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3 Cards in this Set
- Front
- Back
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List the three major components of the electrocardiogram and the associated cardiac electrical events that produce each of these components. |
The P wave: represents the electrical activity in the heart associated with the depolarization of the atrial muscle leading to their contraction.
The QRS complex: produced by the depolarization of the ventricular muscle just prior to its contraction.
The T wave: the result of the depolarization of the ventricular muscle. |
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Describe the sequence of events that results in the contraction and relaxation of cardiac muscle. |
Excitation-contraction coupling refers to the process by which an action potential in the cell membrane (sarcolemma) excites the muscle cell to produce a muscle contraction. The action potential that was generated at the neuromuscular junction will spread out over the sarcolemma and down the T-tubules into the core of the muscle cell. This action potential travels very close to the SR and will open Ca channels, causing the release of Ca from the terminal sisternae of the SR. Cytosolic Ca levels increase due to the plateau phase, causing an abundant amount of Ca to be released from the SR. Extracellular Ca acts as the trigger for the release of Ca from the SR. The Ca will bind to troponin on the thin myofilaments, causing tropomyosin to uncover the myosin binding sites found on actin. Myosin will now be able to attach to the actin and a power stroke occurs. Repolarization is the result of removal of Ca from the cytosol. Ca is actively transported back into the SR by primary active Ca ATPase pump, and Ca is also removed from the cell to the exterior by Ca ATPase and Ca/Na exchanger. Without Ca present in the cytoplasm of the muscle cell, the tropomyosin will cover the myosin binding sites and myosin will be unable to bind to actin and power stroke will not occur. Muscle relaxes. **The amount of free calcium that is available to trigger the process of cross-bridge formation is a direct determinant of the strength of cardiac contraction. More calcium=stronger contraction. |
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Define the refractory period of the heart. |
The refractory period of the cardiac muscle, during which no further muscle contractions can be elicited, lasts almost as long as the mechanical contraction. This is primarily due to the long plateau phase seen in cardiac action potentials. This means that it is impossible to produce tetanic contractions. |
