DSE Biology | Neurons, Synapses and Reflex Arc Lesson-Prep Guide (Tutors)

2026-11-05

Tutor lesson-prep guides · Secondary Biology

Students can draw a neuron but explain myelin as protection. Numbers do the work here: a 100-fold speed difference and a fixed synaptic delay make structure and function one story.

Objectives — Students can: (1) state the gap between resting potential and threshold (-70 mV to -55 mV, a 15 mV rise); (2) explain how myelin raises conduction from about 1 m/s to 100 m/s; (3) label the five parts of a reflex arc and compute synaptic delay (2 synapses x 0.5 ms).

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 with channel proteins	Ions cannot cross freely
Diffusion	High to low concentration	The pump works against the gradient
Coordination	Stimulus versus response	Reflexes bypass conscious brain control

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Diagnostic	Draw one neuron	Label dendron and axon
8-22 min	Membrane potential	Resting, threshold, action potential	Sketch the trace
22-36 min	Synapse	Chemical transmission and delay	Annotate the arrow diagram
36-48 min	Reflex arc	Five parts in order	Sequence and justify
48-56 min	Question practice	Speed comparison and experiments	Follow the steps
56-60 min	Exit check	Three quick questions	Oral answers

03 Teaching script (read it out)

Scene 1: Act 1: resting potential is fifteen millivolts from firing

Write resting potential -70 mV and threshold -55 mV, a gap of 15 mV. Use the pump and leak channels: three sodium out for two potassium in leaves the inside negative. A stimulus opens sodium channels and the inside rises to threshold, triggering an all-or-none action potential. Make every student say the “three sodium, two potassium” line once.

Students mark resting, threshold, depolarisation and repolarisation on the trace.

Scene 2: Act 2: myelin is insulation and jumping, not protection

Compare myelinated and bare axons: about 100 m/s against 1 m/s, a factor of 100. Saltatory conduction regenerates the action potential only at nodes of Ranvier, saving the time spent depolarising each segment. Add synaptic delay: about 0.5 ms per synapse, so a reflex arc with 2 synapses has a floor of 1.0 ms.

Students compute delay for three arcs and explain why a reflex beats a voluntary movement.

Scene 3: Act 3: all five parts of the reflex arc are required

Sequence receptor, sensory neuron, CNS (spinal cord), motor neuron, effector. Use the withdrawal reflex and ask what happens if the sensory neuron is cut: no sensation and no response. Cut the motor neuron and sensation remains but movement is lost. Synapses are one-way, so the order cannot run backwards.

In pairs, one names a cut, the other predicts the outcome, and they correct each other.

04 Question types and worked steps

Type	Example	Steps	Answer
Membrane potential calculation	Resting -70, threshold -55	Threshold minus resting, keep the signs	15 mV
Conduction speed comparison	Myelinated 100, bare 1	Divide for the factor, then give the mechanism	100x
Reflex arc sequencing	Withdrawal reflex	Receptor to effector, number each station	Five parts in order
Experiment design	Cut the dorsal root	State variable, control and observed indicator	Test sensation and movement separately

Sanity check (teach this): Keep potentials negative: resting -70 mV, threshold -55 mV, a rise of 15 mV, not from zero. The speed factor is 100 divided by 1 equals 100, not “faster by 100 metres”. The delay is 2 synapses times 0.5 ms equals 1.0 ms, before conduction time, so a real response is always slower than this floor.

05 Independent practice (3 questions)

Question	Answer	What to watch
Resting -70 mV, threshold -55 mV: how much must it rise?	15 mV	Negative signs handled
Myelinated 100 m/s, bare 1 m/s: how many times faster?	100 times	Units correct
Two synapses at 0.5 ms each: delay?	1 ms	Conduction time added

06 Common mistakes and what to say

Mistake	Student writes	Say this
Threshold written as positive	Rising from zero to 55	“The inside starts at -70, how far to -55?”
Myelin described as protection	Only writing that it protects the neuron	“Protection does not make a signal 100 times faster.”
Reflex arc missing the CNS	Receptor wired straight to motor neuron	“Where does the signal switch?”
Synapse treated as two-way	Signals travelling backwards	“Which side are the vesicles released into?”

07 Homework (paper, section and question numbers)

Nerve conduction practice set (in-house) — Questions 1-4: potentials and speed calculations
Reflex arc experiment (in-house) — Questions 5-6: cut positions and outcomes

08 Exit check (last 5 minutes)

Question	Answer	Decision
Rise from resting to threshold	15 mV	Wrong: reteach negative values
Mechanism behind myelin speed	Saltatory conduction	Wrong: reteach nodes of Ranvier
Five parts of the reflex arc	Receptor, sensory neuron, CNS, motor neuron, effector	Wrong: reteach order

09 Tutor FAQ

Why is the action potential all-or-none

Once threshold is reached, voltage-gated sodium channels open fully, so amplitude is fixed and stimulus strength changes frequency instead.

Must a reflex pass through the brain

No. The spinal cord integrates it, and the brain receives a copy that produces the sensation, so the response can precede awareness.

Why does synaptic delay matter

It makes transmission one-way and allows summation of inputs, the basis of neural computation, at the cost of speed.

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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.