The C3 myotome mainly supplies neck muscles that bend the head to the side and helps supply the diaphragm. It is usually tested as lateral neck flexion. It shares supply of the levator scapulae and trapezius with C4.
| Nerve root | C3 |
|---|---|
| Area | Lateral neck flexion (ear toward shoulder) |
| How to test | The patient tilts the head toward the shoulder while the examiner resists at the side of the head, then repeats on the other side. Strength is graded 0-5 on the Medical Research Council scale and compared side to side. |
| Muscles |
|
| Nerve Supply | Cervical plexus (ventral ramus of C3); Phrenic nerve (C3-C5); Dorsal scapular nerve (with C4-C5) |
Bends the neck to the side and helps drive the diaphragm for breathing.
C3 is part of the phrenic nerve outflow (C3, C4, C5 keep the diaphragm alive), so injuries at or above C3 often need long-term ventilation. Isolated C3 radiculopathy is rare and mainly causes neck pain rather than obvious weakness. Myotome charts vary slightly between sources, and most muscles receive more than one root, so a single root lesion usually weakens rather than paralyses.
Injury at C3 or above paralyses the diaphragm partly or completely, making breathing dependent on a ventilator or phrenic pacing.
Root compression at C2-C3 causing neck pain with mild weakness of lateral neck flexion.
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