The peripheral nervous system (PNS) is made up of all the nerves that branch out from your brain and spinal cord, forming a communication network between your central nervous system and the rest of your body. It includes 12 pairs of cranial nerves, 31 pairs of spinal nerves, and all of their smaller branches, along with clusters of nerve cell bodies called ganglia and specialized support cells. These structures are organized into two major functional divisions: the somatic nervous system, which handles conscious activities, and the autonomic nervous system, which runs things you never have to think about.
Nerves: The Physical Wiring
The most visible components of the peripheral nervous system are the nerves themselves. A nerve is a bundle of long fibers, each carrying electrical signals either toward the brain or away from it. Some nerves are purely sensory, carrying information from your skin, eyes, or ears inward. Others are purely motor, sending commands outward to muscles. Most of the larger nerves in your body are mixed, containing both types of fibers bundled together.
Each nerve has a layered protective structure, somewhat like a cable. The outermost wrapping is a tough connective tissue sheath that covers the nerve’s entire surface. Beneath that, groups of fibers are bundled into smaller clusters called fascicles, each surrounded by its own protective layer. And within each fascicle, individual fibers have their own delicate connective tissue coating. These layers cushion the nerve, supply it with blood vessels, and help it withstand the stretching and compression that comes with everyday movement.
Cranial and Spinal Nerves
The peripheral nervous system’s nerves fall into two groups based on where they connect to the central nervous system. Twelve pairs of cranial nerves emerge directly from the brain, mostly through small openings in the skull. Each pair has a specific job. The first cranial nerve provides your sense of smell. The second carries visual information from your eyes. Others control eye movement, facial expressions, hearing, balance, taste, and swallowing. The tenth cranial nerve, the vagus nerve, is particularly far-reaching: it regulates heart rate, blood pressure, digestion, breathing, mood, and saliva production, making it the main nerve of the parasympathetic nervous system.
The remaining nerves exit from the spinal cord. There are 31 pairs of spinal nerves: 8 cervical (neck region), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (base of the spine), and 1 coccygeal (tailbone). Each pair serves a specific strip of the body. Shortly after leaving the spine, many of these nerves weave together into networks called plexuses. The brachial plexus, for example, combines nerves from the neck and upper back to supply your arms and hands. The lumbosacral plexus serves your legs and pelvis. These plexuses let multiple spinal nerves coordinate the complex movements of a single limb.
Ganglia and Support Cells
Scattered along peripheral nerves are small clusters of nerve cell bodies called ganglia. These serve as relay stations. Dorsal root ganglia sit just outside the spinal cord and house the cell bodies of sensory neurons, collecting incoming information before it enters the spine. Autonomic ganglia are positioned along chains near the spine or close to target organs, relaying commands to your heart, gut, and glands.
The peripheral nervous system also depends on specialized support cells. The most important are Schwann cells, which wrap around nerve fibers to form a fatty insulating layer called myelin. This insulation dramatically speeds up signal transmission and helps maintain nerve health. Schwann cells also play a critical role in nerve repair after injury, guiding regrowth in ways that brain and spinal cord cells cannot. Satellite cells, another type of support cell, surround nerve cell bodies within ganglia and help regulate the chemical environment around them.
The Somatic Nervous System
The somatic division is the part of the PNS you’re most aware of. It connects your brain and spinal cord to your skin, skeletal muscles, and joints. Sensory fibers in this system carry information about touch, temperature, pain, and body position from receptors throughout your body to the central nervous system. Motor fibers carry commands back out to your muscles, producing voluntary movements like walking, typing, or turning your head.
This system works through a straightforward loop. When you touch something hot, sensory neurons in your fingertips send a signal up through the spinal cord to the brain. The brain (or in some cases the spinal cord alone, for speed) processes that input and sends a motor signal back down and out to the muscles in your arm, telling them to pull your hand away. The entire loop can complete in a fraction of a second.
The Autonomic Nervous System
The autonomic division handles everything your body does without your conscious direction. It connects the central nervous system to internal organs like the heart, stomach, intestines, and glands. You don’t decide to speed up your heart rate during exercise or slow your digestion during a stressful moment. The autonomic system manages all of that, and it does so through two branches that work in opposition.
The sympathetic branch is your “fight or flight” system. When you face a threat or stressor, it increases your heart rate, dilates your airways, redirects blood flow to your muscles, and dilates your pupils. The parasympathetic branch does the opposite, promoting “rest and digest” functions. It slows your heart rate, constricts your pupils, boosts digestion, increases saliva and mucus production, and relaxes the muscles involved in urination and bowel movements. It also manages aspects of sexual arousal. These two branches constantly fine-tune each other to keep your body in balance.
The Enteric Nervous System
Your digestive tract has its own extensive nerve network that some scientists consider a third division of the peripheral nervous system. The enteric nervous system contains between 200 and 600 million neurons embedded in the walls of your gastrointestinal tract. That’s more neurons than your spinal cord contains, which is why it’s sometimes called the “second brain.” It coordinates the muscular contractions that push food through your intestines, regulates digestive secretions, and monitors conditions inside your gut.
The enteric system can operate independently to some extent, managing digestion on its own without moment-to-moment input from the brain. But it stays in constant two-way communication with the central nervous system through both sensory and motor nerve fibers. This connection is why stress can cause nausea or digestive upset, and why gut problems can affect your mood.
Sensory and Motor Pathways
Cutting across all these divisions is a basic distinction in how signals flow. Sensory (afferent) nerve fibers carry information inward, from receptors in your skin, organs, muscles, and joints toward the brain and spinal cord. Motor (efferent) fibers carry commands outward, from the central nervous system to muscles and glands. Every peripheral nerve is built from some combination of these two fiber types.
This two-way traffic is what makes the peripheral nervous system fundamentally different from the brain and spinal cord. The central nervous system processes and integrates information. The peripheral nervous system is the messenger network that delivers raw data inward and finished instructions outward. Without it, the brain would have no way to sense the world or act on it.
When the Peripheral Nervous System Is Damaged
Because peripheral nerves extend throughout the body, they’re vulnerable to injury, compression, and disease. Peripheral neuropathy, a condition in which these nerves are damaged, typically causes numbness, tingling, burning pain, or weakness, usually starting in the feet and hands. Diabetes is one of the most common causes. In people with type 2 diabetes, roughly 19% develop peripheral neuropathy, and that figure rises to about 26% in those who have had diabetes for more than 10 years.
Other causes include physical injuries, autoimmune conditions, infections, toxin exposure, and vitamin deficiencies. The symptoms depend entirely on which types of nerve fibers are affected. Damage to sensory fibers produces numbness or pain. Damage to motor fibers causes muscle weakness or coordination problems. Damage to autonomic fibers can disrupt heart rate, digestion, or bladder control. One advantage the peripheral nervous system has over the central nervous system is that its nerves can sometimes regenerate after injury, guided by Schwann cells that clear debris and form a pathway for new fiber growth. Recovery is slow, often measured in months, but possible in many cases.