Pearson Edexcel International A Level Biology: Neurones, Reflexes and Synapses (YBI11) – Revision Notes

Quick recall for the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology (YBI11) specification, Issue 2, statements 8.1 to 8.7 of Topic 8. This is International A2 (IA2) content: Topics 5 to 8 sit in the A2 half, and nothing is tiered. For full explanations see the study guide, then try the practice questions. Planning tools: the course hub, the checklist and the free diagnostics. Related: skeletal muscle, cardiac control and adrenaline.

8.1 Three neurones

Neurone	Carries impulses	Cell body	Fibre
Sensory	Receptor to CNS	Off to one side, outside the cord	Long
Relay	Inside the CNS	In the CNS	Short
Motor	CNS to effector	In the CNS, with dendrites	Long axon with terminals

• Schwann cell: wraps its membrane round an axon many times.
• Myelin sheath: the lipid-rich layers formed. An electrical insulator.
• Node of Ranvier: the gap between Schwann cells where the axon membrane is exposed.

Must-know distinctions

• Sensory (towards CNS) versus motor (away from CNS).
• Axon (carries impulses away from the cell body) versus dendrite (short, receives).
• Myelin (the sheath) versus node (the gap).

8.2 The response chain

Stimulus, receptor, sensory neurone, coordinator (CNS), motor neurone, effector, response.

• Receptor: converts stimulus energy into an electrical change.
• Effector: a muscle (contracts) or a gland (secretes).
• Synapses sit between neurones and between motor neurone and effector.

8.3 Spinal reflex arc

Part	Where	Contains
Grey matter	Central, H-shaped	Cell bodies of relay and motor neurones, synapses
White matter	Around the grey	Myelinated axons running up and down
Dorsal root	Back	Sensory neurones entering; swelling holds cell bodies
Ventral root	Front	Motor neurones leaving

Reflex arc in steps:

1. Receptor stimulated.
2. Sensory neurone to the cord by the dorsal root.
3. Synapse with a relay neurone in the grey matter.
4. Synapse with a motor neurone.
5. Motor neurone leaves by the ventral root.
6. Effector responds.

Why reflexes are quick: short pathway, few synapses, no conscious thought. They protect the body.

8.4 Action potential

Quantity	Value to quote
Resting potential	About −70 mV
Threshold	About −55 mV
Peak	About +40 mV
Pump	3 Na⁺ out, 2 K⁺ in, uses ATP

Method in steps

1. A stimulus opens a few sodium channels (voltage-gated) and Na⁺ moves in by diffusion.
2. At threshold many more open (positive feedback), giving rapid depolarisation.
3. At the peak the sodium channels shut.
4. Voltage-gated potassium channels open and K⁺ moves out, so the membrane repolarises.
5. The K⁺ channels shut slowly, giving a brief hyperpolarisation.
6. The pump and K⁺ leak channels restore the resting potential.

Key ideas:

• All-or-nothing: below threshold, nothing. Above it, same size every time.
• Strength of stimulus: coded by frequency and number of neurones, not size.
• Refractory period: Na⁺ channels cannot open. Gives one-way travel, separate impulses and a ceiling on frequency.
• Conduction: Na⁺ entering spreads sideways inside the axon, bringing the next region to threshold.

Small worked reminders

• Maximum frequency = 1 / refractory time. A 5.0 ms gap gives 1 / 0.0050 = 200 per second.
• Speed = distance / time. 0.30 m in 6.0 ms is 0.30 / 0.0060 = 50 m/s.
• Amplitude = peak minus resting = 40 − (−70) = 110 mV.
• Reflex time: add the axon times and the synaptic delays. Axon time = length / speed. Two axons of 0.40 m at 40 m/s take 10 ms each, so 20 ms; with two synapses at 1.0 ms each the total is 22 ms.
• Pupil area: diameter 4.0 mm gives π × 2.0² = 12.6 mm². Halving the diameter to 2.0 mm gives π × 1.0² = 3.14 mm², a quarter of the area.

8.5 Saltatory conduction

• Myelin insulates, so ions cross only at nodes.
• Local currents link node to node inside the axon.
• Action potentials occur only at nodes.
• Faster: fewer sites to regenerate the impulse.
• Cheaper: fewer ions to pump back, so less ATP.
• If myelin is lost, impulses slow or fail.

Unmyelinated axon: the depolarisation spreads continuously along the whole membrane, so it is slower.

8.6 (i) Synapse

Parts: presynaptic knob (mitochondria, vesicles of ACh, voltage-gated Ca²⁺ channels), synaptic cleft, postsynaptic membrane (receptors on Na⁺ channels).

Sequence

1. Impulse reaches the knob.
2. Voltage-gated calcium channels open and Ca²⁺ floods in.
3. Vesicles move to the knob membrane, fuse and empty their ACh into the cleft (exocytosis).
4. ACh crosses the gap by diffusion and attaches to receptor sites.
5. Linked Na⁺ channels open, so Na⁺ rushes in and the postsynaptic side depolarises.
6. Threshold reached: a new action potential starts.
7. Acetylcholinesterase breaks ACh into choline and ethanoate; products reabsorbed; ACh resynthesised using ATP.

