Cervical nerve root numbering: why C8 exits between C7 and T1
The neck (cervical spine) has eight nerve roots (the parts joining nerves to the spinal cord) but seven vertebrae, so the C8 root exits between C7 and T1.
Remember
Roots exit above their vertebra up to C7, the C8 root takes the gap between C7 and T1, and from T1 downwards roots exit below their vertebra.
A nerve root is the part that joins a nerve to the spinal cord. A spinal nerve leaves the spine through an intervertebral foramen, which is the gap between two neighbouring vertebrae.
The C1 root leaves between the skull and the C1 vertebra, so each root from C1 to C7 exits above the vertebra of the same number. The C7 root therefore leaves between C6 and C7.
There is no C8 vertebra, yet there are eight cervical roots. So the C8 nerve root takes the gap between C7 and T1. From T1 downwards the pattern switches, and each root exits below its vertebra, so the T1 root leaves between T1 and T2.
An intervertebral disc is the cushioning disc between two vertebrae. In a disc herniation the soft centre of the disc breaks through its tough outer ring and can press on a nearby nerve root. In the neck, the root at risk is the one leaving at that level, so a herniation at the C7 to T1 level most likely compresses the C8 root. A C6 to C7 disc threatens the C7 root, and a C5 to C6 disc threatens the C6 root.
A compressed root causes a radiculopathy, which means symptoms caused by a compressed nerve root. These are pain, pins and needles or weakness in the area that root supplies. For C8, that is the little finger and the medial hand.
Key facts
- C1 to C7
- each root exits above the vertebra of the same number.
- C8
- exits between C7 and T1, because there are eight cervical roots but only seven cervical vertebrae.
- T1 downwards
- each root exits below the vertebra of the same number.
- C7 to T1 disc
- a herniation at this level most likely compresses the C8 root.
- C8 skin area
- the little finger and the medial hand.
- Neighbouring discs
- a C5 to C6 herniation most likely compresses the C6 root, which supplies the lateral forearm and thumb, and a C6 to C7 disc threatens the C7 root.
Exam traps
- A herniation at the C7 to T1 level does not threaten the C7 root, because the C7 root is the one at risk from a C6 to C7 disc, while the C7 to T1 level most likely compresses the C8 root.
- The T1 root does not leave in the gap between C7 and T1, because from T1 downwards roots exit below their vertebra, so the T1 root leaves between T1 and T2.
- A C5 to C6 herniation does not compress the C5 root, because the C5 root leaves at the C4 to C5 gap, where a C4 to C5 herniation would press on it.
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