Corpus: Spinal nerve

Spinal nerves are paired nerves that emerge from the spinal cord and are part of the peripheral nervous system.

Spinal nerves are formed within the spinal canal from the anterior and posterior nerve roots. The anterior roots carry efferent (motor) fibers leaving the spinal cord, while the posterior roots carry afferent (sensory) fibers entering the spinal cord. A dorsal root ganglion is located on the posterior root.

In the upper spinal column, spinal nerves exit the spinal canal immediately through the intervertebral foramina. In the lower part of the spine, they first descend as part of the cauda equina within the dural sac before exiting the spinal canal.

2.1. Branches

Outside the spinal canal, each spinal nerve divides into 3 to 4 branches:

• Anterior branch (ventral ramus): This branch provides sensory and motor innervation to the skin and muscles of the anterior trunk, as well as the skin and muscles of the extremities through nerve plexuses.
• Posterior branch (dorsal ramus): This branch provides sensory and motor innervation to the skin of the back and the muscles of the back.
• Meningeal branch: This retrograde branch provides sensory innervation to the meninges, facet joints, periosteum of the spinal canal, and the posterior part of the intervertebral disc's annulus fibrosus in the respective segment.
• Communicating branches: These branches transmit visceroafferent and visceroefferent nerve fibers, including the white ramus communicans and the gray ramus communicans.

2.2. Number

There are a total of 31 pairs of spinal nerves, which are mixed nerves containing motor, sensory, and autonomic components:

• 8 cervical spinal nerves (C1 to C8)
• 12 thoracic spinal nerves (Th1 to Th12)
• 5 lumbar spinal nerves (L1 to L5)
• 5 sacral spinal nerves (S1 to S5)
• 1 coccygeal spinal nerve (Co)

2.3. Plexus formation

The anterior branches of spinal nerves form nerve plexuses near the origin of the extremities, resulting in the mixing of nerve fibers from different spinal nerves. These plexuses include:

• Cervical plexus
• Brachial plexus
• Lumbosacral plexus

2.4. Special features

Certain branches of spinal nerves have specific anatomical names. For example, the dorsal branch of the first spinal nerve is known as the suboccipital nerve, while the strongest branch of the dorsal branch of the second spinal nerve is called the greater occipital nerve.