Posterior root of spinal nerve

At a glance

English

Posterior root of spinal nerve

Latin (TA2)

Radix posterior nervi spinalis

System

Nerve

Region

Trunk

Side

Paired, left and right

About the posterior root of spinal nerve

The posterior root, also called the dorsal or sensory root, is the incoming half of a spinal nerve. It brings afferent fibres from skin, muscles, joints and internal organs into the spinal cord. There is one on each side for every spinal nerve, with the occasional exception of the first cervical nerve, whose posterior root is small or absent.

Its defining feature is the spinal ganglion (dorsal root ganglion), an oval swelling that usually sits in the intervertebral foramen, just outside the dural sac. It contains the cell bodies of the sensory neurons. These cells are pseudounipolar: one process runs out in the peripheral nerve to a receptor, the other runs inwards through the root to the cord. No synapses take place in the ganglion. Central to the ganglion the root splits into a fan of rootlets that enter the cord along the posterolateral sulcus. Just beyond the ganglion the root joins the anterior root to form the mixed spinal nerve, which then divides into anterior and posterior rami.

Inside the cord the incoming fibres sort themselves by type. Large fibres for touch, vibration and position sense mostly turn up in the posterior columns, while fine fibres for pain and temperature enter the posterior horn and relay there. Roots of the lumbar and sacral nerves are long and run down within the cauda equina, and their ganglia lie in the corresponding foramina, those of the sacral nerves inside the sacral canal.

The area of skin served by a single posterior root is its dermatome, the basis of sensory testing after spinal injury. A disc prolapse or bony narrowing that presses on a posterior root causes pain, tingling and numbness in that dermatome, often with a reduced reflex. The varicella-zoster virus lies dormant in spinal ganglia and, when it reactivates, causes shingles along one dermatome. Surgeons can cut selected posterior rootlets to ease severe spasticity or pain.

Course and supply

Arises from

Peripheral processes of sensory neurons in the spinal ganglion, which lies in or near the intervertebral foramen.

Ends in

Enters the spinal cord as rootlets along the posterolateral sulcus, ending in the posterior horn and posterior columns.

Supplies

Carries all sensory information from the body wall, limbs and viscera into the cord: touch, pain, temperature, position and visceral sensation.