Cardiac Muscle Characteristics:
Cardiac (heart) muscle shares many anatomical and functional characteristics with skeletal muscle and some with smooth muscle.
Like skeletal muscle, it is striated. That is it is arranged in a sliding filament pattern, in myofibrils and sarcomeres. Smooth muscle is not. Rather than being attached to bone, it is attached to connective tissue rings, located around the four heart valves, not in a linear array but circular or semi-spherical.
The myocardial cells, or fibers, are much smaller than skeletal muscle cells; cylindrical cells 15 um in diameter by up to 100 um in length, with one or two nuclei. Some cardiac muscle cells are polyploid, especially in humans.
Division continues after myosin and actin begin to be synthesized, unlike multinucleate skeletal muscle cells or myotubes. Cardiac muscle cells are highly dependent upon aerobic conditions. Consequently they have many more mitochondria, which do not increase in density or shape with exercise training.
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