Synaptic transmissionAQA A-Level Biology: Subtopic test

1. 1
   A neurophysiologist is studying a cholinergic synapse between a presynaptic neurone and a postsynaptic neurone in an autonomic ganglion removed from a mammal. She stimulates the presynaptic neurone so that action potentials arrive at its synaptic knob, and records the response of the postsynaptic neurone.

   (a)

   Action potentials arrive at the synaptic knob. What is the immediate effect of calcium ions entering the knob?

   [1 mark]

   • AVesicles containing acetylcholine fuse with the presynaptic membrane
   • BAcetylcholine binds to receptors on the presynaptic membrane
   • CSodium ion channels open in the postsynaptic membrane
   • DAcetylcholinesterase is released into the synaptic cleft

   (b)

   Acetylcholine remains in the synaptic cleft after binding to receptors. Which statement describes how its effect is ended?

   [1 mark]

   • AIt diffuses back into the presynaptic neurone intact and is reused
   • BIt is broken down by an enzyme in the postsynaptic cytoplasm
   • CIt is hydrolysed in the cleft by acetylcholinesterase and the products are reabsorbed by the presynaptic neurone
   • DIt is carried away by the blood to the liver for destruction

   (c)

   Explain why transmission across the synapse can only occur in one direction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A postsynaptic neurone in a spinal cord preparation has a resting potential of −70 mV and a threshold potential of −55 mV. Each time an excitatory presynaptic neurone releases neurotransmitter, it depolarises the postsynaptic membrane by about 5 mV; this depolarisation fades within a few milliseconds.

   (a)

   One presynaptic neurone releases neurotransmitter four times in 8 ms. Each 5 mV depolarisation arrives before the previous one has faded, so the depolarisations add together (assume that none of the earlier depolarisation has been lost). Which row describes the outcome?

   [1 mark]

   • ANo action potential, because each depolarisation is below threshold
   • BA threshold of −55 mV is reached by spatial summation
   • CThe postsynaptic membrane hyperpolarises because the effects are added
   • DAn action potential is initiated by temporal summation of repeated releases from the same neurone

   (b)

   Instead, four different presynaptic neurones each release neurotransmitter at the same moment onto the postsynaptic neurone. What happens, and why?

   [1 mark]

   • AThe membrane stays at −65 mV because each neurone acts independently
   • BThe membrane depolarises to −50 mV and an action potential is generated, by spatial summation
   • CThe membrane depolarises to −50 mV and an action potential is generated, by temporal summation
   • DThe membrane hyperpolarises to −90 mV because of the combined release

   (c)

   A different presynaptic neurone is inhibitory. Explain how it would reduce the chance of an action potential in the postsynaptic neurone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student prepares a frog nerve–muscle preparation in which a motor neurone is still attached to a skeletal muscle fibre. The motor neurone ends at a neuromuscular junction, which uses the same neurotransmitter as the cholinergic synapse in an autonomic ganglion.

   (a)

   Compare transmission across a cholinergic synapse between two neurones with transmission across a neuromuscular junction.

   [3 marks]

   (b)

   The student bathes the junction in a solution that blocks the voltage-gated calcium ion channels in the presynaptic membrane, then stimulates the motor neurone. Predict and explain the effect on the muscle fibre.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two chemicals are tested on a neuromuscular junction. Drug A has a molecular shape similar to acetylcholine; it binds to acetylcholine receptors on the muscle fibre membrane but does not open the associated sodium ion channels, and acetylcholinesterase does not break it down. Drug B is an insecticide that irreversibly inhibits acetylcholinesterase.

   (a)

   Predict and explain the effect of drug A on the contraction of the muscle. Use your knowledge of transmission across a neuromuscular junction.

   [6 marks]

   (b)

   Predict and explain the effect of drug B on the muscle. Use your knowledge of transmission across a neuromuscular junction.

   [6 marks]

   Total for question 4: 12 marks

End of questions