Neural signalling — IB Diploma Biology HL
IB Biology neural signalling: neurons, resting and action potentials, impulse speed and synapses, plus HL saltatory conduction and summation.
Revision notes
7 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.
An action potential starts when the membrane reaches the threshold potential, about −55 mV, and voltage-gated sodium channels open. Na⁺ diffuses in (depolarization), then voltage-gated potassium channels open and K⁺ diffuses out (repolarization). Local currents of Na⁺ bring the next region to threshold. An oscilloscope trace plots these changes in membrane potential against time.
Nerve impulses travel faster along wider axons and much faster along myelinated fibres. The giant axons of squid conduct faster than smaller non-myelinated fibres, but narrow myelinated fibres are faster still. At HL, myelinated fibres achieve faster impulses by saltatory conduction: the action potential jumps from one node of Ranvier to the next.
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.
Exogenous chemicals from outside the body can alter synaptic transmission: neonicotinoid pesticides block transmission at insect acetylcholine synapses, and cocaine blocks reuptake of dopamine. Inhibitory neurotransmitters hyperpolarize the postsynaptic membrane, producing inhibitory postsynaptic potentials. A postsynaptic neuron sums excitatory and inhibitory inputs and fires only if the threshold potential is reached.
Pain is perceived when neurons with free nerve endings in the skin are stimulated. Their nerve endings have channels for positively charged ions that open in response to high temperature, acid or chemicals such as capsaicin in chilli peppers. Positive ions enter, the threshold is reached and impulses travel to the brain, where pain is perceived.
Exam questions
8 exam-style questions (29 marks), each with its mark scheme.
Answer the questionsFlashcards
22 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.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
What are the two types of fibre that project from the cell body of a neuron?
Show answer
One long axon and multiple shorter dendrites.
How many sodium and potassium ions does one cycle of the sodium–potassium pump move?
Show answer
Three Na⁺ out and two K⁺ in, using one ATP.
Why can a signal only pass one way across a synapse?
Show answer
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?
Show answer
Calcium ions (Ca²⁺).
HL only Where are ion pumps and channels clustered in a myelinated fibre?
Show answer
At the nodes of Ranvier.
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.
Why do myelinated neurons conduct impulses faster?
HL only Myelin insulates the axon, so action potentials can only occur at the nodes of Ranvier, where ion pumps and channels are clustered. Local currents spread under the myelin to the next node, so the impulse jumps from node to node. This saltatory conduction is much faster than depolarizing every part of the membrane in turn.
How does cocaine affect synapses?
HL only Cocaine blocks the reuptake of the neurotransmitter dopamine. It binds to the transporter proteins that normally take dopamine back into the presynaptic neuron, so dopamine stays in the synaptic cleft and keeps stimulating the postsynaptic neurons. This prolonged stimulation of reward pathways in the brain causes euphoria and leads to addiction.
What happens during depolarization and repolarization?
HL only During depolarization, the threshold potential is reached and voltage-gated sodium channels open, so Na⁺ diffuses in and the inside of the axon becomes positive. During repolarization, sodium channels close and voltage-gated potassium channels open, so K⁺ diffuses out and the inside becomes negative again, returning towards the resting potential.
Written and checked against the IB Biology HL specification · Updated October 2026