Misconception 1. Cardiac and skeletal muscle are both striated, and identical in their functional characteristics.
Misconception 2. Cardiac contractions occur only by virtue of central nervous system connections to the heart.
Cardiac and skeletal muscles are striated, arranged in a sliding filament pattern, and in myofibrils and sarcomeres. Myocardial cells, or fibers, are much smaller than skeletal muscle cells; cylindrical cells 15 um in diameter and up to 100 um in length, with one or two nuclei. Cell 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.
Automatic depolarization and contraction (automaticity) is an important characteristic of cardiac muscle, permitting each individual cell to beat on its own under certain conditions. Thus, the central nervous system does not initiate the heart beat!
The sarcoplasmic reticulum and transverse tubular system is less extensive than in skeletal muscle, since most of the Ca++ for depolarization comes from outside the cell. Intercalated discs provide structural strength to hold cells together and permit cell-to-cell communication, making the heart a functional but not an anatomic syncytium.
The autonomic nervous system releases substances which alter rate of contraction (chronotropicity), strength of contraction (inotropicity), and speed of conduction (dromotropicity).
The variable inotropicity of cardiac muscle is unlike skeletal muscle but like smooth muscle. Cardiac muscle has no inherent tone, shows a slow contraction rate, and a long refractory period (Figure 1.01). The latter two characteristics fit well with its physiological function and anatomy. There is no recruitment of fibers as in skeletal muscle; all the fibers contract together.
The length-tension curve of cardiac muscle is like that of skeletal muscle, but narrower in working range. There is much greater passive tension at, and above Lmax, than in skeletal muscle. As the rate of cardiac muscle contraction increases (increased heart rate) the strength of contraction increases (Bowditch effect, Staircase, Treppe).
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