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?

<p>Ventricular repolarization (B)</p> Signup and view all the answers

What are the limb leads measuring in an ECG?

<p>Electrical potential between pairs of limbs (C)</p> Signup and view all the answers

Which neurotransmitter is responsible for increasing heart rate?

<p>Norepinephrine (C)</p> Signup and view all the answers

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

<p>Ventricular contraction (C)</p> Signup and view all the answers

Which heart sound occurs at the start of ventricular systole?

<p>First heart sound (C)</p> Signup and view all the answers

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

<p>Respiratory sinus arrhythmia (C)</p> Signup and view all the answers

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

<p>AV node (D)</p> Signup and view all the answers

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

<p>Changes in extracellular charge (D)</p> Signup and view all the answers

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

<p>S3 heart sound (D)</p> Signup and view all the answers

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

<p>Isovolumic contraction (B)</p> Signup and view all the answers

What effect does norepinephrine have on AV node conduction?

<p>It increases the conduction rate (D)</p> Signup and view all the answers

What happens when atrial and ventricular pressures are nearly equal?

<p>Blood flows through the AV valves (C)</p> Signup and view all the answers

What is a key feature of the cardiac action potential?

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

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

<p>Phase 0 (C)</p> Signup and view all the answers

What primarily causes the resting potential of cardiac cells?

<p>Open potassium channel's permeability (B)</p> Signup and view all the answers

How does calcium contribute to cardiac muscle contraction?

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

What initiates pacemaker activity in the heart?

<p>Increased sodium and calcium permeability (C)</p> Signup and view all the answers

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

<p>β1 receptors (B)</p> Signup and view all the answers

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

<p>Shorter phase 2 duration (D)</p> Signup and view all the answers

What is a characteristic of cardiac muscle cells?

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

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

<p>Increased calcium release from the sarcoplasmic reticulum (A)</p> Signup and view all the answers

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

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

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

<p>The Bowditch staircase phenomenon (D)</p> Signup and view all the answers

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

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

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

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

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

<p>Arterial pressure (A)</p> Signup and view all the answers

What happens to stroke volume when the afterload increases?

<p>It decreases immediately. (D)</p> 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?

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

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

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

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

<p>An increase in preload leads to an increase in stroke volume. (C)</p> 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?

<p>They all increase. (D)</p> Signup and view all the answers

What is the dicrotic notch (incisura) caused by?

<p>The closure of the aortic valve. (D)</p> Signup and view all the answers

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

<p>Arterial pressure. (D)</p> Signup and view all the answers

Flashcards

Cardiac Muscle

Relatively small cells organized in a network, connected by gap junctions for action potential propagation.

Cardiac Action Potential

A long-duration electrical signal in heart cells with phases involving Na+ and Ca++ ion movements.

Phases of Ventricular Action Potential

Five phases (0-4) showing ion channel activity that alters membrane potential.

Phase 0

Na+ channels open, shifting membrane potential to positive.

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Phase 2

Voltage-gated Ca++ channels open, maintaining a near-zero membrane potential for a longer duration.

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Refractory Period

The time following an action potential where a new action potential cannot be initiated.

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Pacemaker Activity

Spontaneous generation of action potentials primarily from SA and AV nodes.

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β1 Receptors

Receptors that increase the force of contraction in heart muscle during sympathetic stimulation.

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SA Node Regulation

The SA node's conduction rate is regulated by parasympathetic (ACh slows heart rate) and sympathetic (norepinephrine increases heart rate) systems.

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Chronotropic Effects

Chronotropic effects refer to changes in heart rate due to external influences like neurotransmitters.

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Dromotropic Effects

Dromotropic effects describe changes in conduction speed of action potentials in the heart.

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Action Potential Spread Rates

Action potential spreads at different rates: SA node (1 m/sec), AV node (0.05-0.1 m/sec), Purkinje fibers (2 m/sec), ventricular muscle (0.3 m/sec).

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Electrocardiogram (ECG)

An ECG detects electrical dipoles produced by changes in extracellular charge during action potentials in the heart.

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P wave

Represents the spread of atrial action potentials during the cardiac cycle.

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QRS complex

Indicates ventricular depolarization, the process that triggers heartbeats.

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ST segment

Period when most ventricular cells are depolarized and no contraction occurs.

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QT interval

Duration of time from the start of the Q wave to the end of the T wave; close to the ventricular action potential duration.

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Atrial fibrillation

A type of arrhythmia where the atria beat irregularly and often rapidly.

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Positive Inotropy

Increased force of contraction in the heart muscle.

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Bowditch Staircase

Phenomenon where faster heart rates result in stronger contractions, independent of autonomic input.

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Sympathetic Nerve Stimulation

Activation that increases cardiac contractility through calcium influx.

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Cardiac Glycosides

Drugs, like digitalis, that increase heart contraction strength by affecting ion pumps.

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Calcium's Role in Contraction

Calcium entering heart cells triggers the release of more calcium for stronger contractions.

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AV valves

Valves between atria and ventricles; include mitral and tricuspid valves.

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S3 heart sound

Sound associated with rapid filling of ventricles during diastole.

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Preload

The resting force of ventricles matching venous return pressure during filling.

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Afterload

The pressure the ventricles must overcome to eject blood.

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Aortic Pressure

Pressure in the aorta, usually higher than in the ventricle during most of the cycle.

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Dicrotic Notch

A small dip in aortic pressure due to backflow when the aortic valve closes.

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Ventricular Volume Changes

Volume decreases during systole and increases during diastole.

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Frank-Starling Law

Describes the relationship between end-diastolic volume and stroke volume.

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Stroke Volume

The amount of blood ejected by the ventricle per contraction.

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Balancing Outputs

Mechanism via Starling's law that balances outputs of systemic and pulmonary circuits.

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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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