1. 1
   An electrode inserted into a squid giant axon is connected to an oscilloscope. At rest the trace is steady at −70 mV. After a stimulus, the potential rises to −55 mV, then very rapidly to +35 mV within 0.5 ms, falls to −80 mV by 2 ms and returns to −70 mV by 4 ms. During a period of repeated stimulation, 10 identical action potentials are recorded in 50 ms.

   (a)

   What causes the rapid rise from −55 mV to +35 mV?

   [1 mark]

   • AVoltage-gated potassium channels open and K⁺ diffuses out
   • BThe sodium–potassium pump moves Na⁺ into the axon
   • CVoltage-gated sodium channels open and Na⁺ diffuses in
   • DCalcium ions diffuse out of the axon

   (b)

   Which explains why the potential falls to −80 mV before returning to −70 mV?

   [1 mark]

   • ASodium channels remain open after the peak
   • BPotassium channels close slowly, so K⁺ continues to diffuse out
   • CThe sodium–potassium pump stops working
   • DChloride ions are pumped out of the axon

   (c)

   Calculate the frequency of impulses and state how the trace would differ if a stronger stimulus were applied.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Botulinum toxin, produced by the bacterium Clostridium botulinum, is taken up by the nerve endings of motor neurons at neuromuscular junctions. It destroys proteins that allow vesicles to fuse with the presynaptic membrane. Impulses still arrive at the nerve endings and calcium ions still enter, but the muscles supplied become paralysed. Tiny doses are injected into facial muscles in cosmetic treatments.

   (a)

   Which neurotransmitter is released at the neuromuscular junctions affected by the toxin?

   [1 mark]

   • AAcetylcholine
   • BDopamine
   • CAdrenaline
   • DGlucagon

   (b)

   By which process is neurotransmitter normally released from the presynaptic membrane?

   [1 mark]

   • AActive transport through pump proteins
   • BOsmosis
   • CDiffusion through the phospholipid bilayer
   • DExocytosis of vesicles

   (c)

   Explain why the toxin causes paralysis of the muscle.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Forager honeybees were fed sugar syrup containing the neonicotinoid pesticide imidacloprid at 0, 1 or 10 µg per kg of syrup and released 1 km from their hive. The percentages that returned to the hive within 2 hours were 92%, 78% and 41% respectively. In mammals, cocaine binds to transporter proteins in presynaptic membranes that normally reabsorb the neurotransmitter dopamine from the synaptic cleft.

   (a)

   Compare the ways in which neonicotinoids and cocaine affect synaptic transmission.

   [3 marks]

   (b)

   Evaluate the evidence from this study that the use of neonicotinoids contributes to declines in bee populations.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Capsaicin, the compound that makes chilli peppers taste hot, binds to receptor proteins in the membranes of free nerve endings in the mouth and skin; the same receptors also open at temperatures above about 43 °C. The fibres of many of these pain neurons are myelinated. In the spinal cord the pain neurons synapse with relay neurons that carry the signal to the brain. The relay neurons also receive synapses from interneurons that release an inhibitory neurotransmitter, and these interneurons are activated when the skin near an injury is rubbed.

   (a)

   Explain how capsaicin leads to nerve impulses being transmitted along a pain neuron.

   [6 marks]

   (b)

   Explain how excitatory and inhibitory neurotransmitters together determine whether a relay neuron passes the pain signal to the brain, and suggest why rubbing the skin near an injury can reduce the pain felt.

   [6 marks]

   Total for question 4: 12 marks

End of questions