The way nerves talk to each other, and communicate, is through the secretion of a chemical called a neurotransmitter, which stimulates the circuit to be activated. And when this regulation of chemical neurotransmission is disturbed, you have the alterations in the functions that those brain areas are supposed to, to mediate. So in a condition like depression, or mania, which occurs in bipolar disorder, you have a disturbance in the neurochemistry in the part of the brain that regulates emotion