discs are where the cells connect to each other and are essentially desmosomes, tight junctions, and gap junctions that allow the passage of ions between the cells, helping to bind the cells together. There is also intercellular connective tissue, which binds the cells together during contraction. Cardiac muscle undergoes aerobic respiration just like other muscle cells. Lipids and carbohydrates are metabolized in the mitochondria to make energy. Cardiac muscle cells have long refractory periods with brief relaxation periods. The relaxation period is necessary to allow the heart to fill with blood for the next cardiac cycle. The refractory period is long in order to prevent tetany of the heart muscle, which isn’t compatible with life. Damaged cardiac muscle cells cannot easily repair or replace themselves if the cell is damage. There are a few cardiac muscle stem cells that can potentially replace dead cells but those that replace dead cells aren’t as functional as the original cells. Dead cells are often replaced by inactive scar tissue.
CONDUCTION SYSTEM OF THE HEART If embryonic heart cells are grown in vitro (in a Petri dish), they can generate their own electrical impulse and contract. When they are connected, they contract together from the faster cell through to the slower cell. The heart can generate its own electrical impulse and the fastest cells lead the way for slower cells. The major components of the cardiac conduction system are the SA node (or sinoatrial node), AV node or atrioventricular node, the bundle branches, and the Purkinje cells. The SA node is where the cardiac conduction cycle begins. It is located in the upper back wall of the right atrium near where the superior vena cava enters the heart. The SA node has the fastest rate of depolarization and is considered the pacemaker of the heart. Impulse spreads from the SA node via internodal pathways through the atria to the AV node. There are three bands of internodal pathways (anterior, middle, and posterior) that lead onto the next node in the electrical pathway. It takes 50 milliseconds to travel between the nodes.
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