B3.3.2 Sliding filament model of muscle contraction Notes

Sarcomeres Shorten as Actin and Myosin Slide Past Each Other

1. The Sarcomere consists of:
   1. Z-Discs (Z-Lines): Define sarcomere boundaries, anchor thin filaments (actin).
   2. Thin Filaments (Actin): Extend from the Z-discs toward the sarcomere center.
   3. Thick Filaments (Myosin): Occupy the central region, interlock with actin filaments.
   4. I-Bands (Light Bands): Contain only actin.
   5. A-Bands (Dark Bands): Overlap region of actin and myosin.
   6. H-Zone: Central part of the A-band where only myosin is present when muscle is relaxed.
2. Z-Discs are pulled closer together to shortening the sarcomere.

The Sliding Filament Mechanism

1. Cross-Bridge Formation
   1. Myosin heads attach to binding sites on actin, forming cross-bridges.
   2. Initiated by Ca²⁺ release (triggered by a nerve impulse).
2. Power Stroke
   1. Myosin heads pivot, pulling actin toward the sarcomere center.
   2. ATP hydrolysis provides the energy for this movement.
3. Detachment
   1. A new ATP binds to the myosin head, causing it to release actin.
   2. Without ATP, the myosin head would remain locked in place (rigor).
4. Resetting the Myosin Head
   1. The myosin head hydrolyzes ATP, returning to its cocked position.
   2. Ready to form another cross-bridge for continuous sliding.

The Role of ATP and Calcium in Muscle Contraction

The Role of ATP and Calcium (Ca²⁺)

1. ATP
   1. Energy source for the power stroke and detachment of myosin heads.
   2. In its absence, myosin remains attached (leading to rigor mortis after death).

1. Calcium Ions (Ca²⁺)
   1. Stored in the sarcoplasmic reticulum and released upon nerve stimulation.
   2. Binds to troponin, shifting tropomyosin to expose actin binding sites.
   3. Allows myosin heads to attach and initiate contraction.

Coordinated Muscle Contraction

1. Simultaneous Sarcomere Shortening
   1. All sarcomeres in a myofibril shorten at once.
   2. Multiple myofibrils in a muscle fiber contract together.
2. Force Generation
   1. Large-scale force is produced by millions of sarcomeres working in unison.
   2. Enables both strong movements (e.g., lifting) and fine movements (e.g., typing).

Applications and Implications

1. Athletics: Knowledge of muscle contraction enhances training methods to optimize strength and endurance.
2. Medical Context
   1. Diseases like muscular dystrophy and myasthenia gravis involve disrupted actin-myosin interaction.
   2. Treatments focus on restoring proper sliding filament function.
3. Engineering & Robotics: Biomimetic designs use principles of muscle contraction to create artificial muscles for prosthetics or robots.