OxfordAQA International AS and A-level Biology: Nerve impulses and synapses (9610) – Revision Notes

Condensed notes for sections 3.4.3.1 (Nerve impulses) and 3.4.3.2 (Synaptic transmission) of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, for International A-level exams May/June 2018 onwards. The specification lists Unit 4, Control, as International A-level only. For full explanations use the study guide. Test yourself with the practice questions, see the course hub and checklist, and find weak spots with the free diagnostics.

Key terms

Term	Meaning
Resting potential	Potential difference across the membrane of an unstimulated neurone, inside negative, about −70 mV
Depolarisation	Inside becomes less negative, then positive, because Na⁺ enters
Repolarisation	Inside returns to negative because K⁺ leaves
Hyperpolarisation	Potential falls below the resting value
Threshold	Potential at which enough voltage-gated Na⁺ channels open to trigger an action potential, about −55 mV
Refractory period	Time after an action potential when a new one cannot start, or needs a bigger stimulus
Saltatory conduction	Action potential appearing to jump between nodes of Ranvier
Summation	Adding up of postsynaptic depolarisations to reach threshold

Myelinated motor neurone

• Carries impulses from the central nervous system to an effector.
• Cell body (nucleus, organelles), long axon, axon terminal.
• Schwann cells wrap the axon in lipid-rich myelin, an insulator.
• Nodes of Ranvier are bare gaps between Schwann cells.

Resting potential

Cause	How it works
Sodium-potassium pump	ATP drives 3 Na⁺ out and 2 K⁺ in
Differential permeability	Many open K⁺ channels, few open Na⁺ channels
Result	K⁺ leaves faster than Na⁺ enters, so the inside is negative

Action potential in steps

Stage	Channels	Ion movement	Potential
Stimulus	A few voltage-gated Na⁺ open	Na⁺ in	Rises to threshold
Depolarisation	Many Na⁺ open (positive feedback)	Na⁺ in	Rises to about +40 mV
Repolarisation	Na⁺ shut, voltage-gated K⁺ open	K⁺ out	Falls
Hyperpolarisation	K⁺ close slowly	K⁺ still leaving	Below −70 mV
Recovery	K⁺ shut; pump and K⁺ leak	Gradients restored	Back to rest

• All-or-nothing: below threshold, nothing. At or above threshold, a full action potential of the same size every time.
• A stronger stimulus raises the frequency of impulses, not their size.
• ATP and the pump: restore the ion distribution after impulses. If ATP supply fails, gradients run down.

Refractory period

• Na⁺ channels cannot open straight after an impulse (absolute). The membrane then needs a larger stimulus (relative).
• Gives discrete impulses, one direction of travel, and a maximum frequency.
• Maximum frequency = 1 / refractory period, with the period in seconds.

Speed of conduction

Factor	Effect	Reason in one line
Myelination	Faster	Saltatory conduction: depolarisation only at nodes
Wider axon	Faster	Less resistance to ion flow inside; less leakage
Higher temperature	Faster, to an optimum	Faster diffusion and ATP supply; denaturation beyond optimum

Speed = distance / time. Convert ms to s and mm or cm to m first.

Cholinergic synapse in steps

1. Action potential depolarises the presynaptic knob.
2. Voltage-gated Ca²⁺ channels open; Ca²⁺ diffuses in.
3. Vesicles fuse with the membrane; ACh released by exocytosis.
4. ACh diffuses across the cleft and binds to receptors.
5. Na⁺ channels open; postsynaptic membrane depolarises; action potential if threshold reached.
6. Acetylcholinesterase hydrolyses ACh to choline and ethanoate.
7. Choline taken back in; ACh resynthesised using ATP.

• Unidirectional: vesicles and Ca²⁺ channels only presynaptic; receptors only postsynaptic.
• Spatial summation: several presynaptic neurones, same moment.
• Temporal summation: one presynaptic neurone, rapid repeats.
• Inhibition: Cl⁻ channels and K⁺ channels open; Cl⁻ in, K⁺ out; hyperpolarisation.

Neuromuscular junction compared

Feature	Neurone synapse	Neuromuscular junction
Postsynaptic cell	Neurone	Muscle fibre
Membrane	Smooth	Folded; more receptors and enzyme
Effect	Excitatory or inhibitory	Excitatory only

Drug and toxin method

1. Which step is changed?
2. Increase or decrease?
3. What happens to ACh in the cleft or to the postsynaptic potential?
4. What happens to the action potential or the muscle?

Substance acts on	Typical prediction
Ca²⁺ channels (blocked)	Less ACh released; less depolarisation
Vesicle fusion (stopped)	No ACh released; no response
Receptors (blocked)	No depolarisation
Acetylcholinesterase (inhibited)	Prolonged stimulation
Choline uptake (blocked)	Less ACh remade during repeated firing
Axon Na⁺ channels (blocked)	No action potential reaches the knob

Worked reminders

Frequency. Refractory period 2.5 ms.

