Phrenic nerve

Nerve: Phrenic nerve
The phrenic nerve and its relations with the vagus nerve. (Phrenic labeled at upper left.)
Plan of the cervical plexus. (Phrenic labeled at bottom right.)
Latin	nervus phrenicus
Gray's	subject #210 928
Innervates	Diaphragm
From	C3-C5 of cervical plexus

The phrenic nerve arises from the third, fourth, and fifth cervical spinal nerves (C3-C5) in humans.

Function

The phrenic nerve is made up mostly of motor nerve fibres for producing contractions of the diaphragm. In addition, it provides sensory innervation for many components of the mediastinum and pleura. There is minor involvement in the upper abdomen, with branches to the celiac plexus.

Path

Found in the middle mediastinum, both phrenic nerves run from C3, C4 and C5 along the anterior scalene muscle deep to the carotid sheath.

• The right phrenic nerve passes over the brachiocephalic artery, posterior to the subclavian vein, and then crosses the root of the right lung anteriorly and then leaves the thorax by passing through the vena cava hiatus opening in the diaphragm at the level of T8. The right phrenic nerve passes over the right atrium.
• The left phrenic nerve passes over the pericardium of the left ventricle and pierces the diaphragm separately.

Both these nerves supply motor fibres to the diaphragm and sensory fibres to the fibrous pericardium, mediastinal pleura, and diaphragmatic peritoneum.

The pericardiacophrenic arteries and veins travel with their respective phrenic nerves.

Clinical relevance

Pain arising from structures served by the phrenic nerve is often "referred" to other somatic regions served by spinal nerves C3-C5. For example, a subphrenic abscess beneath the right diaphragm might cause a patient to feel pain in the right shoulder-Kehr's sign. Irritation of the phrenic nerve (or the tissues supplied by it) leads to the hiccup reflex. A hiccup is a spasmodic contraction of the diaphram, which pulls air against the closed folds of the larynx.

The phrenic nerve must be identified during thoracic surgery and preserved. It passes anterior to the hilum of the corresponding lung, and therefore can be identified easily. Severing the phrenic nerve, or a phrenectomy,^[1] will paralyse that half of the diaphragm. Breathing will be made more difficult but will continue provided the other nerve is intact.

Fortunately, the phrenic nerve arises from the neck (C3-C5) and innervates the diaphragm, which is much lower. Hence, patients who suffer spinal cord injuries below the neck are still able to breathe effectively, despite any paralysis of the lower limbs.

Additional images

Transverse section of thorax, showing relations of pulmonary artery.

The arch of the aorta, and its branches.

Superficial dissection of the right side of the neck, showing the carotid and subclavian arteries.

The right brachial plexus with its short branches, viewed from in front.

External links

• -1402273763 at GPnotebook
• Phrenic+nerve at eMedicine Dictionary
• SUNY Figs 19:04-05 - "Left side of the mediastinum."
• SUNY Figs 25:03-15 - "Diagram of the cervical plexus."

References

1. ^ Hine, Maynard Kiplinger (1975). Review of dentistry: questions and answers (6 ed.). Mosby. pp. pg 421. ISBN 0801621968. http://books.google.com/books?id=OPJpAAAAMAAJ&q=Phrenectomy+-wikipedia&dq=Phrenectomy+-wikipedia&client=firefox-a&pgis=1. Retrieved May 13, 2008. 

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