Anatomy Atlas

Spinal nerves and roots

Nervi spinales
Also: Dorsal root · Ventral root · Dorsal ramus · Ventral ramus · Radiculopathy · Dorsal root ganglion
NerveHigh-yielddraftNeuroMSK

The 31 pairs of mixed nerves formed from dorsal (sensory) and ventral (motor) roots. Knowing where each root exits and what it supplies lets you localize radiculopathy.

Core anatomy

Location

Intervertebral foramina along the length of the vertebral column.

Root values

31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Origin

Dorsal root (sensory; its cell bodies are in the dorsal root ganglion, in the intervertebral foramen) and ventral root (motor; cell bodies in the ventral horn) unite in the foramen to form the short spinal nerve.

Course

Exit rule: C1–C7 exit above the vertebra of the same number; C8 exits between C7 and T1; from T1 down each nerve exits below its same-numbered vertebra. Lower roots descend progressively farther within the canal before exiting (forming the cauda equina).

Branches
  • Dorsal (posterior) ramus: deep back muscles (e.g. Erector spinae), facet joints, and a strip of skin of the back
  • Ventral (anterior) ramus: limbs and the anterolateral trunk; forms the cervical, brachial, lumbar and sacral plexuses, and the intercostal nerves
  • Rami communicantes: white (preganglionic sympathetic, T1–L2 only) and gray (postganglionic, all levels) connect to the sympathetic trunk
  • Meningeal (sinuvertebral) branch: back into the canal (dura, PLL, outer anulus)
Motor

Via ventral roots (myotomes).

Sensory

Via dorsal roots (dermatomes; e.g. C6 thumb, C7 middle finger, C8 little finger, T4 nipple, T10 umbilicus, L1 inguinal region, L4 medial leg, L5 dorsum of foot, S1 lateral foot).

Notes

Lower limb root summary. L3: hip flexion/knee extension, anterior thigh. L4: knee extension, ankle dorsiflexion (tibialis anterior), patellar reflex, medial leg. L5: great-toe extension (EHL), hip abduction, dorsum of foot and first web space, no reliable common reflex. S1: plantarflexion, foot eversion, Achilles reflex, lateral foot and sole.

Muscles innervated

  • Erector spinae — Dorsal (posterior) rami of the spinal nerves, segmentally.

Generated automatically from links in other entries.

Lumbar disc herniation and nerve rootsPosterior view of L4, L5 and S1. Each lumbar root exits below the pedicle of its own vertebra. A posterolateral herniation of the L4–L5 disc compresses the traversing L5 root; a far-lateral herniation at the same level compresses the exiting L4 root.L4L5S1L4–L5 discL5–S1 discDural sac / cauda equinaL4 rootL5 rootS1 root② far-lateral → L4① posterolateral→ L5 (traversing)RootWeaknessReflexSensory lossL4Knee extension, ankle dorsiflexionPatellar ↓Medial leg / ankleL5Great-toe extension, hip abductionNone reliableDorsum of foot, 1st webS1Plantarflexion, eversionAchilles ↓Lateral foot, sole

Swipe sideways to see the whole diagram →

Lumbar disc herniation and nerve roots. Posterior view of L4–S1 (right-sided roots shown). Lumbar roots exit below their own pedicle, so a posterolateral L4–L5 herniation (1) compresses the traversing L5 root and a far-lateral herniation (2) compresses the exiting L4 root.
Original schematic, created for this atlas.
Upper limb dermatomes, anterior viewC4 over the top of the shoulder; C5 lateral arm; C6 lateral forearm, thumb and index finger; C7 middle finger; C8 ring and little fingers and medial hand and distal forearm; T1 medial forearm; T2 medial arm and axilla.C4C5C6C7C8T1T2Lateral(thumb side)Medial(little finger)ASIA key sensory point
Upper limb dermatomes (anterior view). Schematic of the commonly taught pattern. Adjacent dermatomes overlap and published maps disagree at the borders, so check against your textbook. The C6–C8 ASIA key points are tested on the dorsal surface of the digits.
Original schematic, created for this atlas.
Segment of spinal cord with anterior and posterior roots, the spinal ganglion on the posterior root and their union into a spinal nerve
Formation of a spinal nerve: anterior root, posterior root with spinal ganglion, and the posterior division (dorsal ramus).
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 675. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Posterior trunk showing the cutaneous territories of the dorsal rami of the spinal nerves by segment
Cutaneous territories of the dorsal rami on the back.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 802. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Radiculopathy PANCE

Mechanism / lesion

Root compression or inflammation by disc herniation, foraminal stenosis (osteophytes, facet hypertrophy), or rarely tumor or infection.

Classic presentation

Radiating pain, dermatomal sensory loss, myotomal weakness, and reduced reflexes in a single root distribution. Distinguished from peripheral nerve lesions by the pattern of affected muscles and skin.

Herpes zoster PANCE

Mechanism / lesion

Reactivation of varicella-zoster virus latent in the dorsal root ganglion (or cranial nerve ganglia).

Classic presentation

Prodromal pain then a unilateral dermatomal vesicular rash that doesn't cross the midline; possible postherpetic neuralgia.

High-yield PANCE pearls

  • Root exit: C1–C7 above, C8 between C7–T1, T1 and below exit below their vertebra.
  • Dermatomes: C6 thumb, C7 middle finger, C8 little finger, T4 nipple, T10 umbilicus, L4 medial leg, L5 dorsum of foot, S1 lateral foot.
  • Reflexes: biceps C5–C6, brachioradialis C6, triceps C7, patellar L4, Achilles S1.
  • Zoster lives in the dorsal root ganglion.

Imaging anatomy

CT

CT (or CT myelography) for bony foraminal stenosis.

MRI

MRI shows root compression in the foramen or lateral recess.

Key views & landmarks
  • EMG/nerve conduction studies: distinguish radiculopathy from plexopathy or peripheral neuropathy (sensory nerve action potentials are preserved in radiculopathy because the lesion is proximal to the dorsal root ganglion)

Physical exam

Strength / function testing

Map motor weakness by myotome, sensory loss by dermatome, and reflexes by root; compare sides. Nerve tension tests (straight leg raise, Spurling) provoke radicular pain.

Self-quiz

Q1.

Which spinal nerve exits through the intervertebral foramen between C7 and T1?

Show answer

B

There are 8 cervical nerves but 7 cervical vertebrae, so C8 exits between C7 and T1. Below that, each nerve exits below its own vertebra.

Q2.

Where are the cell bodies of sensory neurons carried in the spinal nerves located?

Show answer

B

Sensory (pseudounipolar) neuron cell bodies lie in the dorsal root ganglion, the site where varicella-zoster virus lies latent.

Q3.

Which reflex is most associated with the L4 root?

Show answer

B

The patellar reflex tests L4 (with L3). The Achilles reflex tests S1.

Q4.

White rami communicantes are found at which spinal levels?

Show answer

C

White rami carry preganglionic sympathetic fibers and exist only at T1–L2, where the lateral horn is present. Gray rami (postganglionic) join every spinal nerve.

Q5.

What does the dorsal (posterior) ramus of a typical spinal nerve supply?

Show answer

E

The dorsal ramus supplies the intrinsic back muscles (e.g. erector spinae), the facet joints and a strip of skin of the back. The ventral ramus supplies the limbs and anterolateral trunk and forms the plexuses.

Flashcards

See also