The human nervous system is equipped to sense and respond to continuous change within the body and in its external environment. Homeostasis is a state of relative stability within the body and critical for survival, because the body can only survive within a narrow range of conditions. The nervous system regulates body structures and processes to maintain homeostasis by controlling and regulating the other parts of the body. A deviation from a normal set point acts as a stimulus to a receptor, which sends nerve impulses to a regulating center in the brain. The brain directs an effector to act in such a way that an adaptive response takes place.