Skeletal Muscles

Question 1

How do muscles act?

Answer 1

Muscles act in antagonistic pairs against an incompressible skeleton.

Question 2

Describe the gross structure of skeletal muscles.

Answer 2

• Sarcolemma → T-tubules
• Sarcoplasm → Mitochondira and myofibrils
• Sarcoplasmic reticulum (SR) → Protein pumps

Question 3

Describe the structure of myofibrils.

Answer 3

• Myosin (thick filaments)
• Actin (thin filaments)
• H-band (only myosin)
• A-band (myosin and actin)
• I-band (only actin)
• Sarcomere (section of myofibril between two Z-lines)
• Z-lines (attachment for actin)
• M-lines (attachment for myosin)

Question 4

Describe the sliding filament model?

Answer 4

1. Ca2+ ions released into sarcoplasm
2. Calcium ions bind to troponin molecules, stimulating them to change shape
3. Tropomyosin moves position exposing the myosin binding sites
4. Globular heads of the myosin molecules bind with these sites forming cross bridges
5. ADP and Pi released
6. This pulls actin to centre of sarcomere
7. ATP binds to the myosin heads producing a change in shape that causes the myosin heads to release from the actin filaments
8. ATP hydrolase hydrolyses ATP into ADP and Pi which causes the myosin heads to move back to their original positions

Question 5

Outline the roles of ATP and phosphocreatine.

Answer 5

• ATP is needed for the return movement of myosin heads that causes the actin filaments to slide
• The return of calcium ions back into the sarcoplasmic reticulum occurs via active transport
• The mitochondria present in the muscles fibres are able to aerobically respire and produce ATP but this is slow
• Phosphocreatine is a molecule stored by muscles that can be used for the rapid production of ATP
• A phosphate ion from phosphocreatine is transferred to ADP
  ADP + phosphocreatine → ATP + creatine
• It allows for muscles to continue contracting for a short period of time until the mitochondria are able to supply ATP

Question 6

Fast skeletal muscle fibres:

Answer 6

• Fast muscle fibres contract rapidly
• The myosin heads bind and unbind from the actin-binding sites five times faster than slow muscle fibres
• Require large amounts of calcium ions present to stimulate contraction
• They rely on anaerobic respiration for ATP supply
• They are suited to short bursts of high-intensity activity as they fatigue quickly due to the lactate produced from anaerobic respiration
• Fast muscle fibres have fewer capillaries
• Slow supply of oxygen and glucose for aerobic respiration
• Low amounts of myoglobin are present in fast muscle fibres
• Myoglobin functions as a store of oxygen in muscles and increases the rate of oxygen absorption from the capillaries
• Paler in colour than slow muscle fibres

Question 7

Slow skeletal muscle fibres:

Answer 7

• Slow muscle fibres contract more slowly and are suited to sustained activities
• They rely on aerobic respiration for ATP
• They fatigue less quickly due to less lactate production, making them ideal for endurance
• These muscles have to contract for long periods of time in order to keep the skeleton erect when standing or sitting
• Slow muscle fibres have a denser network of capillaries
• This means they have a short diffusion distance and a good supply of oxygen and glucose for aerobic respiration
• High amounts of myoglobin, haemoglobin and mitochondria are present
• This increases the rate of oxygen supply, oxygen absorption and aerobic respiration
• Slow muscle fibres appear a dark red