All revision notes topics

Synapses and the pupil reflexEdexcel International A Level Biology: Revision notes

Section 1

Structure of a cholinergic synapse

A synapse is a junction between two neurones. The presynaptic neurone ends in a swollen presynaptic knob, separated from the postsynaptic neurone by the synaptic cleft, about 20 nm wide. A cholinergic synapse uses the neurotransmitter acetylcholine (ACh).

Key features:

  • Presynaptic knob: many mitochondria (ATP), many vesicles containing ACh, and voltage-gated calcium ion channels in the membrane
  • Synaptic cleft: contains acetylcholinesterase, an enzyme that breaks down ACh
  • Postsynaptic membrane: receptors that bind ACh, associated with sodium ion channels
Key termssynapsesynaptic cleftneurotransmitteracetylcholine

Section 2

Transmission across the synapse

  1. An action potential arrives at the presynaptic knob.
  2. Voltage-gated calcium ion channels open; Ca²⁺ ions diffuse into the knob.
  3. Calcium ions cause vesicles to fuse with the presynaptic membrane and release ACh by exocytosis.
  4. ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane.
  5. Sodium ion channels open; Na⁺ ions diffuse in and depolarise the membrane.
  6. If threshold is reached, a new action potential is generated in the postsynaptic neurone.
Key termsexocytosisdepolarisationthreshold
Exam tip

Learn the order: action potential, calcium in, vesicles fuse, ACh released, ACh binds, sodium in. Marks are lost by swapping calcium and sodium.

Section 3

Ending the signal, one-way flow and summation

Acetylcholinesterase hydrolyses ACh into choline and ethanoate. These diffuse back into the presynaptic knob and are used, with ATP from mitochondria, to resynthesise ACh. Removing ACh stops the postsynaptic membrane being stimulated continuously.

Synapses are unidirectional: vesicles of ACh are only in the presynaptic knob and receptors are only on the postsynaptic membrane.

One impulse may release too little ACh to reach threshold. Temporal summation: rapid impulses from one presynaptic neurone add together. Spatial summation: impulses from several presynaptic neurones add together.

Key termsacetylcholinesteraseunidirectionaltemporal summationspatial summation
Common mistake

Calcium ions enter the presynaptic knob; sodium ions enter the postsynaptic neurone. Do not mix them up.

Section 4

The pupil reflex pathway

The pupil reflex is a rapid, involuntary response that protects the retina. In bright light:

  1. Photoreceptors in the retina detect the light.
  2. Impulses pass along sensory neurones in the optic nerve to the brain.
  3. Via a relay neurone, impulses pass along motor neurones to the iris.
  4. The effector is the muscle of the iris, which changes the size of the pupil.

The pathway contains synapses, which use neurotransmitters such as acetylcholine.

Key termsreflexeffector

Section 5

Pupil contraction and dilation

The iris has two sets of muscle:

  • Circular muscles form rings around the pupil.
  • Radial muscles run outwards like spokes.

Bright light: circular muscles contract, radial muscles relax, so the pupil constricts and less light enters. This protects the retina from damage.

Dim light: radial muscles contract, circular muscles relax, so the pupil dilates and more light reaches the retina, letting the person see.

The two sets of muscle work as an antagonistic pair.

Key termscircular musclesradial musclesantagonistic
Common mistake

Say which muscle contracts. 'The iris contracts' does not gain credit, and the pupil is a hole, so it cannot contract itself.

Must know

  • A synapse has a presynaptic knob, a cleft of about 20 nm and a postsynaptic membrane with receptors
  • Calcium ions enter the knob, vesicles release ACh, and sodium ions enter the postsynaptic neurone
  • Acetylcholinesterase breaks ACh down into choline and ethanoate
  • Synapses are one-way and allow summation
  • Bright light: circular muscles contract, radial relax, pupil constricts
  • Dim light: radial muscles contract, circular relax, pupil dilates

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Synapses and the pupil reflex

  1. A neurophysiologist is studying a cholinergic synapse between two neurones in a tissue sample. She stimulates the presynaptic neurone and records the electrical changes in the postsynaptic neurone.
    Explain why an impulse can cross this synapse in one direction only.2 marks
  2. In a second experiment, the researcher finds that a single stimulus to the presynaptic neurone does not produce an action potential in the postsynaptic neurone, but several stimuli delivered in rapid succession do.
    Explain why a single stimulus failed to produce an action potential in the postsynaptic neurone, but several rapid stimuli did.2 marks
  3. An optician shines a bright torch into one eye of a patient sitting in a dim room. The pupil quickly becomes smaller. When the torch is switched off and the room lights are dimmed, the pupil gradually becomes wider again.
    Describe how the reflex pathway and the iris muscles bring about the change in pupil size when the bright light is shone into the eye.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).