Thoracic Nerves

The innervation of the thorax is somatic for the body wall and autonomic for the viscera. The somatic nerves are the twelve thoracic spinal nerves and the phrenic. The autonomic nerves are the fibres from the sympathetic chain, and the vagus nerve.

Sympathetic chain

In the thorax, each nerve carries a sympathetic ganglion. The T1 ganglion may be fused with the inferior cervical ganglion. Fibres reach the ganglia through white rami communicantes from the spinal cord. These preganglionic fibres either synapse on ganglion cells in the first ganglion, or pass up or down the sympathetic trunk to synapse in another ganglion, or travel through the splanchnic nerves to prevertebral ganglia in the thorax. Post ganglionic fibres pass to the primary rami in grey rami communicantes to be distributed to the tissues. In the skin sympathetic fibres supply blood vessels, glands and arrector pili muscles. In muscles sympathetic fibres control the diameter of blood vessels. In the viscera the fibres are concerned with vasoconstriction, bronchodilation, motility inhibition and glandular secretion. In the heart sympathetic fibres accelerate the heart rate and dilate the coronary circulation.

Cardiac plexus

The sympathetic supply to the heart is provided through the cardiac plexus, a network of fibres around the aortic arch, pulmonary artery, left and right coronary arteries. From the plexus both atra and ventricles are supplied. The fibres of the cardiac plexus arise in the upper four or five thoracic spinal segments. Many of the preganglionic fibres synapse in the T1-T5 ganglia, but others ascend in the cervical sympathetic chain to synapse in the cervical ganglia. The cardiac fibres from the cervical ganglia descend in the neck to reach the cardiac plexus. Afferent fibres from the heart travel the same course as sympathetic fibres. The afferent fibres therefore enter the T1-T5 spinal segments, particularly on the left side. This visceral afferent input is associated with somatic sensory input in the same spinal cord segments. Pain arising in the cardiac afferents (e.g. from a heart attack), can thus be confused with pain from the somatic afferents. This is described as referred pain. In the case of pain arising from the heart, pain is felt over the distribution of the T1-T5 somatic fibres on the left side, and extending up into the root of the neck.

Pulmonary plexus

Sympathetic fibres from T2-T6 of the thoracic sympathetic chain, and parasympathetic fibres from the vagus form the pulmonary plexus. The sympathetic component is distributed together with the parasympathetic fibres along the bronchi and pulmonary and bronchial vessels. The sympathetic component causes vasoconstriction and bronchodilation. Afferent fibres travelling with the sympathetic fibres innervate the visceral pleura.

Splanchnic nerves

The splanchnic nerves arise from the thoracic sympathetic chain. Preganglionic fibres arise from the T5-T9 ganglia and form the greater splanchnic nerve. The nerve passes through the crura of the diaphragm to end in the celiac ganglion. The lesser splanchnic nerve is formed by fibres from the T10, T11 ganglia. The nerve passes through the diaphragm with the greater to end in the aorticorenal ganglion. The least splanchnic nerve arises from the T12 ganglion and enters the abdomen with the sympathetic trunk to end in the renal plexus.

The Vagus Nerve

The vagus nerves enter the thorax at the thoracic inlet. In the neck the nerves lie in the carotid sheath between the internal jugular vein and the common carotid artery. The right vagus enters the thorax behind the internal jugular vein and anterior to the subclavian artery. The right recurrent laryngeal nerve is given off to ascend up between the esophagus and trachea. The right vagus continues into the superior mediastinum where it gives fibres to the pulmonary and cardiac plexuses (as for the sympathetic fibres, some cardiac fibres of the vagus arise from the nerve in the neck and pass down into the thorax to join the cardiac plexus). The nerve lies behind the right main bronchus and forms together with the left vagus a plexus on the esophagus. The esophageal plexus descends to the diaphragm where it forms topgether with the right vagus the anterior and posterior vagal trunks. The right vagus contributes mostly to the posterior vagal trunk. The left vagus enters the thorax between the left brachiocephalic vein and subclavian artery to cross the left side of the aortic arch. At the lower border of the arch the left recurrent laryngeal nerve recurs around the ligamentum arteriosum to ascend in the neck. The left vagus then gives off branches to the pulmonary and cardiac plexuses before contibuting to the esophageal plexus and the formation of the vagal trunks.

The Phrenic Nerve

The phrenic nerve arises in the neck from C3,4,5 roots. It lies on the scalenus anterior muscle and enters the thorax between the subclavian artery and the subclavian vein. The nerve then courses down the mediastinum lying anterior to the hilum of the lung, and squeezed between the pleura and the pericardium. The nerves at this point supply sensory fibres to the mediastinal pleura and pericardium. The right phrenic nerve then passes through the diaphragm close to the inferior vena cava in the central tendon. The left phrenic passes through the muscular part pf the diaphragm. The nerves supply motor fibres to the diaphragm, and sensory fibres to the diaphragmatic pleura and peritoneum. The phrenic nerve is the only motor supply to the diaphragm.

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