The nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells,(plural form glia), that aid in the function of the neurons.

The nervous system is divided broadly into two categories: the peripheral nervous system and the central nervous system. Neurons generate and conduct impulses between and within the two systems. The peripheral nervous system is composed of sensory neurons and the neurons that connect them to the nerve cord, spinal cord and brain, which make up the central nervous system.

In response to stimuli, sensory neurons generate and propagate signals to the central nervous system which then processes and conducts signals back to the muscles and glands. The neurons of the nervous systems of animals are interconnected in complex arrangements and use electrochemical signals and neurotransmitters to transmit impulses from one neuron to the next. The interaction of the different neurons form neural circuits regulate an organism's perception of the world and what is going on with its body, thus regulating its behavior.

Nervous systems are found in many multicellular animals but differ greatly in complexity between species. The human nervous system can be described both by gross anatomy, (which describes the parts that are large enough to be seen with the naked eye,) and by microanatomy, (which describes the system at a cellular level.) In gross anatomy, the nervous system can be divided into two systems: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system (CNS) is the largest part of the nervous system, and includes the brain and spinal cord.

The spinal cavity holds and protects the spinal cord, while the head contains and protects the brain. The CNS is covered by the meninges, a three layered protective coat. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae.

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