Expansion
C3, C4 and C5 contribute to the phrenic nerve
Mnemonic
“C3, 4, 5 keeps the diaphragm alive”, for the roots of the phrenic nerve.
- The phrenic nerve is the sole motor supply to the diaphragm
- A cord lesion above C3 abolishes diaphragmatic breathing and is immediately fatal without ventilation; a lesion below C5 spares it
- The same roots explain referred pain to the shoulder tip (C4 dermatome) from diaphragmatic irritation, as in a subphrenic abscess, splenic rupture or blood in the peritoneum
The nerve also carries sensory fibres to the central diaphragm, mediastinal pleura and pericardium, which is why pericarditis refers to the shoulder in the same way.
Expansion
The phrenic nerve receives contributions from C3, C4 and C5 and supplies the diaphragm.