- 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?