Podcast
Questions and Answers
What is the primary role of the SA node in the cardiac conduction system?
What is the primary role of the SA node in the cardiac conduction system?
Which phase of the cardiac muscle action potential is characterized by a plateau?
Which phase of the cardiac muscle action potential is characterized by a plateau?
How do the action potentials in cardiac pacemaker cells differ from those in skeletal muscle cells?
How do the action potentials in cardiac pacemaker cells differ from those in skeletal muscle cells?
Which component of the autonomic nervous system primarily influences cardiac function?
Which component of the autonomic nervous system primarily influences cardiac function?
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What is the primary purpose of the conduction system of the heart?
What is the primary purpose of the conduction system of the heart?
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What does the T wave in a normal ECG represent?
What does the T wave in a normal ECG represent?
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Which ion movement is primarily responsible for depolarization in cardiac contractile cells?
Which ion movement is primarily responsible for depolarization in cardiac contractile cells?
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What is automaticity in the context of cardiac function?
What is automaticity in the context of cardiac function?
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What initiates atrial depolarization?
What initiates atrial depolarization?
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What occurs at the AV node after complete atrial depolarization?
What occurs at the AV node after complete atrial depolarization?
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Which wave on an ECG represents atrial depolarization?
Which wave on an ECG represents atrial depolarization?
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Which sequence correctly represents the heart's electrical activity before ventricular contraction?
Which sequence correctly represents the heart's electrical activity before ventricular contraction?
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What is the function of the SA node in the cardiac cycle?
What is the function of the SA node in the cardiac cycle?
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What initiates the action potential in cardiac pacemaker cells?
What initiates the action potential in cardiac pacemaker cells?
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Which part of the heart is known as the pacemaker?
Which part of the heart is known as the pacemaker?
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What happens in the heart during the delay caused by the Atrioventricular (AV) node?
What happens in the heart during the delay caused by the Atrioventricular (AV) node?
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What role does the AV bundle (bundle of His) play in the heart's conduction system?
What role does the AV bundle (bundle of His) play in the heart's conduction system?
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Which part of the conduction system carries impulses toward the apex of the heart?
Which part of the conduction system carries impulses toward the apex of the heart?
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What occurs during repolarization of the action potential in cardiac pacemaker cells?
What occurs during repolarization of the action potential in cardiac pacemaker cells?
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What is the inherent rate of the Atrioventricular (AV) node in the absence of input from the SA node?
What is the inherent rate of the Atrioventricular (AV) node in the absence of input from the SA node?
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How quickly do cardiac pacemaker cells pass impulses across the heart?
How quickly do cardiac pacemaker cells pass impulses across the heart?
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What allows for the ventricular contraction to follow immediately after the impulse reaches the inferior region of the heart?
What allows for the ventricular contraction to follow immediately after the impulse reaches the inferior region of the heart?
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Why do the atrial and ventricular myocytes not contract simultaneously?
Why do the atrial and ventricular myocytes not contract simultaneously?
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What is the primary function of the sympathetic innervation of the heart?
What is the primary function of the sympathetic innervation of the heart?
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Which center is associated with the parasympathetic control of the heart?
Which center is associated with the parasympathetic control of the heart?
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Which nerve is primarily involved in decreasing the heart rate?
Which nerve is primarily involved in decreasing the heart rate?
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The medulla oblongata contains which centers for heart regulation?
The medulla oblongata contains which centers for heart regulation?
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What effect does the parasympathetic nervous system have on heart activity?
What effect does the parasympathetic nervous system have on heart activity?
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The sympathetic trunk ganglion's role in heart function is to?
The sympathetic trunk ganglion's role in heart function is to?
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Which of the following is true regarding the cardioacceleratory center?
Which of the following is true regarding the cardioacceleratory center?
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Which structure does NOT play a primary role in heart innervation?
Which structure does NOT play a primary role in heart innervation?
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What effect does the Cardioinhibitory center have on the SA and AV nodes?
What effect does the Cardioinhibitory center have on the SA and AV nodes?
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Activation of which nervous system results in an increased heart rate?
Activation of which nervous system results in an increased heart rate?
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What does the P wave in an electrocardiogram represent?
