cardiac physiology action potential97

cardiac physiology action potential97 - Cardiopulmonary...

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Cardiopulmonary Physiology Millersville University Dr. Larry Reinking Chapter 4 - Characteristics of Cardiac Muscle Cells. This chapter deals with the properties of heart muscle cells and will serve as background information for future laboratory and lecture sessions. In previous courses, you have learned about the mechanical and electrical properties of skeletal muscle. We will review these concepts and use them as a foundation for learning about heart muscle. As you will see, there are similarities as well as vast differences between these two types of striated muscle. General Terminology and Structure The prefixes myo- and sarco- are frequently used when referring to striated muscle cells and their organelles. Hence, we speak of the sarcoplasm (cytoplasm) or sarcoplasmic reticulum (endoplasmic reticulum). Heart muscle is also called cardiac muscle or myocardium . Skeletal muscle is a syncytium , that is, the individual cells are fused and form a long, multinucleate fiber (called a myofiber ). This arrangement of cells seems to enhance communication and coordination within the muscle. Cardiac myofibers have distinct boundaries between the cells called the intercalated discs . These cell boundaries are very permeable and offer little resistance to cell-cell communication. Hence, cardiac muscle is a functional rather than an anatomical syncytium. The histological appearance of cardiac muscle is also distinct from skeletal muscle in that it is highly branched and has centrally located nuclei: ca rd ia c m u sc le sk e le ta l m u sc le nu c le u s in te rc a la te d d is c Figure 4.1 Striated Muscle Contraction of Striated Muscle As seen above, skeletal muscle and cardiac muscle have a characteristic striated appearance. This appearance is due to a regularly repeating pattern of contractile proteins within the muscle cells. Each contractile unit, known as the sarcomere, is about 2 μ m in length and can be depicted as follows: sarcomere Z-line actin myosin actin myosin cross-bridge resting contracting = Ca 2+ Figure 4.2 Sarcomere Chapter 4 1
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Contraction of striated muscle involves ‘cross-bridging’ between the protein filaments, actin and myosin. Repeated cross-bridging causes these filaments to slide past one another and results in muscle shortening (i.e., the Sliding Filament Theory ). The trigger that promotes cross-bridging is a sudden rise in intracellular calcium levels; at rest there is almost no free calcium in the cytoplasm. Be aware that other regulatory proteins are involved in this process but are not shown in the diagram. ATP, of course, is needed for muscle contraction and appears to be involved in breaking cross-bridges and ‘re-energizing’ these broken cross-bridges. Calcium and Muscle Contraction
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cardiac physiology action potential97 - Cardiopulmonary...

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