Back & Spinal Cord LO 9

9. Distinguish between the parts of a spinal nerve, and understand how spinal nerves are named.

Spinal nerves originate from the gray matter of the spinal cord, arising as ventral and dorsal rootlets from the ventral and dorsal horns of gray matter, respectively. Based on their origin and composition, ventral rootlets are entirely efferent (motor), and dorsal rootlets are entirely afferent (sensory).

Rootlets coalesce to form roots. The ventral root is entirely motor, whereas the dorsal root is entirely sensory.

A notable (and grossly identifiable structure) in this area are the dorsal root ganglia (DRG). These are located in association with the dorsal root proximal to the trunk of spinal nerve, and are collections of cell bodies outside of the PNS.

The ventral & dorsal roots join to form the trunk of the spinal nerve, often referred to as just spinal nerve. At this point, the afferent (from the dorsal root) and efferent (from the ventral root) fibers combine, making the trunk of the spinal nerve a mixed nerve (having both afferent and efferent nerve fibers).

The trunk of the spinal nerve exits the vertebral canal laterally via an intervertebral foramen and almost immediately bifurcates into a dorsal primary ramus (DPR) and a ventral primary ramus (VPR). Both the DPR and VPR are mixed nerves. Primary ramus means 1st branch, and so think of the DPR and VPR as the first branches of the spinal nerve. Each DPR and VPR will typically have numerous distal branches (some of which are named).

The dorsal primary rami (DPR) efferently and afferently serve the deep back (including vertebral joints and deep back muscles) as well as cutaneous innervation for the back. DPR branches rarely form plexuses (networks of nerve fibers from different spinal cord levels).

The ventral primary rami innervate a substantial area including all structures not supplied by DPRs and cranial nerves, including the upper and lower limbs. VPR branches frequently form plexuses (cervical, brachial, lumbar, & sacral).

Spinal nerves are named for the spinal cord segment from which it originates and the vertebral level that the nerve exits the vertebral column.

There are 31 spinal nerve pairs:

  • 8 cervical spinal nerve pairs (C1-C8)
  • 12 thoracic spinal nerve pairs (T1-T12)
  • 5 lumbar spinal nerve pairs (L1-L5)
  • 5 sacral spinal nerve pairs (S1-S5)
  • 1 coccygeal spinal nerve pairs (Co1)