Introduction to spinal nerves

Introduction to spinal nervesTypes of spinal nerves. Afferent and efferent fibers
~ 6 min

In this note, we are going to discuss the anatomy and function of the spinal nerves (nervi spinales).

Spinal nerves (nervi spinales)
Spinal nerves (nervi spinales)

Before diving into it, let’s recall that the nervous system is structurally subdivided into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS).

Spinal nerves (nervi spinales) arise from the spinal cord. There are 8 cervical (C),

Cervical part of the spinal cord (pars cervicalis)
Cervical part of the spinal cord (pars cervicalis)
Cervical spinal nerves (nn. cervicales)
Cervical spinal nerves (nn. cervicales)
  • 12 thoracic (Th),
Thoracic part of the spinal cord (pars thoracica)
Thoracic part of the spinal cord (pars thoracica)
Thoracic spinal nerves (nn. thoracici)
Thoracic spinal nerves (nn. thoracici)
  • 5 lumbar (L),
Lumbar spinal cord (pars lumbalis)
Lumbar spinal cord (pars lumbalis)
Lumbar spinal nerves (nn. lumbales)
Lumbar spinal nerves (nn. lumbales)
  • 5 sacral (S),
Sacral part of the spinal cord (pars sacralis)
Sacral part of the spinal cord (pars sacralis)
Sacral spinal nerves (nn. sacrales)
Sacral spinal nerves (nn. sacrales)
  • and 1 coccygeal (Co) spinal nerves.
Coccygeal part of the spinal cord (pars coccygea)
Coccygeal part of the spinal cord (pars coccygea)
Coccygeal spinal nerve (n. coccygeus)
Coccygeal spinal nerve (n. coccygeus)

Spinal nerves are formed by two roots: the anterior root (radix anterior) and the posterior root (radix dorsalis),

Anterior root (radix anterior)
Anterior root (radix anterior)
Posterior root (radix posterior)
Posterior root (radix posterior)

which merge in the intervertebral foramen.

Spinal nerve (n. spinalis)
Spinal nerve (n. spinalis)

The sensory spinal ganglion (ganglion spinale), which contains cell bodies of peripheral sensory neurons, adheres to the posterior root. In humans, these neurons are pseudounipolar.

Sensory spinal ganglion (ganglion sensorium nervi spinalis / ganglion spinale)
Sensory spinal ganglion (ganglion sensorium nervi spinalis / ganglion spinale)

The peripheral process ends with receptors, and the central process enters the posterior horn of the spinal cord as part of the posterior root of the spinal nerve.

Afferent fibers heading to receptors
Afferent fibers heading to receptors
Posterior horn (cornu posterius)
Posterior horn (cornu posterius)

The anterior root consists of motor or efferent fibers coming out of the anterior horn of the spinal cord.

Anterior horn (cornu anterius)
Anterior horn (cornu anterius)
Efferent fibers heading to the target organs
Efferent fibers heading to the target organs

These fibers carry motor information from the CNS to the target organs, for example, to muscles of the body.

The autonomic (sympathetic) fibers enter into the spinal nerves at the level between the first thoracic to the second lumbar segments (Th1-L2). The cell bodies of these sympathetic neurons are located in the lateral horns of the spinal cord (which can be found in all thoracic and the 1st and 2nd lumbar segments of the spinal cord, and some sources also include the 8th cervical segment.

Lateral horn (cornu laterale) contains cell bodies of sympathetic neurons
Lateral horn (cornu laterale) contains cell bodies of sympathetic neurons

For the most part, the spinal nerves exit the spinal canal through the intervertebral foramen (foramen intervertebrale) under the corresponding vertebra.

Intervertebral foramen (foramen intervertebrale)
Intervertebral foramen (foramen intervertebrale)

Only 7 superior cervical nerves do it the other way around, passing through the intervertebral foramen above the corresponding vertebra.

Spinal nerve roots projections on the vertebral column
Spinal nerve roots projections on the vertebral column

In the sacral region, the nerves exit through the sacral foramina (foramina sacralia).

