Because of the particularity of myocardial cells. For example, cardiomyocytes differentiated in vitro in our experiment can be seen beating in a Petri dish under a microscope. The following is the principle extracted from the internet: the physiological characteristics of myocardial cells are automatic rhythm (autonomy), conductivity, excitability and contractility. 1. Self-discipline characteristics The sinoatrial node has the highest self-discipline, about 100 beats per minute, which is the normal pacing point of heartbeat. The heartbeat rhythm controlled by sinoatrial node is called sinus rhythm. The autonomy of atrioventricular junction is second, about 50 times per minute; Purkinje cells are the lowest, about 25 times per minute. Autonomic tissues other than sinoatrial node are usually controlled by sinoatrial node, and their autonomy is concealed and cannot be shown, which is called potential pacing point.

2. Conduction characteristics Excitement has a strict order in intracardiac conduction. ① The conduction speed of excitation in the atrioventricular junction is the slowest, and the delay time is longer, which needs 0.08? 0. 10 second, this phenomenon is called atrioventricular delay. It makes atria and ventricles not excited and contracted at the same time, but excited and contracted alternately, which is beneficial to ejection and filling of ventricles. ② The conduction speed of excitement in the ventricle is the fastest, and it only takes 0.06 seconds to spread throughout the ventricle, which is beneficial to the synchronous contraction of the ventricle, thus ensuring a certain stroke volume. 3. Excitability characteristics The excitability of myocardium has periodic changes, including effective refractory period, relative refractory period and abnormal period. ① The effective refractory period is the longest, from depolarization to repolarization.

During the third stage of about -60mV. During this period, no matter how powerful the stimulus is, myocardial cells can't be depolarized and excited, that is, they can't produce action potentials. ② Relative refractory period: After the effective refractory period, the membrane potential changed from -60mV? During the period of -80mV, myocardial excitability gradually recovered, but it was still lower than normal, and action potential could only be generated by suprathreshold stimulation. ③ Abnormal period: After the relative refractory period, the membrane potential changed from -80mV? During the period of -90mV, the membrane potential level is close to the threshold potential, and subthreshold stimulation can produce action potential, which indicates that the myocardial excitability in this period is higher than normal. 4. Shrinkage characteristics ① "All or none" shrinkage. Myocardial cells are connected by intercalated discs, and the resistance is extremely low, which makes the excitement easy to transmit and conduct, and makes the myocardium contract like a functional syncytium. Once the excitement is generated, all myocardial cells contract synchronously, that is, "all or nothing" contraction. ② No tension contraction. The effective refractory period of myocardium is particularly long, which is equivalent to the whole systolic period and early diastolic period of myocardial mechanical activity. During this period, no matter what strong stimulation, it can not cause myocardial excitement and contraction, so tonic contraction will not occur. ③ It obviously depends on Ca2+ in extracellular fluid. The sarcoplasmic reticulum of myocardial cells is underdeveloped, and the amount of Ca2+ stored in the terminal pool is small. When blood Ca2+ increases, myocardial contractility increases. Conversely, myocardial contractility is weakened.