Skeletal Muscle Fiber Contraction Regulation

Contraction and Relaxation

  • Skeletal muscles produce movement by pulling on tendons attached to bones.
  • Sarcomere (Relaxed Phase)
    • Actin: Thin filament protein.
    • Myosin: Thick filament.
    • Tropomyosin blocks myosin binding sites on actin.
    • Troponin binds to actin.
      • Troponin I: Binds to actin, holding tropomyosin in place.
      • Troponin T: Binds to tropomyosin.
      • Troponin C: Binds to calcium ions.
  • Sliding Filament Mechanism: Muscle shortening occurs as actin filaments slide past myosin filaments.
    • Myosin heads latch onto actin, forming cross-bridges.
    • II bands shorten.
    • ZZ discs distance shortens.
    • HH zone disappears.
  • Contraction Cycle:
    • Sarcoplasmic reticulum releases calcium into the muscle cell.
    • Calcium binds with troponin, removing tropomyosin.
    • Myosin binds to actin, forming cross-bridges.
    • Action potential moves through TT tubule, calcium channels open.
    • After action potential, calcium channels close.
    • Calcium is pumped back into the sarcoplasmic reticulum allowing tropomyosin to cover binding sites and the muscle relaxes.
  • Types of Muscle Contractions:
    • Isometric: No change in muscle length.
    • Isotonic: Change in muscle length.
      • Concentric: Muscle shortens.
      • Eccentric: Muscle lengthens.
  • Graded Muscle Contraction: Voluntary adjustment of force.

Neural Regulation

  • Motor Unit: A motor neuron and all the muscle fibers it innervates.
    • Smaller muscles (e.g., eye) have smaller motor units.
    • When a motor unit fires, all its muscle fibers contract.
    • Somatic motor neurons directly move skeletal muscles.

Neuromuscular Junctions

  • The neuromuscular junction is the synapse where the axon terminal meets the muscle cell (sarcolemma).
  • Process:
    1. Release of AChACh: Nerve impulse causes synaptic vesicles to release acetylcholine (AChACh) into the synaptic cleft.
    2. Activation of AChACh receptors: AChACh binds to receptors on the sarcolemma, opening ion channels for sodium ions.
    3. Muscle action potential: Sodium entry creates a positive charge, triggering an action potential which releases calcium ions into the sarcoplasm, causing muscle contraction.
    4. Stopping AChACh activity: Acetylcholinesterase (AChEAChE) breaks down AChACh into acetyl and choline, ending muscle contraction.