When at resting state, for a cardiac muscle cell specifically, the positively charged ions are located at the outer side of the cell membrane and the negatively charged ions are located at the inner side of the cell membrane, therefore rendering the cell at a state of equilibrium described as positive outside and negative inside or polarized (Fig. 1.4a). When the cell membrane is stimulated by the outer electric activity, the negatively charged ions move outward whereas the positively charged ions move inward, to alter the state to negative outside and positive inside. This process is called depolarization (Fig. 1.4b). At the recovery phase of cardiac muscle cells, the positively charged ions, again, move back to the outside of the cell membrane, and the negatively charged ions move to the inside. Thereby the cell returns to a state of electrical equilibrium. This process is called repolarization (Fig. 1.4c). When the depolarization wave moves toward the electrodes, the galvo-recorder would detect and record a wave that is upward (positive) (Fig. 1.5a). When the depolarization wave moves away from the electrodes, the galvo-recorder would record a downward (negative) wave (Fig. 1.5b). And when the depolarization wave has some distance from the location of electrodes, a small deflection would be recorded (Fig. 1.5c); that is one of the reasons for low voltage occurrence in the ECG.

The resting potential of cardiac muscle cell is the potential difference between the inside and outside of the cell membrane when the cardiac muscle cell is not stimulated by the outside electrical activities (at the resting state). The theory can be explained as follows: at resting state, the concentration of K+ inside the cell is 30 times higher than that of the outside (the concentration of Na+ outside the cell is 30 times higher than that of the inside). In addition, the cell membrane has a relatively high permeability to K+ and a relatively low permeability to Na+ and organic negatively charged ions A−. As a result, K+ can diffuse from the inside of the membrane to the outside under the concentration difference (concentration gradient), whereas the negatively charged ions A- cannot diffuse with K+ in the opposite direction. With the process of K+ moving out, the membrane would slowly form a potential difference which is negative inside and positive outside. Such potential difference would slow down the process of K+ further moving out, until reaching a point when the potential difference and the concentration difference of K+ balance out. Then the moving stops and this potential difference between the inside and outside of the membrane is called the resting potential (Fig. 1.6). Normally, the resting potential of cardiac muscle cells is −90 mV.

If the cell is stimulated properly on the basis of resting potential, a rapid and transient fluctuation of the membrane potential will be triggered. Such fluctuation in the membrane is called action potential. Action potential is the sign of cardiac excitation.

According to the change of potential, action potential of cardiac muscle cell can be divided into five phases (Fig. 1.7) as phase 0, phase 1, phase 2, phase 3, and phase 4. Its mechanism is as follows. When the cardiac cell receives a certain level of stimulus, the stimulus would trigger the opening of Na+ channel in the cell membrane and increase of Na+ inflow. Under the dual effect of both the electric gradient and the concentration gradient, Na+ move inside the cell membrane rapidly, resulting in a rapid increase of potential inside which is higher than the outside (+30 mV). The cell membrane is then at a positive inside and negative outside depolarized state. This process is the 0 phase of action potential. Na+ channel is fast channel, activation and inactivation both happen in very short time, and when the cell depolarization reaches a peak, the potential inside will decline with the closing and inactivation of Na+ channel, that is, the repolarization process of cardiac muscle. The repolarization process is rather slow, including phase 1, phase 2, and phase 3. At phase 1, the cause for action potential waveform is the outflow of K+. The waveform at phase 2 is relatively flattened, so it is called the plateau phase or the slow recovery state; the mechanism of this plateau is mainly the relatively balanced state of outflow (K+) and inflow (Ca2+) of ions. The action waveform of phase 3 is rather steep. With the inactivation of Ca+ channel and massive opening of K+ channel, the process of repolarization is accelerated apparently (the rapid recovery phase) and eventually recovers to the previous negative inside and positive outside state, otherwise, to the resting state.

The action potential could travel around the cell without attenuation, which is a very important feature. When a spot of cell is stimulated and produces impulse, this part of the cell membrane presents a depolarization state that is “positive inside and negative outside,” whereas the adjacent cell membrane presents a polarized state that is “negative inside and positive outside,” and the potential difference occurs between them (Fig. 1.8). The potential difference renders “local current” between the two parts. When the local current begins to move, it results in the elevation of membrane potential in the adjacent cell membrane (the potential difference between the inside and outside of the membrane deceases). When the membrane potential reaches the threshold potential, it will excite the adjacent part to form action potential. In such case, one part of excitation in the membrane can travel through the whole cell membrane by the local current, producing new action potential successively until the whole cardiac cell is excited.