Nerve impulses and synapsesEdexcel A-Level Biology B: Subtopic test

1. 1
   A researcher records the potential difference across the membrane of a resting mammalian axon using a microelectrode. The inside of the axon is 70 mV negative relative to the outside. The membrane contains sodium-potassium pumps and potassium ion leak channels, but very few sodium ion leak channels.

   (a)

   Which statement describes the action of the sodium-potassium pump?

   [1 mark]

   • AIt moves 2 Na⁺ ions out and 3 K⁺ ions in by active transport
   • BIt moves 3 K⁺ ions out and 2 Na⁺ ions in by active transport
   • CIt moves 3 Na⁺ ions out and 2 K⁺ ions in by active transport
   • DIt moves 3 Na⁺ ions out and 2 K⁺ ions in by facilitated diffusion

   (b)

   What is the main reason the inside of the axon is negative at rest?

   [1 mark]

   • AThere is a higher concentration of Na⁺ inside the axon than outside
   • BThe membrane is impermeable to K⁺ ions
   • CCl⁻ ions are pumped into the axon
   • DK⁺ ions leak out faster than Na⁺ ions leak in, and the pump removes more positive charge than it brings in

   (c)

   Predict what would happen to the resting potential over several hours if the axon was supplied with a poison that stopped ATP production. Explain your answer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A neurone is stimulated and the membrane potential is recorded. The potential rises slowly from −70 mV to about −55 mV, then rises rapidly to +40 mV. It then falls rapidly, overshoots to about −80 mV, and finally returns to −70 mV. The whole event lasts about 3 ms.

   (a)

   What causes the rapid rise in potential from −55 mV to +40 mV?

   [1 mark]

   • ANa⁺ ions entering through open voltage-gated Na⁺ channels
   • BK⁺ ions leaving through open voltage-gated K⁺ channels
   • CThe sodium-potassium pump moving Na⁺ ions into the axon
   • DCl⁻ ions leaving through leak channels

   (b)

   What is the main function of the refractory period in the propagation of impulses?

   [1 mark]

   • AIt allows impulses to travel in both directions along the axon
   • BIt ensures that impulses travel in one direction and remain separate
   • CIt increases the speed of the impulse
   • DIt restores the resting concentration gradients of ions

   (c)

   Explain the fall in potential from +40 mV to below −70 mV.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A myelinated mammalian motor axon conducts impulses at 80 m s⁻¹, whereas a non-myelinated axon of similar diameter conducts impulses at 1 m s⁻¹. In the myelinated axon, Schwann cells wrap the axon in layers of myelin, leaving small gaps, the nodes of Ranvier, about 1.5 mm apart. A motor axon carrying impulses from the spinal cord to a foot muscle is 1.2 m long.

   (a)

   Explain why the myelinated axon conducts impulses faster than the non-myelinated axon.

   [3 marks]

   (b)

   Calculate how much longer an impulse would take to travel along the 1.2 m axon if it were non-myelinated. Explain why a disease that destroys myelin slows conduction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A neurone in the central nervous system has a resting potential of −70 mV and a threshold of −55 mV. It receives inputs from several presynaptic neurones. Some of these synapses are excitatory and release acetylcholine, and others are inhibitory. A single impulse arriving at one excitatory synapse produces a depolarisation of 5 mV that does not trigger an action potential. Several impulses arriving close together in time, or at several synapses at once, do trigger one.

   (a)

   Explain how an impulse arriving at the presynaptic knob of an excitatory acetylcholine synapse leads to depolarisation of the postsynaptic membrane.

   [6 marks]

   (b)

   Explain why a single impulse at one excitatory synapse fails to trigger an action potential while several together succeed, and how inhibitory synapses affect the outcome. Name one other transmitter that can act at excitatory synapses.

   [6 marks]

   Total for question 4: 12 marks

End of questions