Neuroanatomy- Post #4- The Hypothalamus

            The hypothalamus, which serves a number of autonomic, appetitive, and regulatory functions, lies below and in front of the thalamus; it forms the floor and lower walls of the third ventricle. External landmarks of the hypothalamus are the optic chiasm; the tuber cinereum, with its infundibulum extending to the posterior lobe of the hypophysis; and the mamillary bodies lying between the cerebral peduncles.

The hypothalamus can be divided into an anterior portion, the chiasmatic region, including the lamina terminalis; the central hypothalamus, including the tuber cinereum and the infundibulum (the stalk connecting the pituitary to the hypothalamus); and the posterior portion, the mamillary area.

The right and left sides of the hypothalamus each have a medial hypothalamic area that contains many nuclei and a lateral hypothalamic area that contains fiber systems (eg, the medial forebrain bundle) and diffuse lateral nuclei.

            It is through these pathways that the hypothalamus relays afferent and efferent signals. The hypothalamus receives (afferent) inputs from limbic structures, thalamus and cortex, visceral and somatic afferents, and sensors such as osmoreceptors, which permit it to monitor the circulation.

Efferent tracts from the hypothalamus include the hypothalamohypophyseal tract, which runs from the supraoptic and paraventricular nuclei to the neurohypophysis, the mamillotegmental tract going to the tegmentum; and the mamillothalamic tract, from the mamillary nuclei to the anterior thalamic nuclei. There are also the periventricular system, including the dorsal fasciculus to the lower brain levels; the tuberohypophyseal tract, which goes from the tuberal portion of the hypothalamus to the posterior pituitary; and fibers from the septal region, by way of the fornix, to the hippocampus. The table below gives a summary of the major tracts and their roles.

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            The next topic to talk about would be the functions of the hypothalamus. These include: eating, autonomic functions, body temperature regulation, water balance, Circadian Rhythm regulation, anterior pituitary functions, some control of emotions. Naturally problems can occur within each of these categories. So that’s what I’m going to talk about next.

            For example, damage to the feeding center leads to anorexia and severe loss of body weight. This damage can occur either through physical trauma or through lesions within this area. Generally, the first employment route for treatment would be appetite increasing medications. This is used to avoid drastic health problems. Of course, this doesn’t solve the problem. So the next route is surgery. Of course, if lesions are the problem then they are removed before any medication is given, but if the cause is from blunt trauma, then usually medications are used.

Another problem can occur if there is misregulation of water balance. Hypothalamic influence on vasopressin secretion within the posterior pituitary is activated by osmoreceptors within the hypothalamus, particularly in neurons within a "thirst center" located near the supraoptic nucleus. Lack of secretion of vasopressin caused by hypothalamic or pituitary lesions can result in diabetes insipidus, which is characterized by polyuria (increased urine excretion) and polydipsia (increased thirst). Generally this is treated with a combination of surgery and diet management.

Finally, a third pathology can be associated with Circadian Rhythm malfunction. Within the hypothalamus, a specific cell group, the suprachiasmatic nucleus, functions as an intrinsic clock. Cells within this nucleus show circadian rhythms in metabolic and electrical activity, and in neurotransmitter synthesis, and appear to keep the rest of the brain on a day–night cycle. Obviously lesions within this area can lead to a malfunction of all circadian rhythms throughout the body, resulting in many problems.

References

1.)     Molina PE, "Chapter 2. The Hypothalamus & Posterior Pituitary Gland" (Chapter). Molina PE: Endocrine Physiology, 3e: http://www.accessmedicine.com/content.aspx?aID=6169265.

2.)     Hopper AH, Samuels MA, "Chapter 27. The Hypothalamus and Neuroendocrine Disorders" (Chapter). Ropper AH, Samuels MA: Adams and Victor's Principles of Neurology, 9e: http://www.accessmedicine.com/content.aspx?aID=3634491.