HabituationSpec 8.9
In short
Habituation is a decrease in the response to a repeated, harmless stimulus. It happens because fewer calcium channels open in the presynaptic knob, so less neurotransmitter is released and the postsynaptic neurone is less likely to reach threshold. It lets an animal ignore irrelevant stimuli and respond to new, important ones.
Habituation is a form of simple learning in which an organism gradually stops responding to a stimulus that is repeated but has no good or bad consequence. For example, a bird stops flying off at a scarecrow, or you stop noticing the sound of a clock.
- Each repeated stimulus causes fewer voltage-gated Ca²⁺ channels to open in the presynaptic knob.
- Less Ca²⁺ enters, so fewer vesicles release neurotransmitter.
- The postsynaptic membrane is less depolarised and the postsynaptic neurone is less likely to reach threshold.
- The response (for example a muscle contraction) becomes smaller and may stop.
Habituation is useful because it stops an animal wasting energy on irrelevant stimuli and keeps its attention available for new stimuli that may signal food or danger. It differs from fatigue because a different stimulus restores the full response straight away.
Recommended additional practical: investigate habituation to a stimulus. Choose a response that can be scored, for example the withdrawal of a snail’s tentacles when touched with a thin rod, or the number of times a stimulus causes a response in set intervals. Repeat the stimulus at a regular interval; record the response each time; control the strength, interval and position of the stimulus and temperature; test a different stimulus at the end to show it is not fatigue. Ethics: use minimal stimulation, handle animals gently and return them to their habitat.
Quick check
What is the resting potential of an axon, approximately?
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About –70 mV (the inside is negative relative to the outside).
Which ion enters the presynaptic knob and causes vesicles to release acetylcholine?
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Calcium ions (Ca²⁺).
What does acetylcholinesterase do?
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It hydrolyses acetylcholine to choline and ethanoate in the synaptic cleft, ending the stimulation.
What is saltatory conduction?
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Conduction in which the action potential jumps from one node of Ranvier to the next along a myelinated axon.
What happens to a rod cell’s membrane potential when light strikes it?
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It becomes hyperpolarised, because rhodopsin splits and the cation channels close.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026