|Nerve: Phrenic nerve|
|The phrenic nerve and its relations with the vagus nerve. (Phrenic labeled at upper left and right.)|
|Plan of the cervical plexus. (Phrenic labeled at bottom right.)|
|Gray's||subject #210 928|
|From||C3-C5 of cervical plexus|
The phrenic nerve is a nerve that originates in the neck (C3-C5) and passes down between the lung and heart to reach the diaphragm. It is important for breathing, as it passes motor information to the diaphragm and receives sensory information from it. There are two phrenic nerves, a left and a right one.
The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves (C3-C5) in humans. Thus, the phrenic nerve receives innervation from parts of both the cervical plexus and the brachial plexus of nerves.
The phrenic nerves contain motor, sensory, and sympathetic nerve fibers. These nerves provide the only motor supply to the diaphragm as well as sensation to the central tendon. In the thorax, each phrenic nerve supplies the mediastinal pleura and pericardium.
The phrenic nerve descends obliquely with the internal jugular vein across the anterior scalene, deep to the prevertebral layer of deep cervical fascia and the transverse cervical and suprascapular arteries. On the left, the phrenic nerve crosses anterior to the first part of the subclavian artery. On the right, it lies on the anterior scalene muscle and crosses anterior to the 2nd part of the subclavian artery. On both sides, the phrenic nerve runs posterior to the subclavian vein and anterior to the internal thoracic artery as it enters the thorax where it runs anterior to the root of the lung and into the pericardium between the fibrous and parietal layers.
- 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.
The pericardiacophrenic arteries and veins travel with their respective phrenic nerves.
The contribution of the 5th cervical nerve may stem from an accessory phrenic nerve. Most often it is a branch of the nerve to the subclavius and may contain numerous phrenic nerve fibers. If the accessory phrenic nerve is present, it lies lateral to the main nerve and descends posterior and occasionally inferior to the subclavian vein. The accessory phrenic nerve connects to the phrenic nerve in the thorax or the root of the neck.
It also innervates in the lungs.
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 hiccough reflex. A hiccough is a spasmodic contraction of the diaphragm, 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. The right phrenic nerve may be crushed by the vena cava clamp during liver transplantation. Severing the phrenic nerve, or a phrenectomy, will paralyse that half of the diaphragm. Diaphragm paralysis is best demonstrated by sonography. Breathing will be made more difficult but will continue provided the other nerve is intact.
The phrenic nerve arises from the neck (C3-C5) and innervates the diaphragm, which is much lower. Hence, patients suffering spinal cord injuries below the neck are still able to breathe effectively, despite any paralysis of the lower limbs.
Additional images 
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- Moore, Keith L. (1999). Clinically oriented anatomy. Philadelphia: Lippincott Williams & Wilkins. ISBN 0-683-06141-0.
- Vivian C. McAlister, David R. Grant, Andre Roy, William F. Brown, Linda C. Hutton, David J. Leasa, Cameron N. Ghent, James E. Veitch, and William J. Wall. "Right Phrenic Nerve Injury in Orthotopic Liver Transplantation" Transplantation 55.4 (1993): 826-830.
- Hine, Maynard Kiplinger (1975). Review of dentistry: questions and answers (6 ed.). Mosby. p. 421. ISBN 0-8016-2196-8. Retrieved May 13, 2008.
- -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."