Practise › Questions › The action potential: the same size, however hard you push

The action potential: the same size, however hard you push questions

The resting potential and how the sodium-potassium pump and differential permeability establish it, depolarisation, repolarisation and hyperpolarisation in terms of voltage-gated channels, the all-or-nothing principle and frequency coding, the refractory period and its consequences, and the three factors affecting conduction speed.

7 original questions · 25 marks · the action potential: the same size, however hard you push notes · Nervous coordination, receptors, muscles and behaviour

Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the points are much easier to see when you have something of your own to compare against.

• 4 marks · Explain

  Explain how the sodium-potassium pump and the permeability of the axon membrane together establish a resting potential of about −70 mV.

  Mark scheme

  1. B1 the pump hydrolyses ATP to move three sodium ions out for every two potassium ions in, both against their concentration gradients
  2. B1 three positive charges leave for every two that enter, so each cycle of the pump makes the inside slightly more negative
  3. B1 the membrane has many potassium channels open at rest and very few sodium channels, so it is far more permeable to potassium than to sodium
  4. A1 potassium therefore leaks back out down the steep gradient the pump has built, carrying positive charge with it, while large negatively charged proteins are trapped inside and cannot leave

• 4 marks · Describe

  Describe the changes in membrane potential during an action potential, from threshold to the recovery of the resting value, in terms of which voltage-gated channels are open at each point.

  Mark scheme

  1. B1 at about −55 mV voltage-gated sodium channels open and sodium ions diffuse in, so the inside becomes less negative
  2. B1 the depolarisation opens more sodium channels, which is positive feedback, and the potential overshoots zero to reach about +40 mV
  3. B1 at the peak the sodium channels inactivate and the slower voltage-gated potassium channels are now open, so potassium leaves and the potential falls back: repolarisation
  4. A1 the potassium channels are slow to shut, so potassium keeps leaving past the resting value to about −80 mV, and the sodium-potassium pump then restores the resting potential

• 4 marks · Explain

  Explain what is meant by the all-or-nothing principle, and explain how the nervous system distinguishes a light touch from a heavy blow if every action potential is the same size.

  Mark scheme

  1. B1 below threshold no action potential is produced at all, and the small depolarisation simply fades away
  2. B1 at or above threshold a full action potential occurs and reaches the same peak of about +40 mV whatever the size of the stimulus, because the positive feedback runs to completion on its own
  3. B1 a stronger stimulus increases the frequency of impulses along a neurone
  4. A1 a stronger stimulus also recruits more neurones, as receptors with higher thresholds are brought in, and nothing about the amplitude of any impulse changes

• 4 marks · Explain

  Explain what causes the refractory period after an action potential, and explain two consequences it has for the impulses a neurone can carry.

• 4 marks · Calculate

  An action potential is recorded 8.4 cm along an axon from the point of stimulation, 1.2 ms after the stimulus was applied. Calculate the conduction velocity of this axon in metres per second, and calculate how long an impulse would take to travel 1.4 m along an unmyelinated axon conducting at 1.0 m s⁻¹.

• 3 marks · Suggest

  A local anaesthetic injected at a tooth blocks the voltage-gated sodium channels in the sensory neurones nearby. Suggest why the patient feels no pain from the tooth, and suggest why the sense of touch in the lip on that side is lost as well.

• 2 marks · State

  State the resting potential of a typical neurone in millivolts, and state the ratio in which the sodium-potassium pump moves the two ions across the membrane.

Practise the action potential: the same size, however hard you push one question at a time

The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.