The phrenic nerve controls the diaphragm. It originates from the upper portion of the spinal cord in the neck, starting near the C3 vertebra and connecting to the C4 and C5 vertebrae. This single pair of nerves, one on each side of the body, is responsible for every breath you take, firing signals that cause the diaphragm to contract and flatten downward so air fills your lungs.
Where the Phrenic Nerve Starts and Where It Goes
The phrenic nerve’s origin point surprises most people. Your diaphragm sits at the base of your ribcage, but the nerve that controls it begins all the way up in your neck. It exits the spinal cord between the third and fifth cervical vertebrae (the bones just below your skull) and then travels a long path downward through the chest, passing alongside the heart and lungs before reaching the diaphragm’s surface. There’s one phrenic nerve on the left side and one on the right, each controlling its respective half of the diaphragm.
A classic medical school mnemonic captures this: “C3, 4, 5 keeps the diaphragm alive.” That phrase is worth remembering because it explains why neck injuries can stop someone from breathing. If the spinal cord is damaged above the C3 level, the signals that tell the diaphragm to contract never leave the spine. This is why high spinal cord injuries often require mechanical ventilation.
What the Phrenic Nerve Actually Does
The phrenic nerve carries two types of signals. Motor signals travel from the brain down through the spinal cord and along the nerve to the diaphragm, triggering the muscle contractions that drive breathing. These signals fire automatically, without you thinking about it, roughly 12 to 20 times per minute at rest. Sensory signals travel the opposite direction, carrying information from the diaphragm and the lining around the lungs and heart back up to the brain. This is why irritation of the diaphragm can produce pain that you feel in your shoulder, a phenomenon called referred pain. The brain interprets signals arriving from the phrenic nerve as coming from the shoulder region because the nerve shares spinal cord roots with shoulder nerves.
The phrenic nerve also plays a role in hiccups. During a hiccup, the phrenic nerve fires an involuntary signal that causes a sudden, sharp contraction of the diaphragm. At the same time, a separate nerve triggers the vocal cords to snap shut, producing the characteristic “hic” sound. The phrenic nerve serves as the main output pathway of this reflex, which is why conditions that irritate the nerve can cause persistent hiccups.
What Happens When the Phrenic Nerve Is Damaged
When one phrenic nerve stops working, the diaphragm on that side becomes paralyzed. It no longer contracts during inhalation and instead gets pushed upward by abdominal pressure while the healthy side pulls down. This unilateral paralysis can reduce lung capacity by about 50 percent. Many people with one-sided damage can compensate well enough for daily activities, though exercise tolerance drops significantly.
When both phrenic nerves are damaged, the consequences are far more severe. Bilateral diaphragmatic paralysis can reduce lung capacity by 70 to 80 percent. Breathing becomes heavily dependent on the accessory muscles in the neck and between the ribs, which aren’t designed to sustain normal breathing long-term. People with bilateral paralysis often need mechanical ventilation, particularly during sleep when the accessory muscles relax.
Common causes of phrenic nerve damage include cardiac and thoracic surgery (the nerve’s path near the heart makes it vulnerable during procedures), trauma to the neck or chest, tumors that compress the nerve along its route, and neurological conditions that affect nerve function more broadly. In some cases, the cause is never identified.
How Phrenic Nerve Problems Are Diagnosed
The most straightforward test for phrenic nerve function is called the sniff test. During this exam, you lie on a table under a fluoroscope (a real-time X-ray) while breathing normally, then taking deep breaths, then sniffing sharply through your nose. The sniff is the key part: it demands a quick, forceful diaphragm contraction. A healthy diaphragm moves downward during a sniff. A paralyzed diaphragm does the opposite, rising upward as the functioning side and accessory muscles create negative pressure in the chest. Doctors can see this paradoxical movement clearly on the fluoroscope and use it to confirm which side is affected.
Nerve conduction studies can also measure how well electrical signals travel along the phrenic nerve, helping to distinguish between nerve damage and problems with the diaphragm muscle itself.
Diaphragm Pacing for Spinal Cord Injuries
For people with high spinal cord injuries whose phrenic nerves are intact below the injury site but cut off from brain signals above it, a device called a diaphragm pacing system can restore breathing without a ventilator. The FDA-approved NeuRx system works by implanting small electrodes directly into the diaphragm muscle. These electrodes deliver rhythmic electrical pulses that mimic the brain’s natural signals, causing the diaphragm to contract and relax in a normal breathing pattern.
The key requirement is that the phrenic nerve and diaphragm muscle must still be capable of responding to stimulation. If the nerve itself is destroyed or the muscle has atrophied beyond recovery, pacing won’t work. For eligible patients, though, the system can replace or significantly reduce dependence on mechanical ventilation, improving mobility, speech, and quality of life. The device is approved for adults 18 and older with stable high spinal cord injuries.