The major nerves found in the superior mediastinum are as follows:

Phrenic nerves - originate from spinal cord segments C3 - C5, pass inferiorly over the anterior scalene muscle on each side and anteroinferiorly over the root of the lung to innervate each dome of the diaphragm;
Vagus nerve - the tenth cranial nerve (CN X);
Left recurrent laryngeal nerve and right recurrent laryngeal nerve - arise from the vagus nerve on each side.

Vagus nerves

The vagus nerves (CN X) pass through the superior mediastinum and posterior mediastinum before entering the abdominal cavity. As the vagus nerves pass through the superior mediastinum they give branches to the esophageal plexus (innervating the esophagus), cardiac plexus (innervating the heart), and pulmonary plexus (innervating the lungs). As they pass through the thorax, the vagus nerves provide parasympathetic innervation to the thoracic viscera and carry visceral afferents from the viscera.

The right vagus nerve enters the superior mediastinum and is located between the right brachiocephalic vein and brachiocephalic trunk. It descends in a posterior direction towards the trachea, crosses over its lateral surface and then travels posteriorly to the root of the right lung to reach the esophagus. Just before reaching the esophagus, the arch of the azygos vein crosses the right vagus nerve.
The left vagus nerve enters the superior mediastinum behind the left brachiocephalic vein and between the left common carotid artery and left subclavian artery. Upon entering the superior mediastinum it lies deep to the mediastinal part of the parietal pleura and crosses the left side of the aortic arch. It further descends in a posterior direction behind the root of the left lung, and reaches the esophagus in the posterior mediastinum.

Left recurrent laryngeal nerve

The left vagus nerve gives rise to the left recurrent laryngeal nerve, which emerges from it at the inferior margin of the aortic arch. This nerve descends down the arch of the aorta, passing posterior to the ligamentum arteriosum, and ascends via the tracheoesophageal groove (a groove between the trachea and esophagus) to innervate structures in the larynx.

Phrenic nerves

The phrenic nerves arise from the cervical part of the spinal cord, mainly from the 4th, but also from the 3rd and 5th cervical segments (C3 - C5). As the nerves descend through the thorax, they provide motor and sensory innervation to the diaphragm and its associated membranes. They also provide somatic afferent fibers to the mediastinal pleura, fibrous pericardium, and parietal layer of the serous pericardium.

The right phrenic nerve and left phrenic nerve both enter the superior mediastinum lateral to the vagus nerves and slightly posterior to the initial parts of the brachiocephalic veins. The right phrenic nerve descends on the right side of the right brachiocephalic vein and superior vena cava, while the left phrenic nerve descends across the left lateral side of the aortic arch, passing superficially to the left vagus nerve and the left superior intercostal nerve.

The phrenic nerves continue descending into the middle mediastinum, where they pass along right and left sides of the pericardium anterior to the roots of the lungs. The right phrenic nerve leaves the thorax via the caval foramen of the diaphragm passing together with the inferior vena cava. The left phrenic nerve pierces the diaphragm near the apex of the heart.

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Nerves of superior mediastinum