Groups of spinal nerves
The peripheral nervous system comprises 31 pairs of spinal nerves, which are topographically divided into several groups depending on the segmental level of the spinal cord from which they emerge. The distribution by regions 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 represented by 1 pair.
This metameric (segmental) organization reflects the general structural pattern of the human body, where each nerve supplies a specific "floor" (metamere).
Formation of a spinal nerve
Each nerve trunk is formed by the fusion of two roots that connect to the spinal cord via specialized rootlets — fila radicularia.
- Dorsal root — radix dorsalis (sensoria). It is exclusively sensory. Its key feature is the presence of the spinal ganglion, ganglion spinale (sensorium). As a rule, dorsal roots are visually thicker than ventral roots, with the only exception being the C₁ segment.
- Ventral root — radix ventralis (motoria). It contains only motor fibers and has no ganglia along its course.
The fusion of these roots occurs strictly lateral to the spinal ganglion. It is at this point that the single mixed trunk of the spinal nerve is formed — truncus nervi spinalis.
Topography and course of the roots
The direction in which the roots travel from the spinal cord to the intervertebral foramen directly depends on the level of the segment:
- In the cervical region, they lie almost horizontally.
- The lower the level, the more obliquely downward the roots are directed.
- In the lumbar, sacral, and coccygeal regions, they descend strictly vertically downward. This exact anatomical feature leads to the formation of the so-called cauda equina — cauda equina.
Along their path, the roots travel in the subarachnoid space, where they are gently surrounded by the pia mater. As they approach the intervertebral foramina, they receive a covering from all three meninges.
Location of spinal ganglia
The localization of spinal ganglia also has regional characteristics:
- The absolute 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 external to the dura mater.
- The coccygeal nerve ganglion is unique in that it is located directly inside the cavity of the dura mater. Furthermore, this ganglion may sometimes be entirely absent.
Branching of the spinal nerve
Immediately after exiting through the intervertebral foramen, the single nerve trunk divides into several functionally important branches.
Main branches:
- Ventral branch — r. ventralis (anterior).
- Dorsal branch — r. dorsalis (posterior).
Both of these branches remain mixed in nature, carrying both motor and sensory fibers.
Connection with the autonomic (vegetative) nervous system is provided by communicating branches:
- White communicating branch (r. communicans albus) — carries myelinated preganglionic fibers to the sympathetic trunk. Present only at levels from C₈ to L₂–L₃.
- Gray communicating branch (r. communicans griseus) — returns unmyelinated postganglionic fibers. These branches are present in absolutely all spinal nerves.
In addition, a meningeal branch (r. meningeus) always departs from the trunk. Its task is to return back into the vertebral canal to innervate the dura mater, periosteum, and blood vessels. Among other things, this branch carries sympathetic fibers.
Briefly: answers to topical questions
What do the ventral branches of the spinal nerves innervate?
The ventral branches of the spinal nerves contain sensory and motor fibers and participate in the innervation of various body areas, forming nerves and plexuses.
- Cervical nerves — form cutaneous, muscular, and mixed nerves of the neck, forming the cervical plexus (plexus cervicalis).
- Thoracic nerves — form the intercostal nerves (nn. intercostales) and the subcostal nerve (n. subcostalis); their cutaneous branches innervate the skin of the anterior chest wall and anterior abdominal wall.
- Lumbar and sacral nerves — form the lumbosacral plexus (plexus lumbosacralis).
What do the dorsal branches of the spinal nerves innervate?
The dorsal branches of the spinal nerves contain sensory and motor fibers, providing innervation to the skin and muscles.
- Cervical nerves — participate in the formation of cutaneous, muscular, and mixed nerves of the neck, and innervate the occipital region.
- Lumbar nerves (L₁–L₃) — give off lateral cutaneous branches (superior cluneal nerves, nn. clunium superiores), which innervate the skin of the gluteal region.
- Lumbar nerves (L₄–L₅) — innervate only muscles and do not innervate the skin.
What types of nerve fibers make up the trunk of a spinal nerve?
The trunk of a spinal nerve is mixed and includes several types of nerve fibers.
- Sensory fibers — come from the dorsal root (radix dorsalis).
- Motor fibers — come from the ventral root (radix ventralis).
- Sympathetic fibers — enter through communicating branches with the sympathetic trunk (myelinated preganglionic and unmyelinated postganglionic fibers).
Which branches of the spinal nerves form the cervical plexus?
The cervical plexus (plexus cervicalis) is formed from the ventral branches of spinal nerves C₁–C₄. Sources also indicate the participation of C₅ in the formation of descending branches, in particular the phrenic nerve.
To avoid confusing the number of spinal nerves with the number of vertebrae, remember the rule "Plus one in the neck, minus the coccyx": 8 cervical nerves (with 7 vertebrae), 12 thoracic, 5 lumbar, 5 sacral, and only 1 coccygeal (with 3–5 coccygeal vertebrae).
Class and exam questions
What is the main difference between the ventral and dorsal roots?
The ventral root (radix ventralis) contains only motor fibers and has no ganglia. The dorsal root (radix dorsalis) carries sensory fibers, bears the spinal ganglion, and is usually thicker than the ventral root.
Where exactly is the mixed nerve trunk formed?
The fusion of the ventral and dorsal roots into a single trunk (truncus nervi spinalis) occurs strictly lateral (external) to the spinal ganglion.
How does the white communicating ramus differ from the gray one?
The white ramus communicans carries myelinated (preganglionic) fibers and originates only from segments C₈–L₃. The gray ramus communicans carries unmyelinated (postganglionic) fibers and connects to all spinal nerves without exception.
What does the meningeal branch innervate?
The meningeal branch returns into the vertebral canal and provides innervation to the dura mater, vertebral periosteum, and local blood vessels.
What else to study on this topic
- Formation of nerve plexuses by the anterior rami of spinal nerves
- Segmental skin innervation (dermatomes)
- Histological structure of the spinal ganglion (ganglion spinale)
- Topography and functional role of cauda equina elements
- Clinical manifestations of individual branch and root lesions
Study the complete topic in the Anatomy course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Spinal Nerves" plus 1,363 more pages for this subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 295 video lessons by subject
Other topics in the "PNS" section
Free video lessons: AnatomyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.