Action potentials and saltatory conductionEdexcel International A Level Biology: Subtopic test

1. 1
   A neuroscientist stimulates an axon in a laboratory and records the potential difference across its membrane. At rest the inside of the axon is at −70 mV relative to the outside. After a stimulus the potential difference rises rapidly to +40 mV and then falls back to −70 mV.

   (a)

   Which event causes the rapid rise from −70 mV towards +40 mV?

   [1 mark]

   • AVoltage-gated potassium ion channels open and potassium ions diffuse out
   • BVoltage-gated sodium ion channels open and sodium ions diffuse into the axon
   • CThe sodium-potassium pump moves three sodium ions out of the axon
   • DChloride ions are actively transported into the axon

   (b)

   Which event causes the potential difference to fall from +40 mV back towards the resting value?

   [1 mark]

   • AMore voltage-gated sodium ion channels open
   • BSodium ions are pumped into the axon by active transport
   • CVoltage-gated sodium ion channels close and voltage-gated potassium ion channels open
   • DThe axon membrane becomes impermeable to all ions

   (c)

   Explain why the inside of the axon is negative relative to the outside at rest.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Tetrodotoxin (TTX), a poison found in pufferfish, blocks the voltage-gated sodium ion channels in axon membranes. A different laboratory chemical blocks only the voltage-gated potassium ion channels.

   (a)

   What is the effect of TTX on an axon?

   [1 mark]

   • AThe axon membrane stays depolarised permanently
   • BThe axon fails to repolarise after depolarisation
   • CThe speed of conduction increases
   • DThe axon membrane cannot depolarise, so no action potential is generated

   (b)

   What is the effect of the chemical that blocks only the voltage-gated potassium ion channels?

   [1 mark]

   • ARepolarisation is slower, so the membrane stays depolarised for longer
   • BDepolarisation cannot occur because sodium ion channels cannot open
   • CThe axon is permanently hyperpolarised
   • DAction potentials travel faster

   (c)

   Explain why a person poisoned with TTX becomes paralysed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two axons of the same diameter and 0.50 m long are compared. Axon A is myelinated and conducts impulses at 50 m s⁻¹. Axon B is unmyelinated and conducts impulses at 1.0 m s⁻¹.

   (a)

   Explain why axon A conducts impulses faster than axon B.

   [3 marks]

   (b)

   Calculate the time taken for an impulse to travel along each axon and how many times faster axon A is. Suggest one advantage to a mammal of having myelinated motor neurones.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A neurophysiologist records the membrane potential at one point on a myelinated axon of a mammalian motor neurone. At rest it is −70 mV. After a stimulus it reaches a threshold of −55 mV, rises to +40 mV, falls to −80 mV and then returns to −70 mV. During the first 2 ms after the peak, a second stimulus, however strong, cannot trigger another action potential.

   (a)

   Explain the changes in the membrane potential during the action potential.

   [6 marks]

   (b)

   Explain how the action potential is conducted along this axon in one direction only and at high speed.

   [6 marks]

   Total for question 4: 12 marks

End of questions