The shape of the lesson
By the end of the lesson, learners can
- 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.
- Jumping from node to node: Press Send impulses and race the two neurones.
- The refractory period sets the frequency: Slide the stimulus strength up from 0.3 and watch the impulses per second readout rise.
- Rest: pump 3 Na+ out / 2 K+ in; K+ leaks out.
- Up: Na+ channels open; down: K+ channels open.
- Refractory period: discrete, one-way impulses; caps frequency.
- Myelin: depolarise only at nodes, so the impulse jumps.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, All or nothing, every time.. A neurone at rest holds its inside at about -70 mV. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: 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. Project tab 3 of the lab (Control and coordination lab, resting and action potentials) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Jumping from node to node | Press Send impulses and race the two neurones. Project tab 4 of the lab (Control and coordination lab, saltatory conduction) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: The refractory period sets the frequency | Slide the stimulus strength up from 0.3 and watch the impulses per second readout rise. Project tab 5 of the lab (Control and coordination lab, refractory period and frequency) and let a student drive it while the class predicts. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The refractory period sets the frequency, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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. |
| 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. |
Resting and action potentials: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Explain how the resting potential of a neurone is maintained.
Describe and explain the changes in membrane potential during an action potential.
An action potential trace shows the potential rising from -70 mV to +35 mV in 0.5 ms. Calculate the mean rate of depolarisation, and state which ions are responsible.
Explain why impulses travel faster along a myelinated axon than along an unmyelinated axon of the same diameter.
Explain the importance of the refractory period in nerve transmission.
A weak and a strong touch on the skin both produce action potentials of +40 mV in the same sensory neurone. Suggest how the brain can tell the difference.
Answer key and marking notes
- The Na+/K+ pump uses ATP to actively transport 3 Na+ out for every 2 K+ in.
- This sets up concentration gradients: more Na+ outside, more K+ inside.
- The membrane is more permeable to K+ (some K+ channels open) than to Na+, so K+ diffuses out.
- Together with large negative ions (proteins) inside, the inside stays negative, at about -70 mV.
- A stimulus depolarises the membrane to threshold (about -55 mV).
- Voltage-gated Na+ channels open; Na+ diffuses in, making the inside positive (to about +40 mV); positive feedback opens more channels.
- Na+ channels close and voltage-gated K+ channels open; K+ diffuses out: repolarisation.
- K+ channels are slow to close, so the potential overshoots: hyperpolarisation (about -80 mV).
- K+ channels close; the pump and K+ leak restore the resting potential during the refractory period.
- Change = 35 - (-70) = 105 mV.
- Rate = 105 ÷ 0.5 = 210 mV ms-1.
- Sodium ions diffusing in through voltage-gated Na+ channels.
- Myelin (Schwann cell membranes) is an electrical insulator, so ions cannot move across the membrane where it is present.
- Depolarisation / action potentials happen only at the nodes of Ranvier.
- Local circuits of current pass from node to node, so the impulse jumps: saltatory conduction.
- Fewer regions of membrane must depolarise along the axon, so the impulse travels faster.
- It ensures action potentials are discrete / separate and do not merge.
- It ensures impulses travel in one direction only, as the membrane behind the impulse cannot be depolarised.
- It limits the maximum frequency of impulses, so stimulus strength can be coded by frequency up to that limit.
- The strong touch produces a higher frequency of action potentials.
- It may also stimulate more receptors / more sensory neurones (the size of each action potential is the same: all-or-nothing).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
How many sodium ions and potassium ions are moved by one cycle of the Na+/K+ pump?
- A3 Na+ out and 2 K+ in
- B2 Na+ out and 3 K+ in
- C3 Na+ in and 2 K+ out
- D1 Na+ out and 1 K+ in
The graph shows an action potential. At which point are voltage-gated Na+ channels open and Na+ diffusing into the axon fastest?
- AA
- BB
- CC
- DD
What is meant by the threshold of a neurone?
- Athe potential that must be reached before voltage-gated Na+ channels open and an action potential starts
- Bthe highest potential reached during an action potential
- Cthe potential of the membrane at rest
- Dthe time during which a second action potential cannot be generated
In a myelinated axon, where do ions cross the membrane during the passage of an impulse?
