Nervous system, network of specialized tissue that coordinates and controls the various activites of the body, both voluntary and involuntary. The nervous system can be divided into 2 main parts. The central nervous system consists of the brain and spinal cord. It stores and processes information and sends messages to muscles and glands. The peripheral nervous system consists of 12 pairs of cranial nerves located in and near the medulla oblongata at the rear of the brain and 31 pairs of spinal nerves originating in the spinal cord. It carries messages to and from the central nervous system. A third system, the autonomic nervous system, normally considered part of the peripheral nervous system, controls involuntary actions such as heartbeat and digestion. It can be divided into 2 parts: the sympathetic system, which speeds up the heartbeat and prepares the body for “fight-or-flight,” and the parasympathetic system, which slows down the heartbeat and controls the body's vegetative functions. The balance between the 2 systems is regulated by the central nervous system. The nervous system transmits messages by means of highly specialized nerve cells, called neurons. Tubelike extensions called axons and dendrites branch out from the neuron cell body. Axons, which vary greatly in length and speed of conduction, carry messages. An axon from one neuron may transmit impulses to as many as 1,000 other neurons. Dendrites receive impulses from axons. Adjacent neurons communicate through specialized contact points, called synapses. Messages are also carried from axon to dendrite by chemical messengers known as neurotransmitters across the synaptic gap. The elaborate circuitry involved in synaptic contact is responsible for much of behavior, from simple reflex reactions such as the “knee-jerk response” to complex thought-communication patterns.