Chapter 9

    Regulating body temperature and making sure we eat and drink involves complex processes in which the brain tracks conditions inside the body and corrects imbalances. Sometimes this is handled without our being conscious of changes, but sometimes the brain triggers behaviors needed to maintain the body.

    Homeostasis keeps body variables like temperatures within a fixed range or at a set point, by reacting to negative feedback. Allostasis is a similar concept, but works in anticipation of changes to prevent body variables from getting too far from the appropriate range.

Temperature

Mammals and birds are endothermic, meaning we generate our own heat. Basal metabolism, used to keep temperature constant, uses about ⅔ of a mammal’s energy, and is generated by brown adipose cells, which are a type of fat cell.

For cooling, mammals use evaporation. This can be done through sweat, licking oneself, or panting. For heating, shivering, saving heat for internal organs, and fluffing fur add warmth. Behavioral methods like changing clothes (for humans), getting more or less active, and huddling with others also help.

These changes are controlled by the anterior hypothalamus and the nearby preoptic area, referred to collectively as the POA/AH. This region receives input from temperature receptors in the skin and throughout the body, as well as in the hypothalamus itself. It  also receives chemical information from the immune system. It sends output to the raphe nucleus in the hindbrain, which affects shivering, sweating, changes in heart rate and blood flow, and metabolism changes. In the case of an infection, the hypothalamus maintains a higher set point temperature on purpose, because bacteria do not grow as well and the immune system works better at higher temperatures.

Hunger

The brain gets messages from the mouth, stomach, intestines, fat cells, and more to regulate eating. Even when not especially hungry, people enjoy tasting food and the act of chewing. The signal to end a meal is stomach distension, meaning stretching. The vagus nerve conveys stomach and small intestine distension information to the brain.

Our body uses hormones to regulate hunger. Insulin enables glucose to enter cells, a process that suppresses hunger. Insulin levels are regulated by the lateral hypothalamus. Low insulin levels, like in type 1 diabetes, make it hard for glucose to enter cells, and blood sugar levels rise too high because glucose remains in the blood. Untreated, people will lose weight because even though they are eating, their cells are not getting nutrition.

Glycagon converts glycogen, a starch stored in the liver, back to glucose when we start to get hungry.

Leptin is a hormone created by fat cells, so high levels in the body indicate more fat. Leptin levels signal to the brain how much we have in reserve. People with lower leptin levels will feel the need to eat more and be less active, but high levels of leptin don’t have as strong an effect on decreasing eating and increasing activity. Evolutionarily, we’re better adapted to avoid starvation than to avoid overeating.

The arcuate nucleus in the hypothalamus controls appetite. It contains neurons sensitive to hunger signals and neurons sensitive to satiety signals. Input includes insulin and leptin levels, as well as signals from the amygdala, basal forebrain, and thalamus. Based on the information, the arcuate nucleus can inhibit or excite the paraventricular nucleus, which in turn can inhibit the lateral hypothalamus, which is responsible for eating. The paraventricular nucleus will stop inhibiting the lateral hypothalamus in the case of hunger, or inhibit it in the case of satiety. The amygdala can signal directly to the lateral hypothalamus too, including inhibition in the case of illness or as a response to a food that previously caused illness, or excitement in response to tasty food. The lateral hypothalamus controls insulin secretion and changes responsiveness to taste. It can increase food-seeking behaviors.

Hunger-sensitive neurons have output to many areas of the brain, used in the case of starvation. Animals that are starving will do whatever they need to get food, and cease to do most other activities. In people, this includes sacrificing other goals.