OCR A Level Biology A: Neuronal communication (H420) – Revision Notes

These notes condense OCR A Level Biology A (H420) section 5.1.3, Neuronal communication, learning outcomes (a) to (d), from specification Version 4.1 (April 2026). Read the study guide for full explanations, then test yourself with the practice questions. A short diagnostic shows where to start, the checklist tracks progress and the course hub lists the other units.

5.1.3(a) Receptors and transducers

• A sensory receptor is specific to one type of stimulus.
• A transducer converts one form of energy into another. Receptors convert the energy of the stimulus into electrical energy, as a change in membrane potential.
• The change in potential across a receptor is the generator potential. It is graded: a bigger stimulus gives a bigger generator potential.
• A nerve impulse starts only if the generator potential reaches threshold.

Pacinian corpuscle in steps

1. Pressure deforms the lamellae (connective tissue rings separated by viscous gel).
2. The neurone ending membrane stretches and stretch-mediated sodium channels widen.
3. Sodium ions diffuse in, giving a generator potential.
4. At threshold, voltage-gated sodium channels open and an action potential passes along the sensory neurone.

Receptor	Detects	Converts
Pacinian corpuscle	Pressure, vibration	Mechanical to electrical
Thermoreceptor	Temperature	Thermal to electrical
Chemoreceptor	Chemicals	Chemical to electrical
Photoreceptor	Light	Light to electrical

5.1.3(b) Neurone types

	Sensory	Relay	Motor
Direction	Receptor to central nervous system	Within central nervous system	Central nervous system to effector
Cell body	Side branch off the fibre	In central nervous system	In central nervous system
Dendron or dendrites	One long dendron	Many short dendrites	Many dendrites
Axon	Short	Short	Long

Myelinated and non-myelinated

	Myelinated	Non-myelinated
Covering	Schwann cells wrapped around the axon, forming a lipid-rich insulating sheath	Not wrapped in layers of myelin
Gaps	Nodes of Ranvier, where channels are concentrated	None
Conduction	Saltatory, from node to node	Continuous along the whole membrane
Speed	Fast	Slow

5.1.3(c) Impulses

Resting potential

Feature	Role
Sodium-potassium pump	Uses ATP to move 3 Na⁺ out and 2 K⁺ in
Open K⁺ channels	K⁺ diffuses out down its concentration gradient
Few open Na⁺ channels	Na⁺ cannot diffuse back in quickly
Result	Inside about −70 mV relative to outside

Action potential

Stage	Channels and ions	Potential
Stimulus	Some voltage-gated Na⁺ channels open	Rises from −70 mV
Threshold	Many more Na⁺ channels open (positive feedback)	About −55 mV
Depolarisation	Na⁺ floods in	Up to about +40 mV
Repolarisation	Na⁺ channels close, voltage-gated K⁺ channels open, K⁺ leaves	Falls
Hyperpolarisation	K⁺ channels slow to close	Below −70 mV
Recovery	Pump and leak channels	Back to −70 mV

Method in steps: explaining speed in a myelinated axon

1. Myelin insulates, so no current leaks out through the membrane beneath it.
2. Depolarisation happens only at the nodes of Ranvier.
3. Local circuits carry the current from one node to the next.
4. The action potential appears to jump between nodes, so fewer points depolarise per unit length.
5. Speed = distance / time, so fewer depolarisations means a shorter time.

Must-know distinctions

• Generator potential or action potential. The generator potential is graded and local. The action potential is all-or-nothing and travels.
• Depolarisation or repolarisation. Depolarisation is Na⁺ entering. Repolarisation is K⁺ leaving.
• Refractory period. Na⁺ channels cannot reopen. It makes the impulse one-way, separates impulses and limits frequency.
• Frequency. All action potentials in a neurone are the same size. Stimulus strength is coded by the frequency of impulses.

5.1.3(d) Synapses

Cholinergic synapse in order

Step	Event
1	Action potential depolarises the presynaptic knob
2	Calcium channels (voltage-gated) open and Ca²⁺ flows into the knob
3	Vesicles merge with the membrane and discharge acetylcholine (exocytosis)
4	Acetylcholine crosses the cleft and attaches to receptors
5	Sodium channels open, Na⁺ enters and the postsynaptic membrane depolarises
6	Acetylcholinesterase breaks down acetylcholine into choline and ethanoic acid
7	Products re-enter the knob and are resynthesised using ATP from mitochondria

Roles of synapses

• One-way transmission, because only the presynaptic side releases the transmitter and only the postsynaptic side holds receptors.
• Spatial summation: several presynaptic neurones act together.
• Temporal summation: repeated impulses from one neurone add up.
• Excitatory synapse: depolarises the postsynaptic membrane. Inhibitory synapse: hyperpolarises it (for example by opening chloride ion channels), so threshold is harder to reach.
• Summation filters out weak signals and combines many inputs.

