Neurones and nerve impulsesEdexcel A-Level Biology A: Subtopic test

1. 1
   A student touches a hot kettle and pulls her hand away before she feels any pain. The withdrawal reflex involves three types of neurone linking the receptors in her skin to the muscles in her arm.

   (a)

   Which type of neurone carries impulses from the receptors in her skin towards her spinal cord?

   [1 mark]

   • AMotor neurone
   • BRelay neurone
   • CSchwann cell
   • DSensory neurone

   (b)

   Which feature is characteristic of a motor neurone?

   [1 mark]

   • AA cell body outside the CNS and a short axon
   • BA cell body in the CNS and a long axon that carries impulses to an effector
   • CA short axon that connects two neurones within the CNS
   • DA long dendron that carries impulses from a receptor

   (c)

   Describe the role of Schwann cells in the myelination of an axon.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student studies the giant axon of a squid using microelectrodes. The potential difference across the axon membrane is -70 mV at rest. When the axon is stimulated, the potential rises rapidly to +40 mV and then falls back, dipping below -70 mV briefly before returning to its resting value.

   (a)

   Which movement of ions is mainly responsible for the rapid rise in potential to +40 mV?

   [1 mark]

   • ASodium ions diffuse into the axon through voltage-gated channels
   • BPotassium ions diffuse into the axon through channels
   • CSodium ions are actively transported out of the axon
   • DChloride ions diffuse out of the axon

   (b)

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

   [1 mark]

   • ASodium ions diffuse out through voltage-gated sodium channels
   • BPotassium ions diffuse into the axon
   • CPotassium ions diffuse out through voltage-gated potassium channels
   • DSodium ions are pumped into the axon by the sodium-potassium pump

   (c)

   A weak stimulus fails to produce an action potential. Explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Motor neurones carry impulses from the spinal cord to the muscles of the foot, a distance of 0.84 m. In a myelinated motor neurone, the action potential travels at 70 m s⁻¹. In an unmyelinated neurone of the same length the action potential travels at 1.4 m s⁻¹.

   (a)

   Calculate the time taken, in milliseconds, for an action potential to travel along the myelinated neurone, and state how many times faster this is than in the unmyelinated neurone.

   [3 marks]

   (b)

   Explain why action potentials travel faster along myelinated neurones than along unmyelinated neurones.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A 34-year-old patient is diagnosed with Guillain-Barré syndrome, in which the immune system damages the myelin sheaths around neurones in the peripheral nervous system. She has muscle weakness and slowed reflexes. Her doctor explains how nerve impulses are normally generated and conducted.

   (a)

   Explain how an action potential is generated and then conducted along an axon, in terms of changes in membrane permeability to sodium and potassium ions.

   [6 marks]

   (b)

   Explain how myelination increases the speed of conduction and why loss of myelin leads to slow reflexes and muscle weakness.

   [6 marks]

   Total for question 4: 12 marks

End of questions