Nerve impulses as action potentials propagated along nerve fibres
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Question
What is the resting potential of a neuron?
Answer
About **−70 mV**, with the inside of the axon **negative** compared with the outside.
Question
Define an action potential.
Answer
A rapid, temporary **reversal of the membrane potential** (from about −70 mV up to +40 mV and back) that travels along the axon as a nerve impulse.
Question
What happens during depolarisation?
Answer
Voltage-gated **Na⁺ channels open** and **Na⁺ ions rush IN**, so the inside becomes positive and the membrane potential rises to about +40 mV.
Question
What happens during repolarisation?
Answer
**K⁺ channels open** and **K⁺ ions move OUT**, so the inside becomes negative again and the membrane potential falls back towards −70 mV.
Question
Which ion drives depolarisation, and in which direction?
Answer
**Sodium (Na⁺)**, moving **INTO** the axon.
Question
Which ion drives repolarisation, and in which direction?
Answer
**Potassium (K⁺)**, moving **OUT of** the axon.
Question
What is the threshold?
Answer
The membrane potential a stimulus must reach to trigger an action potential. Below it nothing fires; at or above it a full action potential fires.
Question
What does the all-or-none principle mean?
Answer
An action potential fires **fully or not at all** — every one is the **same size**, regardless of how strong the stimulus is.
Question
How does a stronger stimulus affect the response of a neuron?
Answer
It makes the neuron fire action potentials **more frequently** — it does **not** make each action potential bigger.
Question
How does an action potential travel along an axon?
Answer
Each region **depolarises the next region**, regenerating the impulse so it stays the **same size** all the way along.
Question
Why does a nerve impulse travel in only one direction?
Answer
The region just behind the impulse is briefly **recovering (refractory)** and cannot fire again straight away, so the impulse moves forward only.
Question
On an action-potential trace, what does the rising part show?
Answer
**Depolarisation** — Na⁺ ions entering the axon (membrane potential rising towards +40 mV).
Question
On an action-potential trace, what does the falling part show?
Answer
**Repolarisation** — K⁺ ions leaving the axon (membrane potential falling towards −70 mV).
Question
What restores the resting potential after an action potential?
Answer
The **Na⁺/K⁺ pump**, which pumps Na⁺ out and K⁺ in to re-establish the resting ion balance.
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