- Aonly at the nodes of Ranvier
- Bonly under the Schwann cells
- Calong the whole length of the axon membrane
- Donly at the cell body and the axon terminals
Why does an impulse normally travel in only one direction along an axon?
- Athe region just behind the impulse is in its refractory period
- Bthe Na+/K+ pump pushes the impulse forwards
- Cmyelin is present only on one side of each node
- Dvoltage-gated Na+ channels are found only at the axon terminal
Two electrodes are placed 0.12 m apart on a myelinated axon. An action potential reaches the second electrode 1.6 ms after it passes the first. What is the conduction speed?
- A75 m s−1
- B7.5 m s−1
- C0.075 m s−1
- D0.19 m s−1
A neurone is treated with a respiratory inhibitor that stops ATP production. It can still fire a number of action potentials before it fails. Which explains this?
- Aaction potentials depend on existing ion gradients; without ATP the pump stops and the gradients slowly run down
- Baction potentials need ATP to open the voltage-gated channels, which takes several minutes to run out
- Cthe inhibitor blocks the K+ channels at once, so the neurone can no longer repolarise
- Dthe neurone switches to using the resting potential itself as a source of ATP
In multiple sclerosis, patches of myelin are lost from neurones. Which is the most likely effect on impulses in an affected neurone?
- Aconduction slows or fails, because depolarisation must now occur along the bare membrane between nodes
- Bconduction speeds up, because ions can now cross the membrane everywhere
- Cimpulses become larger, because more Na+ can enter
- Dimpulses travel in both directions, because the refractory period is lost
Quiz key
Each cycle uses one ATP to move 3 Na+ out and 2 K+ in, a net loss of positive charge from the cell. Reversing the numbers would make the inside more positive. Moving Na+ in and K+ out is the opposite direction to the real gradients. A 1:1 exchange would not move any net charge.
A is on the steep rising phase (depolarisation), when voltage-gated Na+ channels are open and Na+ is flooding in. At B, the peak, the Na+ channels are closing. C is on the falling phase (repolarisation), caused by K+ leaving through K+ channels. D is the hyperpolarisation, when K+ channels are slow to close.
Threshold is the depolarisation (about -55 mV) at which enough voltage-gated Na+ channels open for an action potential to follow. The peak of about +40 mV is the top of the action potential. About -70 mV is the resting potential. The time when another action potential cannot be generated is the refractory period.
Myelin stops ions crossing the membrane where it covers the axon, so depolarisation happens only at the nodes of Ranvier and the impulse jumps between them. Under the Schwann cells is exactly where ions cannot cross. Ion movement along the whole length is what happens in unmyelinated axons. The cell body and terminals are not the only places: the impulse must be regenerated all along the axon at each node.
Local circuits spread both ways, but the membrane behind the impulse is refractory (its Na+ channels cannot reopen), so only the membrane ahead can be depolarised. The pump restores gradients; it does not propel impulses. Myelin covers the internodes evenly. Voltage-gated Na+ channels are found all along the axon (at the nodes in myelinated axons), not only at the terminal.
1.6 ms = 0.0016 s, so speed = 0.12 ÷ 0.0016 = 75 m s−1, a typical value for a myelinated neurone. 7.5 comes from a power-of-ten slip converting milliseconds. 0.075 divides by 1.6 without converting ms to s. 0.19 multiplies 0.12 by 1.6 instead of dividing.
Ions move through voltage-gated channels by diffusion, using gradients already set up, so each action potential needs no ATP directly. Without ATP the Na+/K+ pump stops, so each impulse erodes the gradients a little until no more can be generated. The channels are opened by voltage, not ATP. The inhibitor targets respiration, not the K+ channels. A membrane potential cannot be turned into ATP by the neurone.
Without myelin, local circuits can no longer jump from node to node; the bare membrane, which has few Na+ channels, must be depolarised along its length, so impulses slow down or are blocked. More places for ions to cross makes conduction slower, not faster. Action potentials are all-or-nothing, so they do not become larger. Loss of myelin does not remove the refractory period.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.