the main role of a sinoatrial node cell is to initiate action potentials of the heart, so that it can pass throughout the heart and cause contraction. an action potential is a change in voltage (membrane potential) across the membrane of the cell, produced by the movement of charged atoms (ions). non-pacemaker cells (including the ventricular and atrial cells) have a period, immediately after an action potential, where the membrane potential remains relatively constant; this is known as a resting membrane potential. this resting phase (see cardiac action potential, phase 4) ends when another action potential reaches the cell. this produces a positive change in membrane potential (known as depolarisation), which initiates the start of the next action potential. pacemaker cells, however, don’t have this resting phase. instead, immediately after one action potential, the membrane potential of these cells begins to depolarise again automatically, this is known as the pacemaker potential. once the pacemaker potential reaches a set value, known as the threshold value, it then produces an action potential. other cells within the heart (including the purkinje fibers and atrioventricular node; avn) can also initiate action potentials; however, they do so at a slower rate and therefore, if the sa node is working, it usually beats the avn to it.