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

Question 10

Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contractions. [3]

Answer 10

1. (Less/no) tropomyosin moved from binding site
   OR
   Shape of tropomyosin not changed so binding site not exposed;
2. (Fewer/no) actinomyosin bridges formed;
3. Myosin head does not move
   OR
   Myosin does not pull actin (filaments)
   OR
   (Less/no) ATP hydrolyse (activation)

Question 11

In muscles, pyruvate is converted to lactate during prolonged exercise.

Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration. [2]

Answer 11

1. Regenerates NAD
   OR
   Oxidises reduced NAD;
2. (So) glycolysis continues

Question 12

Explain what causes the decrease in length of the H zone and I band. [2]

Answer 12

1. Actin filaments slide in between myosin;
2. Pull Z lines closer

Question 13

The myosin-ATP hydrolyse of fast muscle fibres has a faster rate of reaction than that in slow fibres. Use your knowledge of the mechanis, of muscle contraction to explain how this will help fast muscle fibres to contract faster than slow fibres. [4]

Answer 13

1. Overall rate of contraction limited by rate of ATP hydrolysis;
2. ATP hydrolysis converts ATP to ADP (and Pi);
   ATP hydrolysis provides energy for (2 from):
3. Myosin heads move back to original position;
4. Cross bridges between actin and myosin;
5. Power stroke / movement of myosin heads / pulling of actin

Question 14

The blood leaving an active muscle with a high percentage of fast muscle fibres contained a higher concentration of lactate than that leaving a muscle with a high percentage of slow muscle fibres. Explain why. [2]

Answer 14

1. Lactate produced in anaerobic respiration;
2. Fast have fewer mitochondria (and so rely more on anaerobic respiration)

Question 15

Explain the role of glycogen granules in skeletal muscle. [2]

Answer 15

1. As a store of glucose
   OR
   To be hydrolysed to glucose;
2. For respiration / to provide ATP

Question 16

During vigorous exercise, the pH of skeletal muscle tissue falls. The fall in pH leads to a reduction in the ability of the calcium ions to stimulate muscle contractions.

Suggest how. [3]

Answer 16

1. Lower pH changes shape/tertiary structure of tropomyosin;
2. Fewer calcium ions can bind to tropomyosin;
3. Fewer tropomyosin molecules move away;
4. Fewer binding sites on actin revealed;
5. Fewer cross-bridges form
   OR
   Fewer myosin heads can bind

Question 17

What is the role of phosphocreatine in providing energy during muscle contraction? [2]

Answer 17

1. Provides phosphate;
2. To make ATP