OCR A Level Biology A: Neuronal communication (H420)

This guide covers OCR A Level Biology A (H420) section 5.1.3, Neuronal communication, learning outcomes (a) to (d), from the OCR specification Version 4.1 (April 2026), first assessment 2025. It sits in Module 5, Communication, homeostasis and energy. The specification text for 5.1.3 carries no separate tier or stage labels, so everything here is for the full A Level course.

Use the revision notes for quick recall and the practice questions to test yourself. To see where this unit sits in the course, open the course hub or tick it off on the printable checklist. If you are unsure where to start, try a short diagnostic first.

What 5.1.3 asks for

Outcome	What you must be able to do
5.1.3(a)	Explain the roles of mammalian sensory receptors in converting different stimuli into nerve impulses, including the Pacinian corpuscle and the idea of a transducer
5.1.3(b)	Describe the structure and functions of sensory, relay and motor neurones, and the differences between myelinated and non-myelinated neurones
5.1.3(c)	Explain how the resting potential is established and maintained, how an action potential is generated (including positive feedback) and transmitted in a myelinated neurone, and why the frequency of impulses matters
5.1.3(d)	Describe a cholinergic synapse, the action of neurotransmitters, the importance of synapses in summation, and inhibitory and excitatory synapses

5.1.3(a) Sensory receptors as transducers

A sensory receptor responds to one type of stimulus and converts its energy into an electrical signal. This conversion is why a receptor is called a transducer. The signal starts as a change in the potential difference across the receptor’s membrane, called a generator potential. If the generator potential is large enough, it triggers an action potential in a sensory neurone.

Receptor	Stimulus	Energy converted
Pacinian corpuscle	Pressure and vibration	Mechanical to electrical
Thermoreceptor	Temperature change	Thermal to electrical
Chemoreceptor	Chemicals	Chemical to electrical
Photoreceptor	Light	Light to electrical

The Pacinian corpuscle

A Pacinian corpuscle is a pressure receptor found in the skin. The ending of a sensory neurone lies in the centre, wrapped in concentric rings (lamellae) of connective tissue. Viscous gel separates the rings. The membrane of the neurone ending contains stretch-mediated sodium ion channels.

1. Pressure deforms the lamellae and stretches the membrane of the neurone ending.
2. The stretch-mediated sodium channels widen.
3. Sodium ions diffuse in down their electrochemical gradient, and the membrane depolarises. This is the generator potential.
4. A bigger pressure opens more channels, so the generator potential is bigger.
5. If the generator potential reaches the threshold, voltage-gated sodium channels open and an action potential travels along the sensory neurone.

Worked example 1. A resting neurone ending is at −70 mV and its threshold is −55 mV. Pressure of 0.5 kPa gives a membrane potential of −64 mV, 1.0 kPa gives −58 mV and 1.5 kPa gives −52 mV. Which pressures trigger an action potential?

depolarisation needed = −55 − (−70) = 15 mV
0.5 kPa: −64 − (−70) = 6 mV   (below 15 mV)
1.0 kPa: −58 − (−70) = 12 mV  (below 15 mV)
1.5 kPa: −52 − (−70) = 18 mV  (above 15 mV)

Only 1.5 kPa triggers an action potential. The first two pressures give a generator potential that is graded with the stimulus but too small to reach threshold, so the sensory neurone stays silent.

5.1.3(b) Neurone structure and function

All three types have a cell body (with the nucleus), dendrites or a dendron, and an axon. The differences relate to position in the reflex pathway.

Feature	Sensory neurone	Relay neurone	Motor neurone
Function	Carries impulses from a receptor to the central nervous system	Connects sensory and motor neurones inside the central nervous system	Takes impulses out of the central nervous system to a muscle or gland
Cell body	Off to one side, between a long dendron and a short axon	In the central nervous system, with many short processes	In the central nervous system, at one end of the cell
Dendrites	One long dendron from the receptor	Many short dendrites	Many short dendrites around the cell body
Axon	Short	Short	Long, ending at a muscle or gland

Myelinated and non-myelinated neurones

In a myelinated neurone, Schwann cells wrap around the axon many times. Their membranes contain lipid, and the layers form the myelin sheath, which is an electrical insulator. The sheath is not continuous. Short gaps between neighbouring Schwann cells, the nodes of Ranvier, leave the axon membrane exposed. Voltage-gated ion channels are concentrated at these nodes.

In a non-myelinated neurone the axon is not wrapped in layers of myelin, so the membrane is exposed along its whole length and conduction is slower.

5.1.3(c) Nerve impulses

The resting potential

A neurone not carrying an impulse has an inside that is negative compared with the outside, at about −70 mV. It is established and maintained by:

• The sodium-potassium pump, which uses ATP to move 3 sodium ions out and 2 potassium ions in. This leaves a net loss of positive charge from the axon.
• Potassium ion channels that are open at rest, so potassium ions diffuse out down their concentration gradient.
• Few open sodium channels at rest, so sodium ions diffuse back in only slowly.

The membrane is much more permeable to potassium ions than to sodium ions. Positive charge therefore leaves faster than it returns, and the inside stays negative. The pump keeps replacing the ions that diffuse down their gradients, which maintains the resting potential.

Generating an action potential

An action potential is a brief reversal of the potential difference across the membrane.

