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?"
Fig. 5 Quoted from this book, written in Japanese;https://www.amazon.co.jp/dp/4784931813/
We apologize that some of the annotations in Fig. 5 are in Japanese, but we could not find a clearer figure. Here, English translation of the Japanese annotations in the upper panel of Fig. 5;
- 膜電位:Membrane potential
- オーバーシュート:Overshooting (Overpolarization)
And, here is for the Japanese annotations in the upper panel of Fig. 5;
- 電流量： Intensity of the current, it means the strength of Ion each current;
For example"ナトリウム電流"（Blue curve) represents the Intensity of the current due to sodium ions.
- ↑外向き：Outward. That means the direction from the inside of the cell toward the outside,
- ↓内向き：Inward. Direction from outside to inside of cell.
This Fig. 5 is essentially equivalent to Fig. 3, but Fig.5 gives the impression that "the Ca ion current seems insufficient to balance the K ion current at Phase 2" in the two phases.