Topic 6G: Skeletal Muscles

Question 1

Damage to the myelin sheaths of neurones can lead to problems controlling the contraction of muscles.

Suggest and explain one reason why. [3]

Answer 1

1. No saltatory conduction;
2. (Nerve) impulses slowed;
3. Neuromuscular junction

Question 2

Cannabinoids are hydrophobic molecules that can easily pass into neurones.

Explain why. [1]

Answer 2

1. Lipid soluble / pass through phospholipid bilayer

Question 3

Cannabinoid receptors are found in the pre-synaptic membrane of neuromuscular junctions. When a cannabinoid binds to its receptor, it closes calcium ion channels.

Suggest how cannabinoids could prevent muscle contraction. [4]

Answer 3

1. Prevents entry of calcium ions (into synaptic knobs);
2. (Synaptic) vesicles don’t fuse with membrane / vesicles don’t release neurotransmitter;
3. Neurotransmitter does not diffuse across synapse / does not bind to receptors (on post-synaptic membrane);
4. Sodium ion channels don’t open / no action potential/depolarisation

Question 4

People who have McArdle’s disease produce less ATP than healthy people. As a result, they are not able to maintain strong muscle contraction during exercise.

Use your knowledge of the sliding filament theory to suggest why. [3]

Answer 4

ATP is needed:
1. To break actinomyosin (bridges);
2. To move the myosin head;
3. (So) actin (filaments) are moved (inwards)

Question 5

MG is an autoimmune disease caused when antibodies bind to the sarcolemma of a neuromuscular junction. This can weaken contraction of muscles.

Suggest and explain how MG can weaken contraction of muscles. [2]

Don’t include details of my of irbil or muscle contraction in your answer.

Answer 5

1. Less/more neurotransmitter binds to receptors;
2. Fewer/no sodium ions enter to reach threshold
   OR
   Less/no depolarisation
   OR
   Less/no action potentials

Question 6

MG is an autoimmune disease caused when antibodies bind to the sarcolemma of a neuromuscular junction. This can weaken contraction of muscles. Mestinon is a drug that inhibits the enzyme acetylcholinesterase.

Mestinon can help in the treatment of MG. Explain how. [3]

Answer 6

1. Less/no acetylcholine/neurotransmitter broken down;
2. (More) acetylcholine attaches to receptors;
3. Depolarisation (of sarcolemma) occurs
   OR
   Action potentials produced
   OR
   (Enough) sodium ions enter to reach threshold

Question 7

When the myofibrils contracts, which of the A-band, I-band or H-zone will remain unchanged in length? [1]

Answer 7

1. A-band

Question 8

When the myofibrils contracts, which of the A-band, I-band or H-zone will decrease in length? [1]

Answer 8

1. I-band and H-zone

Question 9

Explain why someone who has more slow fibres in their muscles can exercise for a longer period of time than someone with less slow fibres. [3]

Answer 9

1. (More aerobic respiration) produces more ATP;
2. Anaerobic respiration delayed;
3. Less/no lactate

(Exam questions up to slide 100)