Henry Ford College 12

Muscle Contraction Process

Excitation-Contraction Coupling: The process begins with an electrical impulse (action potential) from a motor neuron that travels to the axon terminal, releasing acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the sarcolemma, opening sodium channels and depolarizing the muscle fiber, which propagates along the sarcolemma and down the T-tubules. This triggers calcium release from the sarcoplasmic reticulum (SR). Calcium binds to troponin on actin, moving tropomyosin away from active sites.

Contraction Phase: Myosin heads, fueled by ATP, bind to the exposed active sites on actin, forming cross-bridges. They perform a power stroke, pulling actin filaments toward the center of the sarcomere, shortening the muscle fiber. As the myosin heads release ADP, a new ATP binds for another cycle, continuing as long as calcium and ATP are available.

Relaxation Phase: When the action potential ends, ACh is degraded by acetylcholinesterase, halting depolarization. Calcium is reabsorbed into the SR, causing it to disassociate from troponin. Tropomyosin re-covers active sites, preventing further myosin binding, leading to muscle relaxation and return to resting length.

Structural Overview of Muscle Fiber

  • Sarcomere: Basic unit of contraction, defined from Z-line to Z-line.

  • Myofibrils: Cylindrical structures of sarcomeres in series.

  • Fascicle: A bundle of muscle fibers surrounded by perimysium; multiple fascicles form a muscle surrounded by epimysium.

  • Myoblasts: Immature muscle cells contributing to multinucleated fibers.

Muscle Cell Organization

Muscle cells are categorized as:

  • Skeletal Muscle: Multinucleated, striated, voluntary.

  • Cardiac Muscle: Usually uninucleated, striated, involuntary.

  • Smooth Muscle: Uninucleated, non-striated, involuntary.

Types of Muscle Contractions

  • Isometric: Muscle exerts force without length change (e.g., holding weight).

  • Isotonic: Muscle changes length with constant tension:

    • Concentric: Muscle shortens (e.g., lifting weight).

    • Eccentric: Muscle lengthens (e.g., lowering weight).

Muscle Energy Sources

  • ATP: Main energy source for contraction and relaxation.

  • Creatine Phosphate: Energy reserve replenishing ATP during intense activity.

  • Anaerobic Respiration: Quick energy for short efforts (leads to lactic acid).

  • Aerobic Respiration: Longer activities using oxygen for high ATP yield with less fatigue.

Muscle Fatigue and Recovery

Muscle fatigue occurs due to lactic acid accumulation and depletion of ATP/creatine phosphate. Recovery involves lactate clearance and restoration of energy substrates, aided by rest and nutrition.