What Is the Peripheral Nervous System? Functions and Types

The nervous system has two major parts. The brain and spinal cord form the central nervous system, the body’s control center. Everything else belongs to the peripheral nervous system, or PNS: all the nerves outside the brain and spinal cord. The PNS connects the central nervous system to the rest of the body. It carries sensory information from the skin, muscles, and internal organs to the brain and spinal cord, and it carries motor commands back out to muscles, glands, and organs. Without it, the brain could neither feel the body nor control it.

A network that reaches everywhere

The PNS includes every neural structure outside the central nervous system: cranial nerves, spinal nerves, peripheral nerves, and two kinds of ganglia. If the brain and spinal cord are a city’s central exchange, the PNS is the network of cables running to every house. The comparison is useful, though real nerves carry signals in both directions and also contain relay stations along the way.

There are 12 pairs of cranial nerves, which arise directly from the brain or brainstem. They handle sensation and movement in the head and neck, and several also help regulate internal organs such as the heart, lungs, and digestive system. There are 31 pairs of spinal nerves emerging from the spinal cord, carrying sensory and motor information to and from the neck, trunk, arms, and legs.

Peripheral nerves are bundles of thousands of individual nerve fibers, or axons. Some carry sensory information toward the central nervous system, others carry motor commands away from it, and many contain both kinds of fibers. Ganglia are clusters of nerve cell bodies outside the central nervous system. Sensory ganglia hold the cell bodies of sensory neurons, while autonomic ganglia relay signals that regulate involuntary body functions.

Traffic in both directions

The PNS works in two directions, which is why it doesn’t only control movement. On the sensory, or afferent, side, receptors detect touch, temperature, pain, vibration, body position, light, and sound, and that information travels through peripheral nerves to the spinal cord and brain. On the motor, or efferent, side, once the brain has processed the information, commands travel out through peripheral nerves to muscles and glands. These commands let you move, hold your posture, speak, and respond to changes around you.

Voluntary and automatic divisions

The PNS is usually divided into two functional systems, so peripheral nerves are not all under voluntary control. The somatic nervous system handles voluntary movement. It carries sensory information from the skin, joints, and skeletal muscles and sends motor commands to skeletal muscles, as when you walk, write, speak, or pick something up.

The autonomic nervous system regulates involuntary functions. It controls smooth muscle, cardiac muscle, and glands without conscious effort, managing heart rate, blood pressure, digestion, breathing, pupil size, and sweating. It is further divided into the sympathetic, parasympathetic, and enteric nervous systems.

Why it matters

Every sensation, every voluntary movement, and every automatic body function depends on healthy communication through peripheral nerves. Many conditions can disrupt it, including peripheral neuropathy, carpal tunnel syndrome, Bell’s palsy, Guillain–Barré syndrome, sciatica, and traumatic nerve injuries. Depending on which nerves are involved, these can cause numbness, weakness, pain, tingling, or loss of normal muscle function.

This article is educational background, not a diagnostic guide. Persistent numbness, tingling, or weakness, or sudden facial weakness, should be evaluated by a clinician.

Related: What Is the Central Nervous System? · What Is a Neuron? · How Do Neurons Communicate? · What Is a Reflex Arc? · What Is the Autonomic Nervous System?

References

1. Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2020.
2. Akinrodoye MA, Lui F. Neuroanatomy, Somatic Nervous System. StatPearls. Updated November 7, 2022.
3. Anatomy, Autonomic Nervous System. StatPearls. NCBI Bookshelf.