Step by Step
Rest
Resting membrane potential — -70 mV
At rest, the membrane sits at -70 mV, maintained by potassium (K+) leaking out and the Na+/K+ ATPase pump actively pushing sodium (Na+) out.
Depol
Depolarization — Na+ rushes in
A sufficient stimulus opens Na+ channels, and Na+ rushes into the cell, driving the membrane potential from -70 mV up to +30 mV at its peak. Once threshold (-55 mV) is reached, the action potential fires automatically and is always the same size — this is the all-or-none principle. Stimulus intensity is instead encoded by firing frequency, not by the size of any individual action potential.
Repol
Repolarization — K+ rushes out
Na+ channels inactivate, and K+ channels open, allowing K+ to rush out of the cell, returning the membrane potential back toward -70 mV. During this phase, the absolute refractory period occurs — Na+ channels are inactivated, making a new action potential impossible no matter how strong the stimulus.
Refr
After-hyperpolarization and the relative refractory period
K+ channels are slow to close, briefly driving the membrane potential below -70 mV (to around -80 mV) before returning to rest. During the relative refractory period that follows, a new action potential is possible, but only with a larger-than-normal stimulus.
Once the membrane potential reaches the -55 mV threshold, the resulting action potential is always exactly the same size, regardless of how much stronger the triggering stimulus was — this all-or-none principle is why the nervous system encodes stimulus intensity through firing frequency instead.
Applied Walkthrough
1
A student is confused about how the nervous system can distinguish between a light touch and a firm press, if every action potential is exactly the same size regardless of stimulus strength.
2
Ask: if action potential size can't vary, how is stimulus intensity actually communicated? Through frequency, not amplitude — a stronger stimulus causes the neuron to fire action potentials more rapidly (a higher frequency), while a weaker stimulus produces the same-sized action potentials, just fired less often.
3
This is exactly what the all-or-none principle means in practice: once threshold is reached, the action potential itself is a fixed, uniform event — it's the rate of firing, not the size of any single spike, that varies with stimulus strength.
4
This distinction is important because it explains a genuinely counterintuitive fact about neural signaling — nerve cells communicate intensity information without ever varying the size of their basic signaling unit, relying entirely on how often that unit repeats.
Exam Application
Exams test the correct sequence and ion movements of each action potential phase (resting: -70mV; depolarization: Na+ in, up to +30mV; repolarization: K+ out, back toward -70mV; after-hyperpolarization: briefly below -70mV), the threshold value (-55mV), the all-or-none principle, and the difference between absolute and relative refractory periods.
⚠ Common Trap
The most common trap is assuming a stronger stimulus produces a bigger action potential. Due to the all-or-none principle, every action potential that reaches threshold is exactly the same size — stimulus intensity is instead encoded through firing frequency, not amplitude.
✓ Quick Self-Check
1. What is the resting membrane potential, and what maintains it?
-70 mV, maintained by K+ leaking out and the Na+/K+ ATPase actively pumping Na+ out.
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2. What happens during depolarization, and what is the threshold value?
Na+ channels open and Na+ rushes in, driving the potential up toward +30 mV; the threshold is -55 mV.
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3. What happens during repolarization?
Na+ channels inactivate and K+ channels open, allowing K+ to rush out, returning the potential toward -70 mV.
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4. What does the all-or-none principle mean, and how does the nervous system encode stimulus intensity instead?
Every action potential that reaches threshold is the same size; stimulus intensity is instead encoded by firing frequency.
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5. What is the difference between the absolute and relative refractory periods?
During the absolute refractory period, no new action potential is possible (Na+ channels inactivated); during the relative refractory period, a new action potential is possible but requires a larger-than-normal stimulus.
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