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The nervous system is an organ system that coordinates voluntary and involuntary actions and responses by transmitting signals between different parts of our bodies.
Central to the functioning of the nervous system is an extensive network of specialized cells called neurons. Neurons feature many thin projecting fibers called axons, which penetrate deep into tissues. They are able to communicate with other cells by chemical or electrical means at synapses. Neuronal function is supported by neuroglia, specialized cells which provide nutrition, mechanical support, and protection.
In most animals, including humans, the nervous system consists of two parts: central and peripheral. The central nervous system (CNS) is composed of the brain, spinal cord, and cerebellum. The peripheral nervous system (PNS) consists of sensory neurons, motor neurons, and neurons that communicate either between subdivisions of the PNS or connect the PNS to the CNS
The nervous system has three broad functions: sensory input, information processing, and motor output. In the PNS, sensory receptor neurons respond to physical stimuli in our environment, like touch or temperature, and send signals that inform the CNS of the state of the body and the external environment. This sensory information is then processed by the CNS, predominantly by the brain.
After information is processed, motor neurons return signals to the muscles and glands of the PNS, which responds with motor output. Central neurons, which in humans greatly outnumber the sensory and motor neurons, make all of their input and output connections with other neurons. The connections of these neurons form neural circuits that are responsible for our perceptions of the world and determine our behavior. Along with neurons, the nervous system relies on the function of other specialized cells called glial cells, or glia, that provide structural and metabolic support to the nervous system.