What does the P wave in an electrocardiogram represent?
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Which part of the ECG reflects the period between the beginning of atrial excitation and the beginning of ventricular excitation?
Which part of the ECG reflects the period between the beginning of atrial excitation and the beginning of ventricular excitation?
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What does the QRS complex signify in the electrocardiogram?
What does the QRS complex signify in the electrocardiogram?
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Which electrical event is recorded as the T wave in an ECG?
Which electrical event is recorded as the T wave in an ECG?
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What is the primary purpose of an electrocardiograph?
What is the primary purpose of an electrocardiograph?
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What does the S-T segment reflect in the electrocardiogram?
What does the S-T segment reflect in the electrocardiogram?
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What leads to the electrocardiogram measuring voltage differences?
What leads to the electrocardiogram measuring voltage differences?
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Which interval measures the time from the start of ventricular depolarization to ventricular repolarization?
Which interval measures the time from the start of ventricular depolarization to ventricular repolarization?
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Which statement about the electrocardiogram is incorrect?
Which statement about the electrocardiogram is incorrect?
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What is typically the most common configuration used in electrocardiography?
What is typically the most common configuration used in electrocardiography?
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What does the P-R interval in an electrocardiogram primarily indicate?
What does the P-R interval in an electrocardiogram primarily indicate?
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Which of the following accurately describes the Q-T interval on an ECG?
Which of the following accurately describes the Q-T interval on an ECG?
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During which phase does the QRS complex occur on an ECG tracing?
During which phase does the QRS complex occur on an ECG tracing?
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Which segment of the ECG indicates the period of time when the ventricles are electrically silent?
Which segment of the ECG indicates the period of time when the ventricles are electrically silent?
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What does the T wave represent in the electrocardiogram's sequence of electrical activity?
What does the T wave represent in the electrocardiogram's sequence of electrical activity?
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What is the significance of the plateau phase in the action potential of cardiac contractile cells?
What is the significance of the plateau phase in the action potential of cardiac contractile cells?
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Which of the following correctly describes the role of the autonomic nervous system in cardiac function regulation?
Which of the following correctly describes the role of the autonomic nervous system in cardiac function regulation?
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What role do cardiac pacemaker cells play in the conduction system of the heart?
What role do cardiac pacemaker cells play in the conduction system of the heart?
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How do cardiac contractile cells differ from skeletal muscle cells in terms of action potential generation?
How do cardiac contractile cells differ from skeletal muscle cells in terms of action potential generation?
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Which sequence appropriately describes the conduction pathway of electrical impulses in the heart before ventricular contraction?
Which sequence appropriately describes the conduction pathway of electrical impulses in the heart before ventricular contraction?
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What does a normal ECG waveform indicate about the atrial and ventricular activity?
What does a normal ECG waveform indicate about the atrial and ventricular activity?
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What is the primary function of the AV node in the cardiac conduction system?
What is the primary function of the AV node in the cardiac conduction system?
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What effect does the sympathetic nervous system typically have on cardiac muscle cells?
What effect does the sympathetic nervous system typically have on cardiac muscle cells?
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What occurs in the heart shortly after depolarization from the pacemaker potential reaches threshold?
What occurs in the heart shortly after depolarization from the pacemaker potential reaches threshold?
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During which sequence does the cardiac impulse pass through the conduction system?
During which sequence does the cardiac impulse pass through the conduction system?
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What is responsible for the delay of approximately 0.1 seconds at the Atrioventricular (AV) node?
What is responsible for the delay of approximately 0.1 seconds at the Atrioventricular (AV) node?
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What is the inherent pacemaker rate of the Sinoatrial (SA) node under normal conditions?
What is the inherent pacemaker rate of the Sinoatrial (SA) node under normal conditions?
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What is the effect of the long absolute refractory period in cardiac muscle?
What is the effect of the long absolute refractory period in cardiac muscle?
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What initiates ventricular contraction following depolarization in the heart?
What initiates ventricular contraction following depolarization in the heart?
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What causes the repolarization phase in cardiac muscle contraction?
What causes the repolarization phase in cardiac muscle contraction?
