Cardiac pacemaker controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarizes the other potential pacemaker cells (AV node) to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses to the pace set by the cells of the SA node. This is the normal conduction of electrical activity in the
Cardiac excitation-contraction coupling into the cell. Secondly Ca indirectly activates proteins, called myofilaments, resulting in muscle contraction. The two main myofilaments in cardiac (and skeletal) muscle are actin and myosin. Ca binds to a protein called troponin, which is bound to the actin filament. This binding causes a shape change in the troponin which exposes areas on the actin, to which the head of the myosin filament binds. The binding of the myosin head to actin is known as a cross-bridge. A molecule, called adenosine triphosphate (ATP) which is produced by an intracellular structure called a mitochondrion, is then used, as a source