The cardiac nerves serve to adapt the automatic activity of the heart to the requirements of the body. Excitation of the cardiac branches of sympathetic nerves increases stroke volume, the speed of conduction of stimuli, cardiac excitability, and pulse frequency. The cardiac branches of the vagus nerve parasympathetic have the opposite effect. Both these nervous pathways also contain afferent fibers along which, among other messages, pain stimuli (angina pectoris) travel and are projected into the left arm.
The cardiac sympathetic nerves contain mainly post-ganglionic neurons; while the parasympathetic nerves (vagus) branches contain mainly pre-ganglionic ones. The post-ganglionic nerve cells lie partly in the walls of the atria beneath the epicardium, and their nerve fibers run to the impulse-conducting system.