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Which structure serves as the only electrical connection between the atria and ventricles?
Which structure serves as the only electrical connection between the atria and ventricles?
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What effect does the inactivation of Ca2+ channels and opening of K+ channels have during cardiac action potential?
What effect does the inactivation of Ca2+ channels and opening of K+ channels have during cardiac action potential?
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How does cardiac muscle contraction differ from skeletal muscle contraction in terms of cell connectivity?
How does cardiac muscle contraction differ from skeletal muscle contraction in terms of cell connectivity?
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Which ion is responsible for the plateau phase of depolarization in cardiac muscle?
Which ion is responsible for the plateau phase of depolarization in cardiac muscle?
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Which of the following structures depolarizes at a slower intrinsic rate when the SA node input is absent?
Which of the following structures depolarizes at a slower intrinsic rate when the SA node input is absent?
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What role does Ca2+ play in cardiac muscle contraction?
What role does Ca2+ play in cardiac muscle contraction?
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Which node is characterized as the primary pacemaker of the heart, generating impulses most rapidly?
Which node is characterized as the primary pacemaker of the heart, generating impulses most rapidly?
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What role do the right and left bundle branches play in the heart's conduction system?
What role do the right and left bundle branches play in the heart's conduction system?
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What is the significance of the automaticity of cardiac muscle cells?
What is the significance of the automaticity of cardiac muscle cells?
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Which feature is unique to the cardiac muscle contraction mechanism?
Which feature is unique to the cardiac muscle contraction mechanism?
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What primarily prevents tetanic contractions in the heart?
What primarily prevents tetanic contractions in the heart?
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During which phase does the cardiac muscle measure the longest duration of action potential?
During which phase does the cardiac muscle measure the longest duration of action potential?
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What is a key difference in the action potentials of cardiac and skeletal muscle cells?
What is a key difference in the action potentials of cardiac and skeletal muscle cells?
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What does the P-R interval in an electrocardiogram signify?
What does the P-R interval in an electrocardiogram signify?
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Which component of the electrocardiogram indicates ventricular repolarization?
Which component of the electrocardiogram indicates ventricular repolarization?
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What does the QRS complex not represent in an electrocardiogram?
What does the QRS complex not represent in an electrocardiogram?
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What impact does the S-T segment have on understanding the ECG?
What impact does the S-T segment have on understanding the ECG?
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Which description best captures the fundamental characteristic of an electrocardiogram?
Which description best captures the fundamental characteristic of an electrocardiogram?
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What is the role of electrodes in the electrocardiograph system?
What is the role of electrodes in the electrocardiograph system?
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Which interval measures the time from the start of ventricular depolarization to the start of ventricular repolarization?
Which interval measures the time from the start of ventricular depolarization to the start of ventricular repolarization?
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In a standard 12 lead ECG, which wave is primarily associated with the depolarization of the SA node?
In a standard 12 lead ECG, which wave is primarily associated with the depolarization of the SA node?
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What does a typical electrocardiogram NOT reflect?
What does a typical electrocardiogram NOT reflect?
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What best characterizes the action potentials detected by an electrocardiograph?
What best characterizes the action potentials detected by an electrocardiograph?
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What does the completion of ventricular depolarization signify in an ECG tracing?
What does the completion of ventricular depolarization signify in an ECG tracing?
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Atrial repolarization occurs simultaneously with ventricular depolarization.
Atrial repolarization occurs simultaneously with ventricular depolarization.
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What wave on an ECG represents ventricular repolarization?
What wave on an ECG represents ventricular repolarization?
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What substance do ischemic cardiac cells primarily produce due to anaerobic respiration?
What substance do ischemic cardiac cells primarily produce due to anaerobic respiration?
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Cardiac muscle cells have a high ability for anaerobic respiration.
Cardiac muscle cells have a high ability for anaerobic respiration.
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The beginning of ventricular depolarization causes the ________ complex in an ECG reading.
The beginning of ventricular depolarization causes the ________ complex in an ECG reading.
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Match the following phases of cardiac electrical activity with their corresponding ECG features:
Match the following phases of cardiac electrical activity with their corresponding ECG features:
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What term describes the cells in the heart that initiate pacemaker potential?
