<P> Cardiac muscle cells branch freely and are connected by junctions known as intercalated discs which help the synchronized contraction of the muscle . The sarcolemma (membrane) from adjacent cells bind together at the intercalated discs . They consist of desmosomes, specialized linking proteoglycans, tight junctions, and large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction . Intercellular connective tissue also helps to strongly bind the cells together, in order to withstand the forces of contraction . </P> <P> Cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates . Oxygen from the lungs attaches to haemoglobin and is also stored in the myoglobin, so that a plentiful supply of oxygen is available . Lipids, and glycogen are also stored within the sarcoplasm and these are broken down by mitochondria to release ATP . The cells undergo twitch - type contractions with long refractory periods followed by brief relaxation periods when the heart fills with blood for the next cycle . </P> <P> It is not very well known how the electric signal moves in the atria . It seems that it moves in a radial way, but Bachmann's bundle and coronary sinus muscle play a role in conduction between the two atria, which have a nearly simultaneous systole . While in the ventricles, the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium . </P> <P> If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction . When two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction . As more cells are joined together, the fastest cell continues to assume control of the rate . A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system . The components of the cardiac conduction system include the atrial and ventricular syncytium, the sinoatrial node, the atrioventricular node, the bundle of His (atrioventricular bundle), the bundle branches, and the Purkinje cells . </P>

Electrical impulses are conducted efficiently through cardia muscle via