The sympathetic branch of the autonomic nervous system releases norepinephrine and the parasympathetic branch releases acetylcholine. The inotropicity of cardiac muscle is unlike skeletal muscle but like smooth muscle. Cardiac muscle has no inherent tone, shows a slow contraction rate, and long refractory period. 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, as occurs with increased heart rate, the strength of contraction increases (Bowditch effect, Treppe, staircase).
Anatomical and Functional Characteristics of Cardiac Muscle
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