The peripheral nervous system (PNS) is the vast network of nerves that branches out from your brain and spinal cord to reach every other part of your body. It serves as the communication highway between your central nervous system (the brain and spinal cord) and your skin, muscles, organs, and glands. Without it, your brain would have no way to sense the outside world or tell your body what to do. The PNS includes 12 pairs of cranial nerves emerging from the brain and 31 pairs of spinal nerves emerging from the spinal cord, giving it an enormous reach.
How the PNS Is Organized
The peripheral nervous system has two main divisions, each handling a fundamentally different kind of work. The somatic nervous system controls everything you do on purpose: walking, picking up a cup, turning your head. The autonomic nervous system handles everything your body does without you thinking about it: digesting food, regulating your heartbeat, adjusting blood pressure. A third division, the enteric nervous system, is embedded in the walls of your digestive tract and operates with a surprising degree of independence.
Within each of these divisions, signals travel in two directions. Sensory (afferent) nerves carry information from your body inward toward the brain and spinal cord: the feel of a hot surface, the position of your knee, the stretch of a full stomach. Motor (efferent) nerves carry commands outward from the brain and spinal cord to muscles and organs, telling them what to do.
The Somatic Nervous System
This is the branch you’re most aware of in daily life. It connects the brain and spinal cord to your skeletal muscles and to the sensory receptors in your skin, joints, and muscles. When you decide to reach for your phone, motor signals travel through somatic nerves to contract the right muscles in the right sequence. When you touch something rough, sensory signals travel back through somatic nerves to inform your brain about the texture.
Somatic sensory nerves carry several types of information. Larger nerve fibers handle fine touch, joint position, and vibration, which is how you can tell where your hand is without looking at it. Smaller fibers handle pain and temperature, alerting you when something is too hot or when tissue is being damaged. All of this input happens continuously and mostly below conscious awareness, keeping your brain updated on what your body is doing and what’s happening around it.
The Autonomic Nervous System
The autonomic nervous system runs your body’s behind-the-scenes operations: heart rate, blood pressure, breathing, digestion, and sexual arousal. It has two branches that generally work in opposition to keep your body in balance.
The sympathetic branch triggers what’s commonly called the “fight or flight” response. When activated, it raises your heart rate, increases blood pressure, constricts blood vessels, and diverts energy away from digestion. This is the system that kicks in when you’re startled, exercising hard, or under stress. It also releases stored sugar into your bloodstream so your muscles have fuel to work with.
The parasympathetic branch does the opposite, promoting “rest and digest” functions. It slows the heart, stimulates digestion and saliva production, and generally nudges the body back toward a calm, energy-conserving state. The vagus nerve, which runs from the brainstem down through the chest and abdomen, is the primary carrier of parasympathetic signals. It slows heart rate, enhances gut movement, and promotes secretion of digestive enzymes.
These two branches don’t take turns in a simple on-off pattern. They’re both active at all times, constantly adjusting their relative influence to match what your body needs moment to moment.
The Enteric Nervous System
Your gut contains more nerve cells than your entire spinal cord. This extensive network, called the enteric nervous system, lines the walls of the digestive tract from the esophagus to the colon, and extends to accessory organs like the pancreas and gallbladder. It’s sometimes called the “second brain” because it can operate largely on its own, without direct instructions from the brain or spinal cord. Isolated segments of the gut can maintain normal wave-like contractions (peristalsis) even when completely disconnected from the rest of the nervous system.
The enteric nervous system has its own sensory neurons that monitor mechanical stretch and chemical conditions inside the gut, local circuit neurons that process this information, and motor neurons that control the smooth muscle in the gut wall and regulate secretions like stomach acid, mucus, and bile. These neurons are organized into two main networks: one that controls gut muscle movement and another, just beneath the inner lining, that monitors chemical conditions and manages glandular secretion.
Physical Structures: Ganglia and Plexuses
Scattered throughout the peripheral nervous system are clusters of nerve cell bodies called ganglia. These serve as relay and processing stations outside the brain and spinal cord. Sensory ganglia, located along the spinal cord (dorsal root ganglia) and along certain cranial nerves, house the cell bodies of sensory neurons that pick up information from the skin, muscles, and organs.
Autonomic ganglia serve the involuntary nervous system. The sympathetic chain ganglia form a row along both sides of the vertebral column, including three ganglia in the neck region. Additional sympathetic ganglia sit in front of the spine. Parasympathetic terminal ganglia are found in the head, neck, chest, abdomen, and pelvis, often embedded directly in the walls of the organs they control.
A plexus is a network of interconnected nerve fibers. Major nerve plexuses exist at several points in the body where spinal nerves branch and recombine before heading to their final destinations. The enteric plexus, woven into the walls of the intestines, is one of the most complex examples.
How PNS Nerves Repair Themselves
One of the most important differences between the peripheral and central nervous systems is the ability to heal. Peripheral nerves can and do regenerate after injury, something central nervous system neurons generally cannot do. When a peripheral nerve is cut or crushed, the portion beyond the injury breaks down, but the nerve fiber can regrow along the original path, potentially restoring function. Recovery tends to be best when the injury is close to its target, such as in the hand or foot, because the nerve has a shorter distance to regrow.
This regenerative ability is a major reason why some hand injuries, for example, recover sensation and movement over weeks or months, while spinal cord injuries typically cause permanent damage. The peripheral environment actively supports regrowth in ways the central nervous system does not, though severe or long-gap injuries can still result in incomplete recovery.
When Peripheral Nerves Are Damaged
Damage to peripheral nerves is called peripheral neuropathy, and it’s surprisingly common. It can result from diabetes (the most frequent cause), autoimmune conditions, infections, physical injury, exposure to toxins, or chemotherapy. Some forms are inherited, like Charcot-Marie-Tooth disease, which affects both sensory and motor nerves in the arms, hands, legs, and feet.
Symptoms depend on which types of nerve fibers are affected:
- Sensory nerve damage causes tingling, numbness, pain, inability to feel vibrations or light touch (especially in the hands and feet), and loss of position sense. That last symptom can make it difficult to walk, button a shirt, or maintain balance with your eyes closed.
- Motor nerve damage causes muscle weakness, painful cramps, visible muscle twitching under the skin, and gradual muscle wasting.
- Autonomic nerve damage can disrupt heart rate regulation, blood pressure control, digestion, and sweating.
Neuropathic pain is often worse at night and can be triggered by stimuli that wouldn’t normally hurt. Some people experience significant pain from the light touch of bedsheets against their skin, a phenomenon called allodynia, where pain receptors fire spontaneously or the spinal cord amplifies normal signals into pain. Symptoms range from mild to severe but are rarely life-threatening.
Autoimmune conditions like Guillain-Barré syndrome attack peripheral nerves directly, causing rapidly progressing weakness. Chronic inflammatory demyelinating polyneuropathy is a related condition where the immune system damages the insulating coating around nerve fibers, slowing signal transmission.