Chapter 12, Problem 1RQ

Which of the following is the rapid upstroke in the action potential?

A. Phase 0

B. Phase 1

C. Phase 2

D. Phase 3

Summary Introduction

Introduction:

An action potential is generated and propagated by the heart for its contraction. Action potentials are the electrical currents that move across the membrane of the cardiac cells. These are identical to the action potential of neurons, cardiac muscles, and skeletal muscles. The primary ions responsible for the electrical differences are potassium, calcium, and sodium.

Answer to Problem 1RQ

Correct answer:

Phase 0.

Explanation of Solution

Justification for the correct answer:

Option (a) is, ‘Phase 0.’ It is the phase of rapid depolarization. The ventricular muscle fibers are activated by the electrical impulses initiated by the sinoatrial node. A change in the resting membrane is brought about by this. It allows the inward flow of sodium ions inside the cells. At the end of depolarization, the voltage is +30 mV (millivolts). This leads to a production of rapid upstroke in the action potential. Hence, option (a) is correct.

Justification for the incorrect answers:

Option (b) is, ‘Phase 1.’ Phase 1 stage is associated with the repolarization of the membrane. The potassium ion channels get opened just after phase 0, which allows the inflow of potassium ions. So, it is an incorrect option.

Option (c) is, ‘Phase 2.’ Phase 2 stage is the plateau stage. This stage is marked by the inflow of calcium ions, which slows the outward flow of potassium ions. So, it is an incorrect option.

Option (d) is, ‘Phase 3.’ Phase 3 stage is the final rapid polarization. The inward flow of the calcium ions stops. The rate of repolarization increases as the outward flow of potassium ions accelerates. So, it is an incorrect option.

Hence, options (b), (c), and (d) are incorrect.

Conclusion

Thus, it can be concluded that the rapid upstroke in the action potential takes place in phase 0 of the action potential. The sinoatrial node initiates the electrical impulse which activates the ventricular muscle fibers.