Back and vertebral column
Introduced| English | Meaning |
|---|---|
| conus medullaris | The tapered lower end of the spinal cord, near L1–L2 in adults. |
| cauda equina | The bundle of lumbar and sacral nerve roots below the end of the spinal cord. |
| lumbar puncture | Inserting a needle between the lumbar vertebrae below the end of the spinal cord to sample cerebrospinal fluid. |
| cauda equina syndrome | Compression of the nerve roots below the spinal cord, causing saddle numbness, bladder and bowel problems and leg weakness; a surgical emergency. |
A column that moves
- The spine is a stack of 33 vertebrae that shields the spinal cord and lets nerves out at 31 pairs of levels.
- It trades stability for movement, which makes it the site of most back pain and every spinal cord emergency.
Counting the vertebrae
- 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused) and about 4 coccygeal (fused): 33 bones, 24 of which move.
- The neck and lower back curve forwards (lordosis); the thoracic and sacral parts curve backwards (kyphosis).
Match each region to its number of vertebrae.
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7 + 12 + 5 = 24 movable vertebrae above the fused sacrum and coccyx.
How many vertebrae are movable?
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7 cervical + 12 thoracic + 5 lumbar = 24; the sacral and coccygeal ones are fused.
Where the cord ends
- In adults the cord ends at L1–2, as the conus medullaris 脊髓圆锥. Below that, the canal holds only nerve roots: the cauda equina 马尾.
- That is why a lumbar puncture 腰椎穿刺 at L3/4 or L4/5 is safe: the needle meets floating roots, not cord.
Why is lumbar puncture done at L3/4 or L4/5?
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Below the conus the needle meets floating cauda equina roots, which move aside, rather than the cord.
Bone level is not cord level
- In the embryo cord and canal are the same length; then the spine grows faster. So roots run downwards before they exit, more steeply lower down.
- The gap grows down the spine: about one segment in the neck, two in the upper thorax, three or more lower down. A T10 vertebral injury damages the L1–2 cord segments.
A fracture of the T10 vertebra damages the T10 cord segment.
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The cord is shorter than the spine; behind the T10 vertebra lie about the L1–2 cord segments.
Worked example: the dermatome ladder
- Thumb C6, nipple T4, umbilicus T10, big toe L5, the skin around the anus S3–5. Five rungs turn a sensory examination into a level.
- Together with the bone-to-cord gap, the ladder tells you both the cord segment injured and which vertebra to image.
Put these dermatome landmarks in order from the highest cord segment to the lowest.
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Five rungs that are easy to test and spread from neck to sacrum.
Worked example: an emergency at the end of the cord
- (Fictional patient) Back pain, numbness of the "saddle" area and an inability to pass urine. A large disc at L4–5 is squeezing the roots of the cauda equina, including the sacral ones.
- These are roots, not cord, so the signs are bladder, bowel and saddle numbness: cauda equina syndrome 马尾综合征. Decompression is needed within hours.
Which combination should make you think of cauda equina syndrome?
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Compressed sacral roots give bladder, bowel and saddle signs; decompression is needed within hours.
Three common slips
- Bone level as cord level. The gap grows down the spine.
- The wrong root. The posterior ligament is strongest in the midline, so discs bulge to the side and press on the next root down: an L4/5 disc compresses L5.
- Moving sacral bones. The 5 sacral and the coccygeal vertebrae are fused.
An L4/5 disc bulges to the side. Which root is usually compressed?
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The L4 root has already left above the disc, so a side bulge catches the next root down, L5.
Recap
- 33 vertebrae, 24 movable, with alternating curves.
- The cord ends at L1–2; below is the cauda equina, so lumbar puncture is safe at L3/4.
- Bone level ≠ cord level; five dermatome rungs read the level; saddle numbness with retention is an emergency.