Drugs and the nervous systemEdexcel A-Level Biology B: Subtopic test

1. 1
   Nicotine from tobacco smoke reaches the brain and the junctions between motor neurones and muscles, where acetylcholine is the transmitter. The shape of the nicotine molecule is similar to that of acetylcholine.

   (a)

   How does nicotine affect an acetylcholine synapse?

   [1 mark]

   • AIt blocks the acetylcholine receptors
   • BIt inhibits the release of acetylcholine from vesicles
   • CIt prevents Ca²⁺ ions entering the presynaptic knob
   • DIt binds to acetylcholine receptors and mimics the effect of acetylcholine

   (b)

   Why does nicotine stimulate a postsynaptic neurone for longer than acetylcholine does?

   [1 mark]

   • AIt is not broken down by acetylcholinesterase, so it stays bound to receptors
   • BIt is hydrolysed more quickly than acetylcholine
   • CIt increases the entry of Ca²⁺ ions into the presynaptic knob
   • DIt is released in larger amounts from the presynaptic vesicles

   (c)

   Explain why nicotine can cause a postsynaptic neurone to fire an action potential when no acetylcholine has been released.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A dentist injects lidocaine, a local anaesthetic, close to a sensory nerve in a patient's jaw before filling a tooth. Lidocaine enters the sensory neurones. While the anaesthetic lasts, the patient feels no pain from the tooth, even though the pain receptors there are still stimulated by the drill.

   (a)

   Which protein does lidocaine block?

   [1 mark]

   • AVoltage-gated Ca²⁺ channels
   • BVoltage-gated Na⁺ channels
   • CK⁺ leak channels
   • DAcetylcholine receptors

   (b)

   What happens to the membrane potential of the sensory neurone when the drill stimulates a pain receptor in the presence of lidocaine?

   [1 mark]

   • AIt hyperpolarises to about −90 mV
   • BIt rises rapidly to about +40 mV
   • CIt changes slightly but stays below threshold, so no action potential forms
   • DIt does not change at all

   (c)

   Explain why the patient feels no pain.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A man is bitten by a cobra. Within an hour his muscles weaken and he has difficulty breathing. Cobra venom contains a toxin that binds to acetylcholine receptors on the postsynaptic membrane at neuromuscular junctions without opening the channel. In a laboratory, a motor neurone and its muscle are stimulated electrically, and the maximum force of muscle contraction is recorded. Before the venom is added the force is 5.0 N, and 10 minutes after the venom is added it is 1.0 N.

   (a)

   Explain why the man's muscles became weak and he had difficulty breathing.

   [3 marks]

   (b)

   Calculate the percentage decrease in the force of contraction after 10 minutes. Suggest why giving a drug that inhibits acetylcholinesterase might partly restore the force.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pharmacology class uses a preparation in which a motor neurone is connected to a muscle by a neuromuscular junction that uses acetylcholine. The motor neurone can be stimulated electrically, and the contraction of the muscle is recorded. The class will test three substances separately: nicotine, lidocaine and a cobra venom toxin.

   (a)

   Explain how each of the three substances affects the transmission of impulses in the preparation.

   [6 marks]

   (b)

   Describe how the class could use the preparation, with electrical stimulation and acetylcholine, to show whether a substance acts on the axon or on the acetylcholine receptors.

   [6 marks]

   Total for question 4: 12 marks

End of questions