Anatomy & Physiology I and II

• Step 1 Rapid depolarization : The stage of rapid depolarization in a cardiac muscle cell resembles that in a skeletal muscle fiber. Voltage-gated sodium channels open at threshold, and this action makes the membrane suddenly permeable to Na+. The result is a huge influx of sodium ions and the rapid depolarization of the sarcolemma. The channels involved are called fast sodium channels because they open quickly and remain open for only a few milliseconds. • Step 2 The plateau : As the transmembrane potential approaches +30 mV, the voltage-gated sodium channels close. They will remain closed and inactivated until the membrane potential reaches –60 mV. The cell now begins pumping Na+ out of the cell. However, a net loss of positive charges discontinues because as the sodium channels are closing, voltage-gated calcium channels are opening. These channels open slowly and remain open for a relatively long period (roughly 175 msec) and are called slow calcium channels. While the slow calcium channels are open, calcium ions enter the sarcoplasm. The entrance of positive charges through the calcium channels, in the form of Ca 2+ , roughly balances the loss of positive ions through the active transport of Na + , and the transmembrane potential remains near 0 mV for an extended period. This portion of the action potential curve is called the plateau. The major difference between action potentials in skeletal muscle cells and cardiac muscle fibers is the presence of this plateau. In a skeletal muscle fiber, rapid depolarization is immediately followed by rapid repolarization.

Anatomy & Physiology Study Guide

19.7 The Electrocardiogram An ECG integrates electrical information obtained by placing electrodes at different locations on the body surface. Clinicians can use an ECG to assess the performance of specific nodal, conducting, and contractile components. When a portion of the heart has been damaged by myocardial infarction, the ECG will reveal an abnormal pattern of impulse conduction. To analyze an ECG, measure the size of the voltage changes and determine the duration and temporal relationships of the various components. The times between waves are reported as segments or intervals. ECG analysis is particularly useful in detecting and diagnosing cardiac arrhythmias —abnormal patterns of cardiac electrical activity. Momentary arrhythmias are not inherently dangerous; about five percent of healthy individuals experience a few abnormal heartbeats each day. Clinical signs and symptoms appear when arrhythmias reduce the pumping efficiency of the heart. Serious arrhythmias may indicate injuries to the conduction pathways or pacemakers, damage to the myocardium, exposure to drugs, or abnormalities in the electrolyte composition of extracellular fluids. 19.8 The Action Potential in Cardiac Muscle Cells The resting potential of a ventricular contractile cell is approximately –90 mV, comparable to that of a resting skeletal muscle fiber (–85 mV). When the membrane of the ventricular muscle cell reaches threshold (about –75 mV), an action potential initiates. Threshold is normally reached in a portion of the membrane next to an intercalated disc. The normal stimulus is the energizing of an adjacent muscle cell. Once the threshold has been reached, the action potential begins with three basic steps:

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