Which of the following graphs properly represents the permeability of Na^+ durin

Question

Which of the following graphs properly represents the permeability of Na^+ during the action potential of a myocardial contractile cell? Methoxamine is an alpha-receptor agonist. If you were to administer this drug directly onto the larger veins (those containing a layer of smooth muscle) which of the following effects would you expect? A. A decrease in venous return resulting in a decrease in stroke volume B. Increased stroke volume caused by an increase in venous return C. Decreased force of contraction of the heart D. No significant affect will be observed

Explanation / Answer

4) The action potential of cardiac myocytes can be divided into 5 phases: 0, 1, 2, 3 and 4. In phase 0, rapid sodium influx occurs through fast sodium channels. During phase 1, transient potassium channels open and there is efflux of potassium ions. In phase 2, L type calcium channels open and there is influx of calcium ions along with potassium ion efflux. In phase 3, calcium channels close but potassium channels remain open. In phase 4, both sodium and calcium channels are closed and potassium channels remain open.

Therefore, when we observe the movement of sodium ions during the action potential of cardiac myocytes, the graph resembles the figure given in option A.

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