Arising principally from C4 in the neck, the nerve passes down over the anterior scalene muscle across the dome of the pleura behind the subclavian vein. It runs through the mediastinum in front of the lung root. Each nerve lies in the thorax as far lateral as possible, being in contact laterally with the mediastinal pleura throughout the whole of its course. Their medial relations, however, are asymmetrical.
Right phrenic nerve:
The right phrenic nerve is in contact with venous structures throughout the whole of its course. The right brachiocephalic vein, the superior vena cava, then the right atrium, and the inferior vena cava, lie to its medial side. It reaches the under surface of the diaphragm by passing through the vena caval foramen in the central tendon.
Left phrenic nerve:
The left phrenic nerve has the left common carotid and left subclavian arteries that arise from the arch of the aorta to its medial side. It crosses the arch lateral to the superior intercostal vein and in front of the vagus nerve, and then runs laterally down the pericardium over the left ventricle towards the apex of the heart. It reaches the under surface of the diaphragm by piercing the muscular part just to the left of the pericardium. Note that the left phrenic passes through diaphragmatic muscle but the right through tendon.
Branches of phrenic nerves:
On the under surface of the diaphragm each phrenic nerve splits into three main branches: anterior, lateral and posterior, which radiate from the point of entry, giving off branches as they go. Each nerve is the sole motor supply to its own half of the diaphragm, including the crus. The part of the right crus that lies to the left of the esophagus is supplied by the left phrenic (because these fibers are embryologically left- sided). About two-thirds of the phrenic nerve fibers are motor; the rest are sensory to the diaphragm (except for the most peripheral parts which receive intercostal afferent fibers), and to the mediastinal pleura, the fibrous pericardium, the parietal layer of serous pericardium, and the central parts of the diaphragmatic pleura and peritoneum. The nerve also gives branches to the areolar tissue of the mediastinum but not to viscera; in a word, it is the nerve of the transverse septum. The nerve is supplied with blood by the pericardiophrenic artery (a branch of the internal thoracic) which accompanies it, with companion veins, all the way to the diaphragm. To avoid damage to the nerve supply, incisions in the diaphragm are made as far as possible round the periphery or radiating towards the main nerve entry point.
Role of phrenic nerve in referred pain to the shoulder:
Pain referred from the diaphragmatic peritoneum (C4) is classically felt in the shoulder tip (C4), but pain from thoracic surfaces supplied by the phrenic nerve (pleura, pericardium) is usually located there, albeit vaguely.