What term describes the cells in the heart that initiate pacemaker potential?
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The __________ is known as the primary pacemaker of the heart.
The __________ is known as the primary pacemaker of the heart.
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Match the following cardiac terms with their definitions:
Match the following cardiac terms with their definitions:
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What effect does high H+ concentration have on cardiac function?
What effect does high H+ concentration have on cardiac function?
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Only the SA node can generate pacemaker potentials in the heart.
Only the SA node can generate pacemaker potentials in the heart.
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Identify one common homeostatic imbalance related to the intrinsic conduction system.
Identify one common homeostatic imbalance related to the intrinsic conduction system.
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Which phase of the cardiac muscle action potential is primarily characterized by potassium ion efflux?
Which phase of the cardiac muscle action potential is primarily characterized by potassium ion efflux?
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The autonomic nervous system solely stimulates cardiac pacemaker cells.
The autonomic nervous system solely stimulates cardiac pacemaker cells.
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What is the role of the AV node in the conduction system of the heart?
What is the role of the AV node in the conduction system of the heart?
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The primary function of the __________ system in the heart is to regulate heart rate and force of contraction.
The primary function of the __________ system in the heart is to regulate heart rate and force of contraction.
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Match the following ion movements with their corresponding actions during the cardiac action potential:
Match the following ion movements with their corresponding actions during the cardiac action potential:
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Which component of the cardiac conduction system is known as the heart's pacemaker?
Which component of the cardiac conduction system is known as the heart's pacemaker?
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The T wave on an ECG represents ventricular depolarization.
The T wave on an ECG represents ventricular depolarization.
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What does the QRS complex on an ECG signify?
What does the QRS complex on an ECG signify?
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Automaticity refers to the heart's ability to create an action potential without __________ stimulation.
Automaticity refers to the heart's ability to create an action potential without __________ stimulation.
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What is the primary function of the plateau phase in cardiac contractile cells?
What is the primary function of the plateau phase in cardiac contractile cells?
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Which part of the nervous system is responsible for increasing heart rate and force of contraction?
Which part of the nervous system is responsible for increasing heart rate and force of contraction?
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The cardioacceleratory center is associated with parasympathetic control of the heart.
The cardioacceleratory center is associated with parasympathetic control of the heart.
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What effect does the vagus nerve have on heart rate?
What effect does the vagus nerve have on heart rate?
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The __________ center inhibits the SA and AV nodes.
The __________ center inhibits the SA and AV nodes.
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Match each center with its function regarding heart innervation:
Match each center with its function regarding heart innervation:
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Which of the following statements about the sympathetic trunk ganglion is true?
Which of the following statements about the sympathetic trunk ganglion is true?
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The medulla oblongata contains centers that only have a sympathetic effect on the heart.
The medulla oblongata contains centers that only have a sympathetic effect on the heart.
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Which nervous system activation leads to a decrease in heart rate?
Which nervous system activation leads to a decrease in heart rate?
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The __________ nerves are primarily involved in sympathetic innervation of the heart.
The __________ nerves are primarily involved in sympathetic innervation of the heart.
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What is the primary function of the cardioacceleratory center?
What is the primary function of the cardioacceleratory center?
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What is the inherent rate of the sinoatrial (SA) node in a healthy heart?
What is the inherent rate of the sinoatrial (SA) node in a healthy heart?
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The Atrioventricular (AV) node allows the atria to contract before the ventricles.
The Atrioventricular (AV) node allows the atria to contract before the ventricles.
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What is the role of the subendocardial conducting network?
What is the role of the subendocardial conducting network?
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The sinoatrial node is located in the ________ atrial wall.
The sinoatrial node is located in the ________ atrial wall.
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Match the cardiac conduction system components with their functions:
Match the cardiac conduction system components with their functions:
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Which phase of cardiac action potential is marked by calcium influx?
Which phase of cardiac action potential is marked by calcium influx?
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The AV bundle has a higher rate of impulse generation than the SA node.
The AV bundle has a higher rate of impulse generation than the SA node.
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What is the total time taken for cardiac pacemaker cells to pass impulses across the heart?
What is the total time taken for cardiac pacemaker cells to pass impulses across the heart?
