Action Potential
Joe Wolfensohn
Teacher
Contents
Recall Questions
This topic requires prior knowledge to resting potential. You can test your knowledge on this below.
What is the resting potential of a neurone?
Approximately –70 mV, with the inside of the axon more negative than the outside.
Which protein maintains the resting potential, and how?
The sodium–potassium pump, by actively transporting 3 Na⁺ out and 2 K⁺ in, using ATP.
Why is the inside of the axon negative at rest?
More positive ions (Na⁺) are pumped out than in, and K⁺ diffuses out, creating a net negative charge inside the axon.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains action potential or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Is an Action Potential?
An action potential is a temporary reversal of charge across the axon membrane, caused by a stimulus that opens voltage-gated ion channels. It allows electrical impulses to travel rapidly along neurones.
Stages of an Action Potential
1. Resting potential (–70 mV)
- Maintained by the sodium–potassium pump and K⁺ diffusion out of the neurone. Remember, the membrane is more permeable to K⁺ than Na⁺.
- The neurone membrane is polarised.
2. Depolarisation (Stimulus: +40 mV)
- A stimulus causes some Na⁺ channels to open.
- Na⁺ diffuses in, and begins to depolarise the membrane, making the inside of the neurone less negatively charged.
- If threshold is reached (typically around –55 mV), more Na⁺ channels open, causing a rapid influx of Na⁺. These Na⁺ channels are voltage gated. This is an example of positive feedback.
- Membrane becomes positive inside.
3. Repolarisation
- At around +40 mV, Na⁺ channels close and voltage-gated K⁺ channels open.
- K⁺ diffuses out, making inside negative again.
4. Hyperpolarisation
- Too many K⁺ ions diffuse out.
- Membrane potential becomes more negative than resting (around –80 mV).
- K⁺ channels close, and resting potential is restored.
5. Return to Resting Potential
- The sodium–potassium pump re-establishes resting ion distributions.
- Membrane returns to –70 mV, ready for the next impulse.
Summary
- Resting potential.
- Stimulus causes sodium ion channels to open and sodium ions diffuse in.
- Threshold is reached and more voltage gated sodium ion channels open.
- Depolarisation is occurring as inside the neurone becomes positively charged.
- Sodium ion channels close.
- Potassium ion channels open and potassium ions diffuse out.
- Repolarisation is occurring as inside the neurone becomes negatively charged.
- Hyperpolarisation as too many potassium ions leave the neurone.
- The resting potential is re-established by the sodium-potassium pump.
Transmission of an Action Potential
- Na⁺ enters one region when Na⁺ channels open. The Na⁺ diffuse sideways in the neurone axon, triggering adjacent voltage-gated Na⁺ channels to open.
- The impulse moves along the axon like a wave of depolarisation.
- Behind the wave, repolarisation restores resting potential.
- The refractory period ensures unidirectional transmission and limits frequency of impulses.
Saltatory Conduction (in Myelinated Neurons)
- In myelinated neurones, action potentials occur only at nodes of Ranvier.
- The impulse jumps from node to node – saltatory conduction.
- This is much faster than transmission in non-myelinated neurones where depolarisation has to occur along the full length of the axon membrane.
Summary Table
| Stage | Event | Ion Movement |
| Resting Potential | –70 mV maintained by Na⁺/K⁺ pump and K⁺ diffusion out. | 3 Na⁺ out, 2 K⁺ in (ATP used). Some K⁺ diffuse out. |
| Depolarisation | Na⁺ channels open. | Na⁺ in. |
| Repolarisation | Na⁺ channels close, K⁺ channels open. | K⁺ out. |
| Hyperpolarisation | Too much K⁺ leaves. | K⁺ continues to exit. |
| Recovery | Na⁺/K⁺ pump restores resting state. | 3 Na⁺ out, 2 K⁺ in (ATP used). |
Key Terms
- Action potential: A temporary reversal of membrane potential from negative to positive inside.
- Depolarisation: Entry of Na⁺ causes the inside of the axon to become less negative.
- Repolarisation: Exit of K⁺ restores a negative charge inside.
- Hyperpolarisation: Overshoot in repolarisation; inside becomes too negative.
- Refractory period: Period after an action potential when the membrane can't respond to a new stimulus.
- Saltatory conduction: Jumping of action potentials between nodes of Ranvier in myelinated neurones.
No answer provided.
Exam Tip
Be specific with ion names, channels, and directions of movement.
Vague answers like "ions move" or "channels open" lose marks.
Mention Na⁺ channels, including voltage gated Na⁺ channels, and Na ⁺ diffusing into the neurone during depolarisation.
No answer provided.
The graph shows changes in membrane potential that occur during an action potential.
It also shows changes in the permeability of the axon membrane to sodium and potassium ions.
Explain what causes:
- The change in membrane potential between points A and B
- The change in membrane potential between points B and C
(5 marks)
A to B:
Sodium channels open
Sodium ions enter
B to C:
Sodium channels close
Potassium channels open
Potassium ions leave
Allow 1 mark, in either section of answer, for the movement of ions by diffusion / from high to low concentration
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!