Aside from heart conducting system, which is made up of the sinoatrial and atrioventricular nodes and the Purkinje's fibers, the cardiac muscle receives a rich supply of sympathetic and parasympathetic nerve fibers, the latter contained in the vagus nerves. The sympathetic postganglionic fibers release primarily norepinephrine, and the parasympathetics release primarily acetylcholine. The receptors for norepinephrine on cardiac muscle are mainly betaadrenergic. The hormone epinephrine, from the adrenal medulla, combines with the same receptors as norepinephrine and exerts the same actions on the heart. The receptors for acetylcholine are of the muscarinic type.
Approximately 1 percent of cardiac muscle cells do not function in contraction, but have specialized features that are essential for normal heart excitation. These cells constitute a network known as the conducting system of the heart and are in contact with the cardiac muscle cells via gap junctions. The conducting system initiates the heartbeat and helps spread the impulse rapidly throughout the heart. One final point about cardiac muscle is that certain cells in the atria secrete the family of peptide hormones collectively called atrial natriuretic peptide.