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During repolarization, K+ channels ________, allowing potassium to leave the cell.
During repolarization, K+ channels ________, allowing potassium to leave the cell.
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Why do the atrial and ventricular myocytes not contract simultaneously?
Why do the atrial and ventricular myocytes not contract simultaneously?
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Cardiac muscle can easily rely on anaerobic respiration due to its high mitochondria count.
Cardiac muscle can easily rely on anaerobic respiration due to its high mitochondria count.
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Ischemic cells lead to increased ATP production and proper heart function.
Ischemic cells lead to increased ATP production and proper heart function.
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The SA node, AV node, and Purkinje fibers are all considered pacemakers in the heart.
The SA node, AV node, and Purkinje fibers are all considered pacemakers in the heart.
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Fibrillation refers to slow, regular contractions of the heart muscle.
Fibrillation refers to slow, regular contractions of the heart muscle.
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Cardiac muscle can utilize lactic acid produced by skeletal muscles as a fuel source.
Cardiac muscle can utilize lactic acid produced by skeletal muscles as a fuel source.
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The SA node has an inherent rate of approximately 60 to 100 beats per minute in the absence of external influences.
The SA node has an inherent rate of approximately 60 to 100 beats per minute in the absence of external influences.
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Cardiac pacemaker cells can only generate action potentials when influenced by external neural stimuli.
Cardiac pacemaker cells can only generate action potentials when influenced by external neural stimuli.
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The plateau phase of the cardiac action potential is primarily due to the influx of potassium ions.
The plateau phase of the cardiac action potential is primarily due to the influx of potassium ions.
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The autonomic nervous system does not influence the conduction speed of impulses through the heart's conduction system.
The autonomic nervous system does not influence the conduction speed of impulses through the heart's conduction system.
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Ventricular myocytes contract simultaneously with atrial myocytes during the cardiac cycle.
Ventricular myocytes contract simultaneously with atrial myocytes during the cardiac cycle.
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Pacemaker cells have stable resting membrane potentials due to the opening of slow Na+ channels.
Pacemaker cells have stable resting membrane potentials due to the opening of slow Na+ channels.
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Calcium channels opening leads to a huge influx and the rising phase of the action potential in pacemaker cells.
Calcium channels opening leads to a huge influx and the rising phase of the action potential in pacemaker cells.
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Repolarization in pacemaker cells occurs because of the closure of K+ channels and the opening of Na+ channels.
Repolarization in pacemaker cells occurs because of the closure of K+ channels and the opening of Na+ channels.
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The intrinsic cardiac conduction system is composed entirely of contractile cardiac muscle cells.
The intrinsic cardiac conduction system is composed entirely of contractile cardiac muscle cells.
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During the repolarization phase of action potential, K+ channels open allowing for the efflux of K+ ions.
During the repolarization phase of action potential, K+ channels open allowing for the efflux of K+ ions.
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Study Notes
Cardiac Conduction System
- SA node is the natural pacemaker of the heart, initiating the electrical impulse that triggers each heartbeat.
Cardiac Muscle Action Potential
- Plateau phase is characterized by prolonged calcium influx into the cell, maintaining a prolonged depolarized state.
Cardiac Pacemaker Cells
- Pacemaker cells exhibit spontaneous depolarization due to "funny" current and calcium influx, resulting in a constant rhythm.
- Skeletal muscle cells require external stimulation to trigger action potentials.
Autonomic Nervous System
- Parasympathetic nervous system primarily slows down heart rate through vagus nerve.
- Sympathetic nervous system increases heart rate and contractility through norepinephrine.
Heart's Conduction System
- The heart's conduction system ensures coordinated contraction of the heart chambers.
- It facilitates electrical transmission throughout the heart, ensuring efficient blood pumping.
ECG Interpretation
- T wave represents ventricular repolarization.
Depolarization in Cardiac Contractile Cells
- Sodium influx is primarily responsible for depolarization in cardiac contractile cells.
Automaticity
- Automaticity is the intrinsic ability of specialized cardiac cells to spontaneously generate action potentials, driving the heart's rhythm.
