAN RHYTHM (BIOLOGICAL CLOCK/DIURNAL RHYTHM)
- Suprachiasmatic nucleus of hypothalamus through efferent nerve fibres, sends circadian signals to different parts of the body to maintain circadian rhythm of sleep, hormonal secretion, thirst, hunger, appetite, etc.
- Whenever body is exposed to a new pattern of daylight or darkness rhythm, circadian rhythm also changes.
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