Neural signalling — IB Diploma Biology SL
IB Biology neural signalling: neurons, resting and action potentials, the speed of nerve impulses and synapses.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Neurons are cells within the nervous system that carry electrical impulses. A cell body with the nucleus has fibres projecting from it: one long axon and many shorter dendrites. The resting potential, about −70 mV, is generated by sodium–potassium pumps using ATP to move Na⁺ out and K⁺ in, establishing and maintaining concentration gradients.
A nerve impulse is an action potential that is propagated along a nerve fibre. An action potential is a rapid reversal of the membrane potential, from negative inside to positive inside, followed by a return to the resting potential. Nerve impulses are electrical because they involve movement of positively charged ions, Na⁺ and K⁺, across the membrane.
Impulse speed rises with axon diameter and myelination. Squid giant axons, correlation coefficients and R².
A synapse is a junction between two neurons or between a neuron and an effector cell. Depolarization of the presynaptic membrane lets calcium ions in, which triggers release of neurotransmitter by exocytosis. Acetylcholine diffuses across the synaptic cleft, binds to transmembrane receptors and lets positive ions in, generating an excitatory postsynaptic potential.
Exam questions
4 exam-style questions (12 marks), each with its mark scheme.
Answer the questionsFlashcards
12 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells (interaction and interdependence) on one page, so you can see where this subtopic fits.
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Quick check questions
What are the two types of fibre that project from the cell body of a neuron?
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One long axon and multiple shorter dendrites.
How many sodium and potassium ions does one cycle of the sodium–potassium pump move?
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Three Na⁺ out and two K⁺ in, using one ATP.
Why can a signal only pass one way across a synapse?
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Only the presynaptic neuron releases neurotransmitter, and only the postsynaptic membrane has receptors for it.
What ion enters the presynaptic neuron and triggers neurotransmitter release?
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Calcium ions (Ca²⁺).
Frequently asked questions
Why is the resting potential negative?
The resting potential is negative because sodium–potassium pumps move three Na⁺ out for every two K⁺ in, the membrane lets K⁺ leak out much faster than Na⁺ leaks in, and negatively charged proteins stay inside the cytoplasm. Together these leave the inside of the neuron about 70 mV more negative than the outside.
How does a nerve impulse cross the synapse?
The impulse depolarizes the presynaptic membrane, so calcium ions enter and cause vesicles to release neurotransmitter by exocytosis. The neurotransmitter, such as acetylcholine, diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Positive ions enter and depolarize it; if threshold is reached, a new action potential starts.
Written and checked against the IB Biology SL specification · Updated October 2026