<P> The SA node 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 heart . </P> <P> There are 3 main stages in the generation of an action potential in a pacemaker cell . Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system . This can lead to some confusion . There is no phase 1 or 2, just phases 0, 3, and 4 . </P> <P> The key to the rhythmic firing of pacemaker cells is that, unlike other neurons in the body, these cells will slowly depolarize by themselves and do not need any outside innervation from the autonomic nervous system to fire action potentials . </P> <P> As in all other cells, the resting potential of a pacemaker cell (- 60mV to - 70mV) is caused by a continuous outflow or "leak" of potassium ions through ion channel proteins in the membrane that surrounds the cells . However, in pacemaker cells, this potassium permeability (efflux) decreases as time goes on, causing a slow depolarization . In addition, there is a slow, continuous inward flow of sodium, called the funny current. These two relative ion concentration changes slowly depolarize (make more positive) the inside membrane potential (voltage) of the cell, giving these cells their pacemaker potential . When the membrane potential gets depolarized to about - 40mV it has reached threshold (cells enter phase 0), allowing an action potential to be generated . </P>

The natural pace maker of the heart is made up of skeletal muscle