The phrenic nerve originates from the anterior primary rami of C3-5 on each side and supplies motor innervation to the diaphragm.

It also coveys sensory fibres from the diaphragm, pleura and pericardium (hence referred shoulder tip pain from diaphragmatic irritation).

It descends vertically on the scalenus anterior muscles, and passes the root of the neck beneath the sternocleidomastoid muscle.

The right phrenic nerve enters the thorax behind the subclavian/internal jugular venous junction, whereas the left nerve enters the thorax between the subclavian artery and vein.