ElectroPhysiology of the Heart

Cardiac muscle, like skeletal muscle, is a striated muscle and much of the mechanism of contraction is similar between the two muscle types. The electrophysiology of the two muscles differs: First ,the cardiac action potential is not initiated by neural activity. Instead, specialized cardiac muscle tissue in the heart itself initiates the action potential, which then spreads directly from muscle cell to muscle cell. Neural influences have only a modulatory effect on the heart rate. Second, the duration of the cardiac action potential is quite long. As a result, the full force of cardiac contraction results from a single action potential. The force of contraction is not the same for every beat of the heart and can be modulated by the cardiac nerves. Finally, all cells in the heart contract together as a unitina coordinated fashion with everybeat. Two broad types of cells are found within the heart: (1) contractile cells and (2) conductile cells. Contractile cells are the cells of the working myocardium and constitute the bulk of the muscle cells that makeup the atria and the ventricles. An action potential in anyone of these cells leads to a mechanical contraction of that cell. Conductile cells are specialized muscle cells that are involved with the initiation or propagation of action potentials but have little mechanical capability. critical importance is the sinoatrial (SA)node. The SA node (sometimes called the sinus node) lies in the right atrium near the venacava.