Anterior sacral foramina (foramina sacralia anteriora)
Anterior sacral foramina (foramina sacralia anteriora)
Posterior sacral foramina (foramina sacralia posteriora)
Posterior sacral foramina (foramina sacralia posteriora)

Then each spinal nerve is divided into three branches: the anterior, posterior and meningeal branches.

The meningeal branch (ramus meningeus) goes back into the spinal canal through the intervertebral foramen and innervates the spinal meninges.

The posterior or dorsal ramus (r. posterior) passes posteriorly between the transverse processes of the vertebrae and innervates the post-vertebral muscles and the skin of the back.

Dorsal ramus (r. posterior)
Dorsal ramus (r. posterior)

The anterior or ventral ramus (r. anterior) innervates the skin and muscles of the neck, anterior aspect of the trunk and limbs.

Ventral ramus (r. anterior)
Ventral ramus (r. anterior)

The thoracic anterior rami remain segmental and become the intercostal nerves (nn. intercostales) in the intercostal spaces.

Intercostal nerves (nn. intercostales)
Intercostal nerves (nn. intercostales)
Intercostal nerves (nn. intercostales)
Intercostal nerves (nn. intercostales)

In the other parts of the body, the anterior rami interconnect with each other, forming the cervical plexus,

Cervical plexus (plexus cervicalis)
Cervical plexus (plexus cervicalis)

brachial plexus,

Brachial plexus (plexus brachialis)
Brachial plexus (plexus brachialis)

lumbar plexus,

Lumbar plexus (plexus lumbalis)
Lumbar plexus (plexus lumbalis)

sacral plexus,

Sacral plexus (plexus sacralis)
Sacral plexus (plexus sacralis)

and coccygeal plexuse, which give rise to the peripheral nerves.

Coccygeal plexuses (plexus coccygeus)
Coccygeal plexuses (plexus coccygeus)

Moreover, there are two types of branches that connect the spinal nerves and the sympathetic trunk.

White rami communicantes (rr. communicantes albi) only exit the spinal cord between the levels of T1-L2. They carry preganglionic sympathetic fibers that pass to the ganglia of the sympathetic trunk.

White rami communicantes (r. communicans albus)
White rami communicantes (r. communicans albus)

Gray rami communicantes (rr. communicantes grisei) exist at all levels of the spinal cord. They carry postganglionic sympathetic fibers from the ganglia of the sympathetic trunk to the target organs.

Gray rami communicantes (r. communicans griseus)
Gray rami communicantes (r. communicans griseus)

In the following videos, we will focus in greater detail on specific nerve plexuses and their nerves.

более подробно остановимся на конкретных нервных сплетениях и входящих в них нервах.

Dictionary

Introduction to spinal nerves

Spinal nerves
nervi spinales
Ganglion
ganglion
Nerve
nervus
Cranial nerves
nervi craniales
Cervical part
pars cervicalis
Cervical nerves
nn. cervicales
Thoracic part
pars thoracica
Thoracic nerves
nn. thoracici
Lumbar part
pars lumbalis
Lumbar nerves
nn. lumbales
Sacral part
pars sacralis
Sacral nerves
nn. sacrales
Coccygeal part
pars coccygea
Coccygeal nerve
n. coccygeus
Anterior root
radix anterior
Posterior root
radix posterior
Spinal nerve
n. spinalis
Spinal ganglion
ganglion sensorium nervi spinalis / ganglion spinale
Posterior horn
cornu posterius
Anterior horn
cornu anterius
Lateral horn
cornu laterale
Intervertebral foramen
foramen intervertebrale
Anterior sacral foramina
foramina sacralia anteriora
Posterior sacral foramina
foramina sacralia posteriora
Meningeal branch
ramus meningeus
Posterior branch
r. posteriores
Anterior branch
r. anterior
Intercostal nerves
nn. intercostales
Cervical plexus
plexus cervicalis
Brachial plexus
plexus brachialis
Lumbar plexus
plexus lumbalis
Sacral plexus
plexus sacralis
Coccygeal plexus
plexus coccygeus
White ramus communicans
r. communicans albus
Gray ramus communicans
r. communicans griseus
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