Histology of Skeletal Muscle



- See:
      - A Band :
      - I Band :
      - Chemistry of Muscle

- Discussion:
      - epimysium:
              - outermost layer is the epimysium
              - covers the surface of the muscle and has important roles in force transmission and insulation
              - composed of the fibrillar collagens, types I and III
      - perimysium:
              - second layer of connective tissue
              - structurally divides muscle fibers into functional groups called fascicles
              - composed of the fibrillar collagens, types I and III
      - endomysium:
              - composed of two layers of (type I and type III collagen) that surround individual muscle fibers
              - layers fuse with the perimysium to form a sheetlike structure that inserts into the tendon
              - allows for the longitudinal transmission of force
    - individual muscle fibers are cylindrical in shape, 10 to 50  in diameter;
    - each fiber has a limiting plasma membrane, the sarcolemma.
              - w/ in sarcolemma, there exist a fluid material or sarcoplasm, w/ large numbers of longitudinally oriented myofibrils, & numerous nuclei;
    - basement membrane:
              - attached to the sarcolemma itself
              - basement membrane is composed mostly of type IV and type VI collagen
              - important in transmitting forces generated within the muscle to the tendon, as well as laterally to other muscle fibers
              - type IV and type VI collagens
                       - form meshlike networks that surround the muscle fiber
                       - allow for the lateral transmission of forces generated within activated muscle fiber sarcomeres to the overall ECM
    - sarcoplasm contains a Golgi apparatus, many mitochondria, ribosomes, sarcoplasmic reticulum, glycogen, and lipid droplets;
          - sarcoplasmic reticulum:
                - contains a network of closed sacs of agranular endoplasmic reticulum that courses around myofibrils in a primarily 
                       longitudinal direction;

- Myofibril:
    - each myofibril exhibits periodic cross striations;
    - these striations are alternating light and dark bands of isotropic and anisotropic materials, respectively;
    - sarcomere: spans one Z line to the next Z line;

- Thin and Thick Filaments:
    - thin filaments are approx 50 angstroms in diameter and 2 Ang in length;
    - they extend for 1 Ang on either side of Z line to border of H zone;
    - thick filaments are approx 100 angstroms in diameter and 1.5 Ang in length;
    - they span the midportion of the sarcomere in A Band ;
    - I Band  contains only thin filaments, whereas A Band contains both thick & thin filaments, except in H zone where only thick 
           filaments are present;
    - in cross-section of an area where both thick & thin filaments overlap, each thin filament is surrounded by 3 thick filaments, 
           & each thick filament is surrounded by six thin filaments in a hexagonal arrangement;

- Changes during contraction.
    - when muscle fiber contracts, each fiber & each sarcomere becomes shorter;
    - thick and thin filaments are arranged so that they can slide w/o restriction past each other to change sarcomere length;
    - w/ max contraction, sarcomere shortens 20-50% of its nl resting length.
          - I Band  becomes shorter and the H zone usually disappears.
          - A Band  does not change in length during contraction or relaxation.
          - sarcomere can extend to 120% of its length during passive stretching, and I Band  becomes longer



M-band: a safeguard for sarcomere stability?

Heterogeneity of Z-band structure within a single muscle sarcomere: implications for sarcomere assembly.



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Friday, March 4, 2016 1:56 pm