biol 3410 10/17

Review of Myofibril Structure

  • Review of Skeletal Muscle Mechanics:

    • Focus on the structure of thick and thin filaments.

    • Thick filament protein = Myosin; Thin filament protein = Actin (F actin and G actin).

  • Sarcomere:

    • Fundamental contractile unit of muscle, organized from Z disc to Z disc.

Sliding Filament Theory

  • Examination of interactions between thick (myosin) and thin (actin) filaments during contraction.

  • Poll Questions:

    • Order of muscle structures: Muscle > Fascicle > Muscle Fiber > Myofibril > Thick Filaments.

    • Identification of zones in the sarcomere (H zone, I band, M line).

    • Concepts reinforced through interactive polling.

Overview of Muscle Contraction Mechanism

  • Contraction Process:

    1. Calcium Release:

      • Binds to troponin, moving tropomyosin and exposing actin sites.

    2. ATP Hydrolysis:

      • Myosin heads reoriented into a high-energy state.

    3. Cross-Bridge Formation:

      • Myosin heads attach to actin.

    4. Power Stroke:

      • Myosin heads pull actin filaments, sliding them toward the M-line.

    5. Detachment:

      • New ATP binds causing the myosin head to detach from actin.

  • Muscular Contraction Cycle:

    • Myosin heads work simultaneously in a cycle, maintaining tension and contraction.

  • Importance of ATP:

    • Required for the detachment of myosin from actin and resetting of the contraction cycle.

  • Rigor Mortis Discussion:

    • Lack of ATP postmortem results in sustained muscle contraction due to inability to detach myosin from actin.

Physical Changes During Contraction

  • Sarcomere Features:

    • A band remains the same length, while the I band and H zone shorten during contraction.

  • Functional muscle unit's contraction leads to overall muscle shortening:

    • Filaments slide, pulling Z discs closer together, enabling muscle movement.

Health Case Discussion

  • Patient Symptoms:

    • Weakness linked to high levels of anti-acetylcholine receptor antibodies and muscle fatigue.

    • Symptoms improved with acetylcholinesterase inhibitors.

  • Key Concepts:

    • Importance of acetylcholine for muscle contraction at the neuromuscular junction.

    • Action potentials and neurotransmitter signals are critical for initiating contraction.

Conclusion and Next Steps

  • Focus will continue on muscle contraction signaling and mechanisms in the next lecture.

  • Students expected to reflect on the connection between electrical impulses and muscle contractions.