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]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 13.4, Spinal Nerves and Nerve Plexuses
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