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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.

A dorsal root ganglion is a collection of sensory neuron cell bodies on a dorsal root associated with a spinal nerve. These neurons carry information from peripheral tissues toward the spinal cord, including signals relevant to touch, temperature, pain and proprioception.

The sensory axon typically branches into a peripheral process and a central process; the ganglion is not simply a relay in which every incoming signal must cross a synapse onto another neuron. Its neurons also comprise multiple functional populations. Single-cell RNA sequencing of mouse dorsal root ganglion neurons separated molecularly distinct sensory types without assigning them beforehand solely from a few selected markers.^[ref] Anatomical location, molecular profile and stimulus response provide complementary descriptions of a sensory neuron.

The lumbosacral plexus is the connected system of lumbar plexus and sacral plexus pathways formed from anterior spinal nerve rami of lower spinal nerves. It redistributes sensory and motor axons into peripheral nerves supplying the pelvis and lower limb. Its connecting lumbosacral trunk usually carries contributions from L4 and L5 into the sacral plexus.^[ref]

For example, the femoral nerve draws mainly on upper lumbar levels, while the sciatic nerve combines lower lumbar and sacral contributions. A named peripheral nerve consequently does not correspond one-to-one with a spinal nerve. The nerve plexus reroutes existing axons; its intersections do not imply that the fibres form synapses.

The pudendal nerve is a mixed nerve of the sacral plexus, usually receiving fibers associated with S2–S4 spinal nerves. It carries sensory information from much of the perineum and motor signals to skeletal muscles involved in continence, including the external anal sphincter and external urethral sphincter. Its route includes the pudendal canal.

A description of anatomy is a pattern with variation, not a guarantee that every individual has identical branching. Dissection of 28 cadavers documented additional branches and interconnections near the external anal sphincter in some specimens.^[ref] The study also illustrates the difference between a skeletal muscle being structurally intact and its nerve supply being intact: coordinated contraction depends on both the skeletal muscle and the arriving motor signals.

The sacral plexus is a network of anterior spinal nerve rami, usually incorporating L4–S4 spinal contributions, lying against the anterior surface of the piriformis muscle. It distributes sensory and motor fibres to much of the pelvis and lower limb. Major branches include the sciatic nerve and the pudendal nerve.^[ref]

Its mixed origins explain an otherwise puzzling arrangement: the sciatic nerve carries fibres associated with several spinal nerves, and fibres from one spinal level can leave through different peripheral branches. A nerve plexus is therefore a redistribution network, rather than a new set of neurons inserted into each pathway.

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 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 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.