Synapses and the pupil reflexEdexcel International A Level Biology: Subtopic test

1. 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.

   (a)

   Which event occurs immediately after an action potential arrives at the presynaptic knob?

   [1 mark]

   • AAcetylcholine binds to receptors on the postsynaptic membrane
   • BVoltage-gated calcium ion channels open in the presynaptic membrane
   • CAcetylcholinesterase hydrolyses acetylcholine in the synaptic cleft
   • DSodium ion channels open in the postsynaptic membrane

   (b)

   What happens to acetylcholine in the synaptic cleft after it has stimulated the postsynaptic membrane?

   [1 mark]

   • AIt is absorbed unchanged into the postsynaptic neurone
   • BIt is converted into noradrenaline
   • CIt diffuses back into the presynaptic knob unchanged and is stored in vesicles
   • DIt is hydrolysed by acetylcholinesterase into choline and ethanoate

   (c)

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

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which term describes the build-up of depolarisation in the postsynaptic neurone caused by rapid repeated stimuli from one presynaptic neurone?

   [1 mark]

   • ATemporal summation
   • BSpatial summation
   • CHabituation
   • DSaltatory conduction

   (b)

   Which statement correctly describes how the postsynaptic membrane is depolarised?

   [1 mark]

   • ACalcium ions leave the postsynaptic neurone through voltage-gated channels
   • BSodium ions are actively transported into the neurone by a pump
   • CSodium ions diffuse in through channels that open when acetylcholine binds to receptors
   • DPotassium ions diffuse into the neurone through the acetylcholine receptors

   (c)

   Explain why a single stimulus failed to produce an action potential in the postsynaptic neurone, but several rapid stimuli did.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   Explain the importance of the pupil constricting in bright light, and explain how the iris muscles make the pupil widen when the room lights are dimmed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A revision guide states that the gap between two neurones at a cholinergic synapse is only about 20 nm wide, that the presynaptic knob contains many mitochondria and vesicles, and that the postsynaptic membrane carries specific receptor proteins.

   (a)

   Explain how the structure of a cholinergic synapse is adapted for its function.

   [6 marks]

   (b)

   Describe the sequence of events from the arrival of an action potential at the presynaptic knob to the production of a new action potential, and explain why acetylcholine must then be removed from the cleft.

   [6 marks]

   Total for question 4: 12 marks

End of questions