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 includes 12 pairs of cranial nerves and 31 pairs of spinal nerves, connecting your central nervous system to your skin, muscles, organs, and glands. Think of it as the wiring that lets your brain communicate with everything from your fingertips to your intestines.
How It Connects to the Central Nervous System
Your nervous system has two major parts. The central nervous system (CNS) is the brain and spinal cord, the command center. The peripheral nervous system is everything else: every nerve fiber that extends outward from that command center into your body. These two systems work as a loop. Sensory nerves in the PNS pick up information (a hot stove, a loud sound, a full bladder) and carry it inward to the brain. The brain processes that information and sends instructions back out through motor nerves in the PNS, telling muscles to contract or organs to adjust.
The nerves responsible for carrying information inward are called sensory (afferent) neurons. The ones carrying instructions outward are called motor (efferent) neurons. This two-way traffic is constant. Every sensation you feel and every movement you make depends on signals traveling along peripheral nerves.
The Two Main Divisions
The PNS splits into two broad divisions based on what they control: the somatic nervous system and the autonomic nervous system.
The somatic nervous system handles conscious, voluntary actions. It connects your brain to your skin and skeletal muscles, letting you do things like pick up a cup, walk across a room, or pull your hand away from something sharp. When you decide to move, signals travel from your brain through somatic motor nerves to the muscles involved. When you feel pressure, temperature, or pain on your skin, somatic sensory nerves carry that information back to your brain.
The autonomic nervous system handles unconscious, involuntary processes. It connects your brain and spinal cord to your internal organs: your heart, lungs, stomach, intestines, and glands. It runs automatically and continuously, regulating things like heart rate, blood pressure, digestion, and sweating without you having to think about them.
Sympathetic vs. Parasympathetic
The autonomic nervous system itself has two complementary branches that act like a gas pedal and a brake. The sympathetic branch activates your “fight or flight” response. When you’re stressed or in danger, it speeds up your heart rate, opens your airways, and redirects blood flow to your muscles so you can react quickly.
The parasympathetic branch does the opposite. It governs your body’s “rest and digest” state, lowering your heart rate, reducing the workload on your lungs, and promoting digestion. These two branches are constantly balancing each other. After a stressful moment passes, your parasympathetic system gradually brings everything back to baseline.
The “Second Brain” in Your Gut
There’s a third component of the autonomic nervous system that often gets its own category: the enteric nervous system. This is a complex network of neurons and supporting cells embedded directly in the wall of your gastrointestinal tract. It’s sometimes called the “second brain” because it can coordinate digestive functions, like moving food through your intestines, with a surprising degree of independence from your actual brain. It still communicates with the CNS, but it has enough local circuitry to manage many digestive and defensive functions on its own.
How Fast Nerve Signals Travel
Not all peripheral nerves transmit signals at the same speed. The difference comes down to whether a nerve fiber is coated in myelin, a fatty insulating layer that dramatically speeds up electrical conduction. Large myelinated fibers, the kind that carry touch and motor signals, transmit at roughly 35 to 75 meters per second. Small myelinated fibers are a bit slower, around 20 to 35 meters per second.
Unmyelinated fibers are the slowest, transmitting at just 0.1 to 5 meters per second. These tend to carry dull, lingering pain signals or gentle touch sensations. This is why a stubbed toe produces two waves of pain: a sharp, fast signal carried by myelinated fibers, followed by a slower, throbbing ache carried by unmyelinated ones.
Peripheral Nerves Can Regenerate
One important distinction between the PNS and the central nervous system is healing capacity. Peripheral nerves can regenerate after injury. When a peripheral nerve fiber is damaged, the portion beyond the injury degenerates, but the remaining stump can slowly regrow. The typical rate of regeneration is about 3.2 millimeters per day, which is roughly an inch per week. That sounds slow, and it is. If a nerve in your arm is injured near the shoulder, it could take many months for the regrowing fiber to reach your hand. Recovery depends heavily on the severity and location of the injury, and regrowth doesn’t always result in full restoration of sensation or motor control. Still, this regenerative ability is something the central nervous system largely lacks, which is why spinal cord injuries tend to cause permanent damage while some peripheral nerve injuries can heal over time.
What Happens When Peripheral Nerves Are Damaged
Damage to peripheral nerves is called peripheral neuropathy, and it’s remarkably common. The most frequent cause is diabetes. More than half of people with diabetes develop some form of neuropathy over time. Other causes include autoimmune diseases, infections like Lyme disease and shingles, traumatic injuries, exposure to toxins, and inherited conditions.
Symptoms depend on which types of nerves are affected. When sensory nerves are damaged, you might notice gradual numbness, tingling, or prickling in your hands or feet that spreads upward over time. Some people experience sharp, burning pain or become extremely sensitive to touch, feeling pain from things that normally wouldn’t hurt, like the weight of a blanket on their feet. Damage to motor nerves can cause muscle weakness, loss of coordination, and difficulty with balance. When autonomic nerves are involved, symptoms can be less obvious but equally disruptive: heat intolerance, abnormal sweating, digestive problems, or sudden drops in blood pressure that cause dizziness when standing.
Chemical Messengers in the PNS
Peripheral nerves relay their signals using chemical messengers released at the junctions between nerve cells or between nerves and muscles. The most prominent one in the PNS is acetylcholine, which plays a central role in both divisions. In the somatic system, it’s the chemical that triggers skeletal muscles to contract. In the autonomic system, it helps regulate heart rate, blood pressure, and gut movement. The sympathetic branch also relies heavily on norepinephrine, which is the chemical behind many of the “fight or flight” effects like increased heart rate and heightened alertness.
This chemical signaling is why certain medications and toxins can have such widespread effects on the body. Anything that interferes with acetylcholine, for instance, can simultaneously affect muscle strength, heart rhythm, and digestion, because all three depend on the same messenger working properly across different branches of the peripheral nervous system.