Why is Action Potential Negative?

[sniderwes]

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If a reaction is spontaneous, such as when it depolarizes and goes to equilibrium, I thought it was supposed to be +Voltage. But the potential in the unpolarized state is negative voltage.

I understand that the interior of the membrane is negative compared to the outside, but how is that related to negative voltage?
For example: With half cell reactions +Voltage indicates a spontaneous reaction, -Voltage indicates nonspontaneous.

 

[Teleologist]

In redox it's positive because you're dealing with the standard hydrogen electrode.

 

[Czarcasm]

If a reaction is spontaneous, such as when it depolarizes and goes to equilibrium, I thought it was supposed to be +Voltage. But the potential in the unpolarized state is negative voltage.

I understand that the interior of the membrane is negative compared to the outside, but how is that related to negative voltage?
For example: With half cell reactions +Voltage indicates a spontaneous reaction, -Voltage indicates nonspontaneous.

The distinction you seem to be ignoring is potential versus potential difference (voltage). Potentials can be positive or negative. However, it's the potential difference (voltage) that tells us whether something is spontaneous or not. Like you said, if it's positive it indicates a spontaneous reaction and vice versa. In the case of the cell, the reason why it has a negative cell potential is because we are constantly pumping 3 Na+ out for every 2 K+ in, and therefore we have a net positive charge outside the cell and a net negative charge inside the cell. This changes during the propagation of an action potential because of the influx of sodium down it's electrochemical gradient. However, eventually the cell repolarizes back to it's normal cell potential and becomes negative again.

 

[sniderwes]

The distinction you seem to be ignoring is potential versus potential difference (voltage). Potentials can be positive or negative. However, it's the potential difference (voltage) that tells us whether something is spontaneous or not. Like you said, if it's positive it indicates a spontaneous reaction and vice versa. In the case of the cell, the reason why it has a negative cell potential is because we are constantly pumping 3 Na+ out for every 2 K+ in, and therefore we have a net positive charge outside the cell and a net negative charge inside the cell. This changes during the propagation of an action potential because of the influx of sodium down it's electrochemical gradient. However, eventually the cell repolarizes back to it's normal cell potential and becomes negative again.

Ah, so it's more like the concept of different potentials during a lightning strike: the ground is a + voltage region and the thundercloud is a - voltage region, so the thunder (electrons) go from cloud to ground.

 

[Czarcasm]

Ah, so it's more like the concept of different potentials during a lightning strike: the ground is a + voltage region and the thundercloud is a - voltage region, so the thunder (electrons) go from cloud to ground.

You got it.