Mastering Neuronal Communication (OCR A 5.1.3 a-c): Common OCR A Level Biology Questions Answered
Prior Knowledge to Recap
Before diving into neuronal communication, make sure you're confident with these key concepts:
• Cell membrane structure – understanding the phospholipid bilayer, channel proteins, and carrier proteins is essential for grasping how ions move across neurone membranes
• Diffusion and active transport – knowing how substances move down concentration gradients (diffusion) and against them (active transport) will help you understand resting and action potentials
• Protein structure – particularly how changes in tertiary structure affect protein function, which is crucial for understanding channel proteins in neurones
• Energy and ATP – comprehending how ATP provides energy for active processes like the sodium-potassium pump
• Specialised cells – recognising how cells adapt their structure to their function will help you appreciate the unique features of different neurone types
Links to GCSE Content
This topic builds directly on your GCSE Biology knowledge:
• Nervous system basics – you'll have learned about the central nervous system (CNS), nerves, and simple reflex arcs at GCSE; A level explores the cellular mechanisms behind these processes
• Homeostasis and responses – GCSE introduced how organisms detect and respond to stimuli; now you'll understand exactly how receptors convert stimuli into electrical signals
• The reflex arc – you studied stimulus → receptor → coordinator → effector → response at GCSE; A level examines the neurones and synapses involved in much greater detail
Common Question Types and How to Answer Them
Let me walk you through five frequently asked questions from past OCR papers, showing you exactly what examiners are looking for.
Question 1: Identifying Neurone Types
Question: Which of the diagrams shows a neurone that connects to an effector?
Answer: C ✓
Why this is correct: A neurone that connects to an effector is a motor neurone. The key identifying features are:
Multiple dendrites branching from the cell body (this receives signals from other neurones)
The cell body is located at one end, typically in the CNS
A long axon extends from the cell body towards the effector
The multipolar structure (many branches from the cell body) is characteristic of motor neurones
Common mistake: Students often choose B, thinking the branched endings connecting to something means it's a motor neurone. However, look carefully at where the cell body is positioned and the overall structure - C shows the classic motor neurone arrangement.
Top tip: Learn to recognize the three neurone types instantly:
Sensory: Cell body positioned along the axon (like D)
Relay: Lots of dendrites, short axon, entirely in CNS (like C, but shorter)
Motor: Cell body at one end with dendrites, long axon (C is the answer!)
Question 2: Understanding the Pacinian Corpuscle
Answer: A ✓
Why this is correct: This demonstrates understanding of how sensory receptors work:
A stimulus produces a generator potential (this is the initial depolarisation at the receptor)
If this generator potential exceeds the threshold value, an action potential is generated
This follows the "all-or-nothing" principle
Why the others are wrong:
B is incorrect because action potentials are always the same size - this is the all-or-nothing law! Stronger stimuli don't make bigger action potentials.
C is wrong because pressure makes the membrane MORE permeable to sodium ions, not less (sodium ions need to enter to cause depolarisation)
D is incorrect because the Pacinian corpuscle converts mechanical energy (pressure) into electrical energy (action potentials), not chemical energy
Crucial concept: Stimulus strength is coded by the frequency of action potentials, never their amplitude. Each action potential is identical in size!
Question 3: Neurone Structure Identification
Answer: C ✓
How to work this out systematically:
Step 1: Identify the neurone type
Look at the cell body position - it's positioned along the length of the neurone (in the middle)
This is the defining feature of a sensory neurone
The direction of travel (from receptor toward CNS) confirms this
Step 2: Identify Structure 7
Structure 7 is on the receptor side, carrying impulses toward the cell body
This is a dendron (carries impulses TO the cell body)
Step 3: Identify Structure 8
Structure 8 carries impulses away from the cell body toward the CNS
This is an axon (carries impulses AWAY FROM the cell body)
Therefore: C is correct - dendron, axon, sensory neurone
Key distinction you MUST know:
Dendron/Dendrite = conducts impulses TOWARDS the cell body
Axon = conducts impulses AWAY FROM the cell body
Memory aid: "Dendron Delivers TO the cell body" - both start with D!
