The Phrenic Nerve

Original Author: Darren Puttock
Last Updated: September 20, 2016
Revisions: 18

The phrenic nerve is a bilateral, mixed nerve that originates in the neck and descends through the thorax to reach the diaphragm. As the only source of motor innervation to the diaphragm, this nerve has an important role in breathing.

In this article, we shall look at the anatomy of the phrenic nerve – its anatomical course, motor and sensory functions.


Overview

Nerve roots: Anterior rami of C3, C4 and C5.

Motor functions: Innervates the diaphragm.

Sensory functions: Innervates the central part of the diaphragm, the pericardium and the mediastinal part of the parietal pleura.


Anatomical Course

The phrenic nerve mainly originates from the C4 spinal root, but it also receives contributions from C3 and C5. It also receives some communicating fibres from the cervical plexus.

The nerve begins at the lateral border of the anterior scalene muscle. It then continues inferiorly over the anterior surface of anterior scalene, deep to the prevertebral layer of deep cervical fascia. From here, the course of the phrenic nerve differs between the left and right:

Right Phrenic Nerve:

  • Passes anteriorly to second part of the subclavian artery, and posteriorly to the subclavian vein.
  • Enters the thorax via the superior thoracic aperture.
  • Descends anteriorly to the right lung root, down the right side of the pericardium.
  • Reaches the diaphragm and pieces the muscle to supply the underlying surface.

Left Phrenic Nerve:

  • Passes anteriorly to the first part of the subclavian artery, and posteriorly to the subclavian vein.
  • Enters the thorax via the superior thoracic aperture.
  • Crosses the aortic arch and vagus nerve, and descends anteriorly to the left lung root, down the left side of the pericardium.
  • Reaches the diaphragm and pieces the muscle to supply the underlying surface.
Fig 1.1 - The branches of the cervical plexus. The smaller branches have been removed for simplicity.

Fig 1.0 – The origin of the phrenic nerve from the anterior rami of C3,4 and 5.


Motor Functions

The phrenic nerve provides motor innervation to the diaphragm; the main muscle of respiration. As the phrenic nerve is a bilateral structure, each nerve supplies the ipsilateral side of the diaphragm (i.e. the hemi-diaphragm on the same side as itself).


Sensory Functions

Sensory fibres from the phrenic nerve supply the central part of the diaphragm, including the surrounding pleura and peritoneum. The nerve also supplies sensation to the mediastinal pleura and the pericardium.

Fig 1.2 - The anatomical course of the phrenic nerves, which innervate the diaphragm.

Fig 1.1 – The phrenic nerve provides sensory innervation to the diaphragm, mediastinal pleura and pericardium.

Clinical Relevance: Diaphragmatic Paralysis

Fig 1.3 - Chest x-ray, showing paralysis of the right hemidiaphragm.

Fig 1.2 – Chest x-ray, showing paralysis of the right hemidiaphragm.

The phrenic nerve provides motor innervation to the diaphragm. If the nerve becomes damaged, paralysis of the diaphragm will result. There are numerous causes of phrenic nerve lesions:

  • Mechanical trauma: ligation or damage to the nerve during surgery.
  • Compression: due to a tumour within the chest cavity.
  • Myopathies: such as myasthenia gravis.
  • Neuropathies: such diabetic neuropathy.

Paralysis of the diaphragm produces a paradoxical movement. The affected side of the diaphragm moves upwards during inspiration, and downwards during expiration. A unilateral diaphragmatic paralysis is usually asymptomatic, and is most often an incidental finding on x-ray. If both sides are paralysed, the patient may experience poor exercise tolerance, orthopnoea and fatigue. Lung function tests will show a restrictive deficit.

Management of diaphragmatic paralysis is two-fold. Firstly, the underlying cause must be identified and treated (if possible). The second part of treatment deals with symptomatic relief. This is usually via non-invasive ventilation, such as a CPAP (continuous positive airway pressure) machine.

 

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Quiz

Question 1 / 4
Which muscle is closely related to the origin of the phrenic nerve?

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Question 2 / 4
Which muscle does the phrenic nerve innervate?

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Question 3 / 4
Which of the following areas is not innervated by the phrenic nerve?

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Question 4 / 4
In which dermatome would referred pain from the phrenic nerve be felt?

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