Peripheral nerves are the network of cable-like structures that connect your brain and spinal cord to every other part of your body. They carry electrical signals that let you move, feel, and regulate unconscious functions like heart rate and digestion. Your peripheral nervous system includes 12 pairs of cranial nerves (running from the brain through the head, neck, and upper torso) and 31 pairs of spinal nerves branching out from the spinal cord. Together, these nerves reach your skin, muscles, organs, and glands.
What Peripheral Nerves Actually Do
Peripheral nerves work like a two-way communication system. Sensory nerves carry information inward, delivering signals that let you touch, taste, smell, see, and hear. Motor nerves carry signals outward, telling your muscles to contract and your glands to secrete. Some nerves handle both jobs at once.
This system splits into two broad divisions. The somatic nervous system controls things you do on purpose: walking, picking up a cup, turning your head. It connects your brain and spinal cord to your skin and skeletal muscles and is involved in conscious activity. The autonomic nervous system handles everything you don’t think about: heartbeat, digestion, blood pressure, sweating. It sends motor impulses to smooth muscle, cardiac muscle, and glands, running automatically and continuously without conscious effort.
How a Peripheral Nerve Is Built
A single peripheral nerve is not one fiber but a bundle of many fibers wrapped in protective layers, somewhat like a fiber-optic cable. Three connective tissue layers give it structure and protection:
- Endoneurium: The innermost layer, a gel-like matrix of thin collagen fibrils that surrounds individual nerve fibers and the cells insulating them.
- Perineurium: A sleeve of flat, layered cells that groups nerve fibers into bundles called fascicles. This is the nerve’s main barrier, using tight junctions and active transport to control what gets in and out, maintaining a stable chemical environment inside each fascicle.
- Epineurium: The tough outer coat of collagen that wraps the entire nerve and gives it tensile strength, protecting it from stretching and compression.
These layers matter because they explain why nerves can tolerate a surprising amount of bending and pulling during everyday movement without losing function.
Schwann Cells and the Myelin Sheath
The signature cell of the peripheral nervous system is the Schwann cell. These cells wrap their membrane around a nerve fiber in tight, concentric layers to form a fatty coating called the myelin sheath. This sheath acts as electrical insulation, dramatically increasing the speed at which signals travel. Each Schwann cell covers only one small segment of one nerve fiber, so a single long nerve fiber requires many Schwann cells lined up along its length.
This is different from how insulation works in the brain and spinal cord, where a single cell called an oligodendrocyte can insulate segments of multiple nerve fibers at once. The one-to-one arrangement of Schwann cells in peripheral nerves turns out to be a major advantage when it comes to repair, because Schwann cells actively participate in cleaning up damage and guiding regrowth.
How Signals Travel Along a Nerve
Electrical signals don’t flow smoothly down a nerve fiber like current through a wire. Instead, the myelin sheath forces the signal to jump from gap to gap. These tiny gaps between segments of myelin are called nodes of Ranvier, and they’re packed with channels that allow sodium ions to rush in and regenerate the electrical impulse. Between nodes, the insulating myelin prevents the signal from fading, so it shoots rapidly through the interior of the fiber to the next node, where it gets boosted again.
This jumping pattern, called saltatory conduction, is what makes myelinated nerves so fast. In healthy upper-limb nerves, sensory signals travel at roughly 39 to 58 meters per second, and motor signals move even faster, around 49 to 70 meters per second. That translates to signals crossing the length of your arm in a few thousandths of a second. Unmyelinated fibers, which lack this jumping mechanism, conduct much more slowly because the signal has to propagate continuously along the entire membrane.
What Happens When Peripheral Nerves Are Damaged
Peripheral nerve damage, broadly called peripheral neuropathy, has many causes. The most common is diabetes: roughly half of people with chronic type 1 or type 2 diabetes develop some form of diabetic neuropathy over time. High blood sugar gradually degrades nerve fibers, typically starting in the longest nerves first, which is why numbness and tingling often begin in the feet. Left unmanaged, diabetic neuropathy can lead to foot ulcers and, in severe cases, amputation.
The most common injury to a single nerve is carpal tunnel syndrome, where the median nerve gets compressed as it passes through a narrow channel in the wrist. Other causes of peripheral nerve damage include infections like HIV and hepatitis C, autoimmune conditions such as Guillain-Barré syndrome, physical trauma, and tumor compression.
When a nerve fiber is crushed or severed, the portion beyond the injury breaks down in a process called Wallerian degeneration. The axon fragments within about 36 to 44 hours. Over the next two days, the myelin sheath disintegrates and Schwann cells reorganize into guide channels. By days two to three, immune cells called macrophages arrive to clear debris, peaking around day seven. The entire cleanup wraps up within about two weeks, and only then can new nerve fibers begin growing back along those guide channels.
Why Peripheral Nerves Can Regenerate
One of the most remarkable features of peripheral nerves is that they can regrow after injury. This stands in sharp contrast to the brain and spinal cord, where regeneration is extremely limited. The difference comes down largely to Schwann cells. After an injury, they don’t just passively wait. They strip away their damaged myelin, release chemical signals that recruit immune cells, and then line up to form tubes that guide regrowing nerve fibers back toward their targets. They also produce growth-supporting molecules that nourish the extending fiber tips.
Regeneration is slow. Nerve fibers typically regrow at about 1 millimeter per day, so recovering from an injury in the upper arm might take many months before sensation or movement returns to the hand. The speed and success of recovery depend heavily on how quickly regrowth begins after injury, how far the new fibers need to travel, and whether the guide channels remain intact. This is why surgical repair of a severed nerve is most effective when performed promptly.
Symptoms of Peripheral Nerve Problems
Because peripheral nerves handle sensation, movement, and automatic body functions, damage can show up in very different ways depending on which fibers are affected. Sensory nerve damage typically produces numbness, tingling, burning pain, or a loss of ability to feel temperature changes. You might not notice a cut or burn on your foot, which is why diabetic neuropathy leads to so many foot complications.
Motor nerve damage causes weakness, muscle wasting, or cramping. You might have trouble gripping objects, walking steadily, or performing fine movements. Autonomic nerve damage is less obvious but can cause dizziness when standing, digestive problems, abnormal sweating, or changes in heart rate. Many neuropathies affect a mix of fiber types, producing a combination of these symptoms that often starts gradually and worsens over time. Managing the underlying cause, whether that’s blood sugar control, relieving compression, or treating an infection, is the most effective way to slow or reverse the damage.