A-Level Biology 9700 / Topic 15 / A2

All or nothing, every time.

A neurone at rest holds its inside at about -70 mV. When a stimulus pushes it past threshold, voltage-gated channels open in a fixed sequence, the potential flips to about +40 mV and back in a couple of milliseconds, and the impulse moves on. Myelin makes it jump; the refractory period keeps impulses separate and one-way.

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The key idea

At rest the inside of an axon is about -70 mV relative to the outside. The Na⁺/K⁺ pump uses ATP to move 3 Na⁺ out for every 2 K⁺ in, and the membrane is more permeable to K⁺, which leaks out; the membrane is relatively impermeable to Na⁺, so the inside stays negative. A stimulus that reaches threshold opens voltage-gated Na⁺ channels: Na⁺ floods in (depolarisation, to about +40 mV). Then Na⁺ channels close and voltage-gated K⁺ channels open: K⁺ diffuses out (repolarisation), overshooting slightly (hyperpolarisation). During the refractory period the Na⁺ channels cannot reopen while the resting potential is restored. Local circuits depolarise the next region of membrane. In a myelinated neurone this happens only at the nodes of Ranvier, so the impulse jumps (saltatory conduction), much faster. Because action potentials are all the same size, a stronger stimulus is coded as a higher frequency, and the refractory period sets the maximum frequency.

Fig. 1: an action potential recorded at one point on an axon, with the phases and the refractory period

Section 01

Resting potential, then an action potential.

Press Stimulate with the Size slider at 0.8 and watch the graph and the phase readout: stimulus, depolarisation, repolarisation, hyperpolarisation, refractory period, resting. Watch the orange Na⁺ and blue K⁺ ions pass through their channels. Then drop the size below 0.4 and stimulate again.

Phase	Channels	Ion movement	Potential
resting	Na⁺/K⁺ pump working; some K⁺ channels open	3 Na⁺ pumped out, 2 K⁺ in; K⁺ leaks out	about -70 mV
depolarisation	voltage-gated Na⁺ channels open at threshold (about -55 mV)	Na⁺ diffuses in	rises to about +40 mV
repolarisation	Na⁺ channels close; voltage-gated K⁺ channels open	K⁺ diffuses out	falls back towards -70 mV
hyperpolarisation	K⁺ channels slow to close	extra K⁺ leaves	about -80 mV
refractory period	Na⁺ channels cannot reopen yet	pump and K⁺ leak restore the ion distribution	returns to -70 mV

Why the inside is negative at rest: the pump moves more positive ions out than in, there are many large negatively charged proteins inside, and K⁺ leaks out through open channels while few Na⁺ channels are open.
Positive feedback: some Na⁺ entry depolarises the membrane, which opens more voltage-gated Na⁺ channels, so the rise is very rapid once threshold is passed.
All-or-nothing: a stimulus below threshold dies away; any stimulus above threshold gives an action potential of the same size.
Propagation: Na⁺ that enters diffuses sideways inside the axon (a local circuit), depolarising the next region to threshold, so the action potential moves along.

Examiner trap

During repolarisation K⁺ moves out by diffusion through channels, not by the pump. The Na⁺/K⁺ pump keeps the concentration gradients in place, but the fast changes in an action potential are all caused by ions diffusing through voltage-gated channels.

Section 02

Jumping from node to node.

Press Send impulses and race the two neurones. The impulse in the myelinated axon arrives long before the one in the unmyelinated axon.

Myelin is an insulator: ions cannot cross the membrane where it is covered, so depolarisation can happen only at the nodes of Ranvier, where voltage-gated channels are concentrated.
Local circuits of current flow from one node to the next, so the action potential jumps from node to node: saltatory conduction.
Speed: up to about 100 m s^-1 in myelinated neurones, compared with about 1 m s^-1 in unmyelinated ones of similar diameter.
Economy: ions move only at the nodes, so less ATP is needed to restore the gradients after each impulse.
Diameter: wider axons also conduct faster, because there is less resistance to the flow of ions inside the axon.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks. Each correct answer earns XP and records this skill on your profile.

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In the action potential simulator, what happens when the stimulus size is set below 0.4?

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Which channels are open during the repolarisation phase shown in the simulator?

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Why does the myelinated impulse win the race in the saltatory conduction simulator?

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As the stimulus strength slider is increased in the refractory period simulator, what changes?

Section 03

The refractory period sets the frequency.

Slide the stimulus strength up from 0.3 and watch the impulses per second readout rise. Push it to 1 and notice the impulses get closer together but never merge. Below 0.15 there are no impulses at all.

Information is carried by frequency: every action potential is the same size, so a stronger stimulus produces more impulses per second, not bigger ones.
Maximum frequency: a new action potential cannot be set up in a region until its refractory period is over, so the refractory period sets an upper limit on how often impulses can pass.
Discrete impulses: each action potential is separated from the next, so they cannot merge.
One direction: the membrane just behind an impulse is refractory, so the local circuits can only depolarise the membrane ahead of it.

Examiner trap

A stronger stimulus does not make action potentials bigger or faster. It increases their frequency (and may stimulate more neurones). Refer to the refractory period when explaining the upper limit.

Stage 2 · Exam

Exam-style questions

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Which best explains why the resting potential is negative inside the axon?

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A drug blocks voltage-gated Na⁺ channels. What is the effect on a neurone?

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The refractory period of a neurone is 2 ms. What is the maximum possible frequency of impulses?

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Which is an advantage of saltatory conduction other than speed?

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Six original Cambridge-style questions

The resting potential and the Na⁺/K⁺ pump, depolarisation, repolarisation and hyperpolarisation, the refractory period, saltatory conduction and impulse frequency.