Properties

• One direction only, because only the presynaptic side releases transmitter.
• Synaptic delay.
• Summation: effects from several presynaptic neurones add together.
• Some transmitters inhibit by hyperpolarising the postsynaptic membrane.

8.6 (ii) Pupil

Condition	Circular muscle	Radial muscle	Pupil
Bright light	Contracts	Relaxes	Constricts
Dim light	Relaxes	Contracts	Dilates

• The two muscles are antagonistic.
• Constriction protects the retina.
• Area = π r², so a small change in diameter gives a larger change in area.

8.7 Drugs

Drug	Target	Effect in one line
Nicotine	Postsynaptic ACh receptors	Mimics ACh; not broken down by acetylcholinesterase; prolonged stimulation
Lidocaine	Voltage-gated Na⁺ channels	Blocked; no action potentials; local anaesthetic
Cobra alpha toxin	Postsynaptic ACh receptors	Blocks them; no depolarisation; paralysis
L-DOPA	Brain	Converted to dopamine; treats Parkinson’s symptoms
MDMA	Serotonin synapses	Releases serotonin and blocks reuptake

• Nicotine and alpha toxin act on the same receptors with opposite outcomes (activate versus block).
• Lidocaine acts on the axon, not on the synapse.
• L-DOPA, not dopamine, is given because it crosses into the brain.

Side-by-side comparisons

Feature	Unmyelinated axon	Myelinated axon
Where action potentials occur	Along the whole length	Only at nodes of Ranvier
Speed	Slower	Faster
Ion movement across membrane	Along the whole length	Only at nodes
ATP use for restoring ion gradients	Higher	Lower

Feature	Reflex	Voluntary response
Pathway	Short, few synapses	Longer, involves the brain
Conscious thought	Not needed	Needed
Speed	Very fast	Slower
Value	Protects the body	Flexible and varied

Feature	Nicotine	Cobra alpha toxin
Binds to	ACh receptors	ACh receptors
Channel result	Opens	Stays closed
Postsynaptic membrane	Depolarised	Not depolarised
Muscle	Overstimulated	Paralysed

Reading graphs and tables

• Action potential trace: read the baseline first (resting potential), then the threshold where the line steepens, the peak, the dip below resting (hyperpolarisation) and the return. Quote values with units.
• Time between peaks: the interval between two peaks gives the frequency. Frequency = 1 / interval.
• Drug data: compare each result with the control. Say what changed, quote a figure, then link the change to a channel or receptor.
• Percentage change: (new − old) / old × 100. Show the subtraction before dividing.
• Rates: include the unit, such as m/s or per second, in the final line.

Memory lines

• Reflex path: receptor, sensory, relay, motor, effector.
• Action potential: in with sodium, out with potassium.
• Synapse: calcium in, vesicles fuse, ACh across, sodium in, enzyme clears.
• Pupil: bright light, circular contracts; dim light, radial contracts.
• Drugs: nicotine copies, toxin blocks, lidocaine silences the axon, L-DOPA refills, MDMA floods serotonin.

Quick self-test

1. Which type of neurone has its cell body outside the spinal cord?
2. What are the gaps between Schwann cells called?
3. Which region of the spinal cord holds relay neurone cell bodies?
4. Which ions enter the axon during depolarisation?
5. An axon has a refractory period of 2.0 ms. What is its maximum firing rate?
6. An impulse covers 0.24 m in 4.0 ms. What is the speed?
7. Which ion enters the presynaptic knob before vesicles fuse?
8. Which enzyme removes acetylcholine from the cleft?
9. Which pupil muscle contracts in bright light?
10. What does lidocaine block?
11. Why is L-DOPA given rather than dopamine?
12. What is the percentage decrease if a pupil narrows from 6.0 mm to 3.0 mm in diameter?

Answers

1. Sensory neurone.
2. Nodes of Ranvier.
3. Grey matter.
4. Sodium ions (Na⁺).
5. 1 / 0.0020 = 500 per second.
6. 0.24 / 0.0040 = 60 m/s.
7. Calcium ions (Ca²⁺).
8. Acetylcholinesterase.
9. The circular muscle.
10. Voltage-gated sodium channels.
11. L-DOPA crosses into the brain, where it is converted to dopamine.
12. (6.0 − 3.0) / 6.0 × 100 = 50%.

Where marks are usually lost

• Writing “sodium moves in” without saying it diffuses through voltage-gated channels.
• Leaving out potassium ions when explaining repolarisation.
• Describing a “bigger” action potential for a bigger stimulus.
• Missing calcium ions in the synapse sequence.
• Saying that acetylcholine is “destroyed” instead of naming acetylcholinesterase and its products.
• Describing the impulse as an “electrical message” when the question asks for the ion movements.
• Giving saltatory conduction as “faster because of insulation” without saying that action potentials occur only at nodes.
• Mixing up nicotine (activates receptors) with alpha toxin (blocks them).
• Forgetting units for speed, potential and area.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, Pearson Education Limited. Statements 8.1 to 8.7 of Topic 8.