Hypothalamus

Hypothalamus

Location : Hypothalamus forms floor of diencephalon (a part of forebrain) or floor of third ventricle i.e. diocoel and part of its lateral walls

Structure : It is composed of nervous tissue. It is a neuroendocrine structure ie it performs both nervous and endocrine functions. It is that area of brain where nervous and endocrine systems are integrated . It communicates with anterior lobe of pituitary gland by hypophysial portal blood vessels and with posterior lobe of the pituitary gland by axons of its neurons. The portal vessels and axons pass through a stalk to the pituitary.

Nervous functions of hypothalamus

It is the temperature regulating centre of body ie thermostat.

It has higher nerve centres for controlling emotional reactions, sweating, fatigue, sleep, thirst, hunger (lateral hypothalamus); satiety (ventro-medial hypothalamus).

Endocrine functions of hypothalamus

It has masses of grey matter (hypothalamic nuclei) scattered in white matter. Hypothalamic nuclei produces neurohormones.

Hypothalamic Nuclei : Cell bodies of certain neurons group together to secrete special biochemical substances. This grouping of cell bodies in

Central Nervous System (CNS) is called Nuclei (such grouping in peripheral nervous system is called Ganglia). Some of the hypothalamic nuclei are

Preoptic Nucleus (PON) Suprachiasmatic Nucleus (SCN)

Supraoptic Nucleus (SON) Paraventricular Nucleus (PVN)

Dorsomedialis Nucleus Ventromedialis Nucleus

Arcuate Nucleus

Preoptic Nucleus and Supra chiasmatic Nucleus are called True Biological Clocks of body. (Biological clock is any cell or tissue in our body

which shows a rhythm in its activity. Study of biological rhythms is called chronobiology). Dorsomedialis, Ventromedialis and Arcuate

Nucleus are called Tonic centres of brain. (These produce GnRH i.e. Gonadotropin releasing hormone)

The neurosecretory cells of hypothalamus (ie Nuclei ) when stimulated release hormones called neurohormones via axons into capillaries. These

neurohormones are carried by portal blood to anterior lobe of pituitary gland and stimulate it to release its hormones. On this account, such

hypothalamic hormones are also called Releasing hormones (RH). Certain hypothalamic hormones inhibit the secretion of pituitary hormones.

These are termed Inhibitory hormones or factors (IF). Hypothalamic releasing and inhibitory hormones are described below.

Summary of hypothalamic hormones and pituitary response

Hypothalamic Hormone Response of Pituitary Target Organ

1. Thyrotrophin-releasing hormone Thyroid-stimulating hormone Thyroid (TRH) (TSH) secretion

2. Adrenocorticotrophin-releasing Adrenocorticotrophic hormone hormone (ACTH-RH) (ACTH) secretion Adrenal cortex

3. Follicle stimulating hormone- Follicle stimulating hormone Ovary / Testis releasing hormone (FSH-RH) (FSH) secretion

4. Luteinising hormone-releasing Luteinising hormone (LH) secretion Ovary / Testis hormone (LH-RH)

5. Growth hormone-releasing Growth hormone (GH) or somato- Most tissues

hormone (GH-RH) trophic hormone (STH) secretion

6. Growth hormone release inhibiting Growth hormone secretion -------------- hormone (GH-RIH) or somatostatin inhibited

7. Prolactin-releasing hormone (PRH) Protactin hormone (PH) or Mammary glands luteotrophic hormone (LTH) secretion

8. Protactin release-inhibiting hormone Prolactin secretion inhibited -------------- (P-RIH)

9. Melanocyte-stimulating hormone Melanocyte-stimulating hormone Skin pigment cells releasing hormone (MSH-RH) (MSH) secretion

10. Melanocyte-stimulating hormone Melanocyte-stimulating hormone -------------- release-inhibiting hormone (MSH-RIH) secretion inhibited

Hypothalamic-pituitary system is a direct proof of coordination between endocrine and nervous system. It regulates most of physiological activities of body and maintain homeostasis inside the body.

The ability of endocrine secretion in hypothalamus is to monitor metabolite and hormone level in the body as per the information gathered by

brain. Hypothalamic hormones released in blood come to pituitary through hypothalamo-hypophyseal portal circulation.