Hipothalamus Part IV

Neurotransmitters in the Hypothalamus

The hypothalamus has been referred to as a “pharmacological museum” (Goodman and Gilman,
‘55) by virtue of the plethora of neurotransmitters that it contains. Some of these neurotransmitters are found in the terminals of axons that originate from neurons outside the hypothalamus, but most are synthesized within the hypothalamus itself. The list of putative neurotransmitters includes the “classical” transmitters ACh, GABA, glutamate, serotonin, dopamine, and norepinephrine as well as literally dozens of peptides that have been identified in recent years.
While the overwhelming number of transmitters may create headaches for scientists and students, their presence offers hope for pharmacological intervention in the large number of different medical problems that are thought to involve the hypothalamus.

POINTS TO CONCENTRATE ON WHEN STUDYING:
1) Spatial relationships to surrounding brain structures. Don’t forget that the optic chiasm
   overlies the hypothalamus and therefore that visual disturbances (especially bitemporal
   hemianopsia, to be discussed in the vision lectures, can accompany pathology of the
   hypothalamus or hypophysis.

2) Know the location of each important nucleus or area with respect to the hypothalamic
   subdivisions (supraoptic, tuberal, etc.). This is, of course, helpful in diagnosis of lesions.

3) Functions: I consider all of the information in the handout with regard to functions as
   essential. However, you will not be tested on hormones since this material is covered in
   other courses. Read the section, “Functions of the Hypothalamus” very carefully.

4) Don’t worry about details in the “Neurotransmitters in the hypothalamus” section of the
   handout. Just read it for concepts.

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Hipothalamus Part III

Mammillary Region

       The mammillary part of the hypothalamus consists of the posterior hypothalamic nucleus and the
       prominent mammillary nuclei. The posterior nucleus is a large, ill-defined group of cells that
       may play a role in thermoregulation (see below). The mammillary nuclei are considered to be
       part of the hypothalamus on anatomical grounds, but, unlike the other hypothalamic nuclei, they
       do not appear to be closely related to autonomic and endocrine functions. Instead, the
       mammillary nuclei are believed to play a role in memory, and will be discussed during the limbic
       system lecture. For now, just associate the mammillary nuclei with memory.

       Summary of hypothalamic fiber tracts: There are four major fiber systems in the hypothalamus,
       all of which are bi-directional. The two largest and most clearly defined tracts carry fibers to and
       from the mammillary nuclei: the fornix (Figs. 6 & 9; also see Fig. 6 in Olfactory Pathways and
       Limbic System handout) carries fibers to these nuclei from the hippocampal formation (to be
       described in the limbic system lecture), and the mammillothalamic tract contains fibers running
       to and from the anterior nuclear group of the thalamus (Figs. 5 and 9). The stria terminalis (Fig.
       6) is an afferent pathway to the hypothalamus from the amygdala (also to be described in the
       limbic system lecture). The medial forebrain bundle (Figs. 6, 9) is an ill-defined bundle of
       unmyelinated and thinly myelinated axons which courses through the lateral hypothalamic area.
       This bundle contains a very large number of different ascending and descending components (50
       were distinguished in a recent paper). These include fibers ascending from the monoamine cell
       groups in the brainstem, descending fibers from the olfactory cortex that may play a role in
       appetite control, and descending fibers from other basal forebrain structures. It has been
       implicated in controlling many physiological functions like sleep, and behavioral traits like
       depression and pleasure.

       Functions of the Hypothalamus: Many of the functions of the hypothalamus are of a homeostatic
       nature (maintenance of constant body states). Several of these have already been described in
       conjunction with the description of the nuclei involved. Such functions require the integration of
       activity in many different body systems. A good example is temperature regulation (see Fig. 7
       but do not learn this figure). When body temperature increases, neurons in the anterior part of
       the hypothalamus turn on mechanisms for heat dissipation that include sweating and dilation of
       blood vessels in the skin. When body temperature decreases, neurons in the posterior part of the
       hypothalamus are responsible for heat production through shivering, vasoconstriction in the skin,
       and blockage of perspiration. Lesions in the anterior part can result in hyperthermia (increase in
       body temperature) and lesions in the caudal part can result in hypothermia when the
       environmental temperature is low.

       A recent, very significant finding is a direct projection from the hypothalamus to the
       preganglionic sympathetic neurons in the lateral horn (intermediolateral cell column) of the
       spinal cord and to the preganglionic parasympathetic neurons in the dorsal motor nucleus of the
       vagus. (Recall from Dr. Harting’s lecture that Horner’s syndrome can result from interruption
       of the pathway from the hypothalamus to the lateral horn as it passes thru the brain stem.) The
       cells of origin for these projections are located in many different parts of the hypothalamus. The
       hypothalamus has long been known as the “head ganglion” of the autonomic nervous system, but
       had been assumed to mediate its effects through multisynaptic pathways.

       In addition to autonomic and endocrine functions, the hypothalamus is also involved in the
       control of emotional expression. As described above, bilateral lesions including the
       ventromedial nucleus of the hypothalamus in animals have long been known to produce
       expressions of rage (see Fig. 8 but do not learn this figure). Stimulation of various other
       hypothalamic nuclei produces a variety of other emotions including pleasure and fear.
   

It is important to bear in mind, however, that many other areas of the brain, especially those
collectively termed the limbic system (following lecture), are involved in the control of emotions.

Another aspect of hypothalamic function that is interesting to consider is that it may be the site
where, in many cases, emotional factors influence body functions. It is well known that many
functions that are under autonomic or hormonal control are influenced to a large degree by
emotions. For example, emotional factors can influence or block menstruation, lactation, or
sexual function. Since the hypothalamus is concerned with control of emotions, and regulation
of both hormone release and the autonomic nervous system, it is thought to be involved in the
mediation of such effects.

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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

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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

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