The paired phrenic nerves arise in the neck from C3-5 nerve roots and course down through the thorax, giving off sensory fibres, to reach the diaphragm, where they supply motor and sensory fibres to their respective hemidiaphragm.
Originates mainly from the anterior primary rami of C4, with contributions from C3 and C5
Arising between the anterior and middle scalene muscles, it courses around the lateral border of anterior scalene, moving from its lateral to medial edge
It enters the mediastinum between the subclavian vein, anteriorly, and the subclavian artery, posteriorly
Sensory supply to mediastinal pleura
Left and right phrenic nerves then follow different courses through the thorax: The left courses over the arch of the aorta, anterior to the left Vagus nerve (CN X) and pulmonary artery then runs laterally through the pericardium (sensory supply) across the left ventricle. It reaches the underside of the diaphragm via its muscular portion near the apex of the heart.
Right descends along the lateral surface of the superior vena cava, pericardium of the right atrium (sensory supply), and inferior vena cava. It travels through the caval orifice with IVC to reach the underside of the diaphragm.
Approximately two thirds of fibres to the diaphragm are motor, the remaining third providing sensory supply to diaphragmatic pleura & peritoneum, and the diaphragm itself.
Injury to one of the phrenic nerves results in paralysis of the ipsilateral hemidiaphragm, with the other half unaffected.
Injection of local anaesthetic around the phrenic nerve where it lies anterior to the anterior scalene produces a phrenic nerve block, causing a temporary paralysis of the hemidiaphragm (e.g. – in thoracic surgery)
Irritation of the diaphragmatic peritoneum (e.g. – subphrenic abscess, cholecystitis) classically refers pain to the C4 dermatome- shoulder tip pain.
Quick-look box – Phrenic nerve
C3, 4, 5 ventral rami
· Mediastinal pleura (SENSORY)
· Pericardium (SENSORY)
· Diaphragmatic pleura (SENSORY)
· Diaphragmatic peritoneum (SENSORY)
· Diaphragm (MOTOR)
Remember: “C3, 4, 5 keeps the diaphragm alive!”
The phrenic nerves descend from neck through thorax where they provide sensory supply to adjacent structures before reaching the diaphragm. Here they supply sensory fibres to adjacent pleura and peritoneum, as well as being the sole motor supply to their respective half of the diaphragm. Insult, either traumatic or iatrogenic, to one phrenic nerve therefore will result in paralysis of the diaphragm on the affected side.
Ellis H & Mahadevan V. 2010. Clinical Anatomy 12th ed. Oxford: Wiley-Blackwell
Whitaker RH & Borley NR. 2010. Instant Anatomy 4th ed. Oxford: Wiley-Blackwell
Sinnatamby CS. 2006. Last’s Anatomy 11th ed. China: Churchill Livingstone
Moore KL & Dalley AF. 2006. Clinically Oriented Anatomy 5th ed. Baltimore: Lippincott Williams & Wilkins