Automaticity, or automatic depolarization and contraction, is an important characteristic of cardiac muscle, permitting each individual cell to beat under certain conditions. 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 anatomical syncytium. Purkinje fibers are specialized cardiac muscle cells providing rapid conduction pathways.
The heart muscle cells are surrounded by a collagen network produced by fibroblasts which hold the cells in place and possibly store energy which is released during diastole. The autonomic nervous system (a.n.s.) releases substances which alter rate of contraction (chronotropicity), strength of contraction (inotropicity), and speed of conduction (dromotropicity).
Anatomical and Functional Characteristics of Cardiac Muscle
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