Atrial Depolarization
- SA node initiates atrial depolarization, triggering the contraction of the atria.
AV Node
- AV node delays the transmission of the electrical impulse, allowing for complete atrial depolarization and ventricular filling.
ECG: Atrial Depolarization
- P wave represents atrial depolarization.
Heart's Electrical Activity
- SA node initiates the electrical impulse (P wave) → atrial depolarization → AV node delays transmission → ventricular depolarization (QRS complex) → ventricular contraction.
SA Node in Cardiac Cycle
- SA node sets the heart's rhythm by initiating the electrical impulse that leads to atrial contraction and ventricular filling.
Cardiovascular Physiology
-
The cardiovascular system is responsible for transporting oxygen, nutrients, and hormones throughout the body; it also removes waste products.
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The cardiac muscle action potential has five phases:
- Phase 0: Rapid depolarization due to sodium influx
- Phase 1: Initial repolarization due to potassium efflux
- Phase 2: Plateau phase due to calcium influx
- Phase 3: Rapid repolarization due to potassium efflux
- Phase 4: Resting membrane potential
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Pacemaker cells:
- Their action potentials are initiated by a slow, gradual depolarization called the pacemaker potential
- They exhibit automaticity
- Depolarization is driven by sodium influx through funny channels (If channels) and calcium influx through T-type calcium channels
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Contractile cells:
- Depolarization is initiated by a rapid influx of sodium ions
- The plateau phase is caused by sustained calcium influx
- Repolarization occurs due to potassium efflux
-
Skeletal muscle cells:
- They depend on nervous system stimulation for depolarization
- They do not exhibit automaticity
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The plateau phase of the action potential in cardiac contractile cells is crucial for ensuring a long-lasting contraction, allowing sufficient time for blood ejection from the heart.
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Cardiac vs. Skeletal Muscle Contraction:
- Similarities: Both rely on calcium-mediated actin-myosin cross-bridge formation
- Differences: Cardiac muscle exhibits a longer duration of contraction, a longer refractory period, and is influenced by the autonomic nervous system
Cardiac Conduction System
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The cardiac conduction system is responsible for the coordinated and efficient flow of electrical impulses throughout the heart, ensuring a synchronized contraction of the chambers.
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It ensures atrial contraction precedes ventricular contraction, leading to efficient blood flow from the atria to the ventricles.
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Components:
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Sinoatrial (SA) node: The "pacemaker" of the heart located in the right atrial wall
- It spontaneously generates impulses at a rate of approximately 75 beats per minute (sinus rhythm)
- Has an inherent rate of 100 beats per minute but is regulated by autonomic nerves
-
Atrioventricular (AV) node: Located in the inferior interatrial septum
- Delays impulses by about 0.1 second, allowing the atria to contract before the ventricles
- This delay is due to the smaller diameter and fewer gap junctions of its fibers
- Has an inherent rate of 50 beats per minute in the absence of SA node input
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Atrioventricular (AV) bundle (Bundle of His): Located in the superior interventricular septum
- The only electrical connection between the atria and ventricles
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Right and left bundle branches: Pathway in the interventricular septum
- Carry impulses towards the apex of the heart.
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Subendocardial conducting network (Purkinje fibers):
- Extends from the apex to the ventricular walls
- Depolarizes at 30 beats per minute in the absence of AV node input
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Sinoatrial (SA) node: The "pacemaker" of the heart located in the right atrial wall
The EKG
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Electrocardiography records the electrical activity of the heart, which is represented as a composite of all action potentials generated by the nodal and contractile cells at a given time.
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The EKG is a graphic recording of these electrical events.
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Key waveforms:
- P wave: Reflects atrial depolarization initiated by the SA node
- QRS complex: Represents ventricular depolarization and the repolarization of the atria (which is obscured by the larger ventricular event)
- T wave: Shows ventricular repolarization
- PR interval: Represents the time from the start of atrial excitation to the beginning of ventricular excitation
- ST Segment: Indicates the time when the entire ventricular myocardium is depolarized
- QT interval: Represents the overall duration of ventricular depolarization and repolarization
Autonomic Nervous System
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Sympathetic Nervous System:
- Increases heart rate and contractility
- This is achieved through the release of norepinephrine, which binds to beta-adrenergic receptors in the heart.
