Hipothalamus

HYPOTHALAMUS

Introduction

The hypothalamus is a very small, but extremely important part of the diencephalon that is involved

in the mediation of endocrine, autonomic and behavioral functions. The hypothalamus: (1) controls

the release of 8 major hormones by the hypophysis, and is involved in (2) temperature regulation, (3)

control of food and water intake, (4) sexual behavior and reproduction, (5) control of daily cycles in

physiological state and behavior, and (6) mediation of emotional responses.

A large number of nuclei and fiber tracts have been described in the hypothalamus. Some of

these are ill-defined and have no known function, while others have been studied in detail both

anatomically and physiologically. This handout will attempt to focus your attention on the

significant and interesting aspects of the structure and function of the hypothalamus.

The hypothalamus is the ventral-most part of the diencephalon. As seen in Fig. 2 of the thalamus

handout, the hypothalamus is on either side of the third ventricle, with the hypothalamic sulcus

delineating its dorsal border. The ventral aspect of the hypothalamus is exposed on the base of the

brain (Fig. 1). It extends from the rostral limit of the optic chiasm to the caudal limit of the

mammillary bodies.

Three rostral to caudal regions are distinguished in the hypothalamus that correspond to three

prominent features on its ventral surface: 1) The supraoptic or anterior region at the level of the

optic chiasm, 2) the tuberal or middle region at the level of the tuber cinereum (also known as the

median eminence—the bulge from which the infundibulum extends to the hypophysis), and 3)

the mammillary or posterior region at the level of the mammillary bodies (Fig. 1)

Important components of the 3 rostral to caudal regions of the hypothalamus are as follows:

Supraoptic Region. In the supraoptic region (above the optic chiasm) there are a number of

named nuclei, the most prominent being the supraoptic and paraventricular (Fig. 2A). Neither of

these two nuclei is large, but each is easily recognized because it is made up of relatively large,

deeply staining cells. The cells in these two nuclei secrete vasopressin (ADH, antidiuretic

hormone), oxytocin, and CRH (corticotropin releasing hormone). ADH and oxytocin are

transported down the axons from cells in the supraoptic and paraventricular nuclei through the

infundibulum to the neurohypophysis (posterior pituitary), where they are released into the blood

stream (Fig. 3A). This pathway is termed the supraopticohypophysial tract. Damage to the

anterior hypothalamus blocks the production of ADH, resulting in diabetes insipidus, which is

characterized by rapid water loss from the kidneys. CRH is released by the paraventricular and

taken up by the portal system where it has its action on the anterior lobe of the pituitary. A

recent, interesting development is the finding of a direct projection from the eye to the

suprachiasmatic nucleus of the supraoptic hypothalamic region (Fig. 2A). The hypothalamus is

thought to contain the “biological clock” that regulates certain body functions that vary at

different times of the day (e.g., body temperature, hormone secretion, hunger) or those that vary

over a period of many days (e.g., menstrual cycle). This projection from the retina to the

suprachiasmatic nucleus is thought to supply the clock with day-night information needed for

synchronizing diurnal (daily) rhythms (also known as circadian rhythms). Lesions of the

hypothalamus often disrupt the state of the sleep-waking cycle.

FIG. 1: Ventral surface of the hypothalamus. Dashed lines indicate levels from which cross sections in figure 2A, 2B and 2C were taken.

Lanjut ke part II