Question 4: The Pacinian Corpuscle as a Transducer (Extended Response)
Model Answer: "It converts mechanical energy into electrical energy" ✓
OR: "It converts energy (mechanical) into another/different form of energy (electrical)" ✓
Markscheme guidance - What to write:
You MUST specify BOTH types of energy
ACCEPT "converts one form of energy into another" BUT it's safer to name them
The energy types must be correct: mechanical IN, electrical OUT
What NOT to write:
Don't just write "pressure" - this isn't specific enough about energy type (markscheme says "IGNORE pressure")
Don't say "converts the stimulus" - be specific about ENERGY transformation
Don't say "kinetic" or "chemical" energy - these are wrong
Examiner insight: The markscheme comments reveal that "many candidates understood that the Pacinian corpuscle is described as a transducer because it transforms one form of energy into another, they often negated their answer by naming the wrong form of energy, such as kinetic or chemical."
Model Answers (you only need ONE of these):
Answer 1: "(The increased pressure) causes sodium (ion) channels to open" ✓
OR
Answer 2: "(Temporary) gaps/holes/spaces appear between the phospholipids/in the bilayer" ✓
Markscheme guidance - What to write:
CREDIT "Na⁺ channels" (with the +)
For answer 2, you must specify the PHOSPHOLIPID bilayer
What NOT to write:
Don't write just "Na channels" without the + symbol - markscheme says "DO NOT CREDIT Na channels"
Don't mention "voltage-gated channels" - these respond to voltage, not mechanical pressure!
Don't say the membrane is "weakened" (markscheme says "IGNORE weakened")
Don't say "pores" - use "gaps" or "spaces" instead
Don't say "breaks in the bilayer" - makes it sound permanent and damaged
Don't suggest additional channels are inserted - that's not what happens
Examiner insight: The markscheme reveals this was "frequently poorly understood. The most common correct response was that deformation would open the sodium ion channels. While some candidates appreciated that the bilayer might develop temporary gaps, they did not specify the phospholipid bilayer. Answers that suggested that the voltage gated channels would open, or that the channels, or the plasma membrane, would be damaged or denatured by the pressure exerted upon them did not gain credit."
Model Answer: "If the stimulus is not strong enough/threshold (value) is not reached/depolarisation (of membrane) is insufficient, then it/an action potential is not generated" ✓
OR the reverse: "If threshold is reached/exceeded, an action potential IS generated" ✓
Markscheme guidance - What to write:
State the condition: threshold must be reached/exceeded
State the consequence: action potential either happens or doesn't
ACCEPT "impulses" for "action potentials"
What NOT to write:
Don't refer to the "strength" of an action potential - they're all the same size!
Don't say "the action potential reaches threshold" - it's the STIMULUS/DEPOLARISATION that reaches threshold
Don't give specific numbers (like -55mV) unless the question asks for them
Don't say action potentials vary in size - this contradicts the principle!
Examiner insight: The markscheme notes that "some incorrectly stated that the action potential would have to reach threshold or simply said that the action potential would either happen or it wouldn't. Some referred to the strength of the action potential, thereby negating their answer."
Perfect answer structure: "If [condition about threshold] then [consequence about action potential being generated or not]"
Model Answer earning both marks:
"It is represented by the frequency of the action potentials" ✓
"A high frequency/rate of action potentials shows a strong/intense stimulus" ✓
Markscheme guidance - What to write:
You MUST use the term "frequency" or "frequent" - this is essential!
