Groups of Spinal Nerves
The peripheral nervous system includes 31 pairs of spinal nerves, which are topographically divided into several groups based on the segmental level of the spinal cord from which they originate. The regional distribution is as follows:
- Cervical nerves — nn. cervicales (segments C₁–C₈) — 8 pairs.
- Thoracic nerves — nn. thoracici (segments Th₁–Th₁₂) — 12 pairs.
- Lumbar nerves — nn. lumbales (segments L₁–L₅) — 5 pairs.
- Sacral nerves — nn. sacrales (segments S₁–S₅) — 5 pairs.
- Coccygeal nerve — n. coccygeus (segment Co₁) — usually 1 pair.
This segmental organization reflects the general metameric pattern of the human body, where each nerve supplies a specific structural level (metamere).
Formation of a Spinal Nerve
Each nerve trunk is formed by the fusion of two roots connected to the spinal cord via rootlets (fila radicularia).
- Dorsal root — radix dorsalis (sensoria). This is purely sensory. Its key feature is the presence of the spinal ganglion (ganglion spinale (sensorium)). Typically, dorsal roots are visibly thicker than ventral roots, with the C₁ segment being the sole exception.
- Ventral root — radix ventralis (motoria). Contains only motor fibers and lacks any ganglia along its course.
The fusion of these roots occurs strictly lateral to the spinal ganglion. It is at this exact point that the single mixed trunk of the spinal nerve (truncus nervi spinalis) is formed.
Topography and Course of the Roots
The direction of the roots from the spinal cord to the intervertebral foramen directly depends on their segmental level:
- In the cervical region, they run almost horizontally.
- The lower the level, the more obliquely downward the roots course.
- In the lumbar, sacral, and coccygeal regions, they descend strictly vertically. This anatomical feature leads to the formation of the cauda equina (horse's tail).
Along their path, the roots travel within the subarachnoid space, enveloped by the pia mater. As they approach the intervertebral foramina, they acquire coverings from all three meninges.
Location of Spinal Ganglia
The location of spinal ganglia also has regional variations:
- The vast majority of ganglia lie directly within the intervertebral foramina.
- Lower lumbar ganglia are located partially inside the vertebral canal.
- Sacral ganglia (except for the very last one) lie within the vertebral canal but outside the dura mater.
- The coccygeal nerve ganglion is unique because it is located directly inside the dural sac. Furthermore, this ganglion may sometimes be entirely absent.
Branching of the Spinal Nerve
Immediately after exiting through the intervertebral foramen, the unified nerve trunk divides into several functionally important branches.
Main branches:
- Ventral ramus — r. ventralis (anterior).
- Dorsal ramus — r. dorsalis (posterior).
Both of these rami remain mixed, carrying both motor and sensory fibers.
Communication with the autonomic nervous system is provided by the communicating rami:
- White communicating ramus (r. communicans albus) — carries myelinated preganglionic fibers to the sympathetic trunk. Present only at levels from C₈ to L₂–L₃.
- Gray communicating ramus (r. communicans griseus) — returns unmyelinated postganglionic fibers. These branches are present in all spinal nerves without exception.
In addition, a meningeal branch (r. meningeus) arises from the trunk. Its function is to return to the vertebral canal to innervate the dura mater, periosteum, and blood vessels. This branch also carries sympathetic fibers.