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.
• Dural ligaments (also known as meningovertebral ligaments or ligaments of Hofmann)
  • The nerve root sleeves are tethered to the vertebral column by these ligaments, which are condensations of the epidural fascia
  • The term dural ligament isn't anatomically correct. It is a connection between the meninges and the vertebral column
  • Variably developed at L1-L4, well developed at L5
  • Ventrally: passes from the ventral surface of the dura to the posterior longitudinal ligament
  • Laterally: passes from the lateral surface of the dural sac to the periosteum of the pedicles and the capsule of the facet joint.
  • Posteriorly: attached to the roof of the vertebral canal by occasional weak pseudo-ligamentous connections
• Nerve root sleeves are tethered within the vertebral canal and in the intervertebral foramen.
  • Vertebral canal: the meningovertebral ligaments connect the dural to the posterior longitudinal ligament and the periosteum of the adjacent pedicle.
  • Intervertebral foramen: the root sleeve is surrounded by the circum-neural sheath, indirectly binding the nerve roots and the spinal nerve mainly to the facet joint capsule dorsally, but also to the margins of the foramen. At the outer end of the foramen, there may be a transforaminal ligament - the spinal nerve is below most varieties of transforaminal ligaments but above the inferior type.
• Size of the spinal nerve and nerve roots
  • The intervertebral foramen increases from L1/2 to L4/5.
  • The L5/S1 foramen is smaller than the rest despite the L5 spinal nerve being the largest.
  • The L5 spinal nerve occupies about 25-30% of the foramen while the other nerves occupy between 7-22%
  • The L5 nerve is therefore the most susceptible to foraminal stenosis.

Anomalies of the Nerve Roots

The clinically most significant anomalies of the lumbar nerve roots are aberrant courses and anastomoses between nerve roots. See Trimba et al for a comprehensive review of the topic.

• 

  Classification of conjoined nerve roots.

Type	General Anomaly	n-A	n-B
Type 1	Aberrant courses	1A: two pairs of nerve roots arise from a single dural sleeve	1B: A dural sleeve arises from a low position on the dural sac
Type 2	Variable nerve roots in an intervertebral foramen	2A: A foramen is unoccupied by a nerve, with the foramen above or below containing two sets of roots	2B: A foramen contains a supernumerary set of roots
Type 3	Extradural anastomoses between roots in which a bundle of nerve fibres leaves one dural sleeve to enter an adjacent sleeve. May be superimposed on a type 2

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.

• The incidence of anomalies is about 8.5%
• The anomalies do not always produce symptoms
  • The doubled nerve roots occupy more of the available space in the radicular canal or intervertebral foramen than a single root
  • Any space-occupying lesion is therefore more likely to compress a double nerve root, and symptoms will occur sooner than for a single nerve root compression.
  • Disorders of the lumbar spine are not more common, but symptom development is more common in the presence of a space-occupying lesion.
• Clinical signs
  • Clinical exam might reveal compression of a nerve root, but if that root has an anomalous course, then the lesion won't be at the expected location.
  • In the case of a double nerve root, a single compressive lesion could cause signs that are suggestive of two lesions.
  • Unusual clinical signs should lead to at least considering a nerve root anomaly.
• Intrathecal anastomoses are more common at 11-30%, but these occur proximal to the standard location of compression and so are not of diagnostic significance (but may be important in proximal nerve root surgery).

