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All­or­Nothing Principle. Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the all-or-nothing principle, and it applies to all normal excit- able tissues.

Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the mem- brane. When this situation occurs, the spread of depolar- ization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This “greater than 1” requirement is called the safety factor for propagation."

What he means with "1" is he talking about milivolts? What are the values of the ratio he mentions.

Source of Info: Guyton & Hall Medical Physiology textbook

Quique
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  • Welcome to Biology StackExchange! Ratio of two identical quantities has no units because both quantities have the same units, which then cancel each other. For example: if the action potential is $60;\rm mV$ above the resting potential, and the threshold potential is $15;\rm mV$ above the resting potential, then their ratio is $60;\rm mV/15;\rm mV = 4$. – Domen Dec 27 '21 at 15:40
  • Thank you very much! – Quique Dec 27 '21 at 16:33

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The two quantities of concern are "threshold" and "peak voltage" (both relative to resting potential).

If the ratio "peak voltage"/"threshold" is less than one, then that means peak voltage is less than threshold, which by definition of threshold means an action potential will not propagate.

If the ratio is exactly = 1 then the peak voltage exactly equals threshold. This is a precarious situation: any little randomness in the system would mean the action potential could fail to propagate. That's why they're calling the ratio a "safety factor": if the ratio is, say, "2", then the depolarization is twice what you need to propagate and you could say this is quite safe. If it's 1.01 then there's a little margin built in but it's still likely you could see a failure to propagate.

For an analogy most people encounter in daily life now, let's think of the charge on your cell phone battery in the numerator and the charge you use in a day in the denominator. If the ratio "charge available"/"charge used" is less than one, your cell phone won't make it through the whole day without dying. If the ratio is 2/1 then most days you'll end the day with around 50% left; you could say you have a safety factor of 2 and if you use more than expected some days you'll still be okay.

Bryan Krause
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