2.5 ms = 0.0025 s
1 / 0.0025 = 400 impulses per s

Speed ratio. 0.45 m in 5.0 ms against 0.012 m in 6.0 ms.

0.45 / 0.0050  = 90 m per s
0.012 / 0.0060 = 2.0 m per s
90 / 2.0 = 45 times faster

Pump arithmetic. Each ATP moves 3 Na⁺ out and 2 K⁺ in. For 12 000 Na⁺ to be removed: 12 000 / 3 = 4000 ATP, and 4000 × 2 = 8000 K⁺ are brought in.

Summation with inhibition. Needed rise is 12 mV and each excitatory impulse gives 2.0 mV, so 6 impulses. An inhibitory input that holds the membrane 4 mV lower makes the needed rise 16 mV: 16 / 2.0 = 8 impulses.

Must-know distinctions

Compare	Point to make
Depolarisation and repolarisation	Na⁺ entering makes the inside positive; K⁺ leaving makes it negative again
Repolarisation and hyperpolarisation	Repolarisation returns to rest; hyperpolarisation overshoots below rest
Pump and channels	Channels give the rapid changes; the pump slowly restores the gradients
Spatial and temporal summation	Many presynaptic neurones at once, or one neurone firing repeatedly
Excitatory and inhibitory synapse	Na⁺ in and depolarisation, or Cl⁻ in with K⁺ out and hyperpolarisation
Non-myelinated and myelinated	Depolarisation along the whole membrane, or only at nodes
Acetylcholine and acetylcholinesterase	Transmitter, and the enzyme that hydrolyses it

Synapse structure to sketch

PRESYNAPTIC KNOB            CLEFT (about 20 nm)      POSTSYNAPTIC MEMBRANE
mitochondria                acetylcholinesterase     ACh receptors with
vesicles of ACh   ---ACh--> in the gap  ---------->  Na+ channels
voltage-gated Ca2+ channels

At a neuromuscular junction the postsynaptic membrane belongs to a muscle fibre and is folded.

Phrases that make an answer precise

• “Down its electrochemical gradient through voltage-gated channels” instead of “moves”.
• “Positive feedback” when explaining the rapid rise after threshold.
• “Local circuits between nodes” when explaining saltatory conduction.
• “Exocytosis” for the release of ACh, and “diffusion” for its crossing of the cleft.
• “Hydrolyses” for the action of acetylcholinesterase.
• “Fewer ions to pump back” if asked about the energy advantage of myelin.
• “Presynaptic only” or “postsynaptic only” when explaining one-way transmission.

Quick self-test

1. State the ion movements of one sodium-potassium pump cycle.
2. A neurone has a refractory period of 0.80 ms. Calculate its maximum frequency.
3. An impulse travels 0.36 m in 4.5 ms. Calculate its speed.
4. How long does an impulse take to travel 0.90 m at 60 m per s? Give the answer in ms.
5. Explain why a wider axon conducts faster.
6. Resting potential is −68 mV and threshold is −54 mV. Each excitatory impulse adds 1.8 mV. How many simultaneous impulses are needed?
7. Which type of summation involves one presynaptic neurone?
8. Name the ion that enters the postsynaptic cell at an inhibitory synapse and state the effect.
9. Why is a neuromuscular junction never inhibitory?
10. Conduction speed rises from 20 to 30 m per s when the temperature is raised. Calculate the percentage increase.
11. A drug stops vesicles releasing ACh. Predict the effect on the muscle.
12. Why can an impulse not travel backwards along an axon?

Answers

1. 3 Na⁺ out, 2 K⁺ in, using one ATP.
2. 0.80 ms = 0.00080 s; 1 / 0.00080 = 1250 impulses per s.
3. 0.36 / 0.0045 = 80 m per s.
4. 0.90 / 60 = 0.015 s = 15 ms.
5. Less resistance to ion flow in the cytoplasm and less leakage, so local circuits spread further and depolarise ahead faster.
6. Needed 14 mV; 14 / 1.8 = 7.8; round up to 8 impulses.
7. Temporal summation.
8. Cl⁻ (K⁺ also leaves). The membrane hyperpolarises, so an action potential is less likely.
9. The only transmitter is ACh, which always depolarises the muscle fibre. Inhibition happens earlier, in the central nervous system.
10. (30 − 20) / 20 × 100 = 50 %.
11. No ACh reaches the receptors, so no depolarisation and no contraction.
12. The region behind is refractory, so it cannot be depolarised again.

Where marks are usually lost

• Writing “sodium and potassium move” without saying which way or through what.
• Giving the pump as the cause of the action potential.
• Forgetting positive feedback when explaining the rapid rise.
• Saying the impulse is bigger when the stimulus is stronger.
• Mixing up depolarisation and hyperpolarisation.
• Not converting ms to s in a frequency calculation.
• Rounding down when asked how many impulses are needed.
• Saying “calcium causes release” without saying vesicles fuse with the membrane.
• Giving the drug effect without linking it to the step that changes.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International A-level exams May/June 2018 onwards, published by OxfordAQA. Notes follow sections 3.4.3.1 and 3.4.3.2.