Nerves (Innervation of the Spine)

Fig. 1

Posterior general view

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Posterior view of cervical spinal nerves

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Posterior view of lumbosacral spinal nerves

Descriptive Anatomy of Spinal Nerves

Each spinal nerve is connected to the spinal cord via two roots, ventral and dorsal, a few millimeters long, which unite to form the trunk of the spinal nerve:

the ventral or motor root emerges opposite the ventral horn of the spinal cord;
the dorsal or sensitive root penetrates opposite the dorsal horn of the spinal cord.

The dorsal root gives rise to the spinal ganglion.

The spinal nerve roots form the trunk of the spinal nerve in the intervertebral foramen where it is surrounded by the dura mater. Each spinal nerve gives a spinal meningeal branch or sinuvertebral nerve of Luschka (Figs. 4 and 5).

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Sinuvertebral nerve of Luschka and anterior and posterior plexus (according to Groen); sinuvertebral nerve on posterior view (a), anterior plexus on anterior view (b), posterior plexus on posterior view (c)

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Sinuvertebral nerve of Luschka and anterior and posterior plexus (according to Groen); sinuvertebral nerve on posterior view (a), anterior plexus on anterior view (b), posterior plexus on posterior view (c)

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Anterior ramus of the root on posterior view (according to Groen)

The spinal nerves have a variable course, horizontal in the cervical region, and increasingly oblique in an inferolateral fashion as the spinal cord descends because of the growth discordance between the spinal cord and the spine.

Each spinal nerve is a mixed nerve that contains afferent and efferent, somatic and autonomic fibers:

somatic fibers with centrifugal motor fibers of axons in the ventral horn of the spinal cord and sensory centripetal fiber sensory neurons of the spinal ganglion.
autonomic fibers belong to the sympathetic and parasympathetic systems.

The autonomic efferent (away from) fibers travel in the anterior root and then in the spinal nerve, which they leave by the communicating white branches to make synapses in the paravertebral sympathetic ganglia.

Autonomic afferent (towards) fibers have their cell bodies at the level of the spinal ganglia that they join either directly by the peripheral nerves, or by the communicating gray branches and the paravertebral sympathetic ganglionic chain of visceral origin.

The spinal nerve divides into two terminal branches just outside the foramen and therefore the spinal nerves are sometimes very short (less than 7 mm) (Fig. 6).

a small dorsal branch for the muscles and skin of the dorsal wall of the neck and trunk;
a larger ventral branch for the ventral wall of the trunk and limbs. It forms the different cervical (C1C4), brachial (C5T1), and lumbosacral (T12S5) plexuses by multiple and complex anastomoses.

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Spinal nerve division in cervical area (a) and lumbar area (b)

Relations of the Nerve Roots

At the Lumbar Level [1–6]

The spinal ganglion in L1 measures 7 mm by 5 mm and the size increases to S1 where it reaches a maximum of 13 × 6 mm.

The distance from the axilla of the root to the proximal part of the ganglion varies from 6 mm for L1 to 15 mm for S1. The ganglion is under the pedicle in 90% of cases, in 2% it is medial and in 8% inferolateral.

The meninges are in close relationship with the nerve roots.

Beyond the dural sac, the roots are enveloped by the pia mater, the arachnoid, and the dura mater. The arteries and root veins are transmitted within these sheaths. The dural sac rests on the posterior longitudinal ligament covering vertebral bodies and discs. Posteriorly, the dural sac is in relation with the laminae and yellow ligaments. The epidural space is relatively narrow because the dural sac is in very close contact with the osteoligamentary structures.

This connective space is filled by the epidural membrane that surrounds the dural sac and lines the laminae and pedicles. Anteriorly, the membrane lines the back of the vertebral bodies and attaches to the anterior surface of the deep portion of the posterior longitudinal ligament. The membrane does not cover the posterior portion of the annulus fibrosus because of the posterior longitudinal ligament that extends laterally over the posterior portion of the annulus fibrosus.