Link frequency to stimulus strength
ACCEPT "impulses" for "action potentials"
ACCEPT "rate of generation" as well as frequency
What NOT to write:
Don't say "more action potentials" without mentioning TIME/RATE - this only gets 1 mark maximum
Don't say action potentials travel "faster" - they always travel at the same speed in a given neurone
Don't just describe how impulses pass to the brain without addressing the frequency aspect
Critical point from markscheme: "Max 1 mark if term 'frequent' or derived term NOT used in answer"
Examiner insight: "Good answers showed an appreciation that the information about the strength and intensity of a stimulus is communicated to the brain by way of the frequency of the action potentials. Many commented that a greater stimulus strength would lead to a greater number of action potentials but without reference to a time element, or that they would travel faster."
Example of a 2-mark answer: "A higher frequency of impulses represents a strong stimulus" - this gets both marks because it includes frequency AND links it to stimulus strength.
Question 5: Interpreting Action Potential Graphs
Answer: B ✓
How to analyse this systematically:
Let's identify what's happening at each position on the graph:
Position 1 (at -70mV, resting potential):
Na⁺/K⁺ pump: YES - always operating
Na⁺ channels: NO - closed at rest
K⁺ channels: Some open - membrane is permeable to K⁺ at rest
Position 2 (upward slope, depolarisation):
Na⁺/K⁺ pump: YES - still operating (it never stops!)
Na⁺ channels: YES ← This is what causes depolarisation!
K⁺ channels: NO - still closed
Position 3 (peak at +30mV):
Na⁺/K⁺ pump: YES - still operating
Na⁺ channels: Closing - starting to inactivate
K⁺ channels: Opening - starting to open
Position 4 (downward slope, repolarisation):
Na⁺/K⁺ pump: YES - still operating
Na⁺ channels: NO - now closed
K⁺ channels: YES ← This is what causes repolarisation!
Therefore B is correct because at position 2:
✓ Na⁺/K⁺-pump IS operating (yes)
✓ Voltage-gated Na⁺ channels ARE open (yes) - causing depolarisation
✓ Voltage-gated K⁺ channels are NOT open yet (no)
CRITICAL MISCONCEPTION: Many students think the sodium-potassium pump switches on and off during the action potential. IT DOESN'T! It operates continuously, constantly moving 3Na⁺ out and 2K⁺ in, using ATP. This maintains the concentration gradients that allow the action potential to occur.
How to remember what causes what:
Depolarisation (going UP) = Na⁺ channels OPEN (sodium rushes IN)
Repolarisation (going DOWN) = K⁺ channels OPEN (potassium rushes OUT)
Na⁺/K⁺ pump = ALWAYS working in the background
Exam technique: If you see a graph question like this, trace what's happening at each position:
Is it going up? → Na⁺ channels opening
Is it going down? → K⁺ channels opening
Pump always operating? → YES!
Additional High-Yield Question: Multiple Sclerosis and Nervous Transmission
Answer: D ✓
Why D is INCORRECT (and therefore the right answer to this question):
The nodes of Ranvier are NOT electrical insulators - in fact, it's the opposite! The nodes of Ranvier are the gaps between the myelin sheath where the axon membrane is exposed. This is where depolarisation occurs during saltatory conduction.
What DOES act as an electrical insulator? The myelin sheath itself (formed by Schwann cells) acts as the insulator.
Why the other statements are correct:
A is correct: Breakdown of myelin (as in Multiple Sclerosis) does cause uncoordinated movement
B is correct: Saltatory conduction (jumping between nodes) does increase speed
C is correct: Schwann cells do wrap around the axon to form myelin
Top tip for "NOT correct" questions: Read carefully! You're looking for the FALSE statement. Circle or underline "not" in the question to remind yourself.
Question 6: Understanding Unmyelinated Neurones
Model Answers (1 mark maximum):
Answer 1: "No nodes of Ranvier" ✓
Answer 2: "Shorter local currents/circuits" ✓
Answer 3: "Whole axon needs to be depolarised" ✓
(Award 1 mark for any ONE of these points)
Markscheme guidance - What to write:
IGNORE references to "jumping between nodes" - that's already in the question!
