The nervous system coordinates body functions through its complex network of nerve cells, enabling sensation and movement. The nervous system has two primary parts: CNS, the central nervous system, and PNS, the peripheral nervous system. The CNS is composed of the brain, which acts as the body's control center, processing sensory information, and the spinal cord, which relays signals between the brain and other parts of the body. Extending from the CNS, the PNS reaches the limbs and organs, facilitating communication to the farthest parts of the body. The PNS is divided into the somatic nervous system, which consists of sensory and motor neurons, and the autonomic nervous system, which controls involuntary processes such as breathing and heart rate. The ANS can be subdivided into the sympathetic and parasympathetic nervous systems. The sympathetic system activates the body's response during stress, known as the "fight or flight" response, by increasing alertness and energy consumption. The parasympathetic system promotes rest to conserve energy and support normal bodily functions.