Understanding Skeletal Muscle Contraction

Nov 9, 2024

Contraction of Skeletal Muscles - Chapter 6

Overview

  • Important topic in physiology and for examinations.
  • Approximately 50% of the body is skeletal muscle.
  • There are three types of muscles: skeletal, smooth, and cardiac.
    • Skeletal muscle: 50%
    • Smooth muscle: ~10% (found in the GI tract, excretory system, etc.)
    • Cardiac muscle: ~10% (found in the heart)

Physiological Anatomy of Skeletal Muscle

  • Muscles made up of fibers ranging from 10 to 80 micrometers in diameter.
  • Each muscle fiber typically innervated by one nerve ending.
  • Muscle Structure:
    • Muscle Fasciculus: Bundle of muscle fibers.
    • Muscle Fiber: Composed of muscle fascicles.
    • Myofibril: Smallest unit within a muscle fiber composed of proteins (actin and myosin).

Terminology

  • Sarcolemma: Cell membrane of a muscle fiber.
  • Myofibrils: Composed of actin and myosin proteins.
  • Z Discs: Protein structures that define the boundaries of a sarcomere.
  • Sarcomere: Basic contractile unit of muscle, lies between two Z discs (about 2 micrometers in length).

Muscle Contraction Mechanism

  1. Sliding Filament Theory: Myosin and actin filaments slide past one another during contraction.
  2. Cross Bridges: Projections from myosin filaments that interact with actin.
  3. Calcium Ions: Released from the sarcoplasmic reticulum to initiate contraction by binding to troponin.
  4. ATP: Required for muscle contraction (energy source).

Muscle Fiber Composition

  • Myosin: Thick filaments (~1500 per myofibril).
  • Actin: Thin filaments (~3000 per myofibril).
  • Tropomyosin & Troponin: Proteins that regulate the interaction between actin and myosin.

Energy Sources for Muscle Contraction

  • Adenosine Triphosphate (ATP): Primary energy source.
  • Creatine Phosphate: Provides a high-energy phosphate for rapid ATP regeneration.
  • Glycolysis: Produces ATP even without oxygen (2.5 times faster than oxidative phosphorylation).
  • Oxidative Phosphorylation: Main source of energy for sustained contractions (95% of energy used).

Muscle Fiber Types

  • Fast Fibers (Type II):
    • Large in size.
    • High strength of contraction.
    • Lower blood supply, fewer mitochondria.
  • Slow Fibers (Type I):
    • Smaller in size.
    • Higher blood supply, more mitochondria, and myoglobin (appear red).

Muscle Contraction Types

  • Isometric: Muscle contracts without shortening.
  • Isotonic: Muscle shortens while maintaining tension.

Muscle Fatigue and Recovery

  • Muscle Fatigue: Results from depletion of glycogen and reduced metabolic processes.
  • Recovery: Following periods of inactivity, muscle fibers can undergo atrophy or hypertrophy depending on usage.

Notable Conditions

  • Rigor Mortis: Post-mortem muscle stiffness due to loss of ATP.
  • Duchenne Muscular Dystrophy: Genetic disorder affecting males resulting in muscle weakness and atrophy.

Conclusion

  • Understanding the mechanisms of skeletal muscle contraction is essential for medical students and professionals.
  • Key concepts include muscle fiber composition, contraction mechanisms, energy sources, muscle types, and their physiological roles.