Friday, November 4, 2011

Functional Topography of the Hypothalamus

The functional topography of the hypothalamus is the description of the functions performed by the different parts of this CNS organ, which is located in the diencephalon at the base of the brain. The centers of the hypothalamus influence all processes that are important for the internal environment of the body; they regulate the performance of organs according to the actual physical load, that is, they control the balance of temperature, water, and electrolytes; the activity of the heart; circulation and respiration; metabolism; and the rhythm of sleeping and waking.

Vital functions (such as food intake, gastrointestinal activity and defecation, fluid intake and urination) are controlled from here, and so are the processes essential for the preservation of the species (procreation and sexuality). These vital activities are triggered by physical needs that are perceived as hunger, thirst, or sexual drive. Instinctive activities serving the preservation of the organism are usually accompanied by a strong affective component, such as desire or aversion, joy, anxiety, or anger. Hypothalamic excitation plays an important role in the creation of these emotions.

Estrous cycle and sexual behavior are affected by lesions to the hypothalamus. Lesions to the caudal hypothalamus between the tuber cinereum and the premamillary nucleus lead to adipsia (cessation of spontaneous drinking). More dorsal lesions result in aphagia (refusal to eat), while dorsal stimulation results in hyperphagia (compulsive eating). In the anterior hypothalamus at the level of the optic chiasm lies a region responsible for the control of body temperature. Stimulation in the vicinity of the fornix triggers fits of rage and aggressive behavior (perifornical zone of rage).

Dynamogenic and Trophotropic Zones

Electric stimulation of the hypothalamus evokes autonomic reactions that can be divided into two groups: those associated with regeneration and metabolic processes, and those associated with increased performance in response to the environment. Stimulation responses from specific regions have established a dorsocaudal and lateral region for the dynamogenic mechanisms, the dynamogenic zone, and a ventro-oral region for the mechanisms promoting regeneration, the trophotropic zone. The two regions correspond to the subdivision of the peripheral autonomic nervous system into a sympathetic component (dynamogenic) and a parasympathetic component (trophotropic). In humans, controlled stimulation of the caudal hypothalamus yielded similar results, namely, dilation of pupils, increase in blood pressure, and accelerated respiration.