What nerve innervates the diaphragm and what would happen if it is damaged?

• A. Phrenic nerve (C3-C5); diaphragmatic paralysis and compromised ventilation
• B. Vagus nerve; loss of parasympathetic control of the lungs
• C. Intercostal nerves; reduced chest wall sensation
• D. Sympathetic trunk; tachycardia and vasoconstriction

Answer: (A)

The main idea is that the diaphragm’s motor control comes from the phrenic nerve, which arises from the ventral rami of C3 to C5. This nerve travels into the thorax and provides the sole motor supply to the diaphragm, the primary muscle that contracts to inflate the lungs during inspiration. When the phrenic nerve is damaged, the diaphragm on that side cannot contract, leading to diaphragmatic paralysis and reduced ability to ventilate effectively. Clinically, unilateral phrenic nerve injury often results in less noticeable symptoms at rest because the other side and accessory muscles help, but breathing can worsen with exertion or in certain positions. Bilateral damage, however, can severely impair breathing and may require respiratory support.

The other nerves listed don’t innervate the diaphragm. The vagus nerve mainly provides parasympathetic innervation to the lungs, intercostal nerves mainly supply the chest wall muscles and skin, and the sympathetic trunk governs sympathetic responses rather than motor control of the diaphragm.