Skeletal Muscles

Question 1

The structure of skeletal muscle

Answer 1

Sarcoplasmic reticulum stores and releases Ca2+ ions

Question 2

Myofibrils

Answer 2

Question 3

Describe the banding pattern in striated muscle

Answer 3

• Lightest band is I band, actin only
• Darkest band is overlapping region, actin and myosin
• Medium shading is H zone/band is myosin only.

Question 4

Muscle contraction

Answer 4

During muscle contraction each sarcomere shortens, bringing the Z lines closer together, as the actin is pulled over myosin filaments increasing the amount they overlap.

Question 5

Sliding filament theory

Answer 5

1. Ca2+ are released from the sarcoplasmic reticulum into sarcoplasm.
2. Ca2+ diffuse and bind to troponin changes the 3° structure and cause tropomyosin to move exposing the myosin head binding sites on the actin filament.
3. Myosin heads bind to actin binding sites forming cross bridges.
4. The myosin head “nods/bends” pulling the actin filament a short distance. [POWER STROKE]
5. ADP & Pi are released from the Myosin head.
6. A new ATP binds to myosin head. This breaks the cross bridge and separates it from the actin. ATP is hydrolysed to ADP and Pi by ATP hydrolase, the energy released re-cocks the myosin head. [RECOVERY STROKE]
7. The process repeats pulling the actin along the myosin filament a bit more each time.

Question 6

Describe the function of calcium ions in muscle contraction.

Answer 6

• Ca2+ Binding/changing shape/removing tropomyosin;
• Exposes actin binding sites;
• Myosin head attaches/cross-bridge formation;
• Activates ATP hydrolase;

Question 7

Nerve impulses arriving at the presynaptic membrane at the neuromuscular junction
result in shortening of sarcomeres. Describe how

Answer 7

• Entry of calcium ions
• Vesicles fuse with membrane
• Neurotransmitter diffuses;
• Binds to receptors, postsynaptic membrane;
• Depolarisation
• Release of calcium ions
• Removes tropomyosin
• Exposing binding sites on the actin;
• Myosin binds;
• Myosin head pulls the actin along;
• ATP hydrolase activated;

Question 8

Explain the importance of ATP hydrolase during muscle contraction.

Answer 8

• Hydrolysis of ATP releasing energy;
• used to form / break actinomyosin cross-bridges;

Question 9

What is ATP required for

Answer 9

1. The sliding of the filaments during contraction
2. The active transport of calcium ions into the Sarcoplasmic Reticulum.

A muscle fibre can only store enough ATP to allow sustained contraction for approx. 3-4 seconds.

Regeneration of ATP by anaerobic respiration (glycolysis) takes about 10 secs and by aerobic respiration even longer than this.

Question 10

Phosphocreatine

Answer 10

Muscle fibres can store phosphocreatine, a molecule which can donate a phosphate to ADP, regenerating ATP in the short term.

Phosphocreatine can be regenerated using ATP when it can be supplied via aerobic respiration.

The energy released from the hydrolysis of phosphocreatine is NOT used in muscle contraction directly. The energy released is used in the phosphorylation of ATP, which can then be used in muscle contraction

Question 11

Describe the role of phosphocreatine

Answer 11

• Provides (energy and) phosphate / phosphorylates;
• To make ATP from ADP & Pi;

Question 12

Slow twitch fibres

Answer 12

have a slower rate of contraction.

1. aerobic respiration to regenerate ATP. They have many large mitochondria - to provide ATP for active transport
2. High concentrations of myoglobin - acts as oxygen store
3. large number of capillaries, to provide a good oxygen supply.
4. Less extensive sarcoplasmic reticulum as less calcium ions required at one time.
5. Less glycogen as glucose broken down fully via aerobic respiration.
   - found in the muscles of the legs and those involved in maintaining posture.
   - slow to fatigue.

Question 13

Describe slow twitch muscle fibres

Answer 13

• have lots of mitochondria/ (slow fibres) respire aerobically;
• More myoglobin

Question 14

Fast twitch fibres

Answer 14

• used for rapid, strong contractions;
• Phosphocreatine used up rapidly during contraction/to make ATP;
• Anaerobic respiration involved, more glycogen;
• ATP used to reform phosphocreatine;
• Lots of phosphocreatine in fast twitch fibres;
• No myoglobin
• fast to fatigue due to build up of lactate
• fewer, smaller mitochondria

Question 15

Describe the role of calcium ions in the contraction of a sarcomere.

Answer 15

• interact with/move/touch tropomyosin;
• to reveal binding sites on actin;
• allowing myosin to bind/touch actin / actinomyosin formed;
• activate ATP hydrolase / energy released from ATP;

Question 16

After death, cross bridges between actin and myosin remain firmly bound resulting in rigor mortis. Explain what causes the cross bridges to remain firmly bound.

Answer 16

• respiration stops;
• no ATP produced;
• ATP required for separation of actin and myosin

Question 17

A muscle fibre contracts when it is stimulated by a motor neurone. Describe how transmission occurs across the synapse between a motor neurone and a muscle fibre.

Answer 17

• Ca2+ channels open;
• Ca2+ ions enter (pre-synaptic neurone);
• Vesicles move towards presynaptic membrane;
• Release of acetylcholine;
• Diffusion across cleft;
• (Transmitter) binds to receptors in postsynaptic membrane;
• Na+ channels open / Na+ ions enter (postsynaptic side);

Question 18

Muscle contraction requires ATP. What are the advantages of using aerobic rather than anaerobic respiration to provide ATP in a long-distance race?

Answer 18

• Aerobic respiration releases more energy
• no lactate produced
• Avoids muscle fatigue;
• CO2 easily removed from the body