The Peripheral Nervous System

All the nerves outside the brain and spinal cord: how a nerve is built, sensory and motor fibres, fast and slow fibres, how nerves regrow — and common problems like carpal tunnel and neuropathy.

The peripheral nervous system (PNS) includes the 12 pairs of cranial nerves, the 31 pairs of spinal nerves and all their branches, plus clusters of nerve cell bodies outside the CNS called ganglia.

How a nerve is built

A nerve is a cable containing thousands of axons — not a single cell. Each axon is wrapped (or, for thin fibres, embedded) in Schwann cells, then in a thin connective tissue layer (endoneurium). Axons are bundled into fascicles (wrapped in perineurium), and fascicles into the whole nerve (wrapped in epineurium), along with blood vessels. Most nerves are mixed, carrying sensory and motor fibres together.

Fast and slow fibres

Fibres differ in diameter and myelination, and so in speed. Large, heavily myelinated fibres carry commands to muscles and precise touch and position sense at up to around 120 m/s. Thinner myelinated A-delta fibres carry sharp first pain and cold; thin unmyelinated C fibres carry slow, burning pain, warmth and itch at about a metre per second. This is why stubbing your toe gives a sharp pain followed a moment later by a duller ache.

Nerves can regrow

When a peripheral axon is cut, the part beyond the cut degenerates (Wallerian degeneration), but Schwann cells form tubes that guide the regrowing axon from the stump — at roughly 1 millimetre a day. Surgeons can repair cut nerves, though recovery over long distances is slow and often incomplete.

Common problems

Compression — such as carpal tunnel syndrome, where the median nerve is squeezed at the wrist, causing tingling in the thumb and fingers. Peripheral neuropathy — damage to many nerves, most often from diabetes, which typically starts with numbness and tingling in the feet ("stocking-glove" pattern). Others include vitamin B12 deficiency, chemotherapy, alcohol, inherited neuropathies (Charcot–Marie–Tooth disease) and autoimmune attacks such as Guillain–Barré syndrome.