S5 Biology: Nervous Coordination — Tutor Lesson Plan (60 min, resting potential to reflex, four errors)

2026-11-02

Tutor lesson-prep guides · Secondary biology (DSE)

Students can label a neurone and still lose marks on the graph. This lesson treats the action potential as one story with three fixed numbers, so the labels arrive after the student already knows what the curve is doing and why.

Objectives — the student can describe a neurone as a cell with dendrons, an axon and terminals, explain the resting potential as a charge difference of about minus 70 millivolts maintained by the sodium-potassium pump, describe depolarisation, repolarisation and the refractory period, explain saltatory conduction and why myelination speeds transmission, and explain transmission across a synapse including the delay.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Diffusion	Which way do sodium ions move when a channel opens?	Down the concentration gradient, into the axon
Cell membrane structure	What kind of molecule is the membrane made of?	A phospholipid bilayer with proteins in it
Negative numbers	What is the difference between minus 70 and minus 55?	15 millivolts

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Hook	A hand pulled from a hot pan before the pain arrives	Name the pathway
8-18 min	Scene 1	Neurone parts and the resting potential	Say which side is positive
18-30 min	Scene 2	Action potential: depolarisation and repolarisation	Read the peak off the graph
30-40 min	Scene 3	Refractory period and all-or-nothing	Compute the maximum frequency
40-52 min	Scene 4	Saltatory conduction and the synapse	Explain the 0.5 ms delay
52-60 min	Exit check	Three items in section 08	Marked at once

03 Teaching script (read it out)

Scene 1: A neurone is a cell with a wire

Draw one dendron, one axon, one cell body and the terminals, then say what each part does in one sentence. Dendrons bring information in; the axon carries it away; the terminals pass it on. Say out loud that a nerve is a bundle of axons, not a single wire, because a student who thinks a nerve carries one signal will not understand why a reflex can be fast and simple at the same time.

Ask: “Which part of the neurone does the myelin sheath wrap?” The axon only, and that detail decides the next twenty minutes.

Scene 2: The membrane at rest is a battery

The sodium-potassium pump moves three sodium ions out for every two potassium ions in, so the inside ends up negative, about minus 70 millivolts. The membrane is more permeable to potassium than to sodium, which is why the resting potential sits near the potassium equilibrium potential. Write the number on the board once: minus 70. Then the threshold: minus 55, which is a change of only 15 millivolts, and that small number is the whole reason a signal is easy to start and impossible to stop halfway.

Warn: the pump does not fire the impulse. It only maintains the resting state; the impulse comes from voltage-gated channels.

Scene 3: One impulse, three numbers

At threshold the voltage-gated sodium channels open and sodium enters fast, so the membrane reaches about plus 40 millivolts. Then sodium channels close and potassium channels open, potassium leaves and the voltage falls back past minus 70 to about minus 80 before settling. That overshoot is hyperpolarisation. The refractory period is about 2 milliseconds, so at most 500 impulses per second, which is why the frequency of a signal can only be changed by changing the rate, not the size of each impulse. All-or-nothing means exactly that: a stronger stimulus gives more impulses per second, never a taller one.

Ask: “Why can two impulses not overlap?” Because the membrane is unexcitable during the refractory period, so the signal travels one way.

Scene 4: Speed and the synapse

Myelination wraps the axon in fatty insulation with gaps about a millimetre apart, so the impulse jumps from gap to gap: saltatory conduction. On a one-metre axon a myelinated fibre takes about 10 milliseconds at 100 metres per second, while a bare fibre at 1 metre per second takes 1000 milliseconds, a hundredfold difference. At the synapse the impulse cannot jump, so it releases a neurotransmitter, which diffuses across the cleft and binds to receptors, costing about 0.5 milliseconds per synapse. Three synapses in a reflex arc therefore cost 1.5 milliseconds of pure delay.

Ask: “Why is the delay useful in a reflex?” It allows summation and inhibition, which is how the nervous system decides whether to pass the signal on.

04 Question types and worked steps

Type	Example	Steps	Answer
Resting potential	Explain why the inside of an axon is negative at rest.	Sodium-potassium pump, three out for two in, plus greater potassium permeability	About minus 70 millivolts
Graph reading	Describe what happens between threshold and the peak.	Voltage-gated sodium channels open, sodium enters	Rises to about plus 40 millivolts
Frequency	A neurone has a 2 millisecond refractory period. Find the maximum impulse frequency.	One impulse per refractory period: 1000 divided by 2	500 per second
Synapse	Explain why a signal crosses a synapse in one direction only.	Transmitter is released only on the presynaptic side and receptors sit only on the postsynaptic membrane	One-way valve

Sanity check (teach this): Every answer is checked against three fixed numbers: resting about minus 70, peak about plus 40, refractory about 2 milliseconds. A student answer that puts the peak near zero, or claims a stronger stimulus makes a taller impulse, is wrong even if the wording sounds biological, and the fix is always the same: go back to which channels are open at that moment.

05 Independent practice (3 questions)

Question	Answer	What to watch
State the resting potential and the threshold, and the difference between them.	Minus 70 and minus 55 millivolts, a difference of 15 millivolts.	Small change, big consequence
Explain why the membrane hyperpolarises after the peak.	Potassium channels stay open a little longer, so potassium keeps leaving.	Overshoot to about minus 80
A myelinated axon conducts at 100 metres per second. Find the time to cross a 0.5 metre axon.	5 milliseconds.	Half the length, half the time
Give two differences between transmission along an axon and across a synapse.	Along the axon it is electrical and fast; across a synapse it is chemical, slower, and one-way.	Both answers must name the mechanism

06 Common mistakes and what to say

Mistake	Student writes	Say this
Saying the pump creates the impulse	The pump maintains the resting potential; voltage-gated channels make the impulse	Ask what opens at threshold
Drawing a taller peak for a stronger stimulus	All-or-nothing: frequency codes strength, amplitude does not	Ask what changes when you hit harder
Forgetting the refractory period	It sets the maximum frequency and makes conduction one-way	Ask why impulses cannot overlap
Treating the synapse as electrical	Transmitter, diffusion, receptors, and a delay of about 0.5 ms	Ask what the delay is made of

07 Homework (paper, section and question numbers)

Secondary biology chapter index — Read this chapter with the homeostasis and the coordination chapters
HKDSE Biology Paper 2 practice paper — Nervous system questions; redo section 05 questions 1 to 4 of this guide

08 Exit check (last 5 minutes)

Question	Answer	Decision
State the resting potential and one thing that maintains it.	About minus 70 millivolts, maintained by the sodium-potassium pump.	All correct: move to the graph
What is the maximum impulse frequency for a 2 millisecond refractory period?	500 per second.	Divide 1000 by the period
Why does a synapse slow the signal down?	The transmitter must diffuse across the cleft and bind to receptors.	Name diffusion, not distance

09 Tutor FAQ

Do students need the Nernst equation?

No. DSE asks for the mechanism and the approximate values, so minus 70, plus 40 and 2 milliseconds carry the marks.

Should they memorise plus 40?

Yes, as an approximate peak. It makes graph questions checkable, and it stops the common error of drawing the peak at zero.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.