In front of the intervertebral foramen, the epidural membrane extends laterally to form a perineural envelope around the dural sheath of the roots and spinal nerve. The anterior and posterior vertebral venous plexuses are transmitted in the areolar tissue of the epidural membrane [7].

In the vertebral canal , the dural sac and the sheaths of the nerve roots are attached to the spine by thickening of the epidural fascia or meningovertebral ligament of Hoffmann. These ligaments pass from the anterior dural surface to the posterior longitudinal ligament, from the lateral surface of the dural sac to the periosteum of the pedicles, and to the ceiling of the vertebral canal by fine and fragile dorsal bundles.

At the cervical level, there are 8 pairs of cervical spinal nerves. The cervical spinal nerve is very short (7 mm) and the volume is greater for the last 4 cervical nerves of the upper limb than the first 4 cervical nerves for the neck.

The cervical spinal nerves are formed by the union of the anterior and posterior roots.

The ganglion of C1, when it exists, is located in the vertebral canal and that of C2, behind the C1C2 articulation.

The anterior root is three times smaller than the posterior root. The path in the intradural portion is horizontal from C1 to C4 and the obliquity increases as one descends to the thoracic region. The roots leave the dural sheath by two orifices separated by a vertical septum.

The two roots pass behind the uncinate apophysis and the anterior motor root is anteroinferior to the posterior root and thus protected from disc compression.

The two roots form the spinal nerve at the outer pole of the spinal ganglion which is always in the foramen, behind the vertebral artery.

The cervical spinal nerve occupies only 20–50% of the foramen surface.

The remaining surface is occupied by the fat, the venous plexuses in continuity with those of the epidural space, and by the means of fixity of the spinal nerve, the epidural tissue condenses and constitutes a sheath which does not adhere to the wall of the foramen nor to the dura mater of the roots but only to the posterior longitudinal ligament in front and to the capsule of the interapophyseal joints behind. This sheath becomes adherent more laterally to the spinal nerve.

The fibrous bundles described by Sunderland [8–12] fix the epidural sheath to the sheath of the transverse process and to the transverse and supranuclear process for the nerves C5, C6, and C7. The dissection under a microscope [13] does not allow retention of a true tethering ligament, but connective attachments on the periphery of the nerve.

An experimental study of 50 cervical roots [13] to determine the tensile strength of the roots shows that tearing occurs by forces of 3–15 kg and is always preceded by rupture of the dura mater.

The spinal nerve crosses, with the root artery , the fibrous operculum of the central external orifice of the foramen and loses its dural sheath on the internal face of the operculum.

The sinuvertebral nerve penetrates with the venous plexuses into the foramina from outside inwards through a peripheral orifice distinct from that of the spine nerve which is situated in front of the latter [14, 15].

Arnold’s nerve is the posterior branch of the second cervical nerve with its occipital ascending course. The posterior ganglion of C2 is extradural and located at the posterior aspect of the inferior articular facet of the lateral mass of the atlas. It is adherent to the capsule of the lateral atlantoaxial joint, 6–12 mm inside the vertebral artery. The large occipital nerve has a sinusoidal course and has been studied in detail [14].

It bypasses the inferior oblique muscle, traversing the semi-spinalis and trapezius to become subcutaneous and gives off its sensory terminal branches. The motor collaterals innervate the posterior intertransversarii cervicis muscles, the semi-spinalis capitis, the splenius, and the inferior oblique. The nerve is vulnerable in its passage under the inferior oblique during cervical flexion. Arnold’s nerve anastomoses with the posterior branches of the mastoid branch of the cervical plexus, the articular branch of the facial nerve, and with the posterior cervical branches of C1 and C3.

1.1.1 Anomalies of the Lumbar Roots [16–18] (Fig. 7)

Type 1 abnormalities have an abnormal path (type 1A where two pairs of roots can emerge from a single dural sheath or the lower part of the dural sac). In type 2, the number of roots