The peripheral nervous system (PNS) refers to all the neurons (and their supporting cells, or glia) of the body outside the brain and spinal cord (central nervous system [CNS]). The brain is the organ that decides how a person responds to what happens in the surrounding world. While this is an extremely important function, the brain relies upon the peripheral nervous system, and its information gathering capabilities, to receive information about the world and to send appropriate responses to various body parts, such as muscles and glands. The neurons of the peripheral nervous system do not make complex decisions about the information they carry. The appropriate decisions are made instead in the brain and spinal cord. However, without the peripheral nervous system's ability to bring in sensory information and send out motor information, it would be impossible for a person to walk, talk, ride a bike, or even watch television. Without the ability to take in information and send out responses, the brain would be useless.
Peripheral neurons are of two types, sensory and motor. Sensory (afferent) neurons bring information about the world within and around the body from sense organs to the brain and spinal cord, while motor (efferent) neurons carry messages from the brain and spinal cord out to the muscles and glands. For example, if a mosquito lands on a person's arm, sensory neurons in the skin send a message to the spinal cord and then the brain, where the message is understood, and a reaction formulated. The brain's response may be to use motor neurons to cause muscle contractions resulting in a slap on the skin where the mosquito landed.