6.3.4 Muscular Contraction

1. The Sliding Filament Model:

• During contraction, actin (thin) filaments slide over myosin (thick) filaments, causing the sarcomere to shorten.
• I-bands and H-zones become narrower, while A-bands remain the same length.

2. Steps of Muscular Contraction:

1. Calcium Ion Release:

• An action potential travels along the sarcolemma and into the muscle fibre via T-tubules.
• This stimulates the sarcoplasmic reticulum to release calcium ions $(Ca²⁺)$ into the sarcoplasm.

2. Binding of Calcium to Troponin:

• Calcium binds to troponin, causing it to change shape.
• This pulls tropomyosin away from the actin binding sites, exposing them for myosin head attachment.

3. Cross-Bridge Formation:

• Myosin heads attach to the exposed binding sites on actin, forming cross-bridges.

4. Power Stroke:

• The myosin head pivots, pulling the actin filament towards the centre of the sarcomere.
• ADP and Pi are released during this movement.

5. ATP Binding and Detachment:

• A new ATP molecule binds to the myosin head, causing it to detach from the actin filament.

6. Reactivation of Myosin Head:

• ATP is hydrolysed to ADP and Pi, providing energy to reset the myosin head into its original position, ready for the next cycle.

3. Energy Requirements for Contraction:

• ATP Hydrolysis: ATP is hydrolysed at each myosin head during the power stroke and for resetting the myosin head.
• Calcium Ion Transport: ATP is also required to pump $Ca²⁺$ ions back into the sarcoplasmic reticulum after contraction.

4. ATP Sources in Muscle Cells:

1. Aerobic Respiration:

• Supplies most ATP during prolonged, low-intensity activity.
• Oxygen is stored in myoglobin for immediate use.

2. Anaerobic Respiration:

• Provides ATP when oxygen levels are low, leading to lactic acid production.

3. Phosphocreatine System:

• Phosphocreatine provides phosphate groups for the rapid regeneration of ATP from ADP.
• Ideal for short bursts of intense activity.