C5

• Muscle comprises the majority of the “flesh” or “meat” of an organism and is a primary mural element of tubular organs
• Muscle fibers


  Enclosed by a sarcolemma and fine reticular fibers, contain myofilaments composed of actin and myosin
• Muscle cells


  • Often called muscle fibers or myofibers, elongate cells with spindle-shaped fiber-like profiles
• Functional types of muscle

  • Voluntary or involuntary
  • Smooth (nonstriated) or striated
• Classification of muscle types

  • Smooth muscle - non-striated, involuntary
  • Skeletal muscle - striated, voluntary
  • Cardiac muscle - striated, involuntary
• Distinguishing Characteristics of Muscle Types

  • See Table 2
• Features of Muscle Types

  • See Table 2
• Mature skeletal muscle fibers are elongated, unbranched, cylindrical, multinucleated cells with flattened peripheral nuclei
• Myofilaments of skeletal muscle

  Thin filaments (actin) and thick filaments (myosin)
• Thin filament (actin)


  Measures 6 nm in diameter, contains filamentous actin, tropomyosin, and troponin
• Thick filament (myosin)


  Measures 15 nm in diameter, is a long, golf club-shaped polypeptide molecule
• Appearance of myofibrils in longitudinal section


  Exhibits repeating, linearly arranged, functional subunits called sarcomere with I bands (isotropic) and A bands (anisotropic)
• I band


  Light-staining band containing only thin filaments, called isotropic because they do not rotate polarized light
• Z line


  Separates sarcomeres from their neighbors at each end, major protein is α-actinin which anchors one end of the thin filaments
• A band


  Dark band regions where planes of polarized light are refracted unequally, called anisotropic because it is birefringent capable of rotating polarized light, has a lighter central region termed H band bisected by an M line
• H band


  Lighter staining central region of the A band containing only thick filaments, derived from the German term “Helles” meaning clear
• M line


  Holds the thick filaments in place and marks the center of each sarcomere, derived from the German term “Mittel” meaning intermediate or middle
• Sarcoplasmic reticulum of skeletal muscle cells is specialized to sequester calcium ions
• Triads


  Complex of 2 terminal cisternae and an intervening T tubule, important in initiating muscle contraction
• Motor End-Plate or myoneural junction transmits nerve impulses to muscle cells, initiating muscle contraction
• Components of myoneural junction


  • Presynaptic (neural) component
  • Synaptic cleft
  • Postsynaptic (muscular) component
• Mechanics of Skeletal Muscle Contraction

  According to the sliding filament hypothesis, initiated by and includes a chain of events involving nerve impulse, depolarization, acetylcholine release, and calcium binding to troponin
• Muscle contraction


  1. Release of sequestered Ca2+ from the sarcoplasmic reticulum into the sarcoplasm surrounding the myofibrils

  2. Ca2+ binds to the TnCs of the troponin complexes

  3. Conformational change of each troponin complex

  4. TnIs of the troponin complexes move away from the myosin head-binding sites on the actin filaments

  5. Myosin heads bind to actin

  6. Activation of the ATPase in the myosin heads

  7. Production of energy and ADP from ATP and movement of the myosin heads

  8. Pull the actin filaments toward the center of the sarcomere, resulting in simultaneous shortening of the sarcomeres by shortening of the I bands (A bands do not narrow), resulting in shortening of the myofibrils, resulting in shortening of the entire muscle fiber
• Muscle contraction continues as long as there is an excess Ca2+ present in the sarcoplasm
• Relaxation


  1. All the membranes repolarize, allowing the sarcoplasmic reticulum to resequester Ca2+ from the sarcoplasm by active transport

  2. Ca2+ removes from the TnC and returns the TnI to a position that it inhibits binding of the myosin head to the actin filament
• Cardiac muscle fibers are long, branched cells with one or two ovoid central nuclei. The sarcoplasm near the nuclear pores contains many mitochondria, glycogen granules and some lipofuscin pigment. Mitochondria lie in chains between the myofilaments
• Differences between cardiac muscle and skeletal muscle

  • Cardiac muscle fibers are branched
  • Connective tissue surrounding each muscle fiber is more prominent
  • Nucleus is centrally located and surrounded by a pale staining cytoplasmic region
  • Distinct, transversely oriented, dark staining intercalated disks are scattered throughout the cardiac muscle
  • Cardiac muscle cells have abundant mitochondria, extensive vascular supply and large quantities of myoglobin
  • Cardiac muscle cells are capable of hypertrophy
• Cardiac muscle cells are capable of hypertrophy that accounts for their increased size during growth and is one of the mechanisms by which the heart responds to increased workloads
• Types of Cardiac Muscle Fibers

  • Atrial cardiac muscle fibers are small and have fewer T tubules than ventricular cells
  • Ventricular cardiac muscle fibers are large cells with more T tubules and no granules
• Purkinje fibers are specialized cardiac muscle cells that are modified as impulse-conducting cells. These fibers conduct impulses from the A-V node through the interventricular septum to the ventricles
• Sarcoplasmic Reticulum and T tubule System of the Cardiac Muscles
• Mechanism of Cardiac Muscle Contraction
• Cardiac muscles


  Contain dyads instead of triads due to the expanded cisterna of the sarcoplasmic reticulum
• Mechanism of Cardiac Muscle Contraction

  1. Composition and arrangement of myofilaments are almost identical with skeletal muscle

  2. Skeletal and cardiac muscle contractions are essentially the same at the cellular level
• Smooth Muscle


  • Mature fibers are elongated, spindle-shaped cells with a single central ovoid nucleus
  • Muscle is non-striated, lacks striped appearance, or cross-striations
  • Sarcoplasm contains abundant mitochondria, some rER, and a large Golgi complex
  • Each fiber produces its own basal lamina consisting of proteoglycan-rich material and Type III collagen fibers
• Myofilaments of the Smooth Muscle

  1. Thin filament: Actin filaments anchored by α-actinin dense bodies associated with the plasma membrane

  2. Thick filament: Myosin filaments less stable, not always present in the cytoplasm, have heads along most of their length and bare areas at the ends
• Smooth muscles cells contain poorly organized sarcoplasmic reticulum that participates in the sequestration and release of Ca2+ but does not divide the myofilaments into myofibrillar bundles
• Smooth muscle contraction is influenced by calcium ions, neuronal stimulation, and hormones
• Types of Smooth Muscle Fibers


  • Visceral smooth muscle
  • Vascular smooth muscle
  • Smooth muscle of the iris
• Visceral smooth muscle


  • Derived from splanchno-pleural mesenchyme, found in thoracic, abdominal, and pelvic organs, slow contraction in waves, classed as unitary smooth muscle