DSE Biology | Nervous System Tutor Prep Guide

DSE Biology | Nervous System Tutor Prep Guide | Reflex Arc and Synapses

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can draw a reflex arc, but ask why a reflex beats a voluntary action and they answer that it skips the brain, then go silent when asked what a synapse does. They treat nerves as wires without gates. This lesson starts from the timing numbers so students calculate the delay themselves.

Objectives — Students can: (1) label the five parts of a reflex arc and mark the direction; (2) compute reflex time from path length and conduction speed; (3) explain depolarisation and threshold; (4) state why synaptic transmission is one-way and why it delays the signal.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Cell membrane	phospholipid bilayer and channels	ions cross through channels
Diffusion and active transport	concentration gradients	sodium is pumped out
Stimulus and response	what counts as a stimulus	light, sound, pressure

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Neurone structure	dendrites in, axon out	myelin speeds conduction
8-20 min	Reflex arc	five parts in order	path 0.9 m
20-30 min	Reflex time	length divided by speed	36 ms
30-42 min	Action potential	threshold and depolarisation	-70 to -55 mV
42-52 min	Synapse	chemical, one-way	delay 0.5 ms times 2
52-60 min	Exit check	collect and mark	independent

03 Teaching script (read it out)

Scene 1: First move: draw the arrows before the cells

Five parts: receptor, sensory neurone, central nervous system (spinal cord), motor neurone, effector. Label every arrow with a direction, because the most common exam question is which function is lost when one segment is cut. With a path of 0.9 m at 25 m/s the conduction time alone is 0.9 divided by 25, which is 36 ms. Ask: if the sensory neurone is cut before the cord, is there still sensation? (No, the signal never reaches the centre.)

Task: write the five parts on paper and join them with arrows.

Scene 2: Action potential: the threshold is a gate, not a slope

Resting potential is about -70 mV, negative inside. A stimulus opens sodium channels, the potential difference narrows, and at -55 mV an action potential fires, so depolarisation is 15 mV. The key phrase is all or nothing: above threshold the response is full size, below it there is nothing, and a stronger stimulus only raises the frequency. Potassium then leaves to repolarise, and the sodium-potassium pump restores the ions using ATP. Reminder: never write that current flows through the nerve; write that the action potential propagates. Ask: why does a stronger stimulus not give a bigger action potential? (All-or-nothing law.)

Scene 3: Synapse: delay is the cost, one way is the guarantee

The presynaptic membrane releases neurotransmitter, which diffuses across the cleft and binds receptors on the postsynaptic membrane. Each synapse costs about 0.5 ms, and this arc has 2 synapses, so total time is 36 plus 0.5 times 2, giving 37 ms. Transmission is one-way because transmitter is stored only on the presynaptic side. Drugs and toxins that block receptors are the standard exam context for asking whether the signal continues. Compute: with four synapses the total becomes 38 ms.

04 Question types and worked steps

Type	Example	Steps	Answer
Label a reflex arc	diagram with missing labels	start at the receptor and count	receptor, sensory, centre, motor, effector
Compute reflex time	length and speed given	length divided by speed plus synaptic delay	37 ms
Explain action potential	threshold or all-or-nothing	channels first, then potential	-70 to -55 mV
Damage analysis	one segment cut	follow the arrows	cut motor means sensation but no movement

Sanity check (teach this): Time is positive, so writing 0.036 with ms as the unit means the factor of 1000 was dropped. A reflex time belongs between 20 and 200 ms; over a second suggests a voluntary action or a slip. Potential is in mV, negative at rest and positive after depolarisation. More synapses mean slower, so saying more synapses means faster is reversed. Direction check: signals always run from receptor to effector.

05 Independent practice (3 questions)

Question	Answer	What to watch
Conduction time for 0.90 m at 25 m/s	36 ms	length divided by speed
Total time with two synapses	37 ms	add 0.5 ms twice
Change from resting to threshold	15 mV	-70 to -55

06 Common mistakes and what to say

Mistake	Student writes	Say this
Saying reflexes skip the brain so they are fast	wrong reason	A reflex bypasses the cortex but still passes the spinal cord, and it is fast because the path is short.
Believing more synapses means faster	reversed	Each synapse waits for diffusion, so more synapses means slower.
Scaling action potential with stimulus strength	breaks all-or-nothing	Above threshold the response is full size, a stronger stimulus only fires more often.
Forgetting the factor of 1000	answers 0.036 ms	Seconds to milliseconds needs a factor of 1000, so 0.036 s is 36 ms.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — questions 3 and 6 are the same type
All prep guides — other chapters in this subject
DSE Biology practice papers — nervous system set, questions 1 to 8

08 Exit check (last 5 minutes)

Question	Answer	Decision
Five parts of a reflex arc	receptor, sensory neurone, centre, motor neurone, effector	three correct allows hormones next
Threshold of an action potential	-55 mV	one wrong means reteach
Why synapses are one-way	transmitter only on the presynaptic side	two or more wrong means restart

09 Tutor FAQ

Must a reflex go through the spinal cord?

Most skeletal reflexes are coordinated in the cord, but some such as the pupil reflex are in the brainstem.

What does myelin do?

It makes the action potential jump between nodes, which raises conduction speed sharply.

Is a neurotransmitter a hormone?

No, it acts locally across a cleft, while hormones travel in the blood.

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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.