Lecture 20 NMJ Energetics

Muscle Contraction and Muscle Energetics

Muscle Twitch

  • Response to a single threshold stimulus.

  • Three phases:

    • Latent Period: No muscle response.

    • Contraction Period: Shortening occurs, peak tension is developed.

    • Relaxation Period: Ca²+ re-entered into the SR, tension reduces to zero.

Graded Muscle Cell Response

  • Variations in muscle contraction dependent on stimulus frequency.

  • Mechanisms:

    • Wave summation.

    • Unfused and fused tetanus.

Motor Unit

  • Composed of a motor neuron and muscle fibers it innervates.

  • Small motor units for fine movements; large motor units for weight-bearing muscles.

  • Units contract asynchronously to prevent fatigue.

Types of Contraction

  • Isotonic Contraction: Muscle shortens and moves the load.

  • Isometric Contraction: Muscle tension increases but does not change length.

  • Concentric vs. Eccentric Contractions:

    • Concentric: Muscle shortens.

    • Eccentric: Muscle lengthens.

Muscle Energy Sources

  • ATP required for:

    1. Re-energizing myosin.

    2. Removing Ca²+.

    3. Restoring ion balance.

  • ATP production pathways:

    • Creatine Phosphate (15 seconds).

    • Anaerobic Glycolysis (2 ATP per glucose, ~1 min duration).

    • Aerobic Respiration (36 ATP per glucose, hours duration).

Glycolysis and Lactic Acid

  • Anaerobic glycolysis results in lactic acid, promotes fatigue.

  • The Cori Cycle recycles lactic acid to glucose at a cost of 6 ATP.

Muscle Fiber Types

  1. Slow Oxidative Fibers: Aerobic, fatigue-resistant, rich blood supply, red color.

  2. Fast Glycolytic Fibers: Anaerobic, fatigues rapidly, white color.

  3. Fast Oxidative Fibers: Intermediate characteristics, aerobic, good for mid-distance activities.