IB Biology HLNerves & SignallingPaper 1 & 2~13 min read
Action Potentials
An action potential is not a bigger or smaller signal depending on how hard you were poked. It either happens completely or it does not happen at all — and once it starts, it is unstoppable.
📚 What you need to know
Some ion channels are voltage gated — they open and close in response to changes in the potential across the membrane.
A stimulus opens a few sodium ion channels; Na+ enters and the inside becomes less negative.
At about −55 mV, the threshold potential, voltage gated sodium channels open and a flood of Na+ takes the membrane to about +40 mV. That is depolarisation.
Sodium channels then close and voltage gated potassium channels open, so K+ leaves and the inside becomes negative again. That is repolarisation.
Potassium keeps leaving for a moment too long, so the membrane briefly goes below resting potential — hyperpolarisation.
During the refractory period the membrane cannot be stimulated again, which makes action potentials discrete and forces the impulse to travel one way.
The all-or-nothing principle: there is no such thing as a small or large action potential.
Stimulus size is coded by the frequency of action potentials, not their size.
Impulses spread along the axon because of local currents that bring the next section to threshold.
Voltage gated channels
Not every channel in a neurone membrane is the same. Some are voltage gated, meaning they open and close in response to the electrical potential across the membrane rather than to a chemical.
Voltage gated channels are closed when the membrane is at rest.
They are the channels responsible for generating and transmitting action potentials.
Not all channels are voltage gated. Some potassium channels are simply open at rest — those are the ones that let K+ leak out to build the resting potential in the first place.
Depolarisation
When a neurone is stimulated, this happens:
A small number of sodium ion channels open in the axon membrane.
Sodium ions move into the axon down their concentration gradient (there is more Na+ outside, thanks to the pump).
This reduces the potential difference — the inside becomes less negative.
If enough sodium enters to reach about −55 mV, the threshold potential, the voltage gated sodium channels open and a much larger influx follows.
The charge reverses from −70 mV to around +40 mV, and an action potential has been generated.
The step at threshold is positive feedback and it is the reason an action potential cannot be half-hearted. Sodium coming in makes the inside less negative, which opens more sodium channels, which lets more sodium in. Once it starts it runs all the way to the top by itself.
Repolarisation and after
About 1 ms after the action potential is generated, all the voltage gated sodium channels close.
Voltage gated potassium channels open, so K+ diffuses out of the axon down its concentration gradient.
Losing all that positive charge makes the inside negative again — this is repolarisation.
The potassium channels are slow to close, so a little too much K+ leaves and the membrane potential dips below resting potential. This is hyperpolarisation.
The potassium channels then close and the sodium–potassium pumps restore the resting potential. Only then can the membrane be stimulated again.
The sodium–potassium pumps have been working the whole time, which is why the gradients are still there when everything is over.
Sodium in takes you up, potassium out brings you down, and the pumps tidy up afterwards. The whole event lasts only a few milliseconds.
The all-or-nothing principle
Action potentials are either generated or not generated, depending only on whether threshold is reached. There is no such thing as a small or large action potential.
A weak stimulus opens only a few sodium channels. The membrane does not depolarise enough to reach threshold, so no action potential is generated.
A strong enough stimulus pushes the membrane past threshold, and a full action potential follows every time.
So how does your brain know the difference between a tap and a punch? Through frequency. As the intensity of a stimulus increases, the frequency of action potentials along the neurone increases. A small stimulus might produce one action potential; a large one produces a rapid burst of them.
🧠
Like a light switch, not a dimmer
A switch is on or off — that is all-or-nothing. To signal "brighter", the neurone flicks the switch more often per second. Size never changes; only frequency does.
How the action potential travels
Generating an action potential in one patch of membrane is no use unless it moves. It spreads because of local currents.
Inside the axon
The depolarised section has a high concentration of sodium ions inside, because they have just rushed in.
This creates a concentration gradient between that section and the neighbouring resting section.
Sodium ions diffuse along the inside of the axon into the next section, making it less negative. If it reaches threshold, a new action potential starts there.
Outside the axon
Sodium has left the fluid outside the depolarised section, so there is more sodium outside the resting section than outside the depolarised one.
Sodium ions diffuse along the outside from the resting region back towards the section that has just depolarised.
These movements are the local currents. They cause a wave of depolarisation followed by repolarisation to travel along the axon, which is the propagation of the nerve impulse.
The section on the left cannot fire again yet — it is in its refractory period. That is exactly why the wave can only move to the right.
Why only one direction
The section of membrane behind the action potential has just fired, so it is hyperpolarised and in its refractory period. It cannot be stimulated again yet. The local currents spread in both directions, but only the section ahead is able to respond. That makes each action potential a discrete event and keeps the impulse travelling one way, towards the axon terminal.
Worked examples
WE 1
Describe the events of an action potential
Describe the changes in the permeability of the axon membrane that produce an action potential. (5 marks)
Point 1: the start
A stimulus opens a few sodium ion channels, so Na⁺ enters and the inside becomes less negative.
Point 2: threshold
At about −55 mV the voltage gated sodium channels open, so permeability to Na⁺ rises sharply.
Point 3: depolarisation
A large influx of sodium reverses the potential to about +40 mV.
Point 4: repolarisation
Sodium channels close and voltage gated potassium channels open, so K⁺ diffuses out and the inside becomes negative again.
Point 5: overshoot and reset
Too much potassium leaves, giving hyperpolarisation; the channels then close and the sodium–potassium pumps restore −70 mV.
Na⁺ permeability up, then K⁺ permeability up, then pumps resetthe question says permeability, so name the channels opening and closing rather than just describing the graph
WE 2
Explain the all-or-nothing principle
A neurone is given a stimulus twice: once weakly, once strongly. Explain what is meant by the all-or-nothing principle, and how the neurone signals the difference between the two stimuli. (4 marks)
Point 1: define it
An action potential is either generated fully or not at all — there is no small or large action potential.
Point 2: the weak stimulus
Too few sodium channels open, the membrane does not reach threshold, and no action potential is produced.
Point 3: the strong stimulus
Threshold is passed, so a full action potential of the same size is generated.
Point 4: coding intensity
A stronger stimulus produces action potentials at a higher frequency, and the brain reads frequency as intensity.
Same size every time; only the number per second changesthe phrase "frequency of action potentials" is the marking point — say it in those words
💡 Exam tips
Learn the three numbers: resting −70 mV, threshold −55 mV, peak +40 mV.
Say voltage gated whenever you mention the channels that make an action potential. It separates them from the leak channels.
Attach an ion to every stage: depolarisation = sodium in, repolarisation = potassium out.
If asked why impulses travel one way along an axon, the answer is the refractory period, not the synapse.
Hyperpolarisation happens because potassium channels are slow to close, so say that rather than just naming it.
⚠ Common mistakes
Saying a bigger stimulus gives a bigger action potential. It gives more of them per second.
Saying the sodium–potassium pump causes depolarisation. Depolarisation is diffusion through channels; the pump only resets things.
Mixing up hyperpolarisation and repolarisation. Repolarisation returns towards −70; hyperpolarisation overshoots past it.
Saying the refractory period stops the impulse. It stops it going backwards.
Forgetting the threshold in an explanation. Without it, nothing about all-or-nothing makes sense.
Describing local currents as electricity in the axon. They are ions diffusing sideways.
Up next: Interpreting Oscilloscope Traces — how to read the graph you have just learned, pull numbers off it, and answer the skills questions that come with it.
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