Cardiac muscle is a specialized muscle with high strength and endurance that is present only in the heart, says InnerBody. Cardiac muscle accounts for most of the heart's mass.
Cardiac muscle consists of interlocking fibers, each with a single nucleus, that appears striped under a microscope, reports InnerBody. The dark bands of cardiac muscle are thick myosin proteins, while the light bands are thinner actin proteins. The stronger myosin fibers compact the actin filaments together, causing the muscle to contract. The branching shape of cardiac muscle cells allows each one to connect and communicate with three or four other muscle cells.
The cell membrane at the ends of cardiac muscle cells is specialized to form finger-like protusions, or intercalated disks, notes InnerBody. Intercalated disks allow for the tight connections between cardiac muscle cells that prevent separation during vigorous pumping of the heart. Intercalated disks also promote the fast passage of electrochemical signals between cardiac muscle cells.
One of the most important abilities of cardiac muscle is autorhythmicity, according to InnerBody. Cardiac muscle cells can maintain their own rhythm even without input from the rest of the nervous system. This is due to specialized pacemaker cells that set a steady rhythm and stimulate other cardiac cells to do the same.