Spinal Cord and Nerve Roots

This section covers the spinal cord's gross anatomy and nerve root organisation specifically as it relates to the vertebral column's structure and levels, the cord's own detailed internal anatomy (grey/white matter organisation, ascending/descending tracts) is and is referenced here rather than repeated.

Extent:

• Continuous with the medulla oblongata at the foramen magnum, extending down to approximately L1-L2 in adults (higher, approximately L3, in neonates, given incomplete differential growth at birth).

Cervical enlargement:

• Approximately spinal segments C4-T1, corresponding to the brachial plexus's origin, reflecting the increased grey matter volume required for upper limb innervation.

Lumbosacral enlargement:

• Approximately spinal segments L1-S3, corresponding to the lumbosacral plexus's origin, reflecting the increased grey matter volume required for lower limb innervation.

Conus medullaris:

• The cord's tapering terminal portion, at approximately L1-L2 (adult).

Filum terminale:

• The non-neural fibrous strand anchoring the cord's inferior end to the coccyx, comprising an internal (pial, within the CSF-filled lumbar cistern) and external (fused with the dura, within the epidural space) portion.

Cauda equina:

• The bundle of lumbar and sacral nerve roots descending below the conus medullaris within the lumbar cistern, before exiting at their respective vertebral levels.

• A "spinal segment" refers to the portion of the spinal cord giving rise to one pair of spinal nerve roots. There are 31 segments in total.
• Below the upper cervical region, the segments do not correspond directly to the same-numbered vertebral level, given the cord's relative shortness.

Cervical segments:

• 8 segments (C1-C8). There are 8 cervical spinal nerve pairs despite only 7 cervical vertebrae.
• The C1-C7 nerves exit above their corresponding vertebra, while C8 exits below C7 (above T1).
• This numbering shift is the origin point for the below-vertebra exit pattern seen throughout the rest of the vertebral column.

Thoracic segments:

• 12 segments (T1-T12), each nerve exiting below its correspondingly numbered vertebra (continuing the pattern established at C8).

Lumbar segments:

• 5 segments (L1-L5), each exiting below its correspondingly numbered vertebra.
• Given the cord's termination around L1-L2, the lumbar spinal cord segments themselves are located considerably higher in the vertebral canal than their numbered exit level, found opposite approximately the T10-T12 vertebral bodies.

Sacral segments:

• 5 segments (S1-S5), located within the conus medullaris, found opposite approximately the T12-L1 vertebral bodies.
• This is the most extreme example of the cord segment-to-vertebral level discrepancy, given their exit foramina lying far below (within the sacrum itself) their actual cord segment of origin.

Coccygeal segment:

• 1 segment (Co1), a single small, often vestigial nerve, at the very tip of the conus medullaris.

Anterior (ventral) roots:

• Carry efferent (motor) fibres, formed by the convergence of multiple fine ventral rootlets emerging from the spinal cord's anterolateral surface, the axons originating from lower motor neuron cell bodies within the cord's anterior horn.

Posterior (dorsal) roots:

• Carry afferent (sensory) fibres, similarly formed by the convergence of multiple fine dorsal rootlets, each root bearing a dorsal root ganglion (containing the pseudounipolar sensory neuron cell bodies) just before the root joins with the ventral root.

Dorsal root ganglia:

• Small swellings located just proximal to (or, depending on the level, sometimes within) the intervertebral foramen, containing the sensory neuron cell bodies.
• A key anatomical landmark, since pathology directly affecting the dorsal root ganglion (e.g., varicella-zoster virus reactivation, causing shingles) produces a characteristic dermatomal pain/rash pattern reflecting the specific ganglion/segmental level affected.

Mixed spinal nerves:

• Formed by the union of the dorsal and ventral roots, just distal to the dorsal root ganglion, within (or just before) the intervertebral foramen.
• The resulting mixed spinal nerve is very short (only a few millimetres in length) before it immediately divides into its anterior and posterior rami.
• This brief mixed segment is the only point along the entire pathway where sensory and motor fibres genuinely travel together as a single combined nerve trunk.