Worked reminders

speed = distance / time        0.45 m in 6.0 ms: 0.45 / 0.0060 = 75 m per s
frequency = impulses / time    45 impulses in 1.5 s: 45 / 1.5 = 30 per s
% change = change / original × 100

Key terms

Term	Meaning
Transducer	A receptor that converts stimulus energy into electrical energy
Generator potential	Graded depolarisation of a receptor membrane
Threshold	The membrane potential (about −55 mV) at which positive feedback begins
Depolarisation	The inside of the membrane becomes less negative or positive
Repolarisation	The inside returns towards the resting value
Hyperpolarisation	The inside becomes more negative than the resting potential
Node of Ranvier	Gap in the myelin where the axon membrane is exposed
Saltatory conduction	Action potentials appearing at successive nodes only
Synaptic cleft	The gap between presynaptic and postsynaptic membranes
Exocytosis	Release of vesicle contents by fusion of the vesicle with the cell surface membrane

Summation and threshold in numbers

A postsynaptic neurone starts at −70 mV and fires at −55 mV, so it needs a net 15 mV of depolarisation.

excitatory input of 3 mV, all arriving together:
  4 inputs give 12 mV  (−58 mV, no impulse)
  5 inputs give 15 mV  (−55 mV, threshold reached)
add one inhibitory input of −3 mV at the same time:
  5 excitatory inputs now give 12 mV net, so a sixth is needed

Spatial summation uses many presynaptic neurones at once. Temporal summation uses one neurone firing again before the first effect has faded. Both are needed in the explanation if a question gives you a table of inputs.

Factors that change conduction speed

Factor	Effect on speed	Reason
Myelination	Faster	Depolarisation only at nodes
Larger axon diameter	Faster	Less resistance to local currents
Higher temperature (within limits)	Faster	Ions diffuse faster and more ATP is available for the pump

Synaptic delay is separate. Each synapse adds a delay because diffusion of the transmitter takes time. Pathways with fewer synapses respond faster.

Quick self-test

1. Name the type of energy a Pacinian corpuscle converts, and the type it produces.
2. What does the sodium-potassium pump move, and in what ratio?
3. An impulse crosses 0.08 m of non-myelinated axon in 40 ms. Find the speed.
4. Each excitatory impulse depolarises a postsynaptic membrane by 4 mV. The membrane starts at −70 mV and threshold is −55 mV. How many simultaneous impulses are needed?
5. State the role of positive feedback in generating an action potential.
6. Why can an action potential not travel backwards?
7. A neurone fires 45 impulses in 1.5 s. What is the frequency, and what does the maximum possible frequency depend on?
8. Frequency rises from 50 to 80 impulses per second. Find the percentage increase.
9. Name the enzyme in the cleft of a cholinergic synapse and say what it does.
10. How does an inhibitory synapse make an action potential less likely?

Answers

1. Mechanical energy (pressure) to electrical energy (a generator potential).
2. Three sodium ions out and two potassium ions in, using ATP.
3. 0.08 / 0.040 = 2.0 m per s.
4. 15 / 4 = 3.75, so 4 impulses (−70 + 16 = −54 mV; three give −58 mV).
5. Depolarisation opens more voltage-gated sodium channels, which causes further depolarisation, so the potential rises rapidly to its peak.
6. The region just behind is refractory because sodium channels cannot reopen yet.
7. 45 / 1.5 = 30 per s. The maximum depends on the length of the refractory period.
8. (80 − 50) / 50 × 100 = 60%.
9. Acetylcholinesterase. It hydrolyses acetylcholine into choline and ethanoic acid, so the receptors are cleared.
10. It hyperpolarises the postsynaptic membrane, for example by opening chloride channels, so more depolarisation is needed to reach threshold.

Where marks are usually lost

• Writing “the nerve impulse jumps” without saying it is the depolarisation, at the nodes of Ranvier.
• Leaving out the units or the millisecond conversion in speed calculations.
• Saying sodium ions “pump” into the axon during depolarisation. They diffuse through channels.
• Naming only one of the two ions and one of the two mechanisms in the resting potential.
• Missing the word threshold when explaining all-or-nothing.
• Describing a bigger action potential instead of a higher frequency.
• Writing “neurotransmitter” where the question asks for the ion that triggers release: calcium.
• Forgetting that acetylcholinesterase is needed to end the response.
• Confusing the pump with channels, or confusing hyperpolarisation with repolarisation.

Official syllabus

OCR Level 3 Advanced GCE in Biology A (H420), specification Version 4.1 (April 2026), first assessment 2025, published by OCR. These notes follow section 5.1.3, learning outcomes (a) to (d).