What is an Action Potential?
An action potential is the rapid, all-or-nothing electrical signal that travels down a neuron's axon, letting the nervous system send information over long distances in milliseconds. It results from ion channels opening and closing across the membrane.
An action potential is a brief reversal of a neuron's membrane voltage — from about −70 mV resting potential to +40 mV and back — triggered when depolarization crosses a threshold, typically around −55 mV.
Step-by-step worked examples
A neuron's membrane sits at −70 mV. A stimulus depolarizes it to −50 mV. Does an action potential fire?
Threshold is about −55 mV −50 mV is above (less negative than) threshold Yes — an action potential fires because the threshold was crossed
During depolarization, which ion rushes into the neuron and why?
Voltage-gated sodium (Na+) channels open at threshold Na+ concentration is much higher outside the cell Na+ flows in down its concentration gradient, rapidly making the inside more positive (up to +40 mV)
After the peak, the membrane potential drops below −70 mV briefly. What causes this hyperpolarization?
Voltage-gated potassium (K+) channels open and stay open slightly too long K+ flows out of the cell down its gradient This overshoots resting potential, causing a brief hyperpolarization before returning to −70 mV
Flashcards
Quick quiz
Q1.What is the approximate threshold for firing an action potential?
Q2.Which ion enters the neuron during depolarization?
Q3.What does 'all-or-nothing' mean for action potentials?
Q4.What causes repolarization back toward resting potential?
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Common mistakes
A stronger stimulus creates a bigger action potential. — Correct: Action potentials are all-or-nothing — stronger stimuli increase firing frequency, not amplitude.
Sodium and potassium channels open at the same time. — Correct: Sodium channels open first (depolarization), then potassium channels open (repolarization).
The resting potential is 0 mV. — Correct: The resting potential is about −70 mV, negative inside relative to outside.
Action potentials can occur below threshold. — Correct: Below threshold, no action potential fires at all — this is the all-or-nothing rule.
FAQ
What is an action potential?
A rapid, all-or-nothing electrical signal that travels down a neuron's axon when depolarization crosses threshold.
What is the action potential sequence?
Resting (−70 mV) → depolarization to threshold (−55 mV) → peak (+40 mV) → repolarization → brief hyperpolarization → back to resting.
What are examples of an action potential?
A sensory neuron firing when you touch something hot, or a motor neuron firing to contract a muscle.
How is an action potential triggered?
When enough Na+ channels open and depolarize the membrane past threshold, typically around −55 mV.