Overview

Pairs31 (8 C, 12 T, 5 L, 5 S, 1 Co)

Spinal nerve length≈1 cm before dividing

White ramiT1–L2 only

Grey ramiAll 31 spinal nerves

Each cord segment gives off rows of dorsal and ventral rootlets, which converge into a dorsal root carrying afferent fibres, with the dorsal root ganglion, and a ventral root carrying efferent fibres. This division is the Bell–Magendie law. The roots unite in the intervertebral foramen to form a short (≈1 cm) mixed spinal nerve. The nerve almost immediately divides into:

• a dorsal (posterior) ramus, supplying the deep (intrinsic) back muscles such as erector spinae, the zygapophysial joints and a strip of skin of the back;
• a larger ventral (anterior) ramus, supplying the anterolateral trunk wall and the limbs. Ventral rami of C1–C4, C5–T1, L1–L4 and L4–S4 form the cervical, brachial, lumbar and sacral plexuses. T1–T11 continue as segmental intercostal nerves and T12 as the subcostal nerve.

A small recurrent meningeal (sinuvertebral) nerve re-enters the canal to supply the dura, posterior longitudinal ligament and outer annulus of the disc. Rami communicantes connect each ventral ramus to the sympathetic trunk. White rami carry myelinated preganglionic fibres from T1–L2 only. Grey rami carry unmyelinated postganglionic fibres back to every spinal nerve, for sweat glands, arrector pili and blood vessels.

Spinal nerves C1–C7 exit above the vertebra of the same number, C8 exits between C7 and T1, and from T1 downward each nerve exits below its vertebra. Because the cord is shorter than the canal, cord segments lie above their vertebrae. Approximate rules: cervical segment ≈ vertebra + 1, upper thoracic ≈ +2, lower thoracic ≈ +3. The lumbar segments lie at T10–T12, and the sacral segments at T12–L1.

Parts & subdivisions

Dorsal root and ganglion
Afferent (sensory) fibres, with cell bodies in the DRG: general somatic and general visceral afferent.
Ventral root
Efferent fibres: α and γ motor fibres from the anterior horn, plus preganglionic autonomic fibres (T1–L2 sympathetic; S2–S4 parasympathetic).
Dorsal ramus
Supplies the intrinsic back muscles, facet joints and the overlying skin near the midline. It does not form plexuses.
Ventral ramus
Supplies the anterolateral trunk and the limbs. It forms the cervical, brachial, lumbar and sacral plexuses, or runs segmentally as the intercostal nerves.
White and grey rami communicantes
White rami are myelinated preganglionic fibres from T1–L2 into the sympathetic trunk. Grey rami are unmyelinated postganglionic fibres from the trunk to all 31 spinal nerves.
Recurrent meningeal (sinuvertebral) nerve
Sensory and vasomotor supply to the dura, ligaments, periosteum and outer annulus fibrosus. It is a source of discogenic back pain.

Anatomy

Histology: Each nerve consists of nerve fibres (myelinated and unmyelinated) wrapped in connective tissue sheaths. Endoneurium surrounds each fibre. Perineurium surrounds each fascicle and forms the blood–nerve barrier. Epineurium encloses the whole nerve and is continuous with the dura at the foramen.

Function

Carry all sensory and motor innervation of the body below the head, apart from the territory of the vagus.
Distribute sympathetic postganglionic fibres to skin and vessels via grey rami.
Provide the segmental basis of dermatomes and myotomes.

Clinical significance

Lumbar disc herniation

Most herniations are posterolateral, where the posterior longitudinal ligament is weakest. They compress the traversing root, the one exiting at the level below. An L4–L5 disc therefore compresses L5: weak great-toe extension and dorsiflexion, numbness over the dorsum of the foot, no reflex change. An L5–S1 disc compresses S1: weak plantarflexion, lateral foot numbness, absent ankle reflex. A far-lateral (foraminal) herniation hits the exiting root instead (L4 at L4–L5).

Cervical radiculopathy

Because cervical nerves exit above their vertebrae, a C5–C6 disc compresses C6: lateral forearm and thumb numbness, weak elbow flexion and wrist extension, reduced biceps and brachioradialis reflexes. A C6–C7 disc compresses C7, the most common: middle finger numbness, weak triceps, reduced triceps reflex.

Mnemonics

Spinal-nerve numbers

"Breakfast at 7, lunch at 12, dinner at 5": 7 cervical vertebrae (but 8 nerves), 12 thoracic, 5 lumbar. Then 5 sacral and 1 coccygeal nerves, so 8 + 12 + 5 + 5 + 1 = 31.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.