Innervation of Heart

The parasympathetic preganglionic (vagal) fibers are the axons of cells in the dorsal vagal nucleus. From the vagal cardiac nerves, they relay in ganglia of the cardiac plexus or in intrinsic cardiac ganglia, which are located mainly in the atrial subepicardial tissue along the coronary sulcus and around the roots of the great vessels. The sinoatrial node and the atrioventricular node and bundle have a rich supply of parasympathetic innervation. Ventricular ganglia are scanty, but enough of them exist to cast doubts on the hypothesis that ventricular innervation is purely sympathetic.

The more important afferent and efferent pathways in cardiac innervation are shown in the illustration. The peripheral processes of the afferent pseudounipolar neurons in the posterior root ganglia transmit input from cardiac receptors of various types and from terminal nerve networks in reflexogenic zones, such as those in and around the large cardiac venous openings, the interatrial septum, and the ascending aorta. Some of their central processes are implicated in spinal reflex arcs, whereas others ascend to the dorsal vagal nuclei in the medulla oblongata, the nearby reticular formation, or the hypothalamus and frontal cortex.

The thoracic sympathetic cardiac nerves carry many afferent pain fibers from the heart and great vessels, and this endows them with a clinical interest disproportionate to their small size, because their surgical destruction produces alleviation of angina pectoris. Other cardiac pain afferents run in the middle and inferior cervical sympathetic cardiac nerves; however, after entering the corresponding cervical ganglia, they descend within the sympathetic trunks to the thoracic region before passing through rami communicantes to the upper four or five thoracic spinal nerves.

Afferent vagal fibers from the heart and vessels play an important role in modifying efferent output that adjusts the rate and strength of the heartbeat; usually, they depress cardiac activity. In humans, the afferent vagal information pass through cardiac branches of the recurrent laryngeal nerves to the main vagus nerves, and thus to the brainstem.

Afferent pericardial fibers from the fibrous and parietal serous pericardium are carried mainly in the phrenic nerves, but those from the visceral serous pericardium join the coronary arterial plexuses.

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