Phrenic Nerve

Origin

The point of origin is from ventral rami of C3, C4 and C5 but primarily from C4.

Course

It runs vertically downwards on the anterior surface of the scalenus anterior, which it crosses obliquely from lateral to medial side. Afterward it runs downwards on the cervical pleura to go into the thorax behind first costal cartilage.

Distribution

The phrenic nerve gives:

• Single Motor Supply to the diaphragm (muscle of respiration) and
• Sensory innervation to diaphragmatic pleura, pericardium and subdiaphragmatic pleura.

Accessory Phrenic Nerve

• It’s the branch from nerve to subclavius featuring C5 fibres. It runs Lateral to phrenic nerve and descends behind or occasionally in front of the subclavian vein to join the chief phrenic nerve near the 1st rib.
• In case of accessory phrenic nerve, the fibres of C5 nerve inspite of joining the phrenic nerve at its commencement runs via, nerve to subclavius and after that leaves it as accessory nerve to join the principal phrenic nerve in the thoracic inlet.

Clinical Significance

• Phrenic crush/avulsion: Before the arrival of contemporary antitubercular treatment, phrenic crush utilized to be done to create paralysis of the corresponding half of the dia-phragm to be able to give rest to the diseased lung and so encourage healing.
• In phrenic crush the accessory phrenic nerve, if present, should also be smashed, otherwise C5 fibres will escape and diaphragm will continue to operate.