The heart autonomic innervation modulates cardiac function. Although it has its own independent conducting system, with the sinoatrial (SA) and atrioventricular (AV) nodes, the myocardium is also innervated by the peripheral nervous system through parasympathetic and sympathetic efferent fibers. The parasympathetic innervation is constituted by the vagus nerve branches, with the right vagus nerve branches innervating the SA node and the left vagus nerve the AV node, making the heart beat slower, decreasing the heart rate. Meanwhile, sympathetic nerve fibers to the heart is supplied by the stellate ganglion of the sympathetic trunk, making the heart beat faster when it is stimulated. (The stellate ganglion is made up of the inferior cervical and the first thoracic ganglion).
Important overlap can occur in the anatomical distribution of nerve fibers to the heart. Atrial myocardium is also innervated by vagal efferent fibers, while the ventricular heart muscle is only sparsely innervated by vagal efferent branches. On the other hand, sympathetic fibers are found throughout both atria and ventricles as well as in the SA and AV nodes. As mentioned in the above paragraph, vagal branches stimulation decreases heart rate, which is called negative chronotropy, as it also decreases contractility of heart muscle fibers, which is called negative inotropy. Vagal-regulated inotropic influences are moderate in the atria and relatively weak in the ventricles. Meanwhile, sympathetic presence is strong in both atria and ventricles.
There are also in the heart, vagal and sympathetic nerve fibers, which relay information from both stretch and pain receptors located in the myocardium. Stretch receptors send feedback information about blood volume and arterial pressure, while the pain receptors are usually activated during a coronary arterial branch ischemia, sending pain and discomfort information as feedback. This influences the electrical activity of the heart and can be seen in a cardiac cycle diagram.
