Why is the ordinary cardiac muscle’s target value of the action potential 0mV?

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?"

Fig.1 https://openbooks.lib.msu.edu/neuroscience/back-matter/images-of-animations/

Fig.2 https://www.youtube.com/watch?app=desktop&v=JaL8lYPMscc

Fig.3 https://www.backoftheboxtalks.com/resources/ventricular-action-potential

Fig.4 https://www.youtube.com/watch?v=m-pzyhVmRJM

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
• 脱分極：Depolarization
• オーバーシュート:Overshooting (Overpolarization)
• 再分極：Repolarization

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.

Answer 1

Membrane potentials are determined by the combination of all the membrane-permeable charges, not single ions.

You can use the Goldman Equation to calculate the reversal potential for any combination of ion permeabilities and concentrations.

Sodium and calcium will be the primary contributors to the action potential height, but the kinetics of all the involved voltage gated channels are relevant, too: the Goldman Equation gives you a steady state/equilibrium voltage. The actual membrane potential will move towards this voltage, at a rate depending on the membrane capacitance, but need not reach a given equilibrium potential before permeabilities change which then changes the equilibrium.

I would recommend looking at a plot of the permeabilities to different ions over time and think about these relative to the Goldman Equation. Here's an example plot:

from Sigg et al., 2009 via http://www.vhlab.umn.edu/atlas/conduction-system-tutorial/cardiac-action-potentials.shtml

Answer 2

Just for a Comment;

I know the Goldman equation (https://en.wikipedia.org/wiki/Goldman%E2%80%93Hodgkin%E2%80%93Katz_flux_equation), and I asked this question because I thought maybe this does not explain "0mV" at all or more specified explanations are required;

In fact, the Goldman-Hodgkin-Katz flux equation cannot be adapted to the divalent ion Ca2+, at least in the following formula. And the contribution of Ca2+ appears to not be negligible in this discussion.

At first, I did not quote the Fig.5,belowt because it was written in Japanese, but I additionally quoted this figure as Fig5. Fig.5 shows the changes of each ion current over time, together with the action potentials of the myocardium and the timescale. If you read carefully, you should be able to read the same story from Fig. 3, but Fig.5 should be easier to understand.

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
• 脱分極：Depolarization
• オーバーシュート:Overshooting (Overpolarization)
• 再分極：Repolarization

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.

Fig.5. Quoted from the book, written in Japanese;https://www.amazon.co.jp/dp/4784931813/

(1) In the Fig. 5, a spike-like inward Na current occurs during phase 0 and phase 1. In other words, the Na current suddenly becomes strong and stops immediately. It should be clear that this is the true nature of the depolarization.

The reason why the Na current stops long before it reaches its own equilibrium potential is probably because the Na ion channel closes when the membrane potential reaches about 30 mV. Am I right?

(2) Then, the Ca current is generated just after the Na ions start up, and

• since the equilibrium potential of the Ca ions is in the negative direction, and
• since the period when the Ca current is flowing roughly coincides with the phase 1 period,

we can also see that the Ca current is probably the main cause which compensates "phase 0 -induced overshoot" to 0 mV during the phase 1.

(3) The reason why the Ca current stops at a membrane potential near 0 mV is probably due to the property of the Ca ion channel to close at a membrane potential near that level. Perhaps. Am I right?

(4) And the biggest question is why the membrane potential is kept at 0V in phase 2, even though the K current is flowing and no current is depicted in the figure that is antagonistic to the K current, at least in phase 2? but I really don't understand this.

• Perhaps there is another current, such as Cl ions, if so, it may explained from the Goldman–Hodgkin–Katz flux equation(https://en.wikipedia.org/wiki/Goldman%E2%80%93Hodgkin%E2%80%93Katz_flux_equation)
• or perhaps the K current in the other direction is flowing from a different ion channel.

This Fig. 5 is essentially equivalent to Fig. 3, but Fig.5 gives us the impression that "the Ca ion current seems insufficient to balance the K ion current　at Phase 2.