at L2-3 and C4-5 levels, which is exiitng nerve root and which is traversing
At each spinal level, there are two nerve roots to consider:
- Exiting (foraminal) nerve root: exits through the neural foramen at that level
- Traversing (descending) nerve root: is passing through the canal on its way to exit at the level below
Cervical spine (C4-5 level):
- Exiting nerve root: C5 - exits through the C4-5 foramen
- Traversing nerve root: C6 - descending to exit at C5-6
Lumbar spine (L2-3 level):
- Exiting nerve root: L2 - exits through the L2-3 foramen
- Traversing nerve root: L3 - descending to exit at L3-4
Key rule to remember:
| Region | Exiting root at level X-Y | Traversing root at level X-Y |
|---|---|---|
| Cervical | Lower number (Y = X+1, exits as C-Y... wait) | See below |
| Lumbar | Upper number (X) exits | Lower number (Y) traverses |
Let me clarify the cervical vs lumbar difference more precisely:
Lumbar rule:
At L2-3 disc level - the L2 root has already exited (pedicular), but more precisely, the L2 root exits at the L2-3 foramen and the L3 root traverses the canal past the L2-3 disc.
Cervical rule:
In the cervical spine, nerve roots are numbered by the level above the pedicle (C1 exits above C1 vertebra, C8 exits below C7). So at C4-5:
- C5 exits through the C4-5 foramen
- C6 traverses past the C4-5 disc
Clinical significance:
- A posterolateral (paracentral) disc herniation at a given level typically compresses the traversing root
- A far lateral (foraminal) disc herniation compresses the exiting root
This is why an L4-5 posterolateral herniation classically causes L5 symptoms (not L4), while a far lateral L4-5 herniation affects L4.
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