"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."

This might be a more basic, straightforward question but for some reason, there's not much info on it on the web.

I'm guessing its called the 'safety factor' because it means that not any stimulus can cause an action potential and that the cells remain at equilibrium/their un disturbed state for most of the time, but this is only a guess.

Source of Info: Guyton & Hall Medical Physiology textbook

  • $\begingroup$ Can you please quote from your book? I think you may be missing some context. $\endgroup$
    – Bryan Krause
    Apr 7, 2020 at 16:12

1 Answer 1


Action potentials propagate by depolarization of one section of the membrane causing depolarization of an adjacent section.

It is necessary that this depolarization be greater than threshold. If the amplitude of the action potential were to drop below threshold, it would fail to propagate. Additionally, it would be quite slow and inefficient.

Therefore, the action potential amplitude is far greater than the threshold. This helps speed and builds in a "safety factor" to ensure the action potential propagates - that's what your book is saying.

  • $\begingroup$ Got it, thank you so much but may I ask one last question ? What determines the maximum potential (the peak) reached in action potential ? I read in some student's notes that the the membrane potential never goes above 0 for small nerve fibres. To specify, he wrote : "Overshooting (meaning that the Inside potential becomes larger than zero) happens in large nerve fibers but not in smaller fibers and many central nervous system neurons. " Does that mean that the larger the nerve fibre the higher the peak ? I'm also uncertain whether he means nerves of the CNS overshoot or not $\endgroup$
    – Lia Ahmed
    Apr 7, 2020 at 18:07
  • $\begingroup$ @LiaAhmed That should probably be a separate question $\endgroup$
    – Bryan Krause
    May 7, 2020 at 17:36
  • $\begingroup$ Could you please explain why they use the number one? What is the ratio exactly? $\endgroup$
    – Quique
    Dec 27, 2021 at 6:39

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