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1、Electrophysiology of MuscleSkeletal a Na+ current. The ability of the cell to open the fast Na+ channels during phase 0 is related to the membrane potential at the moment of excitation. If the membrane potential is at its baseline (about -85 mV), all the fast Na+ channels are closed, and excitation

2、will open them all, causing a large influx of Na+ ions. If, however, the membrane potential is less negative, some of the fast Na+ channels will be in an inactivated state insensitive to opening, thus causing a lesser response to excitation of the cell membrane and a lower Vmax. For this reason, if

3、the resting membrane potential becomes too positive, the cell may not be excitable, and conduction through the heart may be delayed, increasing the risk for arrhythmias,Phase 1,Phase 1 of the action potential occurs with the sudden inactivation of the fast Na+ channels. The small transient down-shoo

4、t (more negative) is due to the movement of K+ and Cl- ions. K+ is moving out and Cl- is moving into the cell making the interior temporarily more negative. Both ions are directionally moving in accordance with their respective electrochemical gradients. They are moving through their respective pass

5、ive leak channels,Phase 2 the plateau phaseThe basis of the prolonged absolute refractory period,As the voltage of the cardiac muscle cell changes it causes the slower later opening of L- Calcium channels and also the opening of some special slow delayed rectifier potassium channels (not the usual p

6、assive leak potassium channels or the usual voltage dependent potassium channels seen in neurons and skeletal muscle cells). This plateau phase of the cardiac action potential is sustained by a balance between inward movement of Ca2+ through L-type calcium channels and outward movement of K+ through

7、 the slow delayed rectifier potassium channels,Phase 3,During phase 3 (the rapid repolarization phase) of the action potential, the L-type Ca2+ channels close, while the slow delayed rectifier K+ channels remain open. This ensures a net outward current, corresponding to negative change in membrane p

8、otential, thus allowing more types of K+ channels to open. These are primarily the rapid delayed rectifier K+ channels. This net outward, positive current (equal to loss of positive charge from the cell) causes the cell to repolarize. The delayed rectifier K+ channels close when the membrane potenti

9、al is restored to about -80 to -85 mV. The Na/K pump helps restore the RMP Phase 4,Phase 4 is the usual Resting Membrane Potential,Cardiac Pacemaker Action Potential,Pacemaker cells,The pacemaker cells have automatic rhythm they spontaneously in and of themselves can fire action potentials. Each pac

10、emaker structure has a different automatic firing rate. SA node 60 80 natural pacemaker of the heart because it has the fastest firing frequency AV node 40 60 spontaneous action potentials per minute Bundle of HIS 20 40 action potentials per minute Bundle Branches 10 20 action potentials per minute

11、It is the pacemaker cells that cause the adjacent cardiac muscle cells to reach threshold voltage thus initiating the cardiac muscle cell action potential,Events of the Pacemaker Action Potential,The rate at which the pacemaker potential reaches threshold is the key to understanding the differences in the rate of firing of the different pacemakers,Putting it all Together,Threshold,Subthreshold stimulusweak local depolarization that does not reach thresho

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