What is an action potential?

The brief reversal of the potential difference across the axon membrane that constitutes the nerve impulse.

What is the threshold?

The level of depolarisation, about −55 millivolts, that must be reached before an action potential is triggered.

Describe depolarisation.

Voltage-gated sodium channels open, sodium ions rush in down their electrochemical gradient, and the inside becomes positive, reaching about +40 millivolts.

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The Nervous System

The nerve impulse

3 things to know
Component 3

Overview

Resting potential of an axon, action potential and refractory period.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is the resting potential of an axon?

Key term

How is the resting potential maintained?

Key term

Why does the inside of a resting axon carry a negative charge?

Questions from this subtopic

Each one with the model answer underneath.

1
What is the resting potential of an axon?
Model answer
The potential difference across the membrane of an axon that is not conducting an impulse, about −70 millivolts inside relative to outside.
2
How is the resting potential maintained?
Model answer
The sodium–potassium pump moves three sodium ions out for every two potassium ions in, and the membrane is far more permeable to potassium than to sodium.
3
Why does the inside of a resting axon carry a negative charge?
Model answer
More positive ions are pumped out than in, potassium leaks back out, and large negatively charged proteins remain inside.
4
What is an action potential?
Model answer
The brief reversal of the potential difference across the axon membrane that constitutes the nerve impulse.
5
What is the threshold?
Model answer
The level of depolarisation, about −55 millivolts, that must be reached before an action potential is triggered.
6
Describe depolarisation.
Model answer
Voltage-gated sodium channels open, sodium ions rush in down their electrochemical gradient, and the inside becomes positive, reaching about +40 millivolts.

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