muscle notes

Muscle Physiology Study Notes

Introduction to Muscle Types

  • Three Types of Muscle Tissue
    • Skeletal Muscles
    • Striated muscle attached to bones of the skeleton
    • Functions: Controls body movement
    • Control: Voluntary control; responds to somatic motor neurons
    • Cardiac Muscle
    • Striated muscle found only in the heart
    • Functions: Moves blood through the circulatory system
    • Control: Involuntary control; responds to autonomic innervation, spontaneous contraction, modulated by the endocrine system
    • Smooth Muscle
    • Primary muscle of internal organs and tubes
    • Functions: Influences the movement of material into, out of, and within the body
    • Control: Involuntary control; responds to autonomic innervation, spontaneous contraction, modulated by the endocrine system

Structure and Composition of Skeletal Muscle

  • Skeletal Muscle Characteristics
    • Attached to bones by tendons.
    • Origin: Closest to the trunk or to more stationary bone
    • Insertion: More distal or more mobile attachment
    • Flexors: Bring bones together
    • Extensors: Move bones away
    • Antagonistic Muscle Groups: Flexor-extensor pairs form groups that move bones in opposite directions.

Anatomy of Muscle Fibers

  • Muscle Fibers
    • The basic unit of skeletal muscle, termed muscle fibers
    • Sarcolemma: cell membrane of muscle fibers
    • Sarcoplasm: cytoplasm of muscle fibers
    • Nuclei: Fused cells result in many nuclei within muscle fibers
    • Satellite Cells: Stem cells that can differentiate into muscle for growth or repair
    • Muscle fibers are bundled into fascicles surrounded by connective tissue.
  • Connective Tissue Layers
    • Endomysium: surrounds individual muscle fibers
    • Perimysium: surrounds fascicles
    • Epimysium: surrounds the entire muscle - continuous with tendon connecting muscle to bone.
  • Composition of Muscle Fibers
    • Myofibrils: Contractile structures within muscle fibers
    • Mitochondria: Powerhouse, aids in energy production
    • Glycogen Granules: Stores energy for muscle activity

Myofibrils and Sarcomere Structure

  • Myofibrils
    • Serve as contractile elements of muscle fibers
    • Composed of thick and thin filaments:
    • Thick Filaments: Composed primarily of myosin, which has motor and regulatory functions.
    • Thin Filaments: Composed of actin, tropomyosin, and troponin
  • Sarcomere
    • Basic contractile unit of skeletal muscle
    • Components:
    • Z discs: Define boundaries of each sarcomere
    • I Band: Region containing thin filaments only (actin)
    • A Band: Darker area where thick and thin filaments overlap
    • H Zone: Clear zone in the middle of the A Band containing thick filaments only
    • M Line: Line of proteins that anchor thick filaments together.

Muscle Contraction Mechanisms

  • Sliding Filament Theory
    • Actin and myosin slide past one another, leading to muscle contraction without changing length
    • Mechanism of contraction involves crossbriding between myosin heads and actin
  • Excitation-Contraction Coupling
    • Involves the excitation resulting from a motor neuron’s action potential followed by muscle contraction triggered by Ca2+ from the sarcoplasmic reticulum.
  • Role of Calcium
    • Calcium binds troponin, changing its conformation to pull tropomyosin off the binding sites on actin, allowing myosin to bind and thus interact with actin
    • Repeated as long as ATP is available and binding sites are uncovered

Energy Supply for Muscle Contraction

  • ATP Requirements
    • ATP is necessary for both contraction and relaxation processes in muscle fibers
    • Energy Sources:
    • Phosphocreatine: Provides a rapid source of energy via creatine kinase
    • Carbohydrates:
      • Anaerobic glycolysis: Quick, less efficient, produces lactate
      • Aerobic respiration: More efficient, utilizes the citric acid cycle and electron transport chain

Types of Muscle Fiber

  • Fast-Twitch vs Slow-Twitch Fibers
    • Slow-Twitch Fibers (Type I):
    • Aerobic metabolism primarily, fatigue resistant
    • Fast-Twitch Fibers (Type II):
    • Type IIa (Fast-Twitch Oxidative-Glycolytic): Use both anaerobic and aerobic metabolism
    • Type IIb (Fast-Twitch Glycolytic): Primarily anaerobic, fatigue rapidly

Functional Differences Among Muscle Types

  • Skeletal vs Cardiac vs Smooth
    • Skeletal Muscle: Voluntary, striated, stronger, faster contraction
    • Cardiac Muscle: Involuntary, electrically linked, capable of pacemaker potentials, slower than skeletal muscle
    • Smooth Muscle: Involuntary, non-striated, can sustain contractions for long periods

Muscle Disorders

  • Causes of Muscle Disorders
    • Muscle cramps: Sustained, painful contractions, often from fatigue or overuse
    • Acquired disorders (e.g., Duchenne muscular dystrophy)
    • Inherited disorders (e.g., McArdle’s Disease: glycogen not converted effectively)

Conclusion

  • Understanding the physiology of muscles is crucial for knowledge in areas such as exercise science, physical therapy, and medicine. Each muscle type serves specific roles and functions, influenced by their unique structural characteristics and control mechanisms.