Innervation of the Heart
Both the afferent and efferent fibers of the autonomic nervous system contribute to the overall innervation of the heart in its entirety. This article aims to simplify the literature available in order to create a greater understanding and provide a general overview of the connection between the heart and the nervous system.
The autonomic fibers accumulate among the bundles of the cardiac plexus which preside over cardiac innervation. They intermingle with the cardiac nerves which provide the bulk of the nervous tissue. The cardiac nerves are ramified autonomic nerve fibers and are found at the level of the tracheal bifurcation. Their function is to innervate the electrical conducting system of the heart, the atrial and ventricular myocardium and the coronary vasculature. These nerves are further divided into superficial and deep sections, along with the coronary plexus which is devised of both sympathetic and parasympathetic fibers.
The parasympathetic fibers are contributed by the vagus nerve (CN X) and are classed as preganglionic fibers that synapse upon postganglionic neurons within either a portion of the cardiac plexus or the heart wall itself. Their role during innervation is to decrease the heart rate, to decrease the force of cardiac contractions and to dilate the coronary resistance vessels. Most of the vagal effects are limited to the area around the sinoatrial node.
The sympathetic fibers stem from the intermediolateral cell column of the thoracic cords T1 to T5 before merging into the sympathetic trunk. The preganglionic fibers synapse in the sympathetic ganglia of the cervical and thoracic regions, which in turn pass on postganglionic fibers to the cardiac plexus. Sympathetic innervation directly opposes the parasympathetic system by increasing the heart rate, as well as increasing the heart’s force of contraction and constricting the coronary resistance vessels.
Injuries to the major nerves that supply the cardiac plexus can cause compensatory disorders. For example, if the branches of the vagus nerve which are directly related to the autonomic innervation of the heart are damaged in some way, the heart will lose the ability to regulate its overall heart rate and will result in signs of tachycardia. The same logic applies to the sympathetic system and vice versa.