Power Stroke in Muscle Contraction

Overview

• Topic: Detailed examination of the power stroke in muscle fibers.
• Focus: Myosin head activity during contraction, role of calcium, and energy usage.

Key Concepts

Myosin Head Conformation Stages

1. Cocked Position (Resting State)

   • Myosin hydrolyzes ATP, binds ADP + Phosphate.
   • Blocked by tropomyosin from interacting with actin.

2. Release of Calcium

   • Calcium release (through excitation contraction coupling) opens binding site on actin.
   • Myosin shifts to the second conformation as phosphate is released, altering protein charge.

3. Rigor State

   • ADP is released, forming the strongest crossbridge between actin and myosin.
   • Short-lived due to ATPase activity of myosin.

4. ATP Binding and Hydrolysis

   • ATP binds to myosin, reducing interaction intensity.
   • Hydrolysis of ATP returns myosin to the cocked position, ready for another cycle.

Power Stroke Cycle

• Repetitive: Continues as long as calcium is present in the cytosol.
• Cessation: Calcium pumps lower calcium levels, halting the cycle.

Structural Changes

• Sarcomere Shortening: Thin filaments move closer to M-line, decreasing sarcomere length.
• Tension Development: Repeated power strokes increase actin-myosin interaction, building muscle tension.

Energy Usage

• ATP Consumption: Each power stroke cycle uses a single ATP; the process is energy-intensive due to the high number of myosin heads.

Rigor State vs. Rigor Mortis

• Similarity: Both involve strong actin-myosin binding.
• Difference: In rigor mortis, absence of ATP leads to prolonged rigor state (24-48 hours) until decomposition.

Electrical and Mechanical Events

• Initiation: Single electrical event (EPP to action potential) is rapid (~2 ms).
• Mechanical Dependence: Relies on diffusion of calcium and troponin binding. Longer process.
• Muscle Twitch: Combination of electrical and mechanical events (~100 ms in skeletal muscle).

Conclusion

• The lecture ends with a note about exploring muscle twitch dynamics and skeletal muscle physiology in future videos.