All subjects · Biology · WBI15 (Unit 5)

Synapses, the pupil reflex and habituation

Neurones pass signals to each other across synapses using chemicals. Synapses explain how reflexes such as the pupil reflex work, and why we stop responding to harmless repeated stimuli.

Transmission at a cholinergic synapse

A synapse is a junction between two neurones (or a neurone and an effector), separated by a tiny gap, the synaptic cleft.

1. An action potential arrives at the presynaptic knob.

2. Voltage-gated Ca2+ channels open and Ca2+ diffuses in.

3. Vesicles of acetylcholine (ACh) move to and fuse with the presynaptic membrane, releasing ACh into the cleft by exocytosis.

4. ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane.

5. Na+ channels open, Na+ flows in and the postsynaptic membrane depolarises. If threshold is reached, an action potential starts.

6. Acetylcholinesterase breaks ACh down into choline and ethanoic acid (acetate), which return to the presynaptic knob to be recycled using ATP. This stops continuous stimulation.

Transmission is one way, because only the presynaptic side releases neurotransmitter and only the postsynaptic side has receptors. Inhibitory synapses make the postsynaptic membrane more negative, making an action potential less likely.

The pupil reflex

The iris has circular and radial muscles controlled by the autonomic nervous system.

Bright light: photoreceptors in the retina send impulses along sensory neurones to the brain, then along parasympathetic motor neurones. The circular muscles contract and radial muscles relax: the pupil constricts, protecting the retina.
Dim light: sympathetic neurones make the radial muscles contract and circular muscles relax: the pupil dilates, letting in more light.

Habituation

Habituation is a reduced response to a repeated stimulus that is harmless, such as a snail that stops withdrawing into its shell when touched repeatedly, or ignoring background traffic noise.

With repeated stimulation, fewer Ca2+ channels open in the presynaptic membrane, so less Ca2+ enters, less neurotransmitter is released, and the postsynaptic membrane is less depolarised. Habituation saves energy and lets the animal attend to important stimuli.

Worked example

Explain why an impulse can cross a synapse in only one direction.

Vesicles of neurotransmitter are found only in the presynaptic knob.
Receptors for the neurotransmitter are found only on the postsynaptic membrane.

Answer: Only one side releases transmitter and only the other side can detect it.

Key idea: Action potential → Ca2+ in → vesicles release ACh → ACh binds postsynaptic receptors → Na+ in, depolarisation → acetylcholinesterase breaks down ACh. Bright light: parasympathetic, circular muscles contract, pupil constricts. Habituation: fewer Ca2+ channels open, less transmitter released.

Check yourself

What triggers the release of neurotransmitter from the presynaptic knob?
- Sodium ions leaving the knob
- Calcium ions entering through voltage-gated channels
- Acetylcholinesterase
- Potassium ions entering the cleft
Show answer
Calcium ions entering through voltage-gated channels. Ca2+ influx makes vesicles fuse with the membrane.
What is the role of acetylcholinesterase?
- It makes acetylcholine
- It opens Ca2+ channels
- It carries the impulse along the axon
- It breaks down acetylcholine in the synaptic cleft
Show answer
It breaks down acetylcholine in the synaptic cleft. Removing ACh stops the postsynaptic membrane being continuously stimulated.
Why is synaptic transmission unidirectional?
- The postsynaptic membrane also releases acetylcholine
- Only the postsynaptic membrane has receptors for the neurotransmitter
- The cleft is too narrow for diffusion
- Ca2+ channels are on the postsynaptic membrane only
Show answer
Only the postsynaptic membrane has receptors for the neurotransmitter. Receptors are only on the postsynaptic side.
In bright light, how does the pupil constrict?
- Radial muscles contract, controlled by sympathetic neurones
- The lens becomes thicker
- Circular muscles of the iris contract, controlled by parasympathetic neurones
- The retina shrinks
Show answer
Circular muscles of the iris contract, controlled by parasympathetic neurones. Circular muscle contraction narrows the pupil.
Which muscles contract to dilate the pupil in dim light?
- Radial muscles
- Circular muscles
- Ciliary muscles
- Intercostal muscles
Show answer
Radial muscles. Radial muscles pull the pupil wider.
What happens at the synapse during habituation?
- More neurotransmitter is released
- Acetylcholinesterase stops working
- Fewer Ca2+ channels open, so less neurotransmitter is released
- More receptors are made
Show answer
Fewer Ca2+ channels open, so less neurotransmitter is released. Reduced Ca2+ entry means smaller responses.
Which is an example of habituation?
- A snail stops withdrawing after being touched gently many times
- Pulling your hand away from a flame
- Pupil constricting in bright light
- Salivating at the sight of food after training
Show answer
A snail stops withdrawing after being touched gently many times. A reduced response to a repeated, harmless stimulus.

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Last updated 7 November 2026. Spotted a mistake? Tell us.