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英文原文： CARDIAC ACTION POTENTIAL The cardiac action potential has 5 phases ( Figure 18.1).In nonpacemaker cells, during the resting phase 4, the myocardial membrane potential is maintained at a steady level of approximately −85 to −90 mV by the Na﹢-K﹢ ATPase pump. Phase 0 defines the opening of voltage-gated Na channels and rapid influx of Na+ ions (INa) that changes the membrane potential to about +40 mV, followed by rapid inactivation of the majority of the Na channels. However, a small fraction of sodium channels remains open and continues to carry inward sodium current. This is called late sodium current (INa-L). Brief, transient outward potassium current (Ito) then follows, which leads to the characteristic spike and dome appearance (phase 1). The L-type calcium channel drives the inward calcium current (ICa-L) that defines the beginning of phase 2 of action potential. Phase 2 represents a delicate balance between inward currents (ICa-L, INa-L) and outward potassium currents (slow activating (IKs) and rapidly activating (IKr) delayed rectifier potassium current). During phase 3, there is a gradual decay of the inward ICa-L,and outward potassium current (IKr, IKs, and IK1) predominates. This brings the membrane potential back to baseline (phase 4) with the recovery of fast Na+ channels in preparation for ventricular reexcitation.  翻 译 心脏动作电位 心脏动作电位分5相(图18.1)。在非起搏细胞的静息期4相，心肌膜电位由Na ﹢ - k ﹢ ATP酶泵调控，维持在约-85至−90 mV的稳定水平。0相为电压门控Na通道的开启和Na﹢离子(INa)的快速内流，使膜电位变为+40 mV，然后是Na通道大部分的快速失活。然而，少量的钠离子通道仍然保持开放状态，并继续保持钠离子内流。这就是所谓的晚期钠电流(INa-L)。随后为短暂的一过性钾离子外流(Ito)，会形成特征性的尖峰和圆顶形的图形(1相)。L型钙通道驱动钙离子内流(ICa-L)标志着2期动作电位的开始。2相为内向离子流(ICa-L, INa - L)和外向钾电流(缓慢的激活(IKs)和快速激活的(IKr)延迟整流钾电流)之间的微妙平衡。在3相，有一个逐渐衰退的内向离子流ICa-L，此时外向钾电流(IKr, IKs，和IK1)占优势。这使膜电位回到基线水平(4相)，伴随快速Na+通道的恢复，为心室激动做准备。  图18.1.各型心肌细胞动作电位与体表心电图关系示意图 英文原文： For practical purposes, the cardiac cellular action potential can be divided into 2 subtypes: the slow,calcium-mediated action potential and the fast, sodium channel-mediated action potential (see Figure 18.1).Myocytes in the SA and AV nodes exhibit automaticity due to spontaneous phase 4 depolarization, rendering them as pacemaker cells. Compared to other cardiac cell types, their resting membrane potential is less negative (−60 mV) during the early part of phase 4.Spontaneous phase 4 depolarization results from pacemaker current (IF) that largely carries Na﹢ into the cell. This brings the membrane potential close to threshold potential, resulting in phase 0 depolarization, that is, mediated by ICa-L. This particular action potential is characterized by a slowly rising upstroke. In contrast, atrial, ventricular, and His-Purkinje system myocytes do not have pacemaker current, and phase 0 is mediated by fast INa with rapid upstroke of the action potential. 翻 译 事实上，心肌细胞动作电位可以分为2个亚型:缓慢型和快速型，分别为钙和钠离子通道介导的动作电位(见图18.1)。窦房结和房室结心肌细胞在4相可自发除极化，由此定义为起搏细胞。与其他类型心肌细胞相比, 在4相早期阶段它们的静息膜电位较负(−60 mV)。4期自动除极化是由起搏细胞自发电流(IF)，使Na﹢进入细胞内。这使得膜电位接近阈电位，导致0相除极，这是由Ica－L介导的。这种特殊的动作电位的特点是上行缓慢。相反，心房、心室和希氏束-浦肯野系统的肌细胞没有起搏电流，而0相为快INa介导的快速动作电位形成。 （翻译：赵楠楠；校对：何树楠） |
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