Nerve impulsesAQA A-Level Biology: Subtopic test

1. 1
   A neurone has a resting potential of −70 mV and a threshold potential of −55 mV. In an experiment, three separate stimuli depolarised the membrane to −62 mV, −50 mV and −30 mV. Only two of the stimuli produced action potentials.

   (a)

   Which statement explains why the inside of the axon is negative at rest?

   [1 mark]

   • AThe sodium-potassium pump moves 2 sodium ions in and 3 potassium ions out
   • BThe membrane is impermeable to potassium ions
   • CSodium ions diffuse in faster than potassium ions diffuse out
   • DThe pump moves 3 sodium ions out for every 2 potassium ions in, and more potassium ions diffuse out than sodium ions diffuse in

   (b)

   Which event occurs at the start of depolarisation?

   [1 mark]

   • AVoltage-gated potassium ion channels open and potassium ions diffuse into the axon
   • BThe sodium-potassium pump moves sodium ions into the axon
   • CVoltage-gated sodium ion channels open and sodium ions diffuse into the axon
   • DSodium ions are actively transported out of the axon

   (c)

   Explain why one stimulus produced no action potential and the other two produced action potentials of the same size.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Neurologists measure the speed of conduction along motor neurones. In a healthy adult at 37 °C the speed along one neurone is 60 m s⁻¹. In a patient with multiple sclerosis, a condition that destroys patches of the myelin sheath, the speed along the same neurone is 8 m s⁻¹.

   (a)

   Which term describes the passage of action potentials from one node of Ranvier to the next along a myelinated axon?

   [1 mark]

   • AContinuous conduction
   • BSaltatory conduction
   • CFacilitated diffusion
   • DSpatial summation

   (b)

   Which change would also increase the speed of conduction along an axon?

   [1 mark]

   • AAn increase in axon diameter
   • BA decrease in temperature from 37 °C to 20 °C
   • CLoss of the myelin sheath
   • DAn increase in the number of nodes of Ranvier crossed per millimetre

   (c)

   Explain why the speed of conduction is lower in the patient.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A researcher stimulates an axon repeatedly. After each action potential the axon cannot produce another for 2.0 ms, because the voltage-gated sodium ion channels are closed and the membrane potential has to return to its resting value. The researcher increases the strength of the stimulus.

   (a)

   Explain the importance of the refractory period for the transmission of nerve impulses.

   [3 marks]

   (b)

   Calculate the maximum number of action potentials this axon could transmit in one second. Explain how the axon signals an increase in the strength of a stimulus.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A squid has a non-myelinated giant axon of diameter 0.5 mm that conducts action potentials at about 25 m s⁻¹ at 20 °C. A human motor neurone has a myelinated axon of diameter only 15 µm, but it conducts action potentials at about 100 m s⁻¹ at 37 °C.

   (a)

   Describe and explain how an action potential is generated at one point on the squid axon and how it passes along the axon.

   [6 marks]

   (b)

   Explain why the human axon conducts action potentials faster than the squid axon, even though its diameter is much smaller.

   [6 marks]

   Total for question 4: 12 marks

End of questions