THURSDAY, DAY 4
THE MIND
In a real sense, it is the limbic system that gives humans humanity—the ability to love, to laugh, to cry, to remember.
The system within the brain that controls our emotions and behavior, the limbic system, comprises three main parts, the hypothalamus, the hippocampus, and the amygdala. Several other parts of the brain are also associated with the limbic system, including areas that store associations for different smells and the part of the brain that appears to be responsible for orgasms.
These parts are interconnected by a network of neural pathways, forming a web that produces complex emotions and allows us to interact with the world around us.
The hippocampus, a seahorse-shaped structure (hippocampus comes from the Greek word for seahorse), turns experiences into memories. A damaged or malfunctioning hippocampus can make it impossible for a person to form long-term recollections andimmediately forget the name of a person he or she has just met, for instance.
Fear and sexual arousal are controlled by the amygdala, which comprises two clusters of nerves next to the hippocampus.
The hypothalamus is one of the busiest parts of the limbic system—indeed, of the entire brain. It controls feelings of hunger and thirst and processes our reactions to pain and pleasure. It is the hypothalamus that is thought to generate rolling, uncontrollable laughter. The hypothalamus also sends signals to the rest of the body via the pituitary gland, which pumps hormones into the bloodstream. Both are located at the front of the brain, above and between our eyes.
Because natural pleasures are an important part of (and reason for) survival, the limbic system creates an appetite for things that will provide sensory rewards. A part of the brain called the ventral tegmental area responds to positive experiences—being praised for a job well done, sexual intercourse, ice cream—by releasing a chemical called dopamine, which produces feelings of happiness and contentment.