Topic Overview

Spinal Nerve

General Features

• A spinal nerve is a mixed nerve formed by the union of:

  • Ventral (anterior) root → motor fibers from anterior horn cells.

  • Dorsal (posterior) root → sensory fibers from dorsal root ganglion.

• Total: 31 pairs of spinal nerves.

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

Formation and Branches

1. Roots

   • Dorsal root → sensory, has dorsal root ganglion.

   • Ventral root → motor.

   • Both unite in intervertebral foramen to form spinal nerve.

2. Primary Rami

   • Dorsal ramus → supplies muscles and skin of back.

   • Ventral ramus → supplies muscles and skin of front and limbs, forms major plexuses (cervical, brachial, lumbar, sacral).

3. Other Branches

   • Ramus communicans → connects to sympathetic trunk.

   • Meningeal branch → supplies dura mater, ligaments, periosteum around vertebra.

Functional Components of Spinal Nerves

• Somatic efferent fibers → motor to skeletal muscle.

• Somatic afferent fibers → sensory from skin, joints, muscles.

• Visceral efferent fibers → autonomic to smooth muscle and glands.

• Visceral afferent fibers → sensory from viscera.

Distribution

• Cervical nerves → form cervical and brachial plexus.

• Thoracic nerves → continue as intercostal nerves.

• Lumbar, sacral, coccygeal nerves → form lumbar, sacral, and coccygeal plexuses.

Clinical Anatomy of Spinal Nerves

• Nerve root compression → from disc herniation causes radiculopathy (pain, numbness, weakness).

• Shingles (Herpes zoster) → infection of dorsal root ganglion → painful vesicular eruption along dermatome.

• Spinal nerve injury → results in sensory loss (dermatome) and motor deficit (myotome).

• Referred pain → visceral afferents converge on same spinal segments as somatic afferents.

• Cauda equina syndrome → compression of lower lumbar and sacral nerve roots → paralysis, saddle anesthesia, incontinence.