Sectional Anatomy of the Spine
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Direct answer
A mid-sagittal T2 MRI of the spine reads as stacked sandwiches: vertebral bodies separated by discs (bright hydrated nucleus pulposus within the darker annulus fibrosus), a canal containing the conus medullaris ending at L1-L2 in adults, the cauda equina nerve roots below it floating in cerebrospinal fluid, and the thecal sac tapering to about S2. Axially, the canal is enclosed by the vertebral body anteriorly with the posterior longitudinal ligament, the laminae behind joined by the ligamentum flavum (which connects adjacent laminae and is pierced during epidural access), and the facet joints forming the posterolateral walls beside the neural foramen. Cervical sections are identified by the foramen transversarium carrying the vertebral artery from C6 upwards; the atlas carries no body, only a ring around the dens of the axis. These are the images on which disc prolapse, stenosis and cord compression are actually diagnosed.
What you must remember
- Level rules: spinal cord ends at L1-L2 (conus); thecal sac ends at S2; lumbar puncture is done at L3-L4 or L4-L5, below the conus — the single most quoted safety fact in the batch.
- Segment-vertebral offset: cervical cord segments lie about one level above their vertebrae, upper thoracic two above, lower thoracic three; lumbar and sacral segments cluster at T11-L1 — the arithmetic behind localising a lesion seen on MRI to its cord segment.
- Disc components: annulus fibrosus (type I collagen lamellae) containing nucleus pulposus (type II collagen, proteoglycan, water); discs are avascular after skeletal maturity, nourished by diffusion.
- Ligament stack: anterior and posterior longitudinal ligaments on the body, ligamentum flavum between laminae (yellow, elastic — the one the needle pierces), interspinous and supraspinous posteriorly.
- Foramen transversarium: C1-C6 transmit the vertebral artery, with entry usually at C6; C7's foramen is usually small and artery-free.
- Herniation behaviour: a posterolateral L4-L5 prolapse compresses the traversing L5 root, an L5-S1 prolapse the S1 root — the traversing-root rule that predicts physical signs before the image is seen.
- Craniocervical numbers: atlantodental interval under 3 mm in adults (up to 5 mm in children); the dens is held by the transverse ligament.
- Facet orientation gradient: cervical near-horizontal (dislocation-prone), thoracic coronal, lumbar sagittal (degenerative spondylolisthesis-prone) — explains injury patterns sectionally.
Walking through a cauda equina case
A 47-year-old presents with acute back pain, unilateral sciatica, saddle anaesthesia and urinary retention. The sagittal T2 shows a large L4-L5 disc extrusion with the posterior disc margin indenting the thecal sac. Apply the rule set: at L4-L5 the cord is long gone (it ended at L1-L2), so the compressed elements are roots of the cauda equina — specifically the traversing L5 root on that side, plus, with a big enough fragment, the S2-S4 roots that carry bladder and perineal sensation. That is why retention plus saddle anaesthesia converts an elective microdiscectomy into an emergency. Read the same image's axial cuts: the normal disc should be concave posteriorly; a convex "canal-filling" contour with fat obliteration in the lateral recess clinches it. Then look below the canal — the axial L5 slice shows the hypertrophied flavum buckling inward from both sides in degenerative stenosis, which is why elderly claudicants get symptoms standing ( canal narrows further on extension) and relief sitting flexed over a shopping trolley. One sagittal image, two diseases, both read off sectional anatomy.
High-yield viva angles
Examiners probe the ligamentum flavum constantly: name it as elastic yellow ligament joining the laminae, pierced in the midline by the epidural needle with a characteristic loss-of-resistance pop. The second favourite is the C7 foramen transversarium tease — it exists but rarely carries the vertebral artery, which enters at C6 in about 90% of people. Indian practicals also expect you to defend the lumbar puncture level against the conus level in the same breath: conus at L1-L2, puncture at L3-L4, because below L1 the roots float apart and a needle pushes them aside rather than transecting cord. Finally, be ready with the Pavlov/Torg ratio idea in one line — canal-to-body diameter under about 0.8 on the lateral view suggests a developmentally narrow canal — since cervical stenosis spotters occasionally appear in postgraduate vivas.
Frequently asked questions
At what level does the spinal cord end in adults?
At the conus medullaris around L1-L2, with nerve roots continuing below as the cauda equina within the thecal sac down to about S2.
Which root is compressed by a typical L4-L5 posterolateral disc prolapse?
The traversing L5 root, because the exiting L4 root has already left through the foramen above the prolapse.
What is the function and location of the ligamentum flavum?
An elastic yellow ligament connecting adjacent vertebral laminae, forming the posterolateral canal wall that the epidural needle pierces at its midline gap.
Why is a lumbar puncture performed below L2?
Because the cord terminates at L1-L2; below that the free-floating cauda equina roots are pushed aside by the needle rather than pierced.
What passes through the foramen transversarium?
The vertebral artery and vein with sympathetic fibres, from C6 up to C1; C7 has a foramen but characteristically no vertebral artery.