The Oral Cavity	The Oral Cavity
Hunger and the Hypothalamus	The Oral Cavity, cont'd
Odontology: Function & Phylogeny of Teeth

THE HYPOTHALAMUS AND HUNGER

John K. Young, Ph.D.

OBJECTIVES:

1. Learn the 3 basic divisions of the hypothalamus.

2. Learn the names and locations of 2 circumventricular organs and the characteristics that makes them unique in comparison to other brain structures. State their roles in the control of feeding, drinking, and vomiting.

3. Describe the location of the paraventricular nucleus. State the role of the paraventricular nucleus in the response to stress and in the control of feeding.

4. State the location of the arcuate nucleus. Describe how blood-borne substances may control the activity of the arcuate nucleus. Describe the role of the arcuate nucleus in the control of feeding.

OUTLINE:

I. Main divisions of the hypothalamus: anterior, middle, and posterior

The major clusters of neurons within the hypothalamus are termed nuclei:

Examples of anterior nuclei: suprachiasmatic (SCN) and supraoptic (SON) nuclei

Examples of middle nuclei: paraventricular (PVN) and arcuate nuclei

Examples of posterior nuclei: medial and lateral mammillary nuclei

II. Anterior Portion of the Hypothalamus

A. Suprachiasmatic nucleus (SCN)

1) Superior to optic chiasm, receives input from optic nerves (retino- hypothalamic tract); this input synchronizes the "clock" in the SCN to the exterior day-night cycle

2) projects to other hypothalamic nuclei, sets circadian rhythms: in sleep, hormone secretion, feeding and drinking, etc.

B. The Organum vasculosum lamina terminalis (OVLT) of the so-called preoptic area of the hypothalamus, just anterior to the SCN, is adjacent to unusually permeable capillaries. This allows the OVLT to "sense" and monitor blood constituents.

1) OVLT and the control of body temperature.

2) OVLT and the control of drinking and water balance

Neurons near the OVLT are sensitive to changes in blood constituents such as sodium, water, and hormones such as angiotensin and atrial natriuretic hormone

3) Examples of other circumventricular organs are:

a. the median eminence, just below the hypothalamus

In this picture, radioactively labelled molecules

of insulin (bright points of light) diffuse from

permeable capillaries in the median eminence

to influence the hypothalamic regulation of feeding.

 

b. the area postrema, near the hypoglossal nucleus in the medulla. The area postrema responds to toxic substances in the bloodstream or to signals from the GI tract by inducing VOMITING

III. Middle Portion of the Hypothalamus

A. Paraventricular nucleus (PVN)

1) Neurons that project to the posterior pituitary contain vasopressin and oxytocin (9 amino acid (aa) peptides)

a. Vasopressin (antidiuretic hormone) is released from the posterior pituitary and causes the production of a concentrated urine; secreted to conserve body fluids during water lack or blood loss

Oxytocin is released from the posterior pituitary in response to the suckling stimulus and causes release of milk from the mammary glands

Uterine contractions during labor can be enhanced by oxytocin;

however, the normal source of oxytocin during labor is now thought to be the uterine endometrium

2) Smaller PVN neurons project to other portions of the brain. These neurons project to:

a. Preganglionic neurons in the intermediolateral horn (T1-L2) of the spinal cord which control the sympathetic nervous system

b. Preganglionic neurons in the dorsal motor nucleus of the Vagus (DMN of Xth nerve), which regulate the parasympathetic control of gastrointestinal motility and insulin secretion

c. Neurons in the Nucleus of Tractus Solitarius and Hypoglossal Nucleus which regulate swallowing and tongue movement

B. Arcuate nucleus

1) Some arcuate neurons contain dopamine and project to the median eminence to inhibit pituitary prolactin release.

2) Some neurons contain Neuropeptide Y and project to the paraventricular nucleus to regulate its influence upon feeding.

a. When activated, NPY positive neurons can produce remarkable increases in eating that result in obesity

b. These neurons are overactive in strains of genetically obese rats.

c. These neurons are adjacent to the highly permeable median eminence, and may be regulated by circulating substances, such as:

glucose and insulin

leptin, a hormone produced by fat cells that regulates the development of obesity

By monitoring blood levels of these substances, arcuate neurons are kept informed of whether or not the body has sufficient calories and nutrients, and can modify feeding accordingly

The genetically obese strain of Zucker rats

(rat on the left) has inherited a defective hypothalamic

receptor for leptin and does not restrain feeding or

body weight.

 

Hypothalamic lesions that disinhibit these neurons also result in massive obesity. (Allowing these neurons to act without normal restrictions)