There are two types of cells within the heart: cardiomyocytes (also known as cardiac myocytes or myocardiocytes) and cardiac pacemaker cells. Cardiomyocytes,making up the atria and the ventricles, have to be flexible enough to be shorten or lengthen as it is critical in the proper form during heart beats. Cardiac pacemaker cells, on the other hand,are distributed throughout the heart. They are responsible for the generation and sending out of electrical impulses and its transfer from cell to cell. All these cells are connected by porous cellular junctions called intercalated discs which contain gap junctions and desmosomes. The gap junctions which are protein-lined tunnels allow direct transmission of the depolarizing current from cell to cell across chambers of the heart so that the the cells contract in unison. Because of these gap junctions, the cardiac muscle cells are said to be electrically coupled. Desmosomes hold the cardiac muscle cells together during contraction. Moreover, these intercalated discs permit the diffusion of sodium, potassium and calcium from cell to cell making the depolarization and repolarization easier for the myocardium. This process makes the heart muscle act as a single coordinated unit. Cardiac cycle has two mechanical events: the systole (from mitral valve to aortic valve closure) and diastole (the rest of the cardiac cycle). Assuming a heart rate of 75 beats per minute, cardiac cycle time is 0.8 sec, systole (0.27 sec) and diastole (0.53 sec). It means that a heart beat happens less than a second. This is attributed to the rapid and efficient communication between cardiac muscles which is very vital to the normal function of the heart and normal cardiac cycle. Any delay or quicker systolic and diastolic process may signify heart problem. Heart failure is one example wherein there is decreased ventricular function. Myocardial infarction, arrythmias or vascular disease can all compromise the normal function of the cardiac cycle.
