S5 Biology: Nervous System and Coordination prep guide | Action potentials, synaptic delay and reflex timing

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can draw a neurone, then answer “how is a strong signal shown” with “a bigger action potential”. The key idea of this lesson is that action potentials are fixed in size and strength is coded as frequency.

Objectives — Students can: (1) describe the ion movements behind the resting and action potentials, including polarisation, depolarisation and repolarisation; (2) explain one-way transmission, synaptic delay and transmitter action; (3) calculate reflex response time from conduction speed and synapse number, and explain the refractory period.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Membrane structure	Which part of a phospholipid is hydrophilic	The head
Active transport	Ions moved per cycle of the sodium-potassium pump	3 Na+ out, 2 K+ in
Stimulus and response	Definition of a reflex	A fast response not involving conscious thought

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-12 min	Neurone structure and resting potential	Inside about 70 mV below outside	Negative inside
12-26 min	The full action potential	-70 to -55 to +30 mV	Fixed 100 mV amplitude
26-38 min	Synaptic transmission	20 nm cleft, transmitter diffuses	0.5 ms delay each
38-48 min	Reflex arc timing	1.2 m path, two synapses	About 13 ms in total
48-56 min	Refractory period and frequency coding	Refractory period 2 ms	Up to 500 impulses per second
56-60 min	Exit check	Three questions collected	Done independently

03 Teaching script (read it out)

Scene 1: Resting and action potentials: ion traffic sets the voltage

At rest the sodium-potassium pump moves 3 Na+ out for every 2 K+ in, and the membrane is more permeable to K+, so the inside sits about 70 mV below the outside (recorded as -70 mV).
A stimulus lifting the membrane potential to the threshold of -55 mV, 15 mV above rest, opens voltage-gated sodium channels. Na+ floods in and depolarises the membrane to about +30 mV; potassium channels then open, K+ leaves, the membrane repolarises and briefly overshoots before the pump restores the resting state.
The amplitude is +30 minus -70, which is 100 mV, and it is all-or-nothing. Once threshold is crossed every impulse is the same size, so stimulus strength cannot be encoded in height.
Strength is coded as frequency instead: with a 2 ms refractory period the theoretical maximum is 1000 / 2, which is 500 impulses per second.

Task: mark threshold and peak on a potential graph and work out how many mV the depolarisation spans (15 mV).

Scene 2: The synapse: one-way, delayed and able to amplify

An action potential reaches the presynaptic knob, calcium enters, vesicles fuse with the membrane, transmitter diffuses across the 20 nm cleft, binds receptors on the postsynaptic membrane, opens ion channels and starts a new action potential.
One-way traffic is guaranteed because vesicles sit only in the presynaptic knob and receptors only on the postsynaptic membrane, which is why a reflex arc never runs backwards.
Each synapse adds about 0.5 ms of delay, so the number of synapses in an arc directly sets the response time. The delay is not a defect: it lets signals summate and gives drugs a target.
Excitatory transmitters depolarise and inhibitory ones hyperpolarise the membrane, and the same transmitter can do either depending on the receptor. The transmitter does not decide the effect; the receptor does.

Task: explain one-way transmission with “vesicles in front, receptors behind” and give one example of an inhibitory transmitter.

Scene 3: Reflex timing and the refractory period: numbers make it real

A knee-jerk arc is about 1.2 m long and myelinated axons conduct at roughly 100 m/s, so conduction takes 1.2 / 100 = 12.000 s, which is 12.0 ms.
With two synapses at 0.5 ms each, the total comes to about 13.0 ms, plus the mechanical time for the muscle to contract.
The same path along unmyelinated fibres at 0.5 m/s would need 2,400 ms, which is 200 times slower. That is the value of myelin and saltatory conduction at the nodes of Ranvier, and why multiple sclerosis slows reactions.
The 2 ms refractory period does two jobs: it caps the firing rate at 500 impulses per second and it keeps each action potential travelling one way instead of bouncing back and forth.

Task: with three synapses and a 1.5 m path, find the total conduction plus synaptic time (15 ms + 1.5 ms = 16.5 ms).

04 Question types and worked steps

Type	Example	Steps	Answer
Labelling	Potential graph	Mark resting, threshold, peak, repolarisation	Amplitude 100 mV
Comparison	Myelinated versus bare axons	Saltatory conduction skips membrane segments	200 times faster
Calculation	Response time	Distance / speed, plus synaptic delays	13.0 ms
Explanation	Frequency coding	All-or-nothing amplitude, strength in frequency	Up to 500 per second

Sanity check (teach this): Keep units consistent: speeds in m/s, times finally in ms (multiply by 1000), and convert cm to m before dividing. Amplitude check: the action potential runs from -70 mV to +30 mV, so the amplitude is 100 mV; answering 30 or 70 takes only one side.
Direction check: sodium entering means depolarisation, potassium leaving means repolarisation, and the pair cannot be swapped; a rising membrane potential is always Na+ influx.
Magnitude check: 13 ms is milliseconds, far faster than a blink at 100 to 400 ms; a 13 second answer is a unit error. The 500 per second figure is a theoretical ceiling, since real firing rates are much lower.

05 Independent practice (3 questions)

Question	Answer	What to watch
Amplitude between rest and peak	100 mV	+30 minus -70
Maximum firing rate with a 2 ms refractory period	500 per second	1000 / 2
Conduction time over 1.2 m at 100 m/s	12.0 ms	1.2 / 100 x 1000
How much faster is a myelinated axon	200 times	100 / 0.5

06 Common mistakes and what to say

Mistake	Student writes	Say this
Thinking a stronger stimulus gives a bigger impulse	Amplitude scaling with stimulus	An action potential is all-or-nothing. A louder signal is not taller, just more frequent.
Swapping the Na+ and K+ movements	Calling repolarisation a sodium influx	Sodium in means depolarisation, potassium out means repolarisation. Swap them and the whole answer falls.
Forgetting the synaptic delay	Timing the conduction only	Every synapse adds about half a millisecond, so count the synapses in the arc.
Treating a reflex as conscious	Saying the brain decides a reflex	A reflex arc bypasses the brain, which is why it is fast. The brain can later modify it, but it is not the starting point.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — All chapters
Biology practice paper — Nervous coordination questions 7 to 13

08 Exit check (last 5 minutes)

Question	Answer	Decision
Amplitude of the action potential	100 mV	Correct means hormonal coordination comes next
Why synapses are one-way	Vesicles in front, receptors behind	Wrong means redo synapses
Two jobs of the refractory period	Limits frequency, keeps direction	Two errors means restart the first act

09 Tutor FAQ

Why is the action potential all-or-nothing?

Once threshold is crossed the sodium channels open in a positive feedback loop, so the impulse runs to completion regardless of the original stimulus size.

How does myelin speed conduction?

Myelin insulates the axon so ions exchange only at the nodes of Ranvier, letting the signal jump forward and cutting the time spent recharging the membrane.

How does nervous coordination differ from hormonal?

Nervous signals travel fast, act briefly and follow fixed routes, while hormones travel in blood, act slowly but persist and reach many targets.

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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.