The Peripheral Nervous System

Core Concepts

Nerves and ganglia: the building blocks

A nerve is a bundle of axons in the PNS wrapped in connective tissue: the endoneurium around each axon, the perineurium around each bundle (fascicle), and the epineurium around the whole nerve. Most spinal nerves are mixed, carrying both sensory and motor fibers. A ganglion is a cluster of neuron cell bodies outside the CNS: sensory ganglia (the dorsal root ganglia) hold sensory cell bodies, and autonomic ganglia hold autonomic motor cell bodies. The same cluster inside the CNS is called a nucleus, not a ganglion.

Cranial nerves

Twelve pairs of cranial nerves attach to the brain, mostly at the brainstem, and are numbered from front to back. Some are purely sensory (olfactory I, optic II, vestibulocochlear VIII), some purely motor (oculomotor III, trochlear IV, abducens VI, spinal accessory XI, hypoglossal XII), and some are mixed (trigeminal V, facial VII, glossopharyngeal IX, vagus X). The vagus nerve (X) is especially important because it carries autonomic fibers to most thoracic and abdominal organs. Mnemonics like "Oh, Oh, Oh, To Touch And Feel Very Green Vegetables, AH!" help memorize the names in order.

Spinal nerves and their roots

There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each is formed by a dorsal root (sensory) and a ventral root (motor) joining at the spinal cord, so every spinal nerve is mixed. Sensory cell bodies sit in the dorsal root ganglion, just outside the cord. The cervical nerves C1–C7 exit above their vertebrae while C8 exits below C7; the lumbar and sacral roots descend within the vertebral canal below the cord's end as the cauda equina ("horse's tail") before exiting at their proper levels.

Plexuses

In the cervical, brachial, lumbar, and sacral regions, ventral rami of adjacent spinal nerves interweave into networks called plexuses, then recombine into named peripheral nerves. The cervical plexus (C1–C4) gives the phrenic nerve (C3–C5), which drives the diaphragm. The brachial plexus (C5–T1) produces the major upper-limb nerves: median, ulnar, and radial. The lumbar plexus (L1–L4) gives the femoral and obturator nerves; the sacral plexus (L4–S4) gives the sciatic nerve, the largest nerve in the body. Thoracic nerves mostly stay segmental, running between the ribs as intercostal nerves.

Dermatomes

A dermatome is the skin area supplied by one spinal nerve. Dermatomes overlap considerably, so losing a single nerve root rarely causes complete numbness. Classic landmarks: T4 at the nipple line, T10 at the umbilicus. Dermatome testing helps localize spinal cord or nerve root lesions and track conditions such as shingles.

Reflexes cross the PNS–CNS boundary

Reflex arcs use both systems. In the patellar reflex, tapping the tendon stretches the quadriceps; a muscle spindle sends a signal through a sensory neuron (PNS) into the spinal cord (CNS), where it synapses directly on a motor neuron whose axon returns through the ventral root (PNS) to contract the muscle. The reflex is fast because the decision happens in the spinal cord, not the brain.

Injury and repair

When a peripheral axon is crushed or cut, the segment separated from the cell body degenerates (Wallerian degeneration), but the cell body survives. If the connective tissue tube around the axon is intact, the axon regrows toward its target at roughly 1 mm per day (a commonly cited figure). This is why peripheral nerve injuries often recover better than injuries to the brain or spinal cord, where regeneration is very limited. Recovery is slower when the injury is far from the target or the nerve is severed rather than crushed.

Common Confusions