Hypothalamus

Hypothalamus is a diencephalic structure. It is situatedjust below thalamus in the ventral part of diencephalon. It is formed by groups of nuclei, scattered in the walls and floor of third ventricle. It extends from optic chiasma to mamillary body.

NUCLEI OF HYPOTHALAMUS

Nuclei of hypothalamus are divided into three groups:

1. Anterior or preoptic group

2. Middle or tuberal group

3. Posterior or mamillary group.

CONNECTIONS OF HYPOTHALAMUS

AFFERENT CONNECTIONS

TO HYPOTHALAMUS

1. Medial forebrain bundle: From rhinencephalon (limbic cortex) to preoptic nucleus, lateral nucleus

and mamillary body

2. Fornix: From hippocampus to mamillary body

3. Stria terminalis: From amygdaloid to preoptic nucleus

4. Corticohypothalamic fibers: From prefrontal area (8) and precentral area (6) of cerebral cortex

to the supraoptic and paraventricular nuclei of hypothalamus

5. Pallidohypothalamic fibers: From globus pallidus to diffused areas of hypothalamus

6. Thalamohypothalamic fibers: From dorsomedial and midline nuclei of thalamus to diffused areas of

hypothalamus

7. Reticulohypothalamic fibers: From reticular formation of brainstem to diffused areas of hypothalamus

8. Retinohypothalamic fibers: Fibers from retina to supraoptic, suprachiasmatic and ventromedial

nuclei of hypothalamus.

EFFERENT CONNECTIONS FROM HYPOTHALAMUS

1. Mamillothalamic tract: From mamillary body to anterior thalamic nuclei

2. Mamillotegmental tract: From mamillary body to the tegmental nuclei of midbrain

3. Periventricular fibers: Fibers from posterior, supraoptic and tuberal nuclei of hypothalamus pass

through periventricular gray matter and reach the following:

i. Reticular formation in brainstem and spinal cord

ii. Dorsomedial nucleus of thalamus

iii. Frontal lobe of cerebral cortex

4. Hypothalamohypophyseal tract: From supraoptic and paraventricular nuclei of hypothalamus to

posterior pituitary.

FUNCTIONS OF HYPOTHALAMUS

Hypothalamus is the important part of brain, concerned with homeostasis of the body. It regulates many vital functions of the body like endocrine functions, visceral functions, metabolic activities, hunger, thirst, sleep, wakefulness, emotion, sexual functions, etc.

SECRETION OF POSTERIOR

PITUITARY HORMONES

Hypothalamus is the site of secretion for the posterior pituitary hormones. Antidiuretic hormone (ADH) and oxytocin are secreted by supraoptic and paraventricular nuclei. These two hormones are transported by means of axonic or axoplasmic flow through the fibers of hypothalamohypophyseal tracts to posterior pituitary.

CONTROL OF ANTERIOR PITUITARY

Hypothalamus controls the secretions of anterior pituitary gland by secreting releasing hormones and inhibitory hormones. It secretes seven hormones.

i. Growth hormone-releasing hormone (GHRH)

ii. Growth hormone-releasing polypeptide (GHRP)

iii. Growth hormone-inhibiting hormone (GHIH) or 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 by discrete areas of hypothalamus and transported to anterior pituitary by

the hypothalamohypophyseal portal blood vessels.

CONTROL OF ADRENAL CORTEX

Anterior pituitary regulates adrenal cortex by secreting adrenocorticotropic hormone (ACTH). ACTH secretion is in turn regulated by corticotropin-releasing hormone (CRH), which is secreted by the paraventricular nucleus of hypothalamus.

CONTROL OF ADRENAL MEDULLA

Dorsomedial and posterior hypothamic nuclei are excited by emotional stimuli. These hypothalamic

nuclei, in turn, send impulses to adrenal medulla through sympathetic fibers and cause release of

catecholamines, which are essential to cope up with emotional stress.

REGULATION OF AUTONOMIC NERVOUS SYSTEM

Hypothalamus controls autonomic nervous system (ANS). Sympathetic division of ANS is regulated

by posterior and 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 hypothalamus6. REGULATION OF HEART RATE

Hypothalamus regulates heart rate through vasomotor center in the medulla oblongata. Stimulation of posterior and lateral nuclei of hypothalamus increases the heart rate. Stimulation of preoptic and anterior nuclei decreases the heart rate.

REGULATION OF BLOOD PRESSURE

Hypothalamus regulates the blood pressure by acting on the vasomotor center. Stimulation of posterior and lateral hypothalamic nuclei increases arterial blood pressure and stimulation of preoptic area decreases the blood pressure.

REGULATION OF BODY TEMPERATURE

Body temperature is regulated by hypothalamus, which sets the normal range of body temperature. The set point, under normal physiological conditions is 37°C. Hypothalamus has two centers which regulate the body temperature:

i. Heat loss center that is present in preoptic

nucleus of anterior hypothalamus

ii. Heat gain center that is situated in posterior

hypothalamic nucleus.

REGULATION OF HUNGER AND FOOD INTAKE

Food intake is regulated by two centers present in hypothalamus:

i. Feeding center

ii. Satiety center..

 

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