Nerves of the Lumbar Spine

The lumbar spinal nerves lie in the intervertebral foraminae, and are connected by spinal nerve roots to the spinal cord. The spinal nerves divide into the ventral and dorsal rami outside the vertebral column. The lumbar sympathetic trunks run along the anterolateral lumbar vertebral column, and they communicate with the ventral rami of the lumbar spinal nerves

Lumbar Spinal Nerves

• The lumbar spinal nerves are numbered according to the vertebra beneath which they lie. The L1 spinal nerve lies below the L1 vertebra in the L1/2 intervertebral foramen. The L2 spinal nerve lies below the L2 vertebra in the L2/3 intervertebral foramen etc.
• Each spinal nerve is connected centrally to the spinal cord by a dorsal and ventral root.
  • The joining of the spinal nerve roots to the spinal nerve occurs in the intervertebral foramen.
• The spinal nerve divides peripherally into a larger ventral ramus and smaller dorsal ramus.
  • The branching into the two rami occurs outside the foramen.
• The spinal nerves are short, and no longer than the width of their intervertebral foramen.
  • They are generally a few millimetres, but can be less than 1mm with the roots branching directly into rami without the formation of a proper spinal nerve.

Lumbar Nerve Roots

The spinal cord showing the meninges, its roots, spinal nerve, and branches (rami)
University of Minnesota © 2006

• The dorsal root transmits sensory fibres from the spinal nerve to the spinal cord
• The ventral root transmits mostly motor fibres from the spinal cord to the spinal nerve
  • The ventral root may also transmit some sensory fibres
  • The L1 and L2 ventral roots also transmit preganglionic, sympathetic, efferent fibres
• The spinal cord has its termination opposite the L1/2 intervertebral disc, but the range is T12/L1 to L2/3.
• The lower lumbar and sacral nerve roots run within the vertebral canal largely enclosed in the dural sac.
• In the cauda equina, the lumbar, sacral, and coccygeal nerve roots run freely
• Each root is covered by individual pia mater, continuous with the pia mater of the spinal cord.
• The roots of the cauda equina are within the cerebrospinal fluid (CSF) which flows through the subarachnoid space
• The nerve fibres within the nerve roots run in a single trunk for most of their course
• Rootlets
  • Near the spinal cord they separate into rootlets which are smaller bundles, and attach to the spinal cord.
  • There are between 2-12 rootlets for each root, and are 0.5-1mm in diameter.
  • The ventral root rootlets attach to the ventrolateral aspect of the spinal cord
  • The dorsal root rootlets attach to the dorsolateral sulcus of the cord and along the ventral and dorsal surface of the cord the rootlets form an uninterrupted series of attachments
• Dural sleeve
  • A pair of spinal nerve roots leaves the dural sac just above the level of the intervertebral foramen
  • The nerve roots penetrate the dural sac inferolaterally
  • They take an extension of dura mater and arachnoid mater called the dural sleeve
  • The dural sleeve encloses the nerve roots merges or becomes the epineurium of the spinal nerve
  • The nerve roots are sheathed with pia mater and CSF flows around them as far as the spinal nerve
• Dorsal root ganglion
  • The dorsal root ganglion is an enlargement of the dorsal root
  • It is formed immediately proximal to the dorsal roots junction with the spinal nerve.
  • It contains the cell bodies of the sensory fibres in the dorsal root
  • It lies within the dural sleeve of the nerve roots and occupies the upper, medial part of the intervertebral foramen. It may lie further distally in the foramen with short spinal nerves.
• Nerve root angles leaving the dural sac
  • The angles of each pair of nerve root as it leaves the dural sac varies, getting increasingly acute angles more caudally.
• The L1 nerve root leaves at about 80°, the L2 at about 70°, L3 and L4 at 60°, and L5 at 45°.
• Nerve root vertebral body origins
  • The nerve root sleeves generally arise opposite the back of their respective vertebral bodies (L1 sleeve arises behind the L1 body, etc)
  • However with successively caudal sleeves, they arise increasingly higher behind their vertebral bodies.
  • And so the L5 nerve root arises behind the L4/5 intervertebral disc.

Relations of the Nerve Roots

• The nerve roots are intimately related to the meninges.
  • The roots of the cauda equina are covered in the dural sac. Individual pairs of roots are covered by pia, arachnoid, and dura mater in the nerve root sleeves.
  • Tumours or cysts of the dura or arachnoid can compress the nerve roots
• Radicular arteries and veins run within the root sleeves.
• Dural sac
  • Anteriorly: floor of the vertebral canal, the vertebral bodies, discs, posterior longitudinal ligament, anterior spinal canal arteries and sinuvertebral nerves
  • Posteriorly: roof of the vertebral canal, the laminae, and ligamenta flava
• Epidural space
  • A space between the dural sac and the osseoligamentous boundaries of the vertebral canal
  • Narrow space, almost a potential space.
  • Filled by a thin layer of areolar connective tissue, some call the epidural membrane. This surrounds the dural sac.
  • Relations
    • Dorsally: lines the deep surface of the laminae and pedicles
    • Ventrally: Lines the vertebral bodies, and passes medially deep to the posterior longitudinal ligament, and attaches to the anterior surface of the deep portion of the ligament. It doesn't cover the anulus fibrosis, but blends with the upper and lower borders of the AF just anterior to that of the posterior longitudinal ligament.
    • Laterally: Opposite the intervertebral foramen, it goes laterally forming a circum-neural sheath around the dural sleeve of the nerve roots and spinal nerve
  • The anterior and posterior internal vertebral venous plexuses run within the areolar tissue of the epidural membrane
  • Epidural fat is concentrated around the nerve roots in the intervertebral foramina
• Radicular canals
  • Individual pairs of nerve roots enclosed in their dural sleeves, course to the intervertebral foraminae along radicular canals
  • The nerve roots are related laterally to a pedicle
  • The pass the back of their vertebral body, run downwards, and enter the upper portion of their intervertebral foramen.
  • Dorsally is a lamina and ligamenta flava, and these separate the root sleeve from the overlying facet joints.

Anomalies of the Nerve Roots

A conjoined nerve root is depicted with the lateralised right L3 and L4 nerves exiting in a common region with a horizontal L3 sheath and vertical and elongated L4 sheath.

Dermatomes

Main article: Dermatomes

Resources

See this brilliant visual tour of the lumbar nerve roots by a US physiotherapist.

References

These are study notes taken from Chapter 10 of:

• Bogduk, Nikolai. Clinical and radiological anatomy of the lumbar spine. Edinburgh: Elsevier/Churchill Livingstone, 2012.