IB Biology: Neurons & Nerve Impulses — FAQs, Exam Tips & Common Mistakes

Neurons and nerve impulses covers how nerve cells generate and transmit electrical signals — resting potential, action potentials, saltatory conduction and, at HL, synaptic transmission. According to the IB Biology guide (first assessment 2025), it sits within Theme C: Interaction and interdependence, and appears in Paper 1 multiple-choice and Paper 2 data-based questions.

SL students learn neuron structure, resting potential and the basic action potential graph. HL students add synaptic transmission in real depth — neurotransmitter release, calcium influx, reuptake, and drug effects at synapses — plus the factors affecting conduction speed. Both groups need the all-or-nothing principle, but only HL gets tested on synapse biochemistry.

Resting potential is the roughly -70mV charge difference across a neuron's membrane when it isn't firing. It's maintained mainly by the sodium-potassium pump, which actively moves 3 Na+ out for every 2 K+ in, while the membrane's greater permeability to K+ lets positive charge leak outward, keeping the inside negative.

An action potential is a rapid, all-or-nothing reversal of membrane charge once a stimulus reaches threshold, roughly -55mV. Voltage-gated Na+ channels open first, causing depolarisation to around +30mV; then K+ channels open, causing repolarisation and a brief hyperpolarisation before the sodium-potassium pump restores the -70mV resting state.

Saltatory conduction is when a nerve impulse jumps between the nodes of Ranvier rather than travelling continuously along the axon, and it's only possible because myelin sheaths — formed by Schwann cells — insulate the sections in between. It can speed transmission up considerably compared with an unmyelinated axon of the same diameter.

Synaptic transmission is how an electrical impulse crosses the gap between two neurons using chemical neurotransmitters. An arriving action potential triggers calcium influx, vesicle fusion, and neurotransmitter release into the synaptic cleft, where it binds receptors on the postsynaptic membrane. This full mechanism, including reuptake and drug interference, is HL-only content.

The single biggest mistake is confusing depolarisation with repolarisation, or claiming a stronger stimulus produces a bigger action potential — it doesn't; only the frequency of impulses changes, never the amplitude. Students also muddle voltage-gated channels (found along the axon) with ligand-gated ones (found only at the synapse).

Examiners test the all-or-nothing principle by asking why a stronger stimulus doesn't produce a bigger action potential — the correct answer is that impulse frequency increases, not amplitude. Refractory period questions usually ask why impulses travel in one direction only: Na+ channels just behind the impulse are inactivated, so it can't fire backward.

Yes — most students I've taught find this one of the trickier topics in the Interaction and interdependence theme, mainly because it mixes precise numerical detail (membrane potentials in mV) with a process that has to be explained in strict sequence. It rewards structured, step-by-step revision far more than raw memorising.

There's no fixed mark allocation published by the IB, but in my experience Paper 1 typically includes one multiple-choice item directly testing this content, while Paper 2 often features a data-based question worth roughly 6-8 marks, built around an action potential or oscilloscope-style graph.

Start by drawing the action potential graph from memory with every phase labelled, then explain each stage out loud in the correct order — that sequencing skill is the single biggest predictor of a strong answer I see in exam scripts. Only add HL synapse detail once the core graph is automatic.

A typical question gives an oscilloscope trace and asks you to label resting potential, threshold, depolarisation and repolarisation, then explain what happens if a neurotoxin blocks voltage-gated Na+ channels. The correct answer links channel blockage directly to the neuron's inability to depolarise — not just "the impulse stops".

Class notes cover the syllabus content, but what closes the gap between understanding the theory and scoring well on a data-based question is targeted past-paper practice. On revisionprep.com, the Biology Revision Notes and Topical Worksheets cover neurons and nerve impulses with exam-style questions, at a fraction of the cost of a full private course.