Phrenic nerve
Nervus phrenicus
Also known as: C3–C5 nerve, nerve of the diaphragm
The sole motor nerve to the diaphragm, arising from C3–C5 (mainly C4). It descends on the anterior scalene, through the superior thoracic aperture, and between the pericardium and pleura. It also carries sensation from the central diaphragm, the pericardium and the mediastinal pleura.
Overview
The phrenic nerve arises from the cervical plexus, mainly from C4, with contributions from C3 and C5. It forms at the lateral border of the anterior scalene at about the level of the upper thyroid cartilage. It then descends almost vertically across the front of that muscle, deep to the prevertebral fascia, the transverse cervical and suprascapular arteries, and the internal jugular vein. It enters the thorax between the subclavian artery and vein.
In the mediastinum both nerves pass anterior to the lung root (the vagi pass posterior), lying between the fibrous pericardium and the mediastinal pleura with the pericardiacophrenic vessels:
• The right phrenic nerve runs along the right brachiocephalic vein, the superior vena cava and the right atrium. It reaches the diaphragm at or through the caval opening (T8).
• The left phrenic nerve crosses the aortic arch, anterior to the left vagus, and descends over the left ventricle. It pierces the diaphragm on its own, near the cardiac apex.
It carries motor, sensory and some sympathetic fibres. An accessory phrenic nerve (usually C5, often via the nerve to subclavius) is common and may preserve some diaphragmatic function after a phrenic nerve injury.
- Location
Neck (on the anterior scalene) → superior thoracic aperture → middle mediastinum (on the pericardium) → diaphragm.
Anatomy
- Innervation
- Motor: the whole ipsilateral hemidiaphragm, its only motor supply.
• Sensory: the central part of the diaphragmatic pleura and peritoneum, the mediastinal pleura, the fibrous and parietal serous pericardium, and parts of the liver capsule, gallbladder and IVC.
• The peripheral rim of the diaphragm receives sensory supply from the lower intercostal nerves (T6/7–T11).- Nerve roots
C3, C4, C5 (mainly C4).
- Course
Lateral border of the anterior scalene → descends on its anterior surface (lateral to medial) deep to the prevertebral fascia → between the subclavian artery and vein → anterior to the lung root, between the fibrous pericardium and mediastinal pleura (with the pericardiacophrenic vessels) → diaphragm. The right nerve reaches it via the caval opening; the left pierces the muscular left dome.
- Relations
- In the neck: anterior scalene behind, IJV and SCM in front, and the transverse cervical and suprascapular arteries crossing anteriorly.
• In the thorax: anterior to the lung root, in contrast to the vagus, which is posterior.
• The right nerve is related to the SVC and right atrium; the left to the aortic arch, left vagus and left ventricle.
Function
- Contraction of the diaphragm, the principal muscle of inspiration
- Sensation from the central diaphragm, pericardium and mediastinal pleura
- Motor limb of hiccups (singultus)
Clinical significance
Referred shoulder-tip pain
Irritation of the diaphragm or pericardium is felt over the shoulder tip, the C4 dermatome supplied by the supraclavicular nerves. Examples are blood under the diaphragm from splenic rupture (Kehr sign), subphrenic abscess, cholecystitis (right shoulder) and pericarditis.
Diaphragmatic paralysis
Unilateral phrenic palsy causes an elevated hemidiaphragm on chest X-ray and paradoxical upward movement on a sniff test (fluoroscopy). Causes include:
• tumour, especially lung cancer invading the mediastinum;
• cardiac surgery (cold injury or stretch);
• interscalene brachial plexus block, which almost always gives transient ipsilateral phrenic block;
• trauma.
Bilateral palsy causes orthopnoea.
High cervical cord injury
"C3, 4, 5 keeps the diaphragm alive." Complete cord injury above C3 abolishes diaphragmatic breathing, so the patient needs ventilation or phrenic nerve pacing. Injuries at C5 or below usually preserve spontaneous breathing, although intercostal and abdominal muscle function is lost.
Mnemonics
Roots
"C3, 4, 5 keeps the diaphragm alive."
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.