Y1-BMF-2024 Muscle Structure and Contraction

Muscle Types

  • Skeletal Muscle:

    • Most common, striated, voluntary control.

    • Attached to bones, aiding in movement, posture, and balance.

  • Smooth Muscle:

    • Non-striated, involuntary.

    • Found in organs like blood vessels and intestines; controls content movement.

  • Cardiac Muscle:

    • Striated, involuntary.

    • Located in the heart, responsible for pumping blood.

Classification Systems

  • Two systems:

    • Appearance

      • Striated

      • Unstriated

    • Innervation

      • Voluntary

      • Involuntary

Structure of Skeletal Muscle

  • Muscle Fiber (Cell):

    • Large, multinucleated, and organized in long cylindrical structures.

  • Connective Tissue Layers:

    • Endomysium surrounds individual muscle fibers.

    • Perimysium surrounds bundles of fibers (fascicles).

    • Epimysium surrounds the entire muscle.

  • Myofibrils:

    • Filament bundles within muscle fibers.

    • Composed of sarcomeres, the contractile units.

Sarcomere Structure (GET PIC)

  • Sarcomere: The basic contractile unit between two Z lines.

    • Z Line: Boundary of each sarcomere.

    • I Band: Light area around the Z line with only thin filaments (actin).

    • A Band: Dark area with overlapping thick (myosin) and thin filaments.

    • H Zone: Center of A band with only thick filaments.

    • M Line: Central line of the sarcomere, anchors thick filaments.

Accessory Proteins

  • Alpha-actinin: Maintains the actin lattice

  • Dystrophin: Anchors actin filaments to sarcolemma

  • Titin: Largest known protein, aligns thick filaments and adds elasticity to sarcomeres

Triads

  • Located at the junction of the A and I bands in skeletal muscle, resulting in two triads per sarcomere

Molecular Components of Sarcomere

  1. Thin Filament (Actin):

    • Actin: Contains binding sites for myosin.

    • Tropomyosin: Blocks myosin-binding sites on actin during muscle relaxation.

    • Troponin: Binds calcium to move tropomyosin, exposing binding sites for contraction.

  2. Thick Filament (Myosin):

    • Myosin Molecule: Two intertwined tails and globular heads that form cross-bridges.

    • Heads: Contain an ATPase site (for energy) and an actin-binding site.

Sliding Filament Hypothesis

  • Muscle shortens by the interdigitation of actin and myosin filaments

  • Sarcomere shortens, while A band remains constant

Muscle Contraction Mechanism

  1. Sliding Filament Model:

    • Muscle contraction is caused by actin and myosin filaments sliding past each other.

    • Sarcomere Shortening:

      • I Band and H Zone shorten, while the A Band remains the same.

      • Z Lines move closer as the muscle contracts.

  2. Cross-Bridge Cycle:

    • Step 1: Myosin heads attach to actin, forming cross-bridges.

    • Step 2: Power Stroke occurs when myosin heads pivot, pulling actin filaments toward the M line.

    • Step 3: ATP binds to myosin, causing it to release actin and reset for the next cycle.

    • Step 4: ATP is hydrolyzed, re-energizing the myosin head to start the cycle again.

Control of Contraction

  • Calcium Role:

    • Calcium ions bind to troponin, causing tropomyosin to move and expose binding sites on actin.