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 In the SAN cAMP binds to an ion channel involved in action potential initiation, speeding up the production of the action potential (see sinoatrial node for more detail). cAMP also, activates a protein called protein kinase A (PKA). PKA affects both the DHPR and RyR, increasing the rise in Ca within the contractile cells and therefore increasing rate of muscle contraction. PKA also affects the myofilaments as well as a protein called phospholamban (PLB; see sarcoplasmic reticulum for more details), speeding up the rate of Cadecline in the cell and so speeding up muscle relaxation. Parasympathetic nerves work by releasing