- Myosin:
- largest of myofibrillar proteins, and it makes up > 50% of muscle mass;
- it constitutes thick filament of the contractile unit;
- myosin molecule resembles a thin rod with two small, globular heads;
- heads of chains protrude from thick filament;
- it has significant ATPase activity in presence of ionic calcium, and it is this
portion of molecule that serves as a cross-bridge binding myosin w actin;
- Actin:
-
I Band, or thin filament, is made up of three proteins:
- major component of thin filament is actin, consisting of two polymers;
- G-actin, or globular actin;
- on binding with ATP, it polymerizes to the F form, or fibrous actin;
- F-actin binds calcium tightly;
- has long, thin fibers on its surface, called tropomyosin fibers;
- each tropomyosin strand has a globular molecule of protein troponin
- tropomyosin strands may interfere w/ the cross-bridges between
actin and myosin, which inhibiting contraction of the muscle;
- troponin molecule in some way locks the tropomyosin in place;
- calcium is required to negate this action;
- calcium ions combine w/ troponin molecules, neutralizing their effect
and "unlocking" the tropomyosin, which moves to a neutral position,
allows coupling of actin and myosin, and muscle contraction results;
- A Band :
- consists of entire thick filament, myosin proteins, which are
considered to be stationary;
- it also contains one end of the thin filament;
- under appropriate conditions, discussed later, heads of myosin
molecules form strong bond with units of actin in thin filament;
- I Band :
- consists of the remainder of the thin filament;
- at Z line, it is attached to branches of transverse tubules and is
closely associated w/ terminal cisternae of sarcoplasmic reticulum;
- Sarcoplasmic Reticulum:
- stores calcium in its interior;
- depolarization of muscle membrane along the T tubular system causes a
release of calcium in the closely adjacent sarcoplasmic reticulum;
- these calcium ions are then available to uncouple the
tropomyosin;
- following muscle contraction occurs, there is reuptake of calcium into
sarcoplasmic reticulum and which then allows reactivation of inhibitory
effect of tropomyosin;
- reuptake by the sarcoplasmic reticulum will not occur if there is insufficient
ATP available, which may lead to muscle contraction w/o electrical
stimulation, as is seen in rigor motis;
