Human histo

• Muscle tissue


  The fourth basic tissue type with epithelia, connective tissues, and nervous tissue, composed of cells that optimize the universal cell property of contractility
• Types of muscle tissue

  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle
• Skeletal muscle


  • Contains bundles of very long, multinucleated cells with cross-striations
• Cardiac muscle


  • Has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs
  • Contraction is involuntary, vigorous, and rhythmic
• Smooth muscle

  • Consists of collections of fusiform cells that lack striations
  • Has slow, involuntary contractions
• Sarcoplasm


  The cytoplasm of muscle cells
• Sarcoplasmic reticulum


  The smooth ER in muscle cells
• Sarcolemma


  The muscle cell membrane and its external lamina
• Skeletal muscle fibers


  • Long, cylindrical multinucleated cells with diameters of 10 to 100μm
  • Elongated nuclei are found peripherally just under the sarcolemma
  • A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers
• Connective tissue layers surrounding skeletal muscle

  • Epimysium
  • Perimysium
  • Endomysium
• Epimysium


  An external sheath of dense connective tissue that surrounds the entire muscle
• Perimysium


  A thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle
• Endomysium


  A very thin, delicate layer of reticular fibers and scattered fibroblasts that surrounds the external lamina of individual muscle fibers
• Collagen in these 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
• Myotendinous junctions


  Where skeletal muscles taper at their ends, the epimysium is continuous with the dense regular connective tissue of a tendon
• Skeletal muscle fibers

  • Show cross-striations of alternating light and dark bands
  • The dark bands are called A bands
  • The light bands are called I bands
  • Each I band is bisected by a dark transverse line, the Z disc
  • The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z disc to Z disc and is about 2.5 μm long in resting muscle
• Myofibrils


  Long cylindrical filament bundles running parallel to the long axis of the muscle fiber
• Myofibrils


  • Have a diameter of 1 to 2μm
  • Consist of an end-to-end repetitive arrangement of sarcomeres
  • The lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic pattern of transverse striations
• Myosin


  • A large complex (~500 kDa) with two identical heavy chains and two pairs of light chains
  • Myosin heavy chains are thin, rodlike motor proteins (150 nm long and 2-3 nm thick) twisted together as myosin tails
  • Globular projections containing the four myosin light chains form a head at one end of each heavy chain
  • The myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity)
• Actin filaments


  • Thin, helical filaments that are each 1.0 μm long and 8 nm wide and run between the thick filaments
  • Each G-actin monomer contains a binding site for myosin
  • Actin filaments are anchored perpendicularly on the Z disc by the actin-binding protein a-actinin and exhibit opposite polarity on each side of this disc
• Tropomyosin


  A 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands
• Troponin


  A complex of three subunits: TnT, which attaches to tropomyosin; TnC, which binds Ca2+; and TnI, which regulates the actin-myosin interaction
• Titin


  The largest protein in the body (3700 kDa), with scaffolding and elastic properties, which supports the thick myofilaments and connects them to the Z disc
• The dark bands are called A bands
• Nebulin


  A very large accessory protein (600-900 kDa) that binds each thin myofilament laterally, helps anchor them to α-actinin, and specifies the length of the actin polymers during myogenesis
• A band


  Contains both 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 and contains a myosin-binding protein myomesin that holds the thick filaments in place, and creatine kinase
• 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 that were important in determining the functions of the filaments and other proteins in the myofibril
• Three co plex subunits of troponin

  • TnT
  • TnC
  • TnI
• Sarcolemma is the plasma membrane of skeletal muscle cells
• Triad is an area where transverse tubule meets sarcoplasmic reticulum
• Transverse tubules are invaginations of the sarcolemma into the cell interior; they form a network throughout the cytoplasm
• Transverse tubules are invaginations of the sarcolemma into the cell interior
• Z discs are dense bands at the ends of the sarcomere where the thin filaments attach
• Sarcoplasmic reticulum (SR) is an extensive system of flattened sacs surrounding the myofibrils; it stores calcium ions
• The light bands are called I bands
• the Z disc, a dark transverse line