Intervertebral Disc
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Direct answer
One quarter of the spinal column's height is contributed by the 23 intervertebral discs — none between atlas and axis or within the fused sacrum — each a sandwich of a tough, lamellated annulus fibrosus (type I collagen) surrounding a gelatinous, pressurised nucleus pulposus, the adult remnant of the embryonic notochord. The nucleus is 85–90 per cent water in youth and is completely avascular in the adult, fed by diffusion and cyclic loading ("imbibition") through the adjacent vertebral endplates; only the outer third of the annulus is innervated, by the sinuvertebral nerve. Because the posterior longitudinal ligament is strong in the midline and the posterolateral annulus is weakest, a degenerating nucleus herniates posterolaterally — at L4–L5 and L5–S1 in about 95 per cent of lumbar cases — compressing the traversing nerve root to produce sciatica.
What you must remember
- Structure: annulus fibrosus of 10–20 concentric lamellae whose fibres run obliquely in alternating directions, containing a nucleus pulposus that behaves hydrostatically — squeezing it flattens the disc uniformly in all directions.
- Numbers: 23 discs; they are thickest in the cervical and lumbar regions (and thinnest in the upper thoracic); the L5–S1 disc is among the thickest, and total disc height falls with age — taller in the morning than evening.
- Blood and nerve supply: vessels reach only the outer annulus in the adult; the nucleus is avascular. The posterior annulus and posterior longitudinal ligament are innervated by the sinuvertebral (meningeal) nerve — the anatomical basis of primary discogenic back pain.
- Prolapse direction and level: posterolateral into the canal, sparing the midline posterior longitudinal ligament; L4–L5 and L5–S1 account for the overwhelming majority, the L4–L5 disc typically compressing the traversing L5 root and L5–S1 the S1 root; a far lateral prolapse catches the exiting root instead.
- Cervical counterpart: C5–C6 and C6–C7 are the usual cervical levels; the C5–C6 disc affects the C6 root (weak biceps, depressed biceps jerk, diminished sensation over the thumb).
- Cauda equina red flags: bilateral sciatica, saddle (S2–S4) anaesthesia, urinary retention or incontinence — an emergency requiring decompression within hours, not days.
- Ageing: water and proteoglycan loss shrinks the nucleus, fissures the annulus and narrows disc height — the cascade behind degenerative back pain.
Mapping a sciatica examination
A forty-year-old labourer with pain shooting from the buttock to the lateral calf and dorsum of the great toe is drawing the L5 root on his own leg. The straight-leg-raise (Lasègue's) test stretches the sciatic nerve and the traversing L5 or S1 root; reproduction of radicular pain below 60 degrees is positive, and the crossed straight-leg-raise (pain in the affected leg when the opposite leg is raised) is even more specific for disc herniation. Now separate the roots by examination: an L5 lesion weakens dorsiflexion of the ankle and, most tellingly, extension of the great toe (extensor hallucis longus), with sensory loss over the dorsum of the foot and first web space — but the ankle jerk survives. An S1 lesion weakens plantarflexion and eversion, dulls the lateral border of the foot, and abolishs the ankle jerk — the reflex is the fastest discriminator on the ward round. Night pain, fever, weight loss or bilateral signs demand an MRI before any injection is planned; and a patient who cannot void with saddle numbness goes to theatre that night.
Where students slip
Two slips repeat every year. First, the level-to-root mapping: an L4–L5 disc prolapse compresses the L5 root, not the L4 root — the L4 root has already left through its foramen above the disc; stating "L4–L5 disc, L4 root" is the classic error. Second, the reason for the posterolateral direction: the posterior longitudinal ligament is a strong midline barrier but weak and narrow laterally, so the herniation detours around it — an answer that must mention the ligament to earn full marks. Candidates also forget that the disc is a secondary cartilaginous joint (symphysis) — the classification question hiding inside.
Frequently asked questions
How many intervertebral discs are there, and where are they absent?
Twenty-three — between C2 and the sacrum; there is none between the atlas and axis or within the fused sacrum.
Why does a lumbar disc prolapse posterolaterally rather than directly backwards?
The posterior longitudinal ligament reinforces the midline, while the posterolateral corner of the annulus fibrosus is the weakest and least supported region.
Which root is compressed by an L4–L5 disc prolapse and what is the telling sign?
Typically the traversing L5 root — weak great-toe extension and ankle dorsiflexion with sensory loss on the dorsum of the foot, but a preserved ankle jerk.
What is the sinuvertebral nerve and what does it supply?
A recurrent meningeal branch from the spinal nerve (with a sympathetic contribution) that supplies the posterior annulus, posterior longitudinal ligament and meninges — the mediator of discogenic pain.
Which features make cauda equina syndrome an emergency?
Bilateral leg pain, saddle anaesthesia and sphincter disturbance from compression of the S2–S4 roots; only urgent surgical decompression prevents permanent bladder and sexual dysfunction.