Describe the external features and internal structure of the spinal cord. Describe its ascending and descending tracts and add a note on its lesions.

External Features

• About 45 cm long in the male, 42 cm in the female
• In the newborn it reaches L3, and in the fetus the whole length of the canal — the vertebral column grows faster than the cord
• Two enlargements — cervical (C4–T1) for the brachial plexus, and lumbar (L1–S3) for the lumbosacral plexus
• Ends as the conus medullaris, continued as the filum terminale
• The lumbar and sacral roots below the conus form the cauda equina
• 31 pairs of spinal nerves — 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

Internal Structure

• Grey matter is central and H-shaped, with anterior (motor), posterior (sensory) and lateral horns
• The lateral horn is present only from T1 to L2 (sympathetic) and S2 to S4 (parasympathetic)
• White matter is peripheral, divided into anterior, lateral and posterior funiculi
• Rexed laminae — ten cytoarchitectural layers; lamina II is the substantia gelatinosa, the gate for pain
• The proportion of grey matter is greatest at the enlargements, and of white matter greatest in the cervical region, since all the tracts are present there

Ascending Tracts

• The level of crossing is the whole basis of localising a cord lesion — the dorsal columns cross in the medulla, the spinothalamic in the cord itself

Descending Tracts

Blood Supply

• One anterior spinal artery — from the two vertebral arteries; supplies the anterior two-thirds of the cord
• Two posterior spinal arteries — supply the posterior one-third, including the dorsal columns
• Reinforced by radicular (segmental) arteries; the largest is the artery OF adamkiewicz, usually on the left between T9 and L2
• The upper thoracic cord (T4–T8) is a watershed zone and the most vulnerable to ischaemia

Meninges and Spaces of the Cord

• Dura — a single layer here, separated from the vertebral canal by a true extradural space containing fat and the internal vertebral venous plexus
• The dural sac ends at S2, well below the cord
• Denticulate ligaments — 21 pairs of pial extensions anchoring the cord to the dura between the roots
• Lumbar cistern — the subarachnoid space from L1 to S2, containing the cauda equina and filum terminale in CSF

Applied Aspects

• Brown-SÉQUARD syndrome (hemisection) — ipsilateral loss of motor power and of dorsal column sensation below the lesion, with contralateral loss of pain and temperature one or two segments below. The dissociation follows directly from where each tract crosses
• Syringomyelia — a cavity around the central canal first interrupts the decussating spinothalamic fibres, giving "cape-like" dissociated sensory loss: pain and temperature lost, touch and proprioception preserved
• Subacute combined degeneration (vitamin B12 deficiency) — affects the dorsal columns and the corticospinal tracts, giving sensory ataxia with brisk reflexes and extensor plantars
• Anterior spinal artery syndrome — loss of power and of pain and temperature, with preserved dorsal column sensation, since the posterior third has its own supply
• Cauda equina syndrome — saddle anaesthesia, urinary retention and loss of anal tone; a surgical emergency; a conus lesion gives earlier and more symmetrical sphincter involvement with less radicular pain
• Tabes dorsalis — syphilitic degeneration of the dorsal columns and roots; sensory ataxia with a stamping gait, a positive Romberg sign, lightning pains and Argyll Robertson pupils
• Poliomyelitis destroys the anterior horn cells, giving a pure lower motor neurone paralysis with intact sensation
• Motor neurone disease affects both anterior horn cells and the corticospinal tracts, so upper and lower motor neurone signs coexist without sensory loss — a combination almost unique to it
• Spinal cord compression is a neurological emergency; the sensory level on the trunk indicates the segment, and urgent imaging and decompression are needed
• Sensory levels are memorised by landmark — T4 nipple, T10 umbilicus, L1 groin — and are the quickest way to localise a cord lesion at the bedside
• Autonomic dysreflexia occurs in lesions above T6; a distended bladder or bowel triggers uncontrolled sympathetic outflow with severe hypertension and headache
• The cord occupies only about half the vertebral canal, which is why considerable displacement can occur in a fracture before the cord is damaged
• Whiplash and hyperextension injury in the elderly with cervical spondylosis causes a central cord syndrome, with the arms affected more than the legs