The nervous system is a complex network of nerve cells and nerve fibers spread throughout the body. Its function is to interpret, store, and respond to information received from inside and outside. The central nervous system or CNS, consists of the brain and spinal cord and is responsible for processing information gathered from the rest of the nerves and transmitting instructions to the body. Messages passing to and from the CNS are carried by the nerves of the peripheral nervous system. This system includes twelve pairs of cranial nerves and thirty-one pairs of spinal nerves. The cranial nerves and the spinal nerves control voluntary movements and sensations. The autonomic nervous system, consisting of sympathetic and parasympathetic nerve fibers, controls such involuntary body functions as the heartbeat.