Hipothalamus Part II

Tuberal Region

       The tuberal region (at the level of the tuber cinereum) is commonly divided into medial and
       lateral parts by a plane passing through the fornix (Fig. 2B). The fornix, which will be covered
       in the olfactory-limbic lecture (you may want to look ahead to Fig. 6 of that handout), is a fiber
       tract that originates in the hippocampus within the temporal lobe and courses rostrally along a C-
       shaped course next to the lateral ventricle, then plunges into the hypothalamus where it
       terminates within the mammillary bodies.

       The medial part of the tuberal region (between the fornix and third ventricle) contains 2 important
       nuclei; the lateral part, which is more loosely organized, is termed the lateral hypothalamic area
       (LHA). The medial forebrain bundle, which is described below, courses through the LHA.

       The largest and most prominent of the nuclei in the medial part of the tuberal region is the
       ventromedial nucleus. One important function that has been attributed to the ventromedial nucleus
       is control of eating. Bilateral lesions of the ventromedial nucleus in animals and probably humans
       as well, result in overeating (hyperphagia) and extreme obesity (Fig. 4) as well as a chronically
       irritable mood and increase in aggressive behavior (termed hypothalamic rage). By contrast,
       bilateral lesions in the lateral hypothalamic area result in anorexia (lack of appetite). Animals with
       lesions in this area may die of starvation. As a result of these lesion studies (along with supporting
       stimulation studies), the ventromedial nucleus has been referred to as a satiety center and the lateral
       hypothalamic area as a feeding center. It has been postulated that these opposing centers define a
       “set point” for body weight: the set point theory of weight control. According to this theory, when
       body weight goes below the set point, the lateral hypothalamus is activated and appetite is
       increased ; when body weight goes above the set point, the ventromedial nucleus is activated and
       appetite is decreased. This theory was questioned in the past, but recent evidence has been
       obtained that supports an integrative role of the ventromedial nucleus and lateral hypothalamic area
       in body weight control: their neurons respond to glucose, free fatty acid and insulin levels in a
       manner consistent with the set point theory, and activity levels in the two nuclei display a strict
       reciprocal relationship that is appropriately correlated with the level of hunger or satiety.

A second nucleus of great importance in the tuberal region is the arcuate nucleus (also known as
the infundibular or periventricular nucleus). This nucleus apparently contains many of the
neurons that control the endocrine functions of the adenohypophysis (although cells with this role
are also found in other nuclei of the tuberal region as well as in the supraoptic region and
preoptic area). These neurons do not directly release hormones from their endings like neurons
in the supraoptic region, but rather secrete releasing or release-inhibiting factors (also termed
releasing hormones) that regulate release of hormones by the adenohypophysis (Fig. 3B).
Releasing factors are secreted from terminals of these neurons in the median eminence and
infundibulum where they enter the portal system of vessels which carries them into the
adenohypophysis. Hormones under control of this system include: growth hormone, ACTH,
thyrotropin, the gonadotropins (FSH and LH), and prolactin. Consideration of the functional
roles of these hormones is beyond the scope of this course.

Lanjut ke part III