The cardiac plexus consists of various sympathetic, parasympathetic and afferent fibers and is divided into superficial and deep parts, but this is merely a descriptive division and functionally they are one. Their branches enter the pericardium to accompany the coronary arteries (vasomotor) and also to reach the SA node (cardioinhibitor and cardioaccelerator), the AV node and bundle and the ventricular myocardium. These branches form the coronary and cardiac parts of the plexus. Other branches enter the lung root as the pulmonary part for the supply of bronchial muscle and the blood vessels of the lung.
The superficial part of the cardiac plexus is formed by the union of the inferior cervical cardiac branch of the left vagus and the cardiac branch of the left cervical sympathetic ganglion, i.e. by the two small autonomic nerves which cross the left side of the arch of the aorta between the phrenic and vagus nerves. This part lies in front of the ligamentum arteriosum and becomes continuous with the deep part.
The deep part of the cardiac plexus is larger and receives contributions from the right vagus nerve by its upper and lower cervical cardiac branches, from the left vagus by its superior cervical cardiac branch, and a branch from each recurrent laryngeal nerve, and also sympathetic fibers from the remaining five cervical sympathetic ganglia (the middle and inferior on the left and all three on the right), and from the upper five or six thoracic sympathetic ganglia of both sides. This part lies to the right of the ligamentum arteriosum, in front of the left bronchus at the bifurcation of the pulmonary trunk. The sympathetic fibers are postganglionic from cell bodies in the cervical and upper thoracic ganglia, and the preganglionic cells are in the lateral horn of the upper five or six thoracic segments of the cord. The vagal fibers are preganglionic from cell bodies in the dorsal motor nucleus of the vagus but some are apparently intermingled with cells of the nucleus ambiguus. The vagal fibers relay in the heart wall, not in the cardiac plexus.
The vagi contain afferent fibers concerned with cardiovascular reflexes, and their cell bodies are in the inferior vagal ganglia. Pain fibers run with sympathetic nerves, reaching any of the three cervical and upper two or three thoracic sympathetic ganglia, but most go to the inferior cervical and first thoracic ganglia. The pain fibers pursue the usual pathway to the central nervous system, passing through the sympathetic ganglia to the spinal nerves via white rami communicantes (hence upper thoracic sympathectomy for the relief of cardiac pain); their cell bodies are in the inferior vagal ganglia. The connection with cervical and thoracic spinal nerves presumably explains the referral of cardiac pain to the arm, chest or neck.