1. 1
   A motor neuron controlling a toe muscle has its cell body in the spinal cord and a nerve fibre about 1 m long running to the foot. Inside the neuron the potassium ion concentration is 140 mmol dm⁻³ and the sodium ion concentration is 15 mmol dm⁻³; in the tissue fluid outside they are 5 mmol dm⁻³ and 145 mmol dm⁻³. The resting potential is −70 mV. When ouabain, a drug that inhibits the sodium–potassium pump, is added, the membrane potential slowly becomes less negative over several hours.

   (a)

   Which describes the structure of this neuron?

   [1 mark]

   • AA cell body with one long axon and several shorter dendrites
   • BA cell body with several long axons and one short dendrite
   • CA nucleus in the axon and cytoplasm in the dendrites
   • DDendrites up to 1 m long and a short axon

   (b)

   Which statement helps to explain why the resting potential is negative?

   [1 mark]

   • ATwo sodium ions are pumped out for every three potassium ions pumped in
   • BSodium ions diffuse into the neuron faster than potassium ions diffuse out
   • CThree sodium ions are pumped out for every two potassium ions pumped in
   • DThe pump moves both sodium and potassium ions into the neuron

   (c)

   Explain the effect of ouabain on the membrane potential.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a nerve–muscle preparation from a frog, stimulating the motor nerve makes the muscle contract. When the preparation is bathed in a solution with no calcium ions, stimulating the nerve still produces impulses that reach the nerve endings, but the muscle does not contract. Contraction returns when calcium ions are added back. Adding a few drops of acetylcholine directly to the muscle, without stimulating the nerve, also causes contraction.

   (a)

   What is the role of calcium ions at the presynaptic membrane?

   [1 mark]

   • AThey bind to acetylcholine receptors on the muscle
   • BThey are pumped out of the neuron to generate the resting potential
   • CThey break down acetylcholine in the synaptic cleft
   • DThey enter the neuron when it is depolarised and trigger release of neurotransmitter from vesicles

   (b)

   Why can a signal pass in only one direction across a typical synapse?

   [1 mark]

   • AThe synaptic cleft is too narrow for diffusion in the other direction
   • BNeurotransmitter is released only by the presynaptic neuron and receptors are only on the postsynaptic membrane
   • CImpulses can only travel towards the cell body
   • DThe postsynaptic membrane has no sodium–potassium pumps

   (c)

   Explain how acetylcholine released at the neuromuscular junction generates an excitatory postsynaptic potential.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Conduction speeds were measured in non-myelinated axons of different diameters: 1 µm, 1.0 m s⁻¹; 5 µm, 2.2 m s⁻¹; 20 µm, 4.5 m s⁻¹; 100 µm, 10 m s⁻¹; and the giant axon of a squid, 500 µm, 23 m s⁻¹. For these data the correlation coefficient (r) between diameter and speed is 0.98 and the coefficient of determination (R²) is 0.96. A myelinated axon from a cat, 10 µm in diameter, conducted impulses at 60 m s⁻¹.

   (a)

   Describe the relationship between axon diameter and conduction speed shown by these data, including what the values of r and R² indicate.

   [3 marks]

   (b)

   Compare the conduction speed of the squid giant axon with that of the cat myelinated axon, and suggest reasons for the differences between the two types of axon.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Multiple sclerosis (MS) is a disease in which the myelin sheath around nerve fibres in the central nervous system is gradually damaged. In a nerve conduction study, the mean conduction speed in a group of MS patients was 32 m s⁻¹, compared with 55 m s⁻¹ in a group of healthy volunteers of the same age. Some patients also showed occasional complete failure of impulses to reach the end of the fibre.

   (a)

   Explain how the resting potential of a neuron is generated and maintained.

   [6 marks]

   (b)

   Using the information, discuss the effects of loss of myelin on the transmission of nerve impulses in people with multiple sclerosis.

   [6 marks]

   Total for question 4: 12 marks

End of questions