Biology · Introductory biology · Worked example
Decide which stimuli fire a neuron
A neuron rests at −70 mV and has a threshold of −55 mV. Three separate stimuli depolarize it by 10 mV, 15 mV and 30 mV. Which of them fire an action potential, and how do the action potentials compare? At the peak the membrane reaches +30 mV: how far does the voltage change from rest to peak?
Find the voltage each stimulus reaches
Add each depolarization to the resting potential: −70 + 10 = −60 mV, −70 + 15 = −55 mV and −70 + 30 = −40 mV.
Compare with threshold
−60 mV is below threshold, so that stimulus fades away without firing. −55 mV reaches threshold and −40 mV passes it, so both fire.
Compare the two action potentials
The action potential is all-or-none: both rise to the same peak of +30 mV, whatever the size of the stimulus that started them.
Find the swing from rest to peak
The voltage climbs from −70 mV to +30 mV.
Result
The 10 mV stimulus brings the membrane only to −60 mV, below threshold, so it does not fire. The 15 mV and 30 mV stimuli reach −55 mV and −40 mV, and each fires an identical action potential that peaks at +30 mV, 100 mV above rest.
Your turn
A drug blocks voltage-gated K⁺ channels. Which phase of the action potential changes, and how?
Show the answer and explanation
Repolarization: it slows, so the action potential lasts longer.
K⁺ flowing out through voltage-gated channels returns the membrane toward rest. With them blocked, only leak channels bring it back, and much more slowly.
Keep exploring
The neuron opens in Neuron & action potential with the lesson’s trace. Under Trace, choose the Hodgkin–Huxley model to fire it with your own current pulses: a weak pulse fades, a strong one fires the same spike, and fewer K⁺ channels slow repolarization.
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