Cardiac Muscle

1. Cardiac muscle is a striated muscle, with a sarcomere structure much like that of skeletal muscle. It has small cells (as in smooth muscle), firmly connected end-to-end at the intercalated disks. 2. The action potential in cardiac muscle is long lasting compared to the duration of the contraction, preventing a tetanic contraction. 3. Under normal circumstances, cardiac muscle operates at lengths somewhat less than the optimal length for peak force production, facilitating length-dependent regulation of the muscle activity. 4. A typical cardiac muscle contraction produces less than maximal force, allowing physiologically regulated changes in contractility to adjust the force of the muscle contraction to the body’s current needs. 5. As in skeletal muscle, the speed of shortening of cardiac muscle is inversely related to the force being exerted, as expressed in the force-velocity curve. 6. The contractility of cardiac muscle is changed by inotropic interventions that include changes in the heart rate, the presence of circulating epinephrine, or sympathetic nerve stimulation. 7. Most changes in cardiac muscle contractility are associated with changes in the amount of calcium available to activate the contractile system. 8. Calcium enters a cardiac muscle cell during the plateau of the action potential. This entry promotes the release of internal calcium stores, which are located mainly in the sarcoplasmic reticulum (SR). Primary and secondary active transport systems remove calcium from the cytoplasm. 9. Cardiac muscle derives its energy primarily from the oxidative metabolism of lactic acid and free fatty acids. It has very little capacity for anaerobic metabolism. MUSCLE K E Y C O N C E P T S