ALLOW "more local currents/circuits"
ALLOW "action potentials need to be generated all the way along the axon"
What NOT to write:
Don't just repeat what's in the question (e.g., "because there's no saltatory conduction")
Don't describe what saltatory conduction IS - explain WHY its absence slows transmission
Examiner insight: The markscheme reveals "There were few correct responses for this part of the question which was assessing AO2 with many candidates referring to the impulse not being able to jump from node to node, which is a description of saltatory conduction already stated in the stem of the question. Good responses referred to the need for depolarisation to occur along the whole axon (membrane)."
The key understanding:
WITH myelin: action potential "jumps" between nodes (long distance, fast)
WITHOUT myelin: action potential must depolarise every section (slow, like a Mexican wave along the entire axon)
Top Tips for Exam Success Based on Markscheme Guidance
1. Use precise ion notation:
Write Na⁺ not "Na" - examiners are specific about this!
The markscheme repeatedly states "DO NOT CREDIT Na channels" but "CREDIT Na⁺ channels"
Same for K⁺ - include the charge
2. For transducer questions, ALWAYS specify BOTH energy types:
✓ "Converts mechanical energy into electrical energy"
✗ "Converts energy" (too vague)
✗ "Converts pressure into impulses" (not energy types)
3. Use "frequency" when discussing stimulus intensity:
This term MUST appear to get full marks
"More action potentials" without time reference = only 1 mark
✓ "Higher frequency of action potentials indicates stronger stimulus"
4. Remember: the all-or-nothing law means:
Action potentials are ALL the same size
Never refer to "strength" or "size" of action potentials
Stronger stimulus = more frequent action potentials, not bigger ones
5. Know what NOT to write:
Topic Don't write Do write Ion channels "Na channels" "Na⁺ channels" / "sodium ion channels" Transducers "converts pressure" "converts mechanical energy to electrical energy" Stimulus coding "more action potentials" "higher frequency of action potentials" All-or-nothing "stronger action potentials" "action potentials either occur or don't occur" Membrane changes "membrane breaks" "temporary gaps in phospholipid bilayer"
6. For myelination questions:
Nodes of Ranvier = GAPS in myelin (NOT insulators)
Myelin sheath = the INSULATOR
Saltatory conduction = FASTER (jumping between nodes)
No myelin = SLOWER (whole axon must depolarise)
Understanding Markscheme Comments
The markschemes include "Examiner's Comments" that reveal common mistakes. Here are the most important ones for this topic:
On Pacinian corpuscles as transducers: "Inadequate responses stated that the corpuscle would transform the stimulus into an electrical impulse" - you must talk about ENERGY transformation, not stimulus transformation.
On sodium channel opening: "Answers that suggested that the voltage gated channels would open... did not gain credit" - mechanical pressure opens STRETCH-SENSITIVE channels, not voltage-gated ones.
On the all-or-nothing law: "Some referred to the strength of the action potential, thereby negating their answer" - never talk about action potential strength!
On stimulus intensity: "Many commented that a greater stimulus strength would lead to a greater number of action potentials but without reference to a time element" - you MUST mention frequency/rate.
On saltatory conduction: "Many candidates referring to the impulse not being able to jump from node to node, which is a description of saltatory conduction already stated in the stem of the question" - don't repeat the question; explain the mechanism!
Practice Strategy
To master this topic:
Make flashcards for definitions - especially the precise wording examiners want
Draw and label neurone diagrams - practice until you can identify all three types instantly
Annotate action potential graphs - label each phase with what's happening to which channels
Practice "What NOT to write" - understanding wrong answers helps avoid them!
Use past papers - the markschemes are gold dust for understanding exactly what's required
Remember: examiners want precision, correct terminology, and clear explanations of mechanisms. Use the markscheme guidance to train yourself to write exactly what they're looking for!
Good luck with your revision! 🧠⚡
Remember: This covers section 5.1.3 (a to c) only. Make sure you also revise synaptic transmission (5.1.4) as it follows on directly and is often examined together!