Spinal Cord

Extent:

• Continuous with the medulla oblongata at the foramen magnum, extending down to approximately the L1-L2 vertebral level in adults (the conus medullaris), notably shorter than the vertebral canal itself, given the differential growth rate between the spinal cord and the vertebral column during development (the vertebral column grows faster/longer than the cord after early fetal life), meaning spinal cord segments do not correspond directly to the same-numbered vertebral level below the cervical region (a critical point for both localisation and lumbar puncture safety).

Enlargements:

• Cervical enlargement (approximately C4-T1 spinal segments), corresponds to the brachial plexus origin, reflecting the increased grey matter volume needed to innervate the upper limb.
• Lumbosacral enlargement (approximately L1-S3 spinal segments), corresponds to the lumbosacral plexus origin, reflecting the increased grey matter volume needed to innervate the lower limb.

Conus medullaris:

• The tapering, cone-shaped termination of the spinal cord proper, at approximately L1-L2 in adults (though this level varies somewhat between individuals, and is notably higher in neonates, at approximately L3, given the less-complete differential growth at birth).

Cauda equina:

• The bundle of lumbar and sacral nerve roots (not the spinal cord itself) descending within the vertebral canal below the conus medullaris, resembling a horse's tail, before exiting via their respective intervertebral foramina at the appropriate vertebral level, since these roots must descend a considerable distance from their spinal cord segment of origin (which lies well above their eventual exit level, given the cord's relative shortness) to reach their exit foramen.

Filum terminale:

• A thin, non-neural fibrous strand extending from the conus medullaris's tip, continuing down through the cauda equina to anchor the spinal cord's inferior end to the coccyx (as the filum terminale externum, fused with the dural sac at its termination), providing longitudinal anchoring/stability without neural function itself.

Anterior median fissure:

• A deep longitudinal groove on the spinal cord's anterior surface, partially dividing the cord into left and right halves.

Posterior median sulcus:

• A shallower longitudinal groove on the posterior surface, corresponding internally to the posterior median septum.

Rootlets:

• Multiple small fine nerve rootlets emerging in a linear series along the cord's anterolateral (ventral roots) and posterolateral (dorsal roots) surfaces, converging to form each spinal nerve's ventral and dorsal roots respectively.

Dorsal roots:

• Carry afferent (sensory) fibres into the cord, each with a dorsal root ganglion (containing the sensory neurons' cell bodies) located just before the root joins to form the mixed spinal nerve.

Ventral roots:

• Carry efferent (motor) fibres out of the cord, from motor neuron cell bodies located within the cord's ventral (anterior) horn grey matter.

Grey matter:

• Arranged centrally, in a characteristic "butterfly" or "H" shape on cross-section, containing neuronal cell bodies, organised into functionally distinct horns.

Anterior (ventral) horn:

• Contains lower motor neuron cell bodies, whose axons form the ventral root, innervating skeletal muscle, organised somatotopically (medial motor neurons supply axial/trunk muscles, lateral motor neurons supply distal limb muscles), most prominent/enlarged at the cervical and lumbosacral enlargements.

Posterior (dorsal) horn:

• Receives incoming sensory (afferent) fibres from the dorsal root, containing the cell bodies of second-order sensory neurons (particularly for the pain/temperature pathway) and interneurons.

Lateral horn:

• Present only at T1-L2/L3 spinal levels (thoracolumbar outflow), contains preganglionic sympathetic neuron cell bodies, whose axons exit via the ventral root before separating to form the white ramus communicans, its restricted presence at only these levels is the direct anatomical basis for the sympathetic nervous system's characteristic "thoracolumbar" outflow pattern; a small, less prominent equivalent structure exists at S2-S4 for parasympathetic (pelvic splanchnic) outflow, though this is typically not termed a discrete "lateral horn" in the same formal sense.

White columns (funiculi):

• Arranged peripherally, surrounding the central grey matter, divided into posterior (dorsal), lateral, and anterior (ventral) columns/funiculi, each containing specific ascending and descending tracts, the white matter's relative proportion (compared to grey matter) increases at higher spinal levels, since more ascending fibres have joined and fewer descending fibres have yet terminated as one moves rostrally up the cord.

• Given the differential growth, spinal cord segments and vertebral levels correspond directly only in the upper cervical region; below this, an increasing discrepancy develops, such that:
• Upper cervical cord segments ≈ corresponding vertebral level.
• Lower cervical/upper thoracic cord segments ≈ vertebral level + 1.
• Mid-to-lower thoracic cord segments ≈ vertebral level + 2.
• Lumbar cord segments are found opposite approximately the T10-T12 vertebral bodies.
• Sacral/coccygeal cord segments are found opposite approximately the T12-L1 vertebral bodies (the conus medullaris region).
• This progressively widening discrepancy is precisely why the cauda equina exists, the lumbar and sacral nerve roots must travel increasingly further down the vertebral canal from their cord segment of origin to reach their correct exit foramen.

• The spinal cord is surrounded by the same three meningeal layers as the brain (dura, arachnoid, pia), with some notable structural differences given the different anatomical context.

Epidural space:

• Unlike the cranial epidural "space" (a potential space only), the spinal epidural space is a true, normal anatomical space, lying between the vertebral canal's bony/ligamentous walls and the spinal dura mater, containing fat and a venous plexus (the internal vertebral venous plexus/Batson's plexus), this is the target space for epidural anaesthesia.

Subarachnoid space:

• Between the arachnoid and pia mater, containing CSF, continuous with the cranial subarachnoid space, extends down to approximately the S2 vertebral level (below the spinal cord's own termination at L1-L2), given the dural sac's more inferior termination point relative to the cord itself.

Lumbar cistern:

• The expanded portion of the subarachnoid space between the conus medullaris (L1-L2) and the dural sac's termination (approximately S2), containing the cauda equina nerve roots bathed in CSF, the safe target zone for lumbar puncture, since a needle introduced here (below the cord's own termination) can safely displace the mobile, freely floating cauda equina roots without risking direct spinal cord trauma.

Lumbar puncture:

• Performed within the lumbar cistern (below L1-L2, typically at L3-L4 or L4-L5), exploiting the anatomical gap between the cord's termination and the dural sac's more inferior extent.

Spinal cord compression:

• Can result from a wide range of causes (disc herniation, tumour, epidural abscess, vertebral fracture/collapse) at any level, producing a characteristic combination of deficits below the level of compression, reflecting the interruption of ascending sensory and descending motor pathways at that point, the specific pattern of deficit (which pathways are predominantly affected) depends on which part of the cord's cross-sectional anatomy the compressing lesion involves.

Cord transection:

• Complete transection at any level produces total loss of all sensory and motor function below that level (both sides, given interruption of all ascending and descending pathways), along with autonomic dysfunction (loss of descending sympathetic control below the lesion, and disruption of bladder/bowel/sexual function reflex arcs, depending on the level relative to the sacral parasympathetic outflow), a devastating but anatomically straightforward pattern to understand.