Anatomy of Nerves: Phrenic and Vagus Nerve

Phrenic Nerve

• The phrenic nerve is composed of fibres from C3, 4, and 5, with the fibres from C5 constituting a separate nerve called the accessory phrenic nerve.
• In most cases, the accessory phrenic nerve joins the main part of the phrenic nerve at the level of the 1st rib.
• If the accessory phrenic nerve is present, surgical cutting of the main phrenic nerve will not achieve complete paralysis of the corresponding part of the diaphragm.

Vagus Nerve

• The vagus nerve is the 10th cranial nerve, arising from the medulla oblongata of the brain stem.
• It is a mixed nerve, containing motor, sensory, and parasympathetic fibres.
• It enters the thorax through the thoracic inlet, crossing in front of the 1st part of the subclavian artery.
• The right vagus gives its recurrent laryngeal branch in the thorax.
• The vagus nerve runs its course first in the superior and then in the posterior mediastinum.
• The 2 vagi are formed again from the lower ends of the oesophageal plexuses and descend through the oesophageal opening of the diaphragm to enter the abdomen as the gastric nerves.
• The right vagus forms the posterior gastric nerve, while the left vagus forms the anterior gastric nerve.
• Branches of the vagus nerve in the thorax include cardiac, pulmonary, and oesophageal branches, as well as the left recurrent laryngeal nerve.
• These branches provide parasympathetic fibres to the corresponding plexuses.

Left Recurrent Laryngeal Nerve

• The left recurrent laryngeal nerve arises from the left vagus nerve as it lies on the left anterior aspect of the arch of the aorta.
• It hooks below the arch of the aorta, behind and to the left of the ligamentum arteriosum.
• It ascends deep to the arch of the aorta, in the left side of the groove between the trachea and the oesophagus.
• It leaves the superior mediastinum to enter the root of the neck by passing through the inlet of the thorax.
• Branches in the thorax include cardiac branches to the deep cardiac plexus, and tracheal and oesophageal branches.
• Compression of the vagus nerve by an aortic aneurysm or enlarged tracheobronchial lymph nodes can lead to hoarseness of voice or persistent cough.

Thoracic Part of the Sympathetic Chain

• The sympathetic chain is a trunk of nerve fibres and ganglia that runs along the vertebral column, one on each side.
• The thoracic part of the sympathetic chain gives off branches which form the posterior pulmonary plexus.
• The right vagus shares in the formation of the posterior oesophageal plexus on the back of the oesophagus.
• The left vagus shares in the formation of the anterior oesophageal plexus.