Dorsal Rami

• The L1-L4 dorsal rami
  • Short in length about 5mm
  • Arise at almost right angles from the lumbar spinal nerves
  • Directed backwards towards the upper border of the subjacent transverse process
  • As they approach their transverse processes, they divide into two or three branches. A medial branch and lateral branch are consistently present. The intermediate branch will either arise from the dorsal ramus itself, or the be a branch off the lateral branch.
• L5 dorsal rami
  • Longer and travels over the top of the ala of the sacrum.
  • It forms only a medial branch and a branch that is equivalent to the intermediate branches of the other lumbar dorsal rami.
• Lateral branches
  • Principally distributed to the iliocostalis lumborum muscle
  • The L1-L3 lateral branches can emerge from the dorsolateral border of the iliocostalis lumborum and become cutaneous, piercing the thoracolumbar fascia and descending inferolaterally across the iliac crest to innervate the skin of the buttock over an area extending from the iliac crest to the greater trochanter. When crossing the iliac crest they run parallel to one another with those from lower levels lying most medially. See article on cluneal nerve pain.
  • The lowest and most medial nerve that crosses the iliac crest does so approximately 7-8cm from the midline.
  • Variability in which of the lateral branches become cutaneous
    • In embryos and fetuses L1 is always cutaneous, L2 in 90%, L3 in 70%, and L4 in 40%.
    • In adults it is similar except L4 cutaneous branches are uncommon. L1 exclusively in 60%. L1 and L2 in 27%. L1-L3 in 13%.
• Intermediate branches
  • Only have a muscular distribution to the lumbar fibres of the longissimus muscle. They form an intersegmental plexus within this muscle.
  • The L5 intermediate branch supplies the lowest fibres of longissimus which arise from the L5 transverse process and attach to the medial aspect of the iliac crest.
• Medial branches
  • Distribution to the facet joints, and so are of clinical relevance
  • L1-L4 medial branches of the dorsal rami
    • Travel across the superior aspect of their respective transverse processes, and then pierce the dorsal leaf of the intertransverse ligament at the base of the transverse process.
    • The nerve then runs along the bone at the junction of the root of the transverse process with the root of the superior articular process.
    • Each nerve hooks medially around the base of the superior articular process, and is covered by the mamillo-accessory ligament.
    • It then crosses the vertebral lamina, and divides into multiple branches that supply the multifidus muscle, the interspinous muscle and ligament, and two facet joints.
    • Each medial branch supplies the zygapophysial joints above and below its course
    • An ascending articular branch arises from the nerve just beyond the mamillo-accessory ligament where it starts to cross the lamina.
    • A descending articular branch arises slightly more distally and travels downwards to the subjacent joint.
  • L5 medial branch of the dorsal ramus
    • Similar course to L1-L4, except instead it crosses the ala of the sacrum instead of a transverse process.
    • It runs in the groove that is formed by the junction of the ala and the root of the superior articular process of the sacrum.
    • It then hooks medially around the base of the lumbosacral facet joint.
    • An articular branch supplies the lumbosacral facet joint, and it ramifies in multifidus.
  • Muscular and interspinous ligament dorsal rami medial branches
    • The muscles that move a particular segment are innervated by the nerve of that segment.
    • L1 medial branch supplies only those fibres from the L1 vertebra, etc.
    • The muscles arising from the spinous process and lamina of a lumbar vertebra, are innervated by the medial branch of the dorsal ramus immediately below that vertebra.

Histology

• Facet joint capsules
  • Lumbar facet joint capsules are richly innervated with encapsulated, unencapsulated, and free nerve endings.
  • Lumbar facet joint capsules transmit proprioceptive and nociceptive information
  • Nerves containing neuropeptide Y is more common than substance P and calcitonin gene-related peptide.
• Facet joint subchondral bone
  • Nerve fibres and nerve endings are also present in the subchondral bone of the facet joints
  • The subchondral bone is another location for nociception.
    • Intra-articular inclusions of the facet joints have nerve fibres that contain substance P - unclear if nociceptive or vasoregulatory.
• Interspinous ligaments
  • Nerve fibres are plentiful
  • The nerves give rise to Ruffini endings, pacini-form endings, and free nerve endings
  • Ruffini endings are mechanoreceptors and are more numerous in the lateral surfaces.
  • Pacini-form endings are associated with blood vessels and are uniformly distributed, the function is unclear.
  • Free nerve endings are found near the attachment of the ligament to the spinous processes.
• Supraspinous ligaments and thoraco-lumbar fascia well well innervated and contain nerve fibres, Ruffini endings, and pacini-form endings.
• Ligamentum flavum is sparsely innervated.

Variations

• Occasionally, an articular branch arises from the dorsal ramus proper, and innervates the ventral aspect of the adjacent joint.
• One author found multiple articular branches from the spinal nerve, lateral branch of the dorsal ramus, and the entire length of the medial branch, but this hasn't been observed in other studies. Bogduk suspects the author mis-identified collagen fibres as nerves, as they weren't confirmed histologically.

Ventral Rami

• The ventral rami pierces the ventral leaf of the intertransverse ligament through the intervertebral foramen.
• They lie within the substance of the psoas major muscle in front of the ligaments
• They form plexuses within the muscle
• The L1-L4 ventral rami form the lumbar plexus
• The L4 and L5 ventral rami join to form the lumbosacral trunk
• The L5 ventral ramus crosses the ala of the sacrum, below the L5 transverse process. It can be trapped between these two bones which is called the "far out syndrome'

Dermatomes

Main article: Dermatomes

• Fluoroscopically guided local anaesthetic blocks has allowed a re-appraisal of the classical data on the cutaneous distribution of the lumbar spinal nerves
• The L4, L5, and S1 dermatomes vary between individuals with respect to their total extent, but exhibit a consistent concentric pattern between individuals.
• Each dermatome can extend from the posterior midline of the back, across the buttock, and into the lower limb.
• However, only a minority of individuals has such an extensive distribution for L4 and L5.
• L4: most people have an area centred on the medial aspect of the lower leg
• L5: medial aspect of the foot, across the dorsum of the foot, and the lateral aspect of the lower leg
• S1: more extensive, a band from the posterior sacrum, along the entire length of the lower limb posteriorly, to the lateral aspect of the foot.

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.