1. Stimulus. A stimulus opens some voltage-gated sodium channels. Sodium ions diffuse in and the inside becomes less negative.
2. Threshold. If the membrane reaches about −55 mV, many more voltage-gated sodium channels open. This is positive feedback: depolarisation opens channels, which causes more depolarisation.
3. Depolarisation. Sodium ions flood in and the inside reaches about +40 mV.
4. Repolarisation. Sodium channels close and voltage-gated potassium channels open. Potassium ions diffuse out and the inside goes negative again.
5. Hyperpolarisation. The potassium channels close slowly, so the potential briefly falls below the resting value.
6. Recovery. Potassium channels close and the sodium-potassium pump and leak channels restore −70 mV.

A stimulus below threshold gives no action potential. At or above threshold, the action potential is always the same size. This is the all-or-nothing principle.

The refractory period

Just after an action potential, the voltage-gated sodium channels cannot reopen. During this refractory period no new action potential can start. This stops the impulse spreading backwards, keeps impulses separate and sets a maximum frequency.

Transmission in a myelinated neurone

At a node, the influx of sodium ions creates a local circuit of current that spreads along the axon inside the myelin. Myelin stops ions leaking out, so the current reaches the next node and depolarises it to threshold. The action potential therefore seems to leap between nodes, which is called saltatory conduction. The region behind is refractory, so the impulse moves in one direction.

Worked example 2. An impulse crosses 0.62 m of myelinated axon in 8.0 ms. In a non-myelinated axon, an impulse crosses 0.12 m in 60 ms. Compare the speeds.

myelinated:      0.62 m / 0.0080 s = 77.5 m per s
non-myelinated:  0.12 m / 0.060 s  = 2.0 m per s
77.5 / 2.0 = 38.75, about 39 times

Convert milliseconds to seconds before dividing. Axon diameter and temperature also affect speed: a wider axon has less resistance to the current, and warmer conditions speed diffusion and the supply of ATP.

Why frequency matters

Every action potential has the same size, so a neurone cannot signal a strong stimulus with a bigger impulse. It signals the strength with the frequency of impulses. The brain interprets a higher frequency as a stronger stimulus.

Worked example 3. A neurone fires 36 impulses in 0.60 s with a light touch and 54 impulses in 0.60 s with a firm touch. Find the percentage increase in frequency.

light: 36 / 0.60 = 60 impulses per s
firm:  54 / 0.60 = 90 impulses per s
(90 − 60) / 60 × 100 = 50%

5.1.3(d) Synapses

A synapse is a junction between two neurones, with a gap, the synaptic cleft, between them. Neurones that use acetylcholine are cholinergic.

Structure of a cholinergic synapse

• Presynaptic knob: many mitochondria, vesicles of acetylcholine and voltage-gated calcium ion channels.
• Synaptic cleft: the narrow gap, which contains the enzyme acetylcholinesterase.
• Postsynaptic membrane: receptors with a shape complementary to acetylcholine, linked to sodium ion channels.

Action of the neurotransmitter

1. An action potential arrives and depolarises the presynaptic knob.
2. Voltage-gated calcium channels open and calcium ions diffuse in.
3. Calcium ions trigger vesicles to fuse with the presynaptic membrane and empty acetylcholine into the cleft (exocytosis).
4. Acetylcholine diffuses across the gap and attaches to its receptors in the postsynaptic membrane.
5. Sodium channels open and sodium ions diffuse in, so the postsynaptic membrane depolarises. Reaching threshold starts an action potential.
6. Acetylcholinesterase breaks acetylcholine into choline and ethanoic acid. This clears the receptors and stops the response.
7. Choline and ethanoic acid are taken back into the knob and recombined into acetylcholine using ATP from the mitochondria.

Because only the presynaptic side holds vesicles and only the postsynaptic side holds receptors, transmission is in one direction.

Summation

One impulse often releases too little neurotransmitter to bring the postsynaptic membrane to threshold. Two processes add the effects together:

• Spatial summation: several presynaptic neurones release neurotransmitter onto the same postsynaptic neurone at the same time.
• Temporal summation: a single presynaptic neurone fires again and again in rapid succession, so each release adds to the last before the effect has faded.

Summation lets the postsynaptic neurone combine information from many inputs and ignore weak background signals.

Excitatory and inhibitory synapses

An excitatory synapse depolarises the postsynaptic membrane, moving it towards threshold. An inhibitory synapse makes the postsynaptic membrane more negative (hyperpolarised) so that threshold is harder to reach. In many inhibitory synapses, the neurotransmitter opens chloride ion channels, or potassium ion channels, in the postsynaptic membrane.

Worked example 4. A neurone rests at −70 mV and fires at −55 mV. Each excitatory impulse depolarises the postsynaptic membrane by 1.8 mV and one inhibitory input hyperpolarises it by 2.7 mV. Assume all inputs arrive together and nothing decays. How many excitatory inputs are needed (a) alone, (b) while the inhibitory input is active?

(a) 15 / 1.8 = 8.33, so 9 inputs
(b) 15 + 2.7 = 17.7 mV needed: 17.7 / 1.8 = 9.83, so 10 inputs

Round up each time: eight inputs give 14.4 mV, which does not reach threshold.

Common errors

• Writing that a bigger stimulus gives a bigger action potential. It gives more frequent ones.
• Saying sodium ions “jump” in a myelinated axon. The action potential jumps; ions move through channels at nodes.
• Forgetting that the resting potential needs both the pump and membrane permeability.
• Giving the pump as a one-for-one exchange. It moves 3 sodium ions out for 2 potassium ions in.
• Describing a generator potential as an action potential.

Official syllabus

OCR Level 3 Advanced GCE in Biology A (H420), specification Version 4.1 (April 2026), first assessment 2025, published by OCR. This guide follows section 5.1.3, learning outcomes (a) to (d), including the additional guidance.