histo - midterms

• Muscular tissue


  Composed of cells that optimize the universal cell property of contractility
• Muscle contraction


  • Caused by the sliding interaction of thick myosin filaments along thin actin filaments
  • Drives movement within organ systems, of blood, and of the body as a whole
  • The forces necessary for sliding are generated by other proteins affecting the weak interactions in the bridges between actin and myosin
• Types of muscle tissue

  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle
• Sarcoplasm


  Cytoplasm of muscle cells
• Sarcoplasmic reticulum


  Smooth ER of muscle
• Sarcolemma


  Muscle cell membrane and external lamina
• Skeletal muscle


  • Striated muscle
  • Muscle fibers are long, cylindrical multinucleated cells with diameters of 10-100 μm
  • Elongated nuclei are found peripherally just under the sarcolemma
• Skeletal muscle is generally attached to the bone except for the intrinsic muscles of the tongue, upper esophagus and some of the facial expression muscles
• Micrograph shows a cross section of striated muscle demonstrating connective tissue and cell nuclei
• Connective tissue layers in skeletal muscle

  • Endomysium
  • Perimysium
  • Epimysium
• Skeletal muscle fibers


  • Long cylindrical and multinucleated cells 10-100 um
  • Myoblasts; satellite cells
  • Muscle fibers do not branch
  • On cross-section, fibers appear almost equal in size and nuclei are located at the periphery or beneath the sarcolemma and may be more than 2
• Organizations of the skeletal muscle

  • Epimysium
  • Fascicle
  • Perimysium
  • Muscle fiber
  • Endomysium
  • Myofibril
• An entire skeletal muscle is enclosed within a thick layer of dense connective tissue called the epimysium that is continuous with fascia and the tendon binding muscle to bone
• Large muscles contain several fascicles of muscle tissue, each wrapped in a thin but dense connective tissue layer called the perimysium. Within fascicles individual muscle fibers (elongated multinuclear cells) are surrounded by a delicate connective tissue layer, the endomysium
• Parts of three muscle fibers are separated by very small amounts of endomysium. One fibroblast nucleus (F) is shown. Muscle nuclei (N) are found against the sarcolemma. Along each fiber thousands of dark staining A bands alternate with lighter I bands
• Longitudinally sectioned skeletal muscle fibers show striations of alternating light and dark bands
• The sarcoplasm is highly organized, containing primarily long cylindrical filament bundles called myofibrils that run parallel to the long axis of the fiber
• The dark bands on the myofibrils are called A bands (anisotropic or birefringent in polarized light microscopy); the light bands are called I bands (isotropic, do not alter polarized light)
• In the TEM, each I band is seen to be 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
• The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively, organized within each myofibril in a symmetric pattern containing thousands of each filament type
• Myofibrils


  • Long cylindrical filamentous bundles running parallel to the long axis of the fiber
  • Diameter of 1-2 µm and consists an end-to-end repetitive arrangement of sacromeres
  • A and I banding pattern in sacromeres is due mainly to the regular arrangement of thin and thick filaments
• Parts of a sarcomere

  • H zone
  • M line
  • A bands
  • I bands
  • Z line
• TEM showing one contractile unit (sarcomere) in the long series that comprises a myofibril. In its middle is an electron-dense A band bisected by a narrow, less dense region called the H zone. On each side of the A band are the lighter-stained I bands, each bisected by a dense Z disc which marks one end of the sarcomere. Mitochondria (M), glycogen granules, and small cisternae of SER occur around the Z disc
• Myofilaments


  • Thick filaments
  • Thin filaments
• Thick filaments


  • 1.6 µm long and 15 nm wide; they occupy the A band, the central portion of the sarcomere
  • Composed of myosin, dissociated into two identical heavy chains and two pairs of light chains
  • Bound to proteins of M line and to the Z disc by a large protein called titin (with spring-like domains)
• Thin filaments


  • 1.0 µm long and 8 nm wide
  • Run between the parallel to the thick filaments and have one end attached to the Z line/disc
  • Composed of: F-actin, Tropomyosin, Troponin
• 1. actin


  • Long filamentous polymers containing two strands of globular (G-actin) monomers, 5.6 nm in diameter, twisted around each other in a double helical formation
• Tropomyosin


  • Assembles to form a long polymer located in the groove between the two twisted actin strands
• Troponin


  • A globular complex of three subunits: TnT- attaches to tropomyosin, TnC - binds calcium ions, Tnl -inhibits the actin-myosin interaction
• Types of muscle fibers

  • Type I (slow, red oxidative fibers)
  • Type IIA (fast, intermediate oxidative-glycolytic fibers)
  • Type IIB (fast, white glycolytic fibers)
• Type I (slow, red oxidative) muscle fibers

  • Many mitochondria; abundant myoglobin
  • Oxidative phosphorylation of fatty acids
  • For slow, continuous contraction for prolonged periods such as postural muscles at the back
• Type IIA (fast, intermediate oxidative-glycolytic) muscle fibers

  • Many mitochondria, myoglobin and glycogen
  • Both oxidative and anaerobic glycolysis
  • For rapid contraction and short burst of activity such as in athletes
• Type IIB (fast, white glycolytic) muscle fibers


  • Fibers are specialized for rapid, short-term contraction
  • Rapid contractions lead to rapid fatigue as lactic acid produced by glycolysis accumulates
  • Few mitochondria or capillaries and depending largely on anaerobic metabolism of glucose derived from stored glycogen features that make such fibers appear white
• Cardiac muscle


  • Found in the heart
  • Cardiac muscle fibers are striated, but they consist of individual cylindrical cells, each containing one (or two) central nuclei and linked by adherent and gap junctions at prominent Intercalated discs
  • Exhibit a cross-striated banding pattern comparable to that of skeletal muscle
• Longitudinal sections of cardiac muscle at the light microscope level show nuclei (N) in the center of the muscle fibers and widely spaced intercalated discs (1) that cross the fibers
• Intercalated discs


  • Represent the interface between adjacent muscle cells where many junctional complexes are present
  • Maintain firm cohesion of successive cellular units of the myocardium
  • Transmits the pull of one contractile unit to the next cellular unit along the long axis of the myofibrils
• Desmosomes and fascia adherents

  • Serve to bind cardiac cells firmly together to prevent their pulling apart under constant contractile activity
• Gap junctions


  • Provide ionic continuity between adjacent cells
• Purkinje myocytes


  • Modified cardiac muscle fibers that are specialized for impulse conduction
  • Located in the sinoatrial (SA) node, atrioventricular (AV) node, and internodal tracts and bundle of His