(Solved): Phase 0 : Depolarization Upon stimulation, voltage-gated Na+channels open and initiate rapid depola ...

Phase 0 : Depolarization Upon stimulation, voltage-gated $${\mathrm{Na}}^{+}$$channels open and initiate rapid depolarization via influx (inwara flow) of sodium ions. Because the cytosol is electrically more negative than extracellular fluid and the $${\mathrm{Na}}^{+}$$concentration is higher in the extracellular fluid during resting phase, $${\mathrm{Na}}^{+}$$rapidly crosses the cell membrane and with a few milliseconds, the cell becomes positively charged (approximately $$+20\mathrm{mV}$$ ). As soon as, electrical potential reaches $$+20\mathrm{mV},{\mathrm{Na}}^{+}$$channels automatically inactivate and $${\mathrm{Na}}^{+}$$inflex stops. Phase 1: Early repolarization Membrane depolarization activates the voltage-gated $${\mathrm{K}}^{+}$$channels and a brief efflux of potassium repolarizes the cell slightly. An outward flow of $${\mathrm{K}}^{\text{* }}$$ returns the membrane potential to approximately 0 $$\mathrm{mV}$$. Phase 2: The plateau phase Almost simultaneously with the opening of potassium channels in phase 1 , persistent L-type $${\mathrm{Ca}}^{2+}$$ channels open and there is a small, constant inward current of $${\mathrm{Ca}}^{2?}$$. The influx of calcium is steady and give rise to the plateau phase whose long duration explains why the vast majority of the ventricular myocardium contracts simultaneously (at some point during the contraction). Phase 3: Repolarization Calcium ( $${\mathrm{Ca}}^{2^{?})}$$ ) channels are gradually inactivated (closed) and potassium $$\left({\mathrm{K}}^{+}\right)$$channels continue working and the efflux of potassium repolarizes the cell now exceeding $${\mathrm{Ca}}^2$$ inflow, brings membrane potential back towards resting potential of $$?90\mathrm{mV}$$ to prepare the cell for a new cycle of depolarization. As mentioned before, concentrations of $${\mathrm{Na}}^{+}$$and $${\mathrm{K}}^{+}$$on both sides of myocyte membrane are recovered by means of $$\mathrm{Na}/\mathrm{K}$$ pump. $${\mathrm{Ca}}^{2+}$$ return to extracellular space by sodium calcium exchanger. 6. What phase and event are associated with heart contraction?