- See:
-
A Band :
-
I Band :
-
Chemistry of Muscle
- Discussion:
- 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;
-
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.
