How does the hypothalamus work ?
REGULATION OF HYPOTHALAMIC AND PITUITARY HORMONES
The control of hypothalamic hormone secretion is best appreciated when considered with the respective pituitary hormone that it regulates. This consists of a feed-back (closed loop) system primarily blood borne on which are superimposed other signals from the central nervous system (open loop) mediated by neurotransmitters. The open loop is affected by environmental changes (light, dark, and temperature), stress (pain, fear, and psychic) and intrinsic rhythmicity of the hormone. Thus both external and internal environmental factors are determinants of the activity of these systems.
EFFECTS OF STRUCTURAL DISEASES OF HYPOTHALAMUS
Apart from neuroendocrine regulation, diseases localized to extra-hypothalamic brain regions as well as non-localized CNS disorders can produce neuroendocrine dysfunction.
The extent of endocrine or metabolic disturbance is dependent on the location than on the size of the hypothalamic lesions. Slowly growing lesions tend to be silent, whilst rapidly growing lesions can cause dramatic clinical and laboratory manifestations even when small.
Acute hypothalamic damage results in disturbances of consciousness, temperature regulation and involvement of cardiovascular, gastrointestinal and respiratory function. In contrast, persistent disease in the hypothalamus results in alteration of cognition and complex homeostatic function. A unilateral lesion of the hypothalamus rarely produces significant symptoms because the neuronal projections are not lateralized. Hence, hypothalamic dysfunction is common with diffuse inflammatory or infiltrative disease, with tumors that expand bilaterally and with median eminence disorders.
EFFECT OF FUNCTIONAL DISEASES OF HYPOTHALAMUS
Alteration in the secretion of pituitary hormones can occur in absence of any structural disease in the pituitary gland or CNS due to functional disease of the hypothalamus. These are as follows:
- Hypothamic hypogonadism
- Hypothalamic thyroid dysfunction
- Hypothalamic adrenal dysfunction
- Idiopathic hyperprolactinaemia
- Idiopathic growth hormone deficiency
- Hypothalamic defect in water regulation
Hypothalamic lesions may affect the water regulation leading to “cerebral hypontraemia” and “cerebral hypernatraemia”. This could be due to involvement of the thirst center.
- Hypothalamic obesity: The hypothalamus contains a lateral feeding center and a medial satiety center. Damage to the satiety center causes over-eating and obesity. The nature of the stimulus to the satiety center is still uncertain.
- Hypothalamic behavior disorders: Hypothalamic biogenic amines influence the behavior, mood and sex drive. In some depressed persons, hypothalamic dysfunction results in disturbances in sleep, appetite, sex drive, mood regulation and other autonomic functions.
- Hypothalamic temperature dysfunction:
Hyperthermia or hypothermia may occur due to the sensitivity of hypothalamic neurons to the temperature alterations in the blood perusing them or indirectly by the release of monoamines.