The phrenic nerve is a nerve arising from the cervical, or neck region, of the spine that supplies movement to the diaphragm and some sensation to the chest and upper abdomen. The body contains a left and a right phrenic nerve which follow different paths, though they both begin in the C3, C4, and C5 vertebrae of the neck. Of principle importance is the phrenic nerve’s role in causing the diaphragm to contract, a crucial step in the respiratory process.

The thoracic diaphragm is a dome-shaped sheet of muscle that pushes up beneath the lungs to control the contraction and expansion during respiration, or breathing. During an inhale, the diaphragm contracts to expand the lungs and allow air to fill the space. During an exhale, the diaphragm relaxes and expands against the lungs, causing the lungs to contract and push out the used air. The diaphragm may also exert pressure on the abdominal cavity, helping out process such as the excretion of vomit, feces, or urine. The motor fibers in the phrenic nerves signal to the diaphragm when to contract and relax so that these vital processes can take place.

Once leaving the cervical spine, the right phrenic nerve makes its way underneath the scalene muscles of the neck and passes under the clavicle between the subclavean vein on its top and the brachiocephalic vein on its bottom. It then traverses the root of the right lung, a body of vessels and tissue that connects the lung to the heart and trachea. The right phrenic nerve continues traveling towards the feet until it hits the caval opening, a hiatus, or hole, in the diaphragm at the level of the eight thoracic vertebra.

The left phrenic nerve also travels down along the scalene muscle in the neck. It then passes between the subclavean vein and the subclavean artery into the thoracic cavity. It continues to travel downward over the pericardium covering the left ventricle of the heart and then enters the diaphragm. Both the left and right phrenic nerves are accompanied by the left and right pericardiacophrenic nerves, respectively.

The sensory fibers of the phrenic nerve innervate and supply sensation to the pericardium, a sac that holds the heart, and the mediastinal parietal and diaphragmatic pleura, membrane layers that line the thoracic cavity and the diaphragm. The phrenic nerve also supplies sensation to the peritoneum, a membrane layer that lines the abdominal cavity.

Damage to the phrenic nerve, whether due to trauma in the cervical spine, a surgical accident, problems in the surrounding tissue, or another source of trauma can make breathing difficult or impossible. Usually, if one phrenic nerve is left in tact, the patient will retain his ability to breathe, though it will be more labored. Irritation of the phrenic nerve may cause a hiccup reflex, in which the diaphragm suddenly contracts. Irritation may also cause referred pain the tip of the shoulder blade, a symptom known as Kehr’s sign. Kehr’s sign is often a symptom of a ruptured spleen, or an abscess in the diaphragm or surrounding tissues.