Answer: The phrenic nerve is the main nerve that signals the activity of the diaphragm, which regulates respiratory action.

The action of the respiratory system largely depends on the activity of a muscle called the diaphragm. When the diaphragm contracts, it moves inferiorly, which causes the lungs to expand, resulting in taking air into the lungs.

The diaphragm receives nervous system input via the phrenic nerve. The phrenic nerve originates at the level of the cervical spinal cord, specifically C3, 4 and 5. It is a paired anatomical structure, meaning that we have a left and a right phrenic nerve.

The phrenic nerves carry motor and sympathetic information down to the diaphragm, as well as sensory information upwards toward the brain.

Phrenic nerve injury

During surgery, it is possible to accidentally sever or otherwise injure the phrenic nerve. Often times, in order to prevent this from happening, the surgeon will identify the nerve by eliciting a quick stimulation, which triggers a hiccup-like response. Even if one of the phrenic nerves are destroyed, there is usually no issue with breathing as long as the other nerve is intact.

In most cases of spinal cord damage, the phrenic nerve is unaffected, keeping respiration intact. Since the phrenic nerve originates very anteriorly in the neck (C3-5), a person may be paralyzed from the arms down but still be able to breathe.

Other conditions can damage the phrenic nerve, such as amyotrophic lateral sclerosis (ALS), a tumor that impinges on the nerve, and demyelination as a result of multiple sclerosis.

Phrenic nerve and hiccups

Spasms of the diaphragm due to phrenic nerve activity is believed to be one contributor to hiccups. Although annoying, hiccups are almost always benign and harmless.

However, a man with a 20-year case of incurable and persistent hiccups was found to have a cancerous growth on his phrenic nerve (Intractable hiccups: A rare presentation of phrenic nerve schwannoma).