About three-fourths of the ventricular mass is composed of cardiomyocytes, normally 60- 1 40 μm in length and 1 7-25 μm in diameter
Each cell contains multiple, rodlike cross-banded strands (myofibrils) that run the length of the cell and are composed of serially repeating structures, the sarcomeres.
The cytoplasm between the myofibrils contains other cell constituents, including the single centrally located nucleus, numerous mitochondria, and the intracellular membrane system, the sarcoplasmic reticulum.
The sarcomere, the structural and functional unit of contraction, lies between adjacent Z lin, which are dark repeating bands that are apparent on transmission electron microscopy.
The distance between Z lines varies with the degree of contraction or stretch of the muscle and ranges between 1 . 6 and 2.2 f.lm.
Within the confines of the sarcomere are alternating light and dark bands, giving the myocardial fibers their striated appearance under the light microscope. At the center of the sarcomere is a dark band of constant length, the A band, which is flanked by two lighter bands, the I bands, which are of variable length.
THE CONTRACTILE PROCESS
The sliding filament model for muscle contraction rests on the fundamental observation that both the thick and the thin filaments are constant in overall
length during both contraction and relaxation.
With activation， the actin filaments are propelled farther into the A band. In the process, the A band remains constant in length, whereas the I band shortens and the Z lines move toward one another.
The myosin molecule is a complex, asymmetric fibrous protein with a molecular mass of about 500,000 Da; it has a rodlike portion that is about 150nm ( 1 500 Á) in length with a globular portion (head) at its end.
These globular portions of myo sin form the bridges between the myosin and actin molecules and are the site of ATPase activity.
Actin has a molecular mass of about 47,000Da. The thin filament consists of a double helix of two chains of actin molecules wound about each other on a larger molecule,tropomyosin. A group of regulatory proteins-troponins C,I， and T -are spaced at regular intervals on this filament.
In contrast to myosin, actin lacks intrinsic enzymatic activity but does combine reversibly with myosin in the presence of ATP and Ca'+. The calcium ion activates the myosin ATPase, which in turn breaks down ATP, the energy source for contraction.
The activity of myo sin ATPase determines the rate of forming and breaking of the actomyosin cross-bridges and ultimately the velocity of muscle contraction.
In relaxed muscle, tropomyosin inhibits this interaction.
Titin is a large,flexible, myofibrillar protein that connects myosin to the Z line; its stretching contributes to the elasticity of the heart.