Muscles and Muscle Tissues Notes

Why Understanding Skeletal Muscle Tissue Matters

  • Effective treatment of strained muscles using RICE (Rest, Ice, Compression, Elevation)
  • Understanding benefits of exercise and body movement
  • Knowledge of muscle disorders

Overview of Skeletal Muscles

  • Approximately 640 skeletal muscles in an adult human body.

Functions of Muscles

  • Movement:
    • Voluntary: Conscious control over actions (e.g., picking up objects)
    • Involuntary: Unconscious control (e.g., heartbeat, digestion)
  • Maintenance of posture: Resisting gravity (losing consciousness results in loss of posture control)
  • Maintenance of blood pressure: By controlling blood vessel diameter
  • Stabilizing joints
  • Heat generation:
    • Shivering generates heat
    • Excess heat during exercise leads to sweating and cooling

Types of Muscle Tissue

  • Prefixes: Myo, mys, and sarco relate to muscle (e.g., sarcoplasm is muscle cell cytoplasm)
  • Three main types:
    1. Skeletal
    2. Cardiac
    3. Smooth
  • Skeletal and smooth muscle cells are elongated, termed muscle fibers, able to convert chemical energy (ATP) into mechanical energy

Skeletal Muscle

  • Characteristics:
    • Packaged into skeletal muscles; attached to bones and skin
    • Longest muscle fibers with striations (bands)
    • Voluntary control,
    • Fast contracting but fatigue easily
    • Approx. 40% body weight in males, 32% in females
    • Key terms: skeletal, striated, voluntary
Anatomy of Skeletal Muscle
  • Composed of nerve and blood supply, connective tissue sheaths, and muscle attachments:
    • Epimysium: surrounds entire muscle; blends with fascia
    • Perimysium: surrounds fascicles (bundles of fibers)
    • Endomysium: surrounds each muscle fiber
  • Each muscle fiber has extensive blood supply and requires oxygen and nutrients

Connective Tissue Sheaths

  • Structure supporting and reinforcing muscle:
    • Epimysium (outer)
    • Perimysium (middle around fascicles)
    • Endomysium (inner around fibers)

Muscle Fiber Microanatomy

  • Skeletal muscle fibers: Long, cylindrical, multi-nucleated cells
  • Sarcolemma: Plasma membrane of the muscle fiber
  • Sarcoplasm: Cytoplasm containing glycosomes (glycogen storage) and myoglobin (O2 storage)
  • Myofibrils: Comprise 80% cellular volume, contain myofilaments in sarcomere units
    • Sarcoplasmic Reticulum: Stores calcium
    • T Tubules: Conduct action potentials into muscle fibers

Sliding Filament Model of Contraction

  • Contraction occurs through interaction of actin and myosin filaments
  • Muscle fibers shorten as myosin heads pull actin filaments closer
  • Energetic process:
    • ATP is critical for muscle contraction and cross-bridge cycling

Events at Neuromuscular Junction

  1. Action potential reaches axon terminal
  2. Calcium channels open, triggering acetylcholine (ACh) release
  3. ACh binds to receptors on muscle fiber’s sarcolemma, leading to depolarization
  4. Action potential generated, subsumed by T-tubules, resulting in muscle contraction via excitation-contraction coupling

Muscle Contraction Steps:

  1. Activation by nerve stimulation
  2. ACh release and binding, raising intracellular calcium levels
  3. Myosin heads attach to actin, executing power stroke
  4. Relaxation occurs when stimulation ceases; calcium pumped back into SR

Types of Muscle Fibers

  • Structural and Functional Characteristics of types (slow twitch vs fast twitch)
    • Slow: Endurance, high myoglobin, aerobic metabolism
    • Fast: Quick bursts, anaerobic metabolism, lower myoglobin

Muscle Fatigue Causes

  • Ionic imbalances (K+, Na+, Ca2+)
  • Inorganic phosphate increasing from CP and ATP breakdown can inhibit muscle function

Excess Postexercise Oxygen Consumption (EPOC)

  • Measures oxygen needed to return to pre-exercise state: replenish oxygen reserves, reconvert lactic acid, restore ATP and glycogen reserves.

Clinical Applications

  • Duchenne Muscular Dystrophy (DMD): Genetic, leads to muscle weakening due to defective dystrophin protein, causes degenerative muscle issues.
  • Rigor Mortis: ATP depletion postmortem leads to sustained muscle contraction.
  • Myasthenia Gravis: Autoimmune disease causing muscle weakness.
  • Tetanus: Muscle paralysis due to toxin interference at the neuromuscular junction.

Cardiac and Smooth Muscle

  • Cardiac: Only in heart, involuntary, striated, connected by gap junctions.
  • Smooth: Found in organs, involuntary, non-striated, slower sustained contractions.

Diseases and Disorders

  • Myopathy, Myalgia, Fibromyalgia, Tetanus, etc., showcase varied effects on muscle function.