• Given the differential growth between the spinal cord and the vertebral column, an increasing discrepancy develops between spinal cord segment level and corresponding vertebral level as one moves caudally down the column.
• This relationship is of major practical importance for both clinical localisation and safe procedural technique.

Clinical localisation:

• A clinical examination may reveal a specific pattern of sensory or motor loss corresponding to a particular spinal cord segment (e.g., loss of a specific dermatome or myotome, or a specific reflex). This must be translated to the corresponding vertebral level (where any causative bony/discal/ligamentous pathology would actually be located) using the differential correspondence pattern.
• A clinical deficit corresponding to, for example, the L5 spinal cord segment does not mean the causative pathology is necessarily at the L5 vertebral level. Given the cord's termination pattern, the L5 spinal segment itself lies opposite approximately the T12/L1 vertebral level.
• Nerve root compression syndromes (the most common clinical scenario, e.g., disc herniation) are typically named and clinically localised by the exiting nerve root level rather than by the spinal cord segment of origin.
• The exiting nerve root level does correspond reasonably closely to the adjacent vertebral level in the lumbar spine, given the nearly vertical course of the lower lumbar/sacral roots within the cauda equina.
• This distinction between cord-segment-level and root-exit-level localisation is a frequently confused but clinically important point.

Lumbar puncture reasoning:

• The same cord-vertebral level discrepancy is the direct anatomical justification for performing lumbar puncture safely below the cord's own termination (L1-L2), within the lumbar cistern where only the mobile cauda equina roots (not the cord itself) are present.
• Without this differential growth pattern, safe lumbar CSF access below the cord's termination would not be anatomically possible.

Cord compression:

• Compression of the spinal cord itself (as opposed to nerve root compression) can occur at any level from the foramen magnum down to the cord's termination (L1-L2).
• Causes include disc herniation (particularly central, large herniations), tumour, epidural abscess/haematoma, or vertebral fracture/collapse with canal encroachment.
• It produces a combination of motor, sensory, and (depending on level and the specific pattern of cord cross-sectional involvement) autonomic/bladder-bowel dysfunction below the level of compression.
• The pattern follows the classic spinal cord lesion patterns (Brown-Séquard, central cord, anterior cord syndromes, and complete cord transection).

Nerve root compression:

• Compression of an individual exiting nerve root (most commonly from lateral/posterolateral disc herniation) produces a radiculopathy: pain, sensory disturbance, and/or motor weakness corresponding to the specific single nerve root's dermatomal/myotomal distribution.
• This is a more anatomically localised and typically less severe presentation than frank cord compression. It is still clinically significant and a common cause of presentation to both primary care and specialist spinal services.

Cauda equina syndrome:

• Compression of the cauda equina (the lumbar and sacral nerve roots below the cord's own termination, within the lumbar cistern), most commonly from a large central lumbar disc herniation.
• It produces a characteristic combination of bilateral leg pain/weakness/sensory loss, saddle anaesthesia (perianal/perineal sensory loss, reflecting the S3-S5 root distribution), and bladder/bowel dysfunction (urinary retention with overflow incontinence being the most sensitive and specific early sign).
• It is a genuine surgical emergency, since delayed decompression risks permanent neurological/sphincteric deficit, given the multiple individual nerve roots (rather than the more centrally organised, somewhat more resilient spinal cord tissue itself) being directly compressed.

Conus medullaris syndrome:

• Compression involving the conus medullaris itself (the cord's very terminal segment, at approximately L1-L2) produces a presentation that can overlap significantly with cauda equina syndrome (given the anatomical proximity of these two structures).
• It classically features a more symmetrical, earlier-onset pattern of saddle anaesthesia and bladder/bowel dysfunction, sometimes with relatively preserved lower limb motor/sensory function.
• This reflects the more centrally concentrated sacral segment involvement at the conus, compared to cauda equina syndrome's more peripherally-distributed, often asymmetric multi-root involvement.
• The clinical distinction between these two syndromes can be genuinely difficult given their overlapping presentation. Both represent surgical emergencies requiring the same urgency of recognition and decompression.