IB DP Biology: Neural signalling: neurons, action potentials and synapses (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover section C2.2 (Neural signalling) of the IB Biology guide, first assessment 2025, for DP Biology. Questions 1 to 6 test C2.2.1 to C2.2.7, which are SL and HL. Questions 7 to 12 test C2.2.8 to C2.2.16 and are labelled HL only. Read the study guide first if a topic is new, and use the revision notes to check recall. See also the IB DP Biology hub, the printable checklist and the free diagnostics. All data values are supplied for the question and are fictional.

Questions

1. (a) State the name of the single long fibre that projects from a neuron cell body. [1]

(b) State the name of the several shorter fibres. [1]

(c) State where the nucleus of a neuron is found. [1]

2. Explain how the sodium-potassium pump helps to produce a negative resting potential. [4]

3. An impulse travels 0.72 m along a myelinated fibre in 9.0 ms.

(a) Calculate the speed of the impulse in m s⁻¹. [2]

(b) A non-myelinated fibre conducts at 1.2 m s⁻¹. Calculate the time in ms for an impulse to travel the same distance. [2]

4. A student measured the reflex delay in fibres of different diameter (fictional data).

Fibre diameter (µm)	4	6	8	10	12	14
Reflex delay (ms)	19	16	13	12	9	8

The correlation coefficient r for these data is -0.988.

(a) Describe the correlation. [1]

(b) Calculate R², to 2 significant figures, and state what it shows. [3]

(c) State why the data do not prove that diameter causes the change in delay. [1]

5. Explain how the arrival of an action potential at a presynaptic membrane causes neurotransmitter to enter the synaptic cleft. [4]

6. Acetylcholine is released at a neuromuscular junction.

(a) Explain how acetylcholine produces an excitatory postsynaptic potential. [3]

(b) Explain why a signal passes in one direction only across a synapse. [2]

7. (HL only) An oscilloscope trace from an axon shows a resting potential of -70 mV. There are four identical peaks, each reaching +30 mV, at 3, 11, 19 and 27 ms.

(a) Calculate the amplitude of one action potential. [1]

(b) Calculate the number of impulses per second. [2]

(c) Explain the events at the membrane during the rising phase of a peak. [2]

(d) Explain the events during the falling phase. [2]

8. (HL only) (a) Explain how an action potential at one point on an axon causes an action potential in the next section. [3]

(b) Explain why impulses travel faster in myelinated fibres. [2]

9. (HL only) (a) Explain how a neonicotinoid pesticide affects synaptic transmission in an insect. [2]

(b) Explain how cocaine increases stimulation of postsynaptic neurons. [3]

10. (HL only) A neuron has a resting potential of -70 mV and a threshold of -55 mV. Neurons A, B and C each give an excitatory postsynaptic potential of +5, +7 and +4 mV. Neuron D gives an inhibitory postsynaptic potential of -6 mV.

(a) Determine the membrane potential when A, B and C fire together and state whether an action potential results. [2]

(b) Determine the potential when A, B, C and D all fire and state the outcome. [2]

(c) Explain what happens at the postsynaptic membrane to produce an inhibitory postsynaptic potential. [2]

11. (HL only) (a) Outline how capsaicin from chilli peppers causes the sensation of pain. [3]

(b) State what is meant by an emergent property, using consciousness as the example. [1]

12. (HL only) Axons from the same animal were treated with two toxins (fictional data).

Treatment	Resting potential (mV)	Highest potential reached (mV)	Time to repolarize (ms)
Control	-70	+32	1.5
Toxin A	-70	-58	not applicable
Toxin B	-70	+32	4.0

Toxin A blocks voltage-gated sodium channels. Toxin B blocks voltage-gated potassium channels.

(a) Compare the control with toxin A. [2]

(b) Explain the result for toxin A. [3]

(c) Calculate the percentage increase in repolarization time caused by toxin B. [2]

(d) Explain the result for toxin B. [3]

Answers

1. (a) The axon [1] (b) Dendrites [1] (c) In the cell body (with the cytoplasm) [1] [3] Examiner insight: A state question needs only the name; adding extra description gains nothing, so give the term and move on.

2. The pump uses energy from ATP [1]; it moves 3 sodium ions out and 2 potassium ions in each cycle [1]; so more positive charge leaves than enters [1]; this establishes concentration gradients (high sodium outside, high potassium inside) that, with potassium leaking out through open channels, make the inside negative [1]. [4] Examiner insight: Answers usually earn credit when the 3 to 2 ratio and the direction of each ion are both stated; saying only that the pump “creates a gradient” is too vague.

3. (a) Speed = distance ÷ time = 0.72 ÷ 0.0090 [1]; 80 m s⁻¹ [1] [2] (b) Time = 0.72 ÷ 1.2 = 0.60 s [1]; 600 ms [1] [2] Examiner insight: A frequent loss is leaving the time in ms during the first division; convert to seconds before dividing and give units on each answer.

4. (a) Strong negative correlation [1] [1] (b) R² = (-0.988)² [1]; = 0.98 (to 2 significant figures) [1]; about 98% of the variation in reflex delay is explained by the variation in diameter [1] [3] (c) Correlation shows an association only; other variables (for example temperature or myelination) could also affect delay [1] [1] Examiner insight: Stating R² as a number without saying what it explains about the dependent variable usually leaves the final point unearned.

