Phrenic Nerve C4 — FRCA Primary MCQ

Correct answer: A — Phrenic nerve

The phrenic nerve provides motor innervation to each hemidiaphragm. It is formed from the anterior rami of C3–C5, with C4 making the predominant functional contribution. In the neck, it descends on the anterior surface of scalenus anterior before entering the thorax and passing anterior to the lung root. Injury may cause ipsilateral hemidiaphragmatic paralysis and paradoxical movement. Intercostal nerves supply the intercostal muscles and contribute sensory innervation at the diaphragm's periphery, but they are not its motor nerve. The accessory nerve supplies sternocleidomastoid and trapezius; the long thoracic nerve supplies serratus anterior; and the vagus provides parasympathetic and visceral innervation rather than diaphragmatic motor supply.

Reference: Sindou M, Georgoulis G. Cervical nerve root contribution to diaphragm motor innervation revisited via intraoperative clinical and neurophysiological observations. Clinical Neurophysiology. 2026. https://pubmed.ncbi.nlm.nih.gov/42364299/