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Spinal Nerve Roots

radices nervorum spinalium

Each spinal nerve is formed by the union of a dorsal root (sensory, entering the cord's dorsal horn through the dorsal root entry zone) and a ventral root (motor, exiting the cord's ventral horn through the anterior root). The dorsal root ganglion (DRG) houses the cell bodies of the sensory neurons. The roots unite at or near the intervertebral foramen to form the mixed spinal nerve before dividing into dorsal and ventral primary rami.

Region: Lower Back

Clinical Relevance

Clinical Notes

The nerve root anatomy within the spinal canal determines which root is compressed by a herniated disc. At each lumbar level, the exiting root traverses the lateral recess and the foramen below its pedicle (L4 root exits below L4 pedicle through L4-L5 foramen), while the traversing root crosses the disc level to exit one level below. A posterolateral disc herniation at L4-L5 typically compresses the L5 traversing root; a foraminal herniation compresses the exiting L4 root. Selective nerve root blocks confirm the symptomatic root before surgery.

Pathology

Common Injuries & Conditions

Nerve Root Compression by Disc Herniation

A posterolateral lumbar disc herniation at L4-L5 compresses the L5 traversing nerve root in the lateral recess, producing L5 dermatomal pain (buttock to lateral calf and dorsal foot) and weakness of great toe extension, while a foraminal herniation at the same level compresses the exiting L4 root with a different distribution.