The peripheral nervous system (PNS) consists of all neurons that exist outside the brain and spinal cord. This includes long nerve fibers containing bundles of axons as well as ganglia made of neural cell bodies. The peripheral nervous system connects the central nervous system (CNS) made of the brain and spinal cord to various parts of the body and receives input from the external environment as well.
Functionally, the PNS is divided into sensory (afferent) and motor (efferent) nerves, depending on whether they bring information to the CNS from sensory receptors or carry instructions towards muscles, organs or other effectors. Motor nerves can be further classified as somatic or autonomic nerves, depending on whether the motor activity is under voluntary conscious control.
Anatomically, the PNS can be divided into spinal and cranial nerves, depending on whether they emerge from the spinal cord or the brain and brainstem. Both cranial and spinal nerves can have sensory, motor or mixed functions. The enteric nervous system, surrounding the gastrointestinal tract is another important part of the peripheral nervous system. While it receives signals from the autonomic nervous system, it can function independently as well and contains nearly five times as many neurons as the spinal cord.
The primary function of the peripheral nervous system is to connect the brain and spinal cord to the rest of the body and the external environment. This is accomplished through nerves that carry information from sensory receptors in the eyes, ears, skin, nose and tongue, as well as stretch receptors and nociceptors in muscles, glands and other internal organs. When the CNS integrates these varied signals, and formulates a response, motor nerves of the PNS innervate effector organs and mediate the contraction or relaxation of skeletal, smooth or cardiac muscle.
Thus, the PNS regulates internal homeostasis through the autonomic nervous system, modulating respiration, heart rate, blood pressure, digestion reproduction and immune responses. It can increase or decrease the strength of muscle contractility across the body, whether it is sphincters in the digestive and excretory systems, cardiac muscles in the heart or skeletal muscles for movement. It is necessary for all voluntary action, balance and maintenance of posture and for the release of secretions from most exocrine glands. The PNS innervates the muscles surrounding sense organs, so it is involved in chewing, swallowing, biting and speaking. At the same time, it mediates the response of the body to noxious stimuli, quickly removing the body from the injurious stimulus, whether it is extremes in temperature, pH, or pressure, as well as stretching and compressing forces.