Hypothalamus Science

Mammalian thermoregulation is controlled by the hypothalamus

 

 Mammals that maintain a relatively constant core temperature need an overall control system . The system functions much like the heating,

cooling system in your house that has a thermostat connected to a furnace to produce heat and an air conditioner to remove heat. Such a system will maintain the temperature of your house about a set point by alternately heating or cooling as necessary.
    When the temperature of your blood exceeds 37°C (98.6°F), neurons in the hypothalamus detect the temperature change (see chapters 44 and 46). This leads to stimulation of the heat-losing center in the hypothalamus. Sympathetic nerves from this area cause a dilation of peripheral blood vessels, bringing more blood to the surface to dissipate heat. Other sympathetic nerves stimulate the production of sweat, leading to evaporative cooling. Production of hormones that stimulate metabolism is also inhibited. 

         When your temperature falls below 37°C, an antagonistic set of effects are produced by the hypothalamus. This is under control of the heat-promoting center, which has sympathetic nerves that constrict blood vessels to reduce heat transfer, and inhibit sweating to prevent evaporative cooling. The adrenal medulla is stimulated to produce epinephrine, and the anterior pituitary to produce TSH, both of which stimulate metabolism. In the case of TSH, this is indirect as it stimulates the thyroid to produce thyroxin, which stimulates metabolism (see chapter 46). A combination of epinephrine and sympathetic nerve stimulation of fat tissue can induce thermogenesis to produce more internal heat. Again, as temperature rises, negative feedback to the hypothalamus reduces the heat-producing response.

Fever

Substances that cause a rise in temperature are called p rogens, and they produce the state we call fever. Fever is a result of resetting the body's normal set point to a higher temperature.
 A number of gram-negative bacteria have components in their cell walls called endotoxins that act as pyrogens. Substances produced by circulating white blood cells act as pyrogens as well. Pyrogens act on the hypothalamus to increase the set point. The adaptive value of fever seems to be that increased temperature can inhibit the growth of bacteria.
 Evidence for this comes from the observation that some ectotherms respond to pyrogens as well. When desert iguanas were injected with pyrogenproducing bacteria, they spent more time in the Sun, producing an elevated temperature: They induced fever behaviorally!
These observations have led to a reevaluation of fever as a state that should be treated medically. Fever is a normal response to infection, and treatment to reduce fever may be working against this natural defense system. Extremely high fevers, however, can be dangerous, inducing symptoms ranging from seizures to hallucinations.

Torpor 

Endotherms can also reduce both metabolic rate and body temperature to produce a state of dormancy called torpor. Torpor allows an animal to reduce the need for food intake by reducing metabolism. Some birds, such as the hummingbird, allow their body temperature to drop as much as 25°C at night. This strategy is found in smaller endotherms; larger mammals have o large a mass to allow rapid cooling. Hibernation is an extreme state in which deep torpor lasts for several weeks or even several months. In this case, the animal's temperature may drop as much as 20°C below its normal set point for an extended period of time. The animals that practice hibernation seem to be in the midrange of size; smaller endotherms quickly consume more energy than they can easily store, even by reducing their metabolic rate.
 
 Very large mammals do not appear to hibernate. It was long thought that bears hibernate, but in reality their temperature is reduced only a few degrees. They instead undergo a pronged winter sleep. With their large thermal mass and low rate il
of heat loss, they do not seem to require the additional energy savings of hibernation.
Body heat is equal to heat produced plus heat transferred. Heat is transferred by conduction, convection, radiation, and evaporation. Organisms that generate heat and can maintain a temperature above ambient levels are called endotherms.
Organisms that conform to their surroundings are called ectotherms. Both types can regulate temperature, but ectotherms use mainly behavior. Mammals maintain a consistent body temperature through regulation by the hypothalamus. Two negative feedback loops act to raise or lower temperature as needed.