Spinal nerve

A spinal nerve is a paired peripheral nerve formed by the union of a sensory dorsal root and a motor ventral root near the spinal cord. Humans usually have 31 pairs. Because sensory and motor fibres travel together after the roots unite, a spinal nerve is mixed even though its two roots have different principal signaling directions.^[ref]

The resulting spinal nerve divides into posterior and anterior spinal nerve rami, both of which remain mixed. A dorsal root and a posterior ramus are therefore different structures: one joins the spinal cord, while the other distributes fibres after root union. Anterior spinal nerve rami can reorganize into a nerve plexus, so one named limb nerve can contain fibres from several spinal levels.^[ref]

A ventral root is a bundle of outgoing motor axons leaving the anterior side of the spinal cord. It contains somatic motor fibres and, at appropriate spinal levels, autonomic efferent fibres. It joins a sensory dorsal root to form a mixed spinal nerve.^[ref]

For example, an action potential destined for a limb muscle can leave the spinal cord through a ventral root, then continue through a mixed spinal nerve and an anterior spinal nerve ramus. The directional separation near the spinal cord does not mean every later branch is motor-only. A ventral root is also distinct from the anterior branch formed after the roots unite.

A spinal nerve ramus is a branch formed after a spinal nerve has been assembled from its dorsal root and ventral root. Both posterior and anterior spinal nerve rami carry mixed sensory and motor fibres. Posterior branches principally supply the back; anterior branches supply the anterolateral trunk and, through nerve networks, the limbs.^[ref]

A root and a ramus therefore describe different positions in the branching pathway. Fibres have already mixed before the spinal nerve rami separate. Many anterior branches contribute to a nerve plexus, redistributing axons from several spinal levels into named peripheral nerves. The branch points route existing fibres rather than forming new synapses.

A dorsal root is the bundle of sensory axons entering the posterior side of the spinal cord at a spinal level. The associated dorsal root ganglion contains the soma of many of these sensory neurons. Their peripheral processes receive information from the body, while their central processes enter through the dorsal root.^[ref]

The dorsal root joins the ventral root to form a mixed spinal nerve. This organization separates sensory entry from motor exit close to the spinal cord while allowing both directions to share a peripheral route. The dorsal root ganglion houses cell bodies; it is not normally a chain of sensory-to-sensory synapses.