Phrenic nerve

The phrenic nerve (Latin: nervus phrenicus) is a bilateral, mixed nerve that arises from the cervical plexus and plays an essential role in respiratory regulation. The phrenic nerve originates primarily from the ventral ramus of the fourth cervical spinal nerve (C4) with additional branches from the C3 and C5 spinal nerves. As a mixed nerve, it contains both - motor and sensory - fibers.

Both phrenic nerves transverse the neck and thorax to reach the diaphragm. After arising, they descend along the anterior surface of the corresponding anterior scalene muscles. Within the thorax, both nerves pass on their respective sides of the heart between the mediastinal part of the parietal pleura and fibrous pericardium. However, the right and left phrenic nerves have slightly different courses in the thorax described below.

The phrenic nerves arise at the superior aspects of the lateral borders of the anterior scalene muscles. These nerves descend along the anterior surfaces of the mentioned muscles. They travel underneath the prevertebral layer of the deep cervical fascia, going posterior to the sternocleidomastoid, inferior belly of the omohyoid, transverse cervical and suprascapular arteries, and internal jugular veins. At the root of the neck, the phrenic nerves go anterior to the subclavian arteries and posterior to the subclavian veins.

The right phrenic nerve passes anteriorly over the lateral aspect of the right subclavian artery and enters the thorax via the superior thoracic aperture. It goes along the lateral side of the right brachiocephalic vein and superior vena cava. The right phrenic nerve continues to descend anterior to the right pulmonary hilum. After passing lateral to the right atrium and ventricle of the heart, the right phrenic nerve enters the abdominal cavity by either piercing the diaphragm near its caval foramen or by going through it.

The left phrenic nerve also descends into the thorax via the superior thoracic aperture. It goes anteriorly over the medial aspect of the left subclavian artery and passes anterior to the aortic arch and left vagus nerve (CN X). The left phrenic nerve also goes anterior to its corresponding pulmonary hilum. The left phrenic nerve continues to descend along the left ventricle of the heart and pierces the diaphragm at its central tendon near the apex of the heart.

Once in the abdominal cavity, both nerves divide into three branches - anterior, lateral (anterolateral) and posterior. These branches continue to pass in a radial pattern to innervate the corresponding areas of the diaphragm.

The sensory fibers of the phrenic nerves innervate the pericardium, central part of the diaphragm, mediastinal pleura, diaphragmatic pleura and parietal peritoneum. The innervated section of the parietal peritoneum covers the anterior surface of the pancreas, the abdominal surface of the diaphragm and the diaphragmatic surface of the liver until the gallbladder.

The motor fibers of the phrenic nerves supply only the diaphragm and are the sole sources of its motor innervation. The phrenic nerves are the only nerves that innervate the main respiratory muscle. Also, the phrenic nerves contain sympathetic fibers that connect them to the splanchnic plexuses and sympathetic trunks.

References:

1. Gray, H., & Carter, H. (2021). Gray’s Anatomy (Leatherbound Classics) (Leatherbound Classic Collection) by F.R.S. Henry Gray (2011) Leather Bound (2010th Edition). Barnes & Noble.
2. Tubbs, S. R., Rizk, E., Shoja, M., Loukas, M., Barbaro, N., & Spinner, R. J. (2015). Nerves and Nerve Injuries: Vol 1: History, Embryology, Anatomy, Imaging, and Diagnostics (1st ed.). Academic Press.
3. Rea, P. (2016). Essential Clinically Applied Anatomy of the Peripheral Nervous System in the Head and Neck (1st ed.). Academic Press.