5. The action potential depolarizes the presynaptic membrane [1]; voltage-gated calcium channels open and Ca²⁺ diffuses in [1]; calcium causes vesicles containing neurotransmitter to fuse with the membrane [1]; the neurotransmitter is released by exocytosis into the cleft [1]. [4] Examiner insight: Answers that skip calcium and jump from depolarization to release tend to lose the linking steps that an explain question rewards.

6. (a) Acetylcholine diffuses across the synaptic cleft [1]; it binds to transmembrane receptors on the postsynaptic membrane, opening channels [1]; positive ions (mainly sodium) enter and depolarize the membrane slightly, giving an EPSP [1] [3] (b) Neurotransmitter is released only from the presynaptic membrane [1]; receptors are located only on the postsynaptic membrane [1] [2] Examiner insight: “Receptors are on one side only” earns the second point only if the answer also says release happens from the other side.

7. (a) 30 - (-70) = 100 mV [1] [1] (b) Interval = 8 ms = 0.008 s [1]; 1 ÷ 0.008 = 125 impulses per second [1] [2] (c) Voltage-gated sodium channels open once threshold is reached [1]; sodium ions diffuse in, making the inside positive [1] [2] (d) Sodium channels close [1]; voltage-gated potassium channels open and potassium diffuses out, restoring the negative inside [1] [2] Examiner insight: Naming the ion, the channel type and the direction of movement for each phase is the pattern that attracts credit on trace-interpretation questions.

8. (a) Sodium ions entering at one point spread sideways, forming local currents inside and outside the axon [1]; this depolarizes the next section of membrane [1]; when it reaches threshold, its voltage-gated sodium channels open and a new action potential starts [1] [3] (b) Myelin insulates the axon, so ion movement happens only at the nodes of Ranvier, where pumps and channels are clustered [1]; the action potential jumps from node to node, which is faster than depolarizing the whole length [1] [2] Examiner insight: Answers that say only “myelin speeds up the impulse” restate the question; the node-to-node jump is the idea that explains the speed.

9. (a) The pesticide binds to acetylcholine receptors on the postsynaptic membrane [1]; this blocks normal synaptic transmission, disrupting the insect’s nervous system [1] [2] (b) Cocaine blocks reuptake of dopamine into the presynaptic neuron [1]; dopamine stays in the synaptic cleft for longer [1]; so receptors on the postsynaptic membrane are stimulated for longer [1] [3] Examiner insight: Naming the molecule that is blocked (the receptor or the reuptake transporter) separates the two chemicals; answers that treat them as the same lose the contrast.

10. (a) -70 + 5 + 7 + 4 = -54 mV [1]; this is above the -55 mV threshold, so an action potential results [1] [2] (b) -54 - 6 = -60 mV [1]; below threshold, so no action potential [1] [2] (c) The inhibitory neurotransmitter opens channels that make the inside more negative [1]; the postsynaptic membrane is hyperpolarized [1] [2] Examiner insight: Write the running total and the comparison with threshold explicitly; an examiner can then follow your reasoning even if one addition slips.

11. (a) Free nerve endings in the skin have cation channels that open in response to capsaicin [1]; positive ions enter and threshold is reached [1]; impulses pass to the brain, where pain is perceived [1] [3] (b) A property that arises from the interaction of many parts (here, neurons) and is not found in any single part [1] [1] Examiner insight: Pain is perceived in the brain, not at the skin; answers that place perception at the nerve ending usually lose this point.

12. (a) Resting potential is the same (-70 mV) [1]; toxin A reaches only -58 mV, but the control reaches +32 mV [1] [2] (b) Toxin A blocks voltage-gated sodium channels, so sodium cannot enter [1]; there is no regenerative depolarization, so the potential stays below the -55 mV threshold [1]; no action potential is propagated [1] [3] (c) (4.0 - 1.5) ÷ 1.5 × 100 [1]; 167% [1] [2] (d) Toxin B blocks voltage-gated potassium channels, so less potassium leaves [1]; repolarization is slower [1]; the peak is unchanged because sodium channels still open normally [1] [3] Examiner insight: Use the numbers from the table in a comparison, and link each toxin to the channel it blocks; answers that quote only one column usually leave marks unearned.

Where marks are usually lost

• Giving the ratio of pump movements the wrong way round.
• Omitting calcium from release of neurotransmitter.
• Writing that sodium “enters the axon” without naming the voltage-gated channel.
• Confusing depolarization (sodium in) with repolarization (potassium out).
• Forgetting to convert ms to s in speed calculations.
• Quoting R² with no meaning for the variables.
• Treating correlation as proof of cause.
• Leaving out the comparison with threshold in summation questions.
• Treating hyperpolarization as depolarization.

Next steps

• Review the revision notes and the study guide.
• Go to the IB DP Biology hub and tick off topics on the printable checklist.
• Try all free 10-minute diagnostics at /diagnostics/.
• Book a free trial class.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section C2.2 Neural signalling.