Nervous coordination and muscle contractionAQA A-Level Biology: Topic test

1. 1
   The axon of a mammalian motor neurone is at rest. Sodium ions are at a higher concentration outside the axon than inside, and potassium ions are at a higher concentration inside than outside. At rest the axon membrane has many open potassium ion channels but very few open sodium ion channels, and the inside of the axon is at −70 mV relative to the outside.

   (a)

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

   [1 mark]

   • ASodium ions diffuse into the axon faster than potassium ions diffuse out.
   • BThe sodium–potassium pump moves more potassium ions out of the axon than it moves sodium ions in.
   • CMore potassium ions diffuse out of the axon than sodium ions diffuse in.
   • DSodium ions are actively transported into the axon.

   (b)

   A stimulus depolarises the membrane to the threshold potential. Which event happens next?

   [1 mark]

   • AVoltage-gated sodium ion channels open and sodium ions diffuse into the axon.
   • BVoltage-gated potassium ion channels open and potassium ions diffuse into the axon.
   • CSodium ions are actively transported out of the axon, making the inside more negative.
   • DPotassium ions diffuse out of the axon faster, making the inside more negative than at rest.

   (c)

   A respiratory inhibitor stops the production of ATP in the axon, so the sodium–potassium pump stops working. Explain why the resting potential of the axon would gradually become less negative.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher studies a cholinergic synapse between two neurones in a spinal cord preparation. She first bathes the synapse in a solution that contains no calcium ions and stimulates the presynaptic neurone; no response is recorded in the postsynaptic neurone. She then restores the normal calcium ion concentration and the response returns.

   (a)

   Which process is directly prevented when no calcium ions are present?

   [1 mark]

   • ADiffusion of acetylcholine across the synaptic cleft
   • BFusion of synaptic vesicles with the presynaptic membrane
   • CBinding of acetylcholine to receptors on the postsynaptic membrane
   • DBreakdown of acetylcholine by acetylcholinesterase

   (b)

   Which statement explains why transmission across a normal cholinergic synapse is unidirectional?

   [1 mark]

   • AAcetylcholinesterase is found only on the presynaptic membrane.
   • BVoltage-gated sodium ion channels are found only in the presynaptic knob.
   • CCalcium ions diffuse only into the postsynaptic neurone.
   • DAcetylcholine is released only from the presynaptic neurone and its receptors are only on the postsynaptic membrane.

   (c)

   Explain the importance of acetylcholinesterase in the synaptic cleft when normal conditions are restored.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A sarcomere in a relaxed skeletal muscle fibre is 2.2 µm long, measured from one Z line to the next. The dark band (the A band), which is the length of the myosin filaments, is 1.6 µm long. Each actin filament is 1.0 µm long and is attached at one end to a Z line.

   (a)

   Calculate the length of the I band and the length of the H zone in the relaxed sarcomere. Show your working.

   [3 marks]

   (b)

   The muscle contracts and the sarcomere shortens to 1.8 µm. State the new lengths of the A band, the I band and the H zone, and explain the changes in terms of what the filaments do.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Multiple sclerosis is a disease in which the myelin sheath around axons in the central nervous system is damaged. In one patient, impulses along a motor neurone to the leg muscles now travel much more slowly than normal, and some fail to arrive at all. The patient also finds that the leg muscles weaken quickly.

   (a)

   Explain why the loss of myelin slows the conduction of impulses along the axon.

   [6 marks]

   (b)

   In the spinal cord the patient's affected neurone is presynaptic to another neurone at a cholinergic synapse. Fewer impulses per second now arrive at this synapse. Explain how this could prevent an action potential in the postsynaptic neurone.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   In a pheasant, the breast muscle is used for short, explosive take-offs and is pale in colour, whereas the leg muscle, used for walking all day, is dark red.

   (a)

   Which feature is greater in the leg muscle fibres than in the breast muscle fibres?

   [1 mark]

   • AThe concentration of phosphocreatine
   • BThe store of glycogen
   • CThe rate of anaerobic ATP production
   • DThe concentration of myoglobin

   (b)

   Which of the following is a property of the fast fibres in the breast muscle?

   [1 mark]

   • AA rich blood supply and many mitochondria
   • BThey fatigue quickly and have a large store of phosphocreatine
   • CA high concentration of myoglobin
   • DThey contract slowly and resist fatigue

   (c)

   Explain why the leg muscle fibres can keep contracting for long periods without fatigue.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Skeletal muscle fibres contain actin and myosin filaments. When a muscle fibre is stimulated, calcium ions are released into the sarcoplasm and the fibre contracts using ATP. During the first seconds of intense activity, the concentration of ATP in the fibre stays almost constant even though ATP is being used rapidly.

   (a)

   What is the effect of calcium ions on tropomyosin?

   [1 mark]

   • AThey cause tropomyosin to move, exposing the myosin-binding sites on actin.
   • BThey bind to the myosin heads, so that they detach from actin.
   • CThey hydrolyse ATP directly to release energy for the power stroke.
   • DThey cause tropomyosin to bind to the myosin heads.

   (b)

   What causes a myosin head to detach from actin during the cycle of contraction?

   [1 mark]

   • AADP binds to the myosin head
   • BCalcium ions are removed from the sarcoplasm
   • CATP binds to the myosin head
   • DPhosphocreatine binds to the myosin head

   (c)

   Explain how the concentration of ATP in the fibre is kept almost constant during the first seconds of intense activity.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A myelinated axon has nodes of Ranvier 1.2 mm apart. An action potential travels along it at 48 m s⁻¹. After each action potential the axon has a refractory period of 2.5 ms. A non-myelinated axon of the same diameter conducts action potentials at 0.8 m s⁻¹.

   (a)

   Calculate the time taken for the action potential to travel from one node to the next, in µs. Then calculate the maximum frequency of action potentials in the myelinated axon, assuming that only the refractory period limits it. Show your working.

   [3 marks]

   (b)

   Explain why the speed of conduction is much lower in the non-myelinated axon, and state two changes that would increase the speed in the non-myelinated axon.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Botulinum toxin, produced by a bacterium, blocks the fusion of synaptic vesicles with the presynaptic membrane at neuromuscular junctions. It is injected in tiny doses to relax overactive muscles. A drug used in a different treatment inhibits the enzyme acetylcholinesterase at neuromuscular junctions.

   (a)

   Use the information to explain how botulinum toxin causes a skeletal muscle to relax and stay relaxed.

   [6 marks]

   (b)

   Predict and explain the effect of the acetylcholinesterase inhibitor on a skeletal muscle stimulated by one action potential from its motor neurone.

   [6 marks]

   Total for question 8: 12 marks

End of questions