Cardiac Muscle Physiology Quiz

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Questions and Answers

What does the QRS complex represent?

AV node conduction

Ventricular repolarization

Atrial action potentials

Ventricular depolarization (correct)

Which ECG segment corresponds specifically to the conduction through the AV node?

ST segment

T wave

QT interval

PR segment (correct)

What effect does acetylcholine have on the heart rate?

Has no effect on heart rate

Only affects ventricular contractions

Decreases heart rate (correct)

Increases heart rate

What electrical activity does the T wave represent?

Ventricular repolarization (B) Signup and view all the answers

What are the limb leads measuring in an ECG?

Electrical potential between pairs of limbs (C) Signup and view all the answers

Which neurotransmitter is responsible for increasing heart rate?

Norepinephrine (C) Signup and view all the answers

What does the 'c wave' in cardiac pressure reflect?

Ventricular contraction (C) Signup and view all the answers

Which heart sound occurs at the start of ventricular systole?

First heart sound (C) Signup and view all the answers

Which of the following is an example of a cardiac cycle irregularity detected by ECG?

Respiratory sinus arrhythmia (C) Signup and view all the answers

Which segment of the conduction system has the slowest conduction rate?

AV node (D) Signup and view all the answers

What creates the electrical dipole that can be detected by ECG electrodes?

Changes in extracellular charge (D) Signup and view all the answers

What is associated with rapid filling of the ventricles during diastole?

S3 heart sound (D) Signup and view all the answers

During which phase does the ventricular pressure increase rapidly until the semilunar valves open?

Isovolumic contraction (B) Signup and view all the answers

What effect does norepinephrine have on AV node conduction?

It increases the conduction rate (D) Signup and view all the answers

What happens when atrial and ventricular pressures are nearly equal?

Blood flows through the AV valves (C) Signup and view all the answers

What is a key feature of the cardiac action potential?

Long duration with significant Ca++ entry (D) Signup and view all the answers

Which phase of the ventricular action potential involves the opening of voltage-gated sodium channels?

Phase 0 (C) Signup and view all the answers

What primarily causes the resting potential of cardiac cells?

Open potassium channel's permeability (B) Signup and view all the answers

How does calcium contribute to cardiac muscle contraction?

By increasing cytoplasmic Ca++ levels during phase 2 (C) Signup and view all the answers

What initiates pacemaker activity in the heart?

Increased sodium and calcium permeability (C) Signup and view all the answers

Which receptors are known to increase the force of contraction in the heart?

β1 receptors (B) Signup and view all the answers

What describes the atrial action potential compared to the ventricular action potential?

Shorter phase 2 duration (D) Signup and view all the answers

What is a characteristic of cardiac muscle cells?

Cells linked via gap junctions for action potential propagation (A) Signup and view all the answers

What is the primary mechanism for increased contractility of cardiac muscle fibers due to sympathetic nerve stimulation?

Increased calcium release from the sarcoplasmic reticulum (A) Signup and view all the answers

How do cardiac glycosides like digitalis enhance the force of contraction?

By inhibiting the sodium-calcium exchanger, leading to increased intracellular calcium (B) Signup and view all the answers

What is the relationship between heart rate and the strength of cardiac muscle contraction referred to as?

The Bowditch staircase phenomenon (D) Signup and view all the answers

Which of the following is NOT a mechanism directly contributing to positive inotropy?

Increased activity of the sodium-potassium pump (C) Signup and view all the answers

Which of these does NOT directly contribute to the increased force of contraction in cardiac muscle fiber?

Increased activity of the sodium-calcium exchanger (B) Signup and view all the answers

Which of the following is NOT a direct contributing factor to preload?

Arterial pressure (A) Signup and view all the answers

What happens to stroke volume when the afterload increases?

It decreases immediately. (D) Signup and view all the answers

What is the approximate pressure relationship between the left ventricle and the aorta during the ejection phase of systole?

Aortic pressure is nearly the same as ventricular pressure. (A) Signup and view all the answers

What is the primary function of Starling's law of the heart (Frank-Starling relationship)?

To match cardiac output to venous return. (B) Signup and view all the answers

What is the direct relationship between preload and stroke volume according to Starling's law?

An increase in preload leads to an increase in stroke volume. (C) Signup and view all the answers

According to Starling's law, what happens to left ventricular filling, stroke volume, and left heart output if the right side of the heart pumps more blood than the left?

They all increase. (D) Signup and view all the answers

What is the dicrotic notch (incisura) caused by?

The closure of the aortic valve. (D) Signup and view all the answers

What primarily determines the force against which the ventricle has to work to move blood into the aorta?

Arterial pressure. (D) Signup and view all the answers

Study Notes

Cardiac Muscle

Relatively small cells compared to skeletal muscle fibers
Cells are interconnected end-to-end
Connected by gap junctions for action potential propagation
High oxygen requirement; many mitochondria are present

Cardiac Action Potential

Long duration with significant Ca²⁺ entry into cells
Some heart cells have spontaneous firing
Cells exhibit a refractory period
Resting potential is mainly determined by potassium channels

Phases of Ventricular Action Potential (0-4)

Phase 0: Voltage-gated sodium channels open, Na⁺ entry causing a positive membrane potential
Phase 1: Similar to neuronal action potential; sodium channels close and potassium channels open.
Phase 2: Voltage-gated calcium channels open, allowing Ca²⁺ entry and holding membrane potential near 0V for prolonged periods
Phase 3: Calcium channels close, potassium efflux dominates, returning the potential to resting state
Phase 4: Resting potential

Calcium Entry Significance in Phase 2

Contributes significantly to increasing cytoplasmic Ca²⁺ levels
Contributes to the force of heart muscle contraction
Target for autonomic regulation of force of contraction

Pacemaker Activity (SA and AV nodes)

Phase 4: Potassium permeability decreases, and sodium and calcium permeabilities increase. Membrane potential shifts to become more positive inside.

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Description

Test your understanding of cardiac muscle characteristics, action potentials, and the phases of ventricular action potential. This quiz covers key concepts related to cardiac physiology, including cell structure and calcium entry mechanisms.