Question

Why do some excitable cells have a target of 0mV for the action potential, even with a slight overshoot? Excitable cells such as muscles and nerves have the ability to rapidly change their membrane potential through depolarization. This mechanism is often explained by the influence of the equilibrium potential, as expressed in Nernst's equation, as illustrated in Figure 1,below. In other words, it is commonly stated that the mechanism behind the depolarization of nerve cells is as follows: Originally, potassium ion channels are open, and the equilibrium potential for potassium is the resting membrane potential.However, when sodium ion channels open, the influx of sodium ions brings the membrane potential closer to the equilibrium potential of sodium ions. However, in the case of ordinary cardiac muscle, it appears that even with a momentary slight overshoot, the height of the action potential approaches around 0V. This seems to be the case in specialized cardiac muscle as well. My question is as follows: In the case of ordinary cardiac muscle and specialized cardiac muscle, why does the height of the action potential tend to reach around 0V, even with a momentary slight overshoot, despite being different from the equilibrium potential of any ion?

Answer 1

The height of the action potential in cardiac cells tends to reach around 0V. During depolarization, positively charged ions enter the cell, raising the membrane potential. The plateau phase maintains depolarization, and repolarization occurs when potassium ions leave the cell.

Step-by-step explanation:

Contractile cells demonstrate a much more stable resting phase than conductive cells. While the initial resting potential may be different, the height of the action potential tends to reach around 0V.

During depolarization, voltage-gated channels open, allowing positively charged ions to enter the cell and raise the membrane potential. This rapid influx of ions is followed by a plateau phase, in which few potassium channels are open, and mainly slow calcium channels are open. This allows calcium ions to enter the cell and maintain depolarization.

Once the membrane potential reaches approximately zero, calcium channels close, and potassium channels open, allowing potassium ions to exit the cell and repolarize it. At this point, the membrane potential drops until it reaches resting levels again.