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In the pacemaking cells of the heart (e.g., the sinoatrial node), the pacemaker potential (also called the pacemaker current) is the slow, positive increase in voltage across the cell's membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential. This increase in membrane potential is what causes the cell membrane, which typically maintains a resting membrane potential of -70 mV, to reach the threshold potential and consequently fire the next action potential; thus, the pacemaker potential is what drives the self-generated rhythmic firing (automaticity) of pacemaker cells, and the rate of change (i.e., the slope) of the pacemaker potential is what determines the timing of the next action potential and thus the intrinsic firing rate of the cell. In a healthy sinoatrial node (SAN, a complex tissue within the right atrium containing pacemaker cells that normally determine the intrinsic firing rate for the entire heart), the pacemaker potential is the main determinant of the heart rate. Because the pacemaker potential represents the non-contracting time between heart beats (diastole), it is also called the diastolic depolarization. The amount of net inward current required to move the cell membrane potential during the pacemaker phase is extremely small, in the order of few pAs, but this net flux arises from time to time changing contribution of several currents that flow with different voltage and time dependence. Evidence in support of the active presence of K+, Ca2+, Na+ channels and Na+/K+ exchanger during the pacemaker phase have been variously reported in the literature, but several indications point to the “funny”(If) current as one of the most important.(see funny current). There is now substantial evidence that also sarcoplasmic reticulum (SR) Ca2+-transients participate to the generation of the diastolic depolarization via a process involving the Na–Ca exchanger. ==Distinctions among autonomic foci== In reality, the heart has several pacemakers known as autonomic foci, each which fires at its own intrinsic rate: * ''SA node:'' 60–100 bpm * ''Atrial foci:'' 60–80 bpm * ''Junctional foci:'' 40–60 bpm * ''Ventricular foci:'' 20–40 bpm The potentials will normally travel in order SA node → atrial foci → junctional foci → ventricular foci Pacemaker potentials are fired not only by SA node, but also by the other foci. However, the other firing frequencies are slower than the one of the SA node (as seen above). Normally, all the foci will end up firing at the SA node rate, not their intrinsic rate. The other foci attempt to fire at their intrinsic rate, but they are activated by the SA node before they can fire. This rapid firing causes all the foci to fire faster than their intrinsic rates, a phenomenon known as overdrive-suppression. Thus, in the normal, healthy heart, only the SA node intrinsic rate is observable. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「pacemaker potential」の詳細全文を読む スポンサード リンク
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