The phrenic nerve is a mixed nerve arising from the anterior rami of C3-C5 spinal nerves, which are components of the cervical plexus. It arises in the neck and descends vertically through the thorax to end on the diaphragm. The phrenic nerve is a bilateral nerve, and its left and right counterparts have some important differences in terms of course and relations with surrounding structures.
The main function of the phrenic nerve is to provide the entire motor innervation to the diaphragm, which makes it a vital component in the physiology of breathing. In addition, the phrenic nerve supplies sensory innervation to the central part of the diaphragm and adjacent fascial coverings.
This article will discuss the anatomy and function of the phrenic nerve.
|Origin||C3-C5 spinal nerves|
Sensory: Central part of diaphragm, pericardium, mediastinal pleura, diaphragmatic pleura, diaphragmatic peritoneum
- Origin and course
- Clinical relations
Origin and course
The phrenic nerve is a deep branch of the cervical plexus arising mainly from the C4 spinal nerve, but also receives contributions from C3 and C5 spinal nerves. This fact can be remembered by the mnemonic: C3, 4, 5, keep the diaphragm alive.
The phrenic nerve arises in the neck, at the upper lateral border of the scalenus anterior muscle, and descends along its anterior surface, deep to the prevertebral layer of the deep cervical fascia. The nerve runs posterior to the sternocleidomastoid muscle, the inferior belly of omohyoid muscle, the internal jugular vein, transverse cervical and suprascapular arteries.
From this point on, the pathway of the left and right phrenic nerves has several differences. However, on both sides, the nerves run behind the subclavian vein and anterior to the internal thoracic artery to enter the thoracic cavity through the superior thoracic aperture. In addition, both phrenic nerves are accompanied by the pericardiacophrenic vessels along their course in the thorax.
Left phrenic nerve
- Passes anteriorly over the first part of the left subclavian artery and deep to the thoracic duct.
- Runs anterior to the hilum of the left lung.
- Crosses anterior to the aortic arch and the vagus nerve.
- Passes over the fibrous pericardium of the left ventricle and pierces the diaphragm close to the apex of the heart to reach the inferior (abdominal) surface of the diaphragm.
Right phrenic nerve
- Descends anteriorly over the second part of the right subclavian artery.
- Passes lateral to the right brachiocephalic vein and the superior vena cava.
- Runs anterior to the hilum of the right lung.
- Passes along the fibrous pericardium of the right atrium and pierces the diaphragm near the inferior vena cava opening.
The phrenic nerve is a mixed nerve, carrying motor, sensory and sympathetic fibers. It is the only nerve that provides motor innervation to the diaphragm, with the left and right phrenic nerves innervating their corresponding ipsilateral hemidiaphragms. Thus, the phrenic nerve stimulates the movements of the diaphragm and plays a crucial role in breathing.
The phrenic nerve also provides sensory innervation to the central part of the diaphragm, as well as the pericardium, mediastinal pleura, diaphragmatic pleura and diaphragmatic peritoneum.
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Phrenic nerve damage
Since the phrenic nerve provides the sole motor innervation to the diaphragm, damage of the phrenic nerve can result in paralysis or palsy of the diaphragm. Causes of phrenic nerve damage can include the following:
- Mechanical trauma: such as ligation during surgery or injury.
- Compression: due to a tumour in the chest cavity.
- Neuropathy: such as diabetic neuropathy.
Bilateral damage of the phrenic nerve can be a life-threatening condition, as it results in the paralysis of the entire diaphragm. On the other hand, unilateral damage to either the left or right phrenic nerve, and the subsequent paralysis of one hemidiaphragm, can be an asymptomatic condition and an incidental finding on chest X-ray.
A unilateral paralysis of one hemidiaphragm causes paradoxical movement of the diaphragm during respiration, where the affected side moves upwards during inspiration, and downwards during expiration.
Phrenic nerve: want to learn more about it?
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