Phrenic Nerve

What Is the Phrenic Nerve?

The phrenic nerve is a paired nerve from the neck that carries the signals that make the diaphragm contract, and it is the only motor nerve that muscle has. Because the diaphragm does most of the work of quiet breathing, the phrenic nerves are among the most important nerves for staying alive. The name comes from the Greek word phren, an old term for the diaphragm.

Medical students remember its roots with a short rhyme: "C3, 4 and 5 keep the diaphragm alive." The nerve belongs to the cervical plexus, a network of nerves formed by the upper neck spinal nerves, yet it travels all the way down through the thoracic cavity to reach a muscle that sits at the bottom of the chest.

Explained simply

Think of the phrenic nerve as a long cable running from the neck to the floor of the chest. Your brainstem sends a pulse down the cable with every breath, the diaphragm tightens and pulls down, and air rushes into your lungs. When the pulse stops, the muscle relaxes and you breathe out.

Phrenic Nerve Anatomy: Roots and Fibers

Each phrenic nerve forms in the neck from the front branches (ventral rami) of the third, fourth and fifth cervical spinal nerves, with the fourth giving the largest share. The cell bodies of its motor fibers sit in a column of the spinal cord called the phrenic motor nucleus, which spans the C3 to C5 segments.

What kinds of fibers does it carry?

• Motor fibers to the muscle of its own half of the diaphragm, including the crus on that side.
• Sensory fibers that carry pain and touch from the central part of the diaphragm, the mediastinal pleura, the fibrous and parietal pericardium, and the membranes just above and below the central diaphragm.
• A few sympathetic fibers, picked up from the cervical sympathetic ganglia, that travel along with it.

The accessory phrenic nerve

In some people, a small accessory phrenic nerve adds extra fibers, usually from C5 through the nerve to the subclavius. It runs a separate short course and joins the main phrenic nerve near the first rib or in the upper chest. This normal variation matters to surgeons, because cutting the main trunk may not fully stop the diaphragm on that side.

Phrenic Nerve Location and Course: From the Neck to the Diaphragm

The phrenic nerve runs from the side of the neck, down the front of the anterior scalene muscle, into the chest between the subclavian artery and vein, and then through the mediastinum in front of the root of the lung to the diaphragm. Its path can be followed in three parts.

Phrenic nerve anatomy in the neck

The nerve forms at the upper outer edge of the anterior scalene muscle and slides down and inward across its front surface, so it is the only nerve that runs straight down on that muscle. It lies under the prevertebral layer of deep fascia, behind the sternocleidomastoid muscle and the internal jugular vein, and the transverse cervical and suprascapular arteries cross in front of it.

Entering the chest

At the root of the neck the nerve passes in front of the subclavian artery and behind the subclavian vein, then crosses the internal thoracic artery as it enters the thorax. From here the right and left nerves follow different routes because the large vessels and the heart are not symmetrical.

In the mediastinum

Inside the mediastinum, both nerves run in front of the root of the lung, squeezed between the fibrous pericardium around the heart and the mediastinal layer of the pleura. A small artery and vein, the pericardiacophrenic vessels, travel with each nerve. At the diaphragm, each nerve divides into several branches that spread through its half of the muscle, mostly from the underside.

How Do the Right and Left Phrenic Nerves Differ?

The right phrenic nerve is shorter and runs almost straight down along the great veins, while the left phrenic nerve is longer because it curves over the aortic arch and the bulge of the left side of the heart. Both still end in the same way, by supplying their own half of the diaphragm.

Phrenic nerve anatomy and the heart

The right nerve lies on the right brachiocephalic vein and the superior vena cava, then on the pericardium over the right atrium, and finally beside the inferior vena cava. It reaches the diaphragm at or near the caval opening, described on our page about the openings of the diaphragm. The left nerve descends between the left common carotid and left subclavian arteries, crosses the left side of the aortic arch in front of the left vagus nerve, and runs over the pericardium of the left ventricle before piercing the muscular left dome.

Neither nerve supplies the heart muscle itself. They only pass over the pericardium, the sac around the heart, and carry pain sensation from it.

What Is the Phrenic Nerve Function?

The main function of the phrenic nerve is to carry the motor signals that make the diaphragm contract during inhalation, and its second function is to carry sensation from the central diaphragm, pericardium and nearby pleura. Without it, the diaphragm on that side cannot contract at all.

Motor function in breathing

Groups of nerve cells in the brainstem set the breathing rhythm and send bursts of signals to the phrenic motor neurons in the spinal cord, as described on our control of breathing page. Each burst travels down the phrenic nerves, the diaphragm contracts and flattens, and the chest gets taller, which pulls air in during inhalation. Between bursts the muscle relaxes and quiet exhalation happens passively. The same pathway also produces deeper breaths during exercise, plus sudden diaphragm actions such as the spasm of a hiccup and the build-up of pressure before a cough or sneeze.

Sensory function and referred pain

Because the phrenic nerve enters the spinal cord at the C3 to C5 level, the brain can mistake irritation of the central diaphragm for pain in the skin of the same segments, especially the top of the shoulder. This referred shoulder tip pain is a classic clue that something is irritating the diaphragm from above or below. The outer rim of the diaphragm is different: its feeling travels in the lower intercostal nerves instead.

Which Structures Work With the Phrenic Nerve?

The phrenic nerve works with the diaphragm it drives, the other breathing muscles and their nerves, and the brainstem centers that control it, and it runs beside the vagus nerve for part of its path. Knowing these neighbors makes its role easier to picture.

• Diaphragm: the dome-shaped muscle under the lungs; the phrenic nerve is its only motor supply.
• Intercostal nerves: they drive the intercostal muscles between the ribs and carry feeling from the outer edge of the diaphragm.
• Other breathing muscles: the scalenes and other muscles of respiration help lift the rib cage when breathing becomes harder.
• Vagus nerve: it also runs down the chest, but behind the lung root rather than in front of it, and carries signals to the heart, airways and gut.
• Chemoreceptors: respiratory chemoreceptors sense carbon dioxide and oxygen and tell the brainstem how hard the phrenic nerves should fire.

Together these parts form the pump that moves air through the airways, a process explained step by step on our mechanics of breathing page.

Common Conditions That Affect the Phrenic Nerve

The phrenic nerve can be stretched, compressed, inflamed or cut anywhere along its long path, and the main result is weakness or paralysis of the diaphragm on the affected side. Our respiratory diseases section covers lung conditions more broadly, and anyone with unexplained breathlessness should see a doctor.

• Phrenic nerve injury: may follow surgery on the heart, chest or neck, a neck injury, or a nerve block given near the neck, and is often temporary.
• Compression by a mass: a tumor or enlarged lymph nodes in the chest, for example from lung cancer, can press on the nerve.
• Nerve inflammation: some inflammatory nerve disorders and viral illnesses can weaken one or both phrenic nerves.
• High spinal cord injury: damage at or above the C3 to C5 segments can cut the brainstem off from the phrenic motor neurons.

When one side stops working, a chest X-ray often shows that half of the diaphragm sitting higher than normal, and a fluoroscopy "sniff test" can show it moving the wrong way. Many people with one paralyzed side have few symptoms at rest, while loss of both sides causes marked breathlessness, especially when lying flat. Long-lasting hiccups and diaphragmatic pleurisy with shoulder tip pain also involve this nerve.