Role of the Hypothalamus in Temperature Homeostasis
The primary neural centers governing temperature balance are localized in the hypothalamus. Their main task is to maintain the constancy of the internal environment.
The importance of the hypothalamic region is confirmed experimentally. Transecting the brainstem just below the hypothalamus (along the upper edge of the superior colliculi) severely disrupts normal thermoregulation. As a result, homeothermic (warm-blooded) animals lose the ability to autonomously control temperature and become poikilothermic: their internal temperature begins to depend entirely on environmental conditions.
Targeted destruction of hypothalamic nuclei also leads to thermoregulation failures, while electrode implantation and direct stimulation of these areas cause immediate changes in body temperature.
Heat Production Center (Posterior Hypothalamus)
The center responsible for heat generation and conservation is located in the posterior hypothalamic nuclei (Posterior hypothalamus). It is activated when the body cools, when the brain is perfused with cooled blood, or when signals from cutaneous cold receptors arrive.
Irritation of the posterior nuclei triggers a cascade of protective reactions:
- Vascular reaction: Vasoconstriction of superficial skin vessels occurs, sharply reducing heat loss.
- Cutaneous reaction: Piloerection occurs (goosebumps).
- Muscular reaction: Muscle shivering appears, providing contractile thermogenesis.
- Endocrine reaction: The adrenal glands increase the secretion of adrenaline and corticosteroids, which boosts metabolism and non-shivering thermogenesis.
Destruction of this center leads to a loss of the ability to resist cold, resulting in a rapid drop in body temperature and the development of hypothermia.
Notably, heat production (chemical thermoregulation) directly depends on tissue metabolism. Its intensity is influenced by genetic factors (height, weight, body surface area, gender), autonomic nervous system tone, and hormonal background. In addition, metabolism increases after food intake due to its specific dynamic action.
Heat Loss Center (Anterior Hypothalamus)
The anterior hypothalamic nuclei are responsible for dissipating excess heat. They are excited if the brain is bathed in warmed blood, as well as upon receiving impulses from cutaneous warm receptors during body heating.
Electrical stimulation of the anterior hypothalamus elicits cooling responses:
- Heat polypnea (panteeing/respiratory frequency changes) occurs.
- Vasodilation of superficial skin vessels takes place.
- Body temperature drops.
- Heat production mechanisms (such as muscle shivering) are strictly blocked.
If the anterior nuclei are destroyed, the body loses the ability to maintain isothermy in high ambient temperatures. Even at room temperature, such a subject develops hyperthermia.
Reciprocal Interaction of Centers
Neurohumoral temperature regulation is built on reciprocal (mutually exclusive) relationships between the anterior and posterior hypothalamus. Increased activity of one center automatically inhibits the other.
- During cooling: Neurons of the heat production center (posterior hypothalamus) are excited. Its afferent part sends inhibitory signals to the efferent part of the heat loss center.
- During warming: Neurons of the heat loss center (anterior hypothalamus) are activated. Its afferent part, in turn, suppresses the efferent divisions of the heat production center.
On functional diagrams, this interaction is typically depicted with solid arrows for excitatory influences and crossed-out lines for inhibitory ones.