ECG Electrophysiology Basics 2

Question 1

Which ion is primarily responsible for the rapid depolarization phase (Phase 0) of the ventricular action potential?

Accepted Answer

Sodium (Na+)

Answer

Phase 0 rapid depolarization is caused by a fast inward sodium current through voltage-gated Na+ channels.

Question 2

The QT interval on an ECG represents which electrophysiological event?

Accepted Answer

Total ventricular electrical systole (depolarization + repolarization)

Answer

The QT interval encompasses the entire period of ventricular electrical activity from depolarization (QRS) through repolarization (T wave).

Question 3

What is the normal resting membrane potential of a cardiac myocyte?

Accepted Answer

-90 mV

Answer

The resting membrane potential of a working cardiac myocyte is approximately -90 mV, maintained primarily by K+ conductance.

Question 4

Phase 4 spontaneous depolarization (automaticity) in SA nodal cells is primarily driven by which current?

Accepted Answer

Inward funny current (If)

Answer

The funny current (If), a mixed Na+/K+ inward current activated by hyperpolarization, drives slow diastolic depolarization in pacemaker cells.

Question 5

Which portion of the His-Purkinje system has the slowest intrinsic pacemaker rate?

Accepted Answer

Purkinje fibers

Answer

Purkinje fibers have the slowest intrinsic rate of approximately 20-40 bpm, serving as a last-resort pacemaker.

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