-
Parasympathetic Nervous System:
- Decreases heart rate
- This occurs through the release of acetylcholine which binds to muscarinic receptors in the heart.
Pacemaker Potential
- The SA node, atrial muscle, AV node, and ventricular muscle all have different action potential shapes
- The SA node exhibits pacemaker potential, which is a slow, gradual depolarization leading to an action potential
- This is due to the opening of slow Na+ channels and the closing of K+ channels
Energy Requirements for Cardiac Muscle
- Cardiac muscle has many mitochondria and relies heavily on aerobic respiration
- It has a limited ability to perform anaerobic respiration
- Can readily switch fuel sources for respiration
- Can even utilize lactic acid from skeletal muscles
Homeostatic Imbalance
- Ischemic cells lead to anaerobic respiration
- This produces lactic acid
- High H+ concentration increases Ca2+ concentration leading to mitochondrial damage and decreased ATP production
- Consequently, the gap junctions between cells close resulting in fatal arrhythmias
Homeostatic Imbalances (Continued)
- Defects in the intrinsic conduction system of the heart can lead to various conditions, including arrhythmias, uncoordinated atrial and ventricular contractions, and fibrillation
- Fibrillation is a rapid, irregular contraction that is inefficient for pumping blood, leading to circulatory cessation and brain death
- Defibrillation can be used to treat fibrillation
Cardiac Muscle: Similarities and Differences
- Cardiac muscle shares similarities with skeletal muscle in that it is striated and contains sarcomeres
- However, cardiac muscle is involuntary while skeletal muscle is voluntary
- Cardiac muscle also has a longer refractory period and is more resistant to fatigue compared to skeletal muscle
Intrinsic Conduction System
- The heart has its own intrinsic conduction system, also known as the pacemaker system
- It is a network of non-contractile cells that initiate and distribute impulses, ensuring coordinated depolarization and contraction of the heart
- These pacemaker cells have an unstable resting membrane potential and are referred to as autorhythmic cells
Pacemaker Cells
- Pacemaker cells have an unstable resting membrane potential due to the opening of slow Na+ channels
- This continuous depolarization eventually reaches the threshold leading to the opening of Ca2+ channels
- Ca2+ influx creates the rising phase of the action potential
- Repolarization occurs with the inactivation of Ca2+ channels and the opening of voltage-gated K+ channels
Extrinsic Innervation of the Heart
- The heartbeat can be modified by the autonomic nervous system via cardiac centers in the medulla oblongata
- The sympathetic nervous system increases heart rate and force, while the parasympathetic nervous system decreases heart rate
- The cardioacceleratory center (sympathetic) affects the SA and AV nodes, heart muscle, and coronary arteries
- The cardioinhibitory center (parasympathetic) inhibits the SA and AV nodes via the vagus nerves
Electrocardiography
- The electrocardiograph (ECG or EKG) measures electrical currents generated by the heart
- It provides a graphic recording of this electrical activity
- The ECG represents a composite of all action potentials at a given time, not a single action potential
- Electrodes are placed on different parts of the body to measure voltage differences
- A 12-lead ECG is the most common type
Key Components of an ECG
- P wave: Depolarization of SA node and atria
- QRS complex: Ventricular depolarization and atrial repolarization
- T wave: Ventricular repolarization
- P-R interval: Beginning of atrial excitation to the beginning of ventricular excitation
- S-T segment: Entire ventricular myocardium is depolarized
- Q-T interval: Beginning of ventricular depolarization through ventricular repolarization
ECG Summary
- The electrocardiogram (ECG or EKG) is a composite representation of all action potentials across the nodal and contractile cells at a particular moment
- It displays three prominent waves:
- P wave: Depolarization of the SA node and the subsequent conduction to the atria
- QRS complex: Ventricular depolarization coinciding with atrial repolarization
- T wave: Ventricular repolarization
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Description
Test your knowledge on the cardiac conduction system, including the roles of the SA node, pacemaker cells, and the influence of the autonomic nervous system. Explore how electrical impulses are generated and maintained for coordinated heart contractions.
