Histo.7

• Muscle tissue


  Cells that optimize the universal cell property of contractility

  
• Types of muscle tissue


  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle

  
• Sarcoplasm


  The cytoplasm of muscle cells
• Sarcoplasmic reticulum


  The smooth ER in muscle cells
• Sarcolemma


  The muscle cell membrane and its external lamina
• Skeletal (or striated) muscle


  • Muscle fibers are long, cylindrical multinucleated cells with diameters of 10-100 µm and cross-striations
  • Their contraction is quick, forceful, and usually under voluntary control
  • Elongated nuclei are found peripherally just under the sarcolemma

  
• Skeletal muscle fibers


  • A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers of differentiated skeletal muscle

  
• Organization of a skeletal muscle

  1. Epimysium - external sheath of dense irregular connective tissue surrounding the entire muscle

  2. Perimysium - thin connective tissue layer surrounding each bundle of muscle fibers (fascicle)

  3. Endomysium - very thin, delicate layer of reticular fibers and scattered fibroblasts surrounding the external lamina of individual muscle fibers

  
• Collagens in the connective tissue layers of muscle serve to transmit the mechanical forces generated by the contracting muscle cells/fibers

  
• Individual muscle fibers seldom extend from one end of a muscle to the other

  
• All three layers plus the dense irregular connective tissue of the deep fascia are continuous with the tough connective tissue of a tendon at myotendinous junctions which join the muscle to bone, skin, or another muscle

  
• Longitudinally sectioned skeletal muscle fibers


  • Show striations of alternating light and dark bands

  
• Sarcoplasm


  Highly organized, containing primarily long cylindrical filament bundles called myofibrils that run parallel to the long axis of the fiber
• A bands


  The dark bands on the myofibrils
• I bands


  The light bands on the myofibrils
• Mitochondria and sarcoplasmic reticulum are found between the myofibrils
• Sarcomeres


  The repetitive arrangement within myofibrils
• The lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic pattern of transverse striations

  
• Thick myofilaments


  Composed of myosin, occupy the A band at the middle region of the sarcomere
• Myosin


  • A large complex with two identical heavy chains and two pairs of light chains
  • Myosin heavy chains are thin, rodlike motor proteins twisted together as myosin tails
  • Globular projections containing the four myosin light chains form a head at one end of each heavy chain
• Myosin heads


  Bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity)
• Several hundred myosin molecules are arranged within each thick filament with overlapping rodlike portions and the globular heads directed toward either end
• Thin, helical actin filaments

  Each 1.0-µm long and 8-nm wide, run between the thick filaments
• 1. actin monomers


  Contain a binding site for myosin
• Thin filaments


  Have two tightly associated regulatory proteins: Tropomyosin and Troponin
• I bands


  Consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands
• Actin filaments


  Anchored perpendicularly on the Z disc by the actin-binding protein α-actinin and exhibit opposite polarity on each side of this disc
• Titin


  The largest protein in the body, with scaffolding and elastic properties, which supports the thick myofilaments and connects them to the Z disc
• Nebulin


  Binds each thin myofilament laterally, helps anchor them to α-actinin, and specifies the length of the actin polymers during myogenesis
• A bands


  Contain both the thick filaments and the overlapping portions of thin filaments
• H zone


  A lighter zone in the center of the A band, corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments
• M line


  Bisects the H zone, containing a myosin-binding protein myomesin that holds the thick filaments in place, and creatine kinase
• Despite the many proteins present in sarcomeres, myosin and actin together represent over half of the total protein in striated muscle
• The overlapping arrangement of thin and thick filaments within sarcomeres produces in TEM cross sections hexagonal patterns of structures which were important in determining the functions of the filaments and other proteins in the myofibril
• Sarcoplasmic reticulum


  The membranous smooth ER in skeletal muscle fibers, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils

  
• Transverse or T-tubules


  Tubular infoldings of the sarcolemma that penetrate deeply into the sarcoplasm and encircle each myofibril near the aligned A- and I-band boundaries of sarcomeres
• Triad


  The complex of a T-tubule with two terminal cisternae of sarcoplasmic reticulum
• The triad complex allows depolarization of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum and trigger release of Ca2+ ions into cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres