Histology of Muscular

• Muscle fiber


  Muscle cell
• Sarcolemma


  Cell membrane of muscle cells
• Types of muscle


  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscle


  • Voluntary contraction
  • Presence of striations
• Cardiac muscle


  • Involuntary contraction
  • Presence of striations
• Smooth muscle


  • Involuntary contraction
  • No striations
• Locations of skeletal muscle

  • Gastrocnemius
  • Latissimus dorsi
• Locations of cardiac muscle

  • Myocardium
  • Large blood vessels attached to the heart
• Locations of smooth muscle

  • Walls and parenchyma of visceral organs
  • Walls of blood vessels
  • Skin
• Epi mission


  Dense irregular connective tissue that envelops the entire skeletal muscle
• Perimysium


  Denser and thinner irregular connective tissue layer that extends inwards and divides the interior of the muscle into smaller bundles of muscle fibers (fascicles)
• Endomysium


  Thin layer of particular connective tissue fibers that invests into individual muscle fibers
• Muscle fiber


  Equivalent to a muscle cell, long and cylindrical, multinucleated with peripheral nuclei
• Myofibril


  Subunit of a muscle fiber, composed of smaller myofilaments of actin and myosin
• I band


  Lighter band in the cross-striations of skeletal muscle fibers
• A band


  Darker band in the cross-striations of skeletal muscle fibers
• Z line/Z disc


  Dark line that bisects the I band
• Sarcomere


  Repeating unit from Z line to Z line that makes up the myofibrils
• H zone


  Lighter zone in the center of the A band
• M line


  Thin dark band that bisects the H zone
• Thick filament


  Composed of myosin protein, covers the entire A band
• Thin filament


  Composed of actin protein, can extend from I band to A band
• Titin


  Giant protein that functions as a molecular spring, providing passive elasticity to muscle
• Muscle contraction


  1. Nerve impulse triggers release of acetylcholine

  2. Acetylcholine binds to receptors on motor end plate

  3. Muscle impulse spreads along T-tubules

  4. Calcium released from sarcoplasmic reticulum

  5. Calcium binds to troponin, exposing actin binding sites

  6. Myosin heads attach to actin, forming cross-bridges
• What happens when calcium binds to troponin

  1. Actin active site is exposed

  2. Myosin head attaches to actin active site forming cross-bridge
• Calcium binds to troponin


  Actin active site is exposed
• Myosin head


  Attaches to actin active site forming cross-bridge
• Muscle contraction


  1. Myosin heads pull thin filaments towards sarcomere center

  2. ATP binds to myosin heads causing detachment from thin filaments

  3. Cycle of attachment, pivoting, detachment repeats causing sliding of thick and thin filaments
• Muscle relaxation


  • Calcium ions transported back into sarcoplasmic reticulum
  • Tropomyosin recovers to block actin active sites
  • Filaments slide back to relaxed state
• Relaxed skeletal muscle


  • Sarcomeres at expanded length
  • Titin molecules in stretched, spring-like state
• Contracted skeletal muscle


  • Z discs drawn closer together
  • Titin molecules compressed
• Skeletal muscle fiber types

  • Red/slow twitch
  • White/fast twitch
• Red/slow twitch muscle


  • Contract slowly but can sustain continuous contraction
  • Have richer blood supply and more mitochondria
• White/fast twitch muscle


  • Contract rapidly but fatigue quickly
  • Have fewer mitochondria and less myoglobin
• Skeletal muscle cells are cylindrical with peripheral nuclei and striations
• Cardiac muscle cells are cylindrical, branched, with central nuclei and striations
• Intercalated disc


  Unique junction between cardiac muscle cells allowing rapid spread of contraction
• Smooth muscle


  • Spindle-shaped cells with central nuclei
  • No striations due to irregular arrangement of actin and myosin
• Differences between skeletal, cardiac and smooth muscle

  • Shape
  • Nucleus location
  • Presence of striations
  • Branching
  • Sarcomeres
  • Voluntary/involuntary contraction
• For further details, refer to the textbook "Basic Histology" by Jun Karan