The electrical activity of a single cardiac cell can be recorded with the aid of a microelectrode and demonstrates that the resting potential of a myocyte is-80 t 90 mv. This resting potential is maintained by the accumulation of potassium inside the cell and the removal of sodium from the cell by the energy-requiring Na+,K+-ATPase.
When a myocyte is depolarized to a certain threshold level(threshold potential), an action potential is produced as a result of a complexs of ionic shifts(Fi action divided into five phases. Phase 0 is the rapid initial depolarization and is mediated by an increased permeability of the sarcolemma an increased lowed by to sodium ions. This is fol ing from the early, result The plateau movement of potassium out of the cell mainly determined 2) the potential is ions, but by the inward movement calcium phase of also by the movement sodium, chloride, and ions. Phase constitutes the the action potential and is the result of th movement of potassium out of the cell.

Phase 4 of the action potential represents the outward flow of potassium and the flow of sodium and results in the gradual depolarization of the cell from resting to threshold Fig, the potential and shortly thereafter there is a period of time elicit an action potential. This stimulus fails to is termed the absolute refractory period and is most closely related to the du The of of action appearance of the action potential of sinus and atrioventricular(AV) nodal cells is different from that of the typical myocyte, The normal resting potential of these cells is higher(-60 the initial upstroke of depolarization is slower and calcium dependent, and the phase 4 depolarization is much more pronounced. The olarize(automaticity) until it reaches threshold potential, thus generatin ing cells. The sinus node usually has the fastest phase 4 depolarization and thus functions as the normal pacemaker of the heart, producing a rate of contraction(heart rate) of 60 to 100 beats/min. If the sinus node fails, the AV node has the next fastest pacemaker rate approximately 50 beats/min) The and produce a heart have phase 4 depolarization rate of 30 to 40 beats/min higher pacemakers fail.

when a lower pacemaker focus appropriately fires in the setting of slowing of the higher focus, the firing is termed an escape beat(if single) or an escape rhythm(if sustained).