Ch21 Cardiac Electrophysiology and the Electrocardiogram

The cardiac action potential originates in a group of cells called the sinoatrial (SA) node (see Fig. 21-1), located in the right atrium. These cells depolarize spontaneously and fire action potentials at a regular, intrinsic rate that is usually between 60 and 100 times per minute for an individual at rest. Both parasympathetic and sympathetic neural input can modulate this intrinsic pacemaker activity, or automaticity (see pp. 397–398).

心脏动作电位起源于位于右心房的一组称为窦房结（SA）的细胞（见图21-1）。这些细胞自发地去极化，并以规则的内在速率发射动作电位，对于休息的个体来说，通常在每分钟 60 到 100 次之间。副交感神经和交感神经输入都可以调节这种内在起搏器活动或自动性（见第 397-398 页）。

Because cardiac cells are electrically coupled through gap junctions (Fig. 21-3A), the action potential propagates from cell to cell in the same way that an action potential in nerve conducts along a single long axon. A spontaneous action potential originating in the SA node will conduct from cell to cell throughout the right atrial muscle and spread to the left atrium. The existence of discrete conducting pathways in the atria is stil disputed. About one tenth of a second after its origination, the signal arrives at the atrioventricular (AV) node (see Fig. 21-1). The impulse does not spread directly from the atria to the ventricles because of the presence of a fibrous atrioventricular ring. Instead, the only available pathway is for the impulse to travel from the AV node to the His Purkinje fiber system, a network of specialized conducting cells that carries the signal to the muscle of both ventricles.

由于心肌细胞通过间隙连接进行电耦合（图21-3A），因此动作电位在细胞之间传播的方式与神经中的动作电位沿单个长轴突传导的方式相同。源自 SA 结的自发动作电位将在整个右心房肌肉中从一个细胞传导到另一个细胞，并扩散到左心房。心房中离散传导通路的存在是有争议的。信号在其产生后大约十分之一秒到达房室 （AV） 淋巴结（见图 21-1）。由于存在纤维状房室环，冲动不会直接从心房扩散到心室。取而代之的是，唯一可用的途径是脉冲从房室结传播到他的浦肯野纤维系统，一个由专门的传导细胞组成的网络，将信号传递到两个心室的肌肉。