Anterior root of spinal nerve

At a glance

English

Anterior root of spinal nerve

Latin (TA2)

Radix anterior nervi spinalis

System

Nerve

Region

Trunk

Side

Paired, left and right

About the anterior root of spinal nerve

The anterior root, also called the ventral or motor root, is the outgoing half of a spinal nerve. It carries efferent fibres away from the spinal cord. Each spinal cord segment gives off a pair of anterior roots, one on each side, so there is a root for every spinal nerve from C1 to the coccygeal nerve.

Every root begins as a row of fine rootlets that leave the anterolateral sulcus on the front of the cord. The rootlets gather into a single bundle that runs outwards and slightly downwards inside the dural sac, bathed in cerebrospinal fluid of the subarachnoid space. It then picks up a sleeve of dura and arachnoid and meets the posterior root just beyond the spinal ganglion, within or close to the intervertebral foramen. The two roots blend to form the mixed spinal nerve, which almost at once divides into anterior and posterior rami. Below the end of the cord at about L1 or L2, the lumbar, sacral and coccygeal roots descend steeply as part of the cauda equina before reaching their foramina.

The fibres in the root are axons of nerve cells whose bodies lie in the grey matter of the cord. Large alpha motor neurons and smaller gamma motor neurons in the anterior horn supply skeletal muscle and muscle spindles. In segments T1 to L2, and in S2 to S4, the root also carries preganglionic autonomic fibres from the lateral grey matter: sympathetic fibres that leave through the white rami communicantes, and parasympathetic fibres that form the pelvic splanchnic nerves. There is no ganglion on the anterior root, because its cell bodies are inside the cord.

Damage to an anterior root, for example from a herniated disc, a tumour or avulsion of the root from the cord in a traction injury, weakens the muscles of that segment (its myotome) and reduces the related reflex, but skin sensation is spared if the posterior root is intact. Wasting and twitching of muscle follow loss of the anterior horn cells or their roots.

Course and supply

Arises from

Rootlets leaving the anterolateral sulcus of the spinal cord, carrying axons of anterior horn and lateral horn cells.

Ends in

Joins the posterior root just distal to the spinal ganglion, in or near the intervertebral foramen, to form the spinal nerve.

Supplies

Carries somatic motor fibres to skeletal muscle and, at thoracolumbar and sacral levels, preganglionic autonomic fibres; it carries no ordinary sensory fibres.