The nerves of the mediastinum are somatic, sympathetic or parasympathetic.
Somatic Nerves
The somatic nerves of the mediastinum are the paired phrenic nerves. These nerves arise from the C3 - C5 ventral roots. They pass inferiorly through the neck, curving over the lateral border of the anterior scalene muscle to descend upon its anterior surface. Within the thoracic inlet, the nerves typically lie anterior to the subclavian artery (separated by the anterior scalene) and posterior to the subclavian vein.
The right phrenic nerve passes inferiorly, lateral to the left brachiocephalic vein, the superior vena cava, the right atrium and the inferior vena cava to reach the diaphragm.
The left phrenic nerve is more complicated. It passes near the apex of the left lung, between the left common carotid and left subclavian arteries, before skirting over the superficial side of the aortic arch. Passing anterior to the hilum of the left lung, it descends between the lung and the pericardium overlying the left ventricle to eventually reach the diaphragm.
The phrenic nerves supply somatic motor input to the diaphragm.
Autonomic Nerves
The autonomic nerves are derived from the vagus (parasympathetic) and the thoracic sympathetic chains
Vagus Nerve (X)
The vagus descends in the carotid sheath from the neck, and the left and right vagus nerves have different courses within the mediastinum.
Right Vagus
The right vagus lies posterior to the internal jugular, and enters the thorax anterior to the right subclavian. It descends posterior to the right brachiocephalic artery, and posterior and lateral to the superior vena cava. At the right hilum, it forms the pulmonary plexus which supplies the right lung. This plexus also communicates with the cardiac plexus that supplies the heart. From these plexuses, several fibres continue on to form the oesophageal plexus on the right side of the oesophagus. Several fibres from this plexus continue into the abdomen.
Left Vagus
The left vagus lies posterior to the left internal jugular, and between the left common carotid and left subclavian arteries. It passes over the anterior aspect of the aortic arch and continues posterior to the left hilum. It forms a similar pulmonary plexus and also contributes to the cardiac plexus. The oesophageal plexus is also formed on the left, and the left vagus continues as several small branches into the abdomen.
Sympathetic Chain
The 11 - 12 thoracic ganglia form a sympathetic chain on each side of the thoracic spine, just anterior to the heads of the ribs. These ganglia communicate with each other and with the ventral rami of the spinal cord. Branches from the chain pass to the thoracic aorta as well as to the pulmonary, cardiac and oesophageal plexuses.
