Podcast
Questions and Answers
What percentage of heart muscle is made up of contractile cells?
What percentage of heart muscle is made up of contractile cells?
- 50%
- 75%
- 99% (correct)
- 100%
Which of the following correctly describes the role of auto-rhythmic cells in the heart?
Which of the following correctly describes the role of auto-rhythmic cells in the heart?
- Initiate and distribute electrical activity (correct)
- Act as the primary force generators in the myocardium
- Have more contractile fibers than contractile cells
- Generate significant contractile force for heartbeats
What type of junctions connect cardiac muscle cells and facilitate electrical conduction?
What type of junctions connect cardiac muscle cells and facilitate electrical conduction?
- Tight junctions
- Gap junctions (correct)
- Desmosomes
- Adherens junctions
Which feature is NOT characteristic of myocardial contractile cells?
Which feature is NOT characteristic of myocardial contractile cells?
What specialized channels are found in the sarcolemma of cardiac muscle cells that are not present in skeletal muscle?
What specialized channels are found in the sarcolemma of cardiac muscle cells that are not present in skeletal muscle?
Which statement about the contraction mechanism of cardiac muscle is FALSE?
Which statement about the contraction mechanism of cardiac muscle is FALSE?
How do auto-rhythmic cells differ from contractile cells in the heart?
How do auto-rhythmic cells differ from contractile cells in the heart?
What physiological property primarily enables the electrical activity to be spread throughout the cardiac muscle?
What physiological property primarily enables the electrical activity to be spread throughout the cardiac muscle?
What is the primary function of the working myocardium?
What is the primary function of the working myocardium?
What type of cardiac cell primarily lacks contractile fibers?
What type of cardiac cell primarily lacks contractile fibers?
What is the primary difference in the refractory periods of skeletal and cardiac muscle?
What is the primary difference in the refractory periods of skeletal and cardiac muscle?
Which component is responsible for initiating action potentials in the heart?
Which component is responsible for initiating action potentials in the heart?
What role does Ca2+ play during cardiac muscle contraction?
What role does Ca2+ play during cardiac muscle contraction?
Which structure is NOT part of the intrinsic conduction system of the heart?
Which structure is NOT part of the intrinsic conduction system of the heart?
What is the consequence of cardiac muscle's inability to undergo tetanus?
What is the consequence of cardiac muscle's inability to undergo tetanus?
How does the action potential spread in cardiac contractile cells?
How does the action potential spread in cardiac contractile cells?
Which mechanism is involved in pumping Ca2+ back into the sarcoplasmic reticulum after contraction?
Which mechanism is involved in pumping Ca2+ back into the sarcoplasmic reticulum after contraction?
What effect does the presence of gap junctions have on action potentials in cardiac muscle?
What effect does the presence of gap junctions have on action potentials in cardiac muscle?
Which of the following best describes the contraction phase of cardiac muscle compared to skeletal muscle?
Which of the following best describes the contraction phase of cardiac muscle compared to skeletal muscle?
What is the primary function of the AV node in the conduction system of the heart?
What is the primary function of the AV node in the conduction system of the heart?
Why does the SA node function as the pacemaker of the heart?
Why does the SA node function as the pacemaker of the heart?
What is the role of the fibrous skeleton of the heart?
What is the role of the fibrous skeleton of the heart?
What occurs during the absolute refractory period of a myocyte?
What occurs during the absolute refractory period of a myocyte?
What effect does the delay in conduction at the AV node have on the heart's function?
What effect does the delay in conduction at the AV node have on the heart's function?
What characterizes the conduction speed through the Purkinje fibers?
What characterizes the conduction speed through the Purkinje fibers?
What prevents excessive atrial contraction rates from leading to fast ventricular rates?
What prevents excessive atrial contraction rates from leading to fast ventricular rates?
Which structure in the heart is responsible for creating the fastest electrical impulse?
Which structure in the heart is responsible for creating the fastest electrical impulse?
What physiological mechanism ensures the ventricles remain relaxed while the atria contract?
What physiological mechanism ensures the ventricles remain relaxed while the atria contract?
What effect does sympathetic stimulation have on the heart rate when analyzed at the SA node?
What effect does sympathetic stimulation have on the heart rate when analyzed at the SA node?
Which neurotransmitter is associated with the parasympathetic influence on heart function?
Which neurotransmitter is associated with the parasympathetic influence on heart function?
What describes the dromotropic effect in relation to the autonomic nervous system's influence on the heart?
What describes the dromotropic effect in relation to the autonomic nervous system's influence on the heart?
What characterizes the relative refractory period in myocardial action potential generation?
What characterizes the relative refractory period in myocardial action potential generation?
In the context of cardiac function, a positive chronotropic effect is primarily achieved by which of the following mechanisms?
In the context of cardiac function, a positive chronotropic effect is primarily achieved by which of the following mechanisms?
Which receptor type does noradrenaline primarily bind to in the context of sympathetic stimulation of the heart?
Which receptor type does noradrenaline primarily bind to in the context of sympathetic stimulation of the heart?
What physiological effect does increased vagal tone have on heart function?
What physiological effect does increased vagal tone have on heart function?
Which of the following best defines the term 'inotropic effect' in relation to cardiac function?
Which of the following best defines the term 'inotropic effect' in relation to cardiac function?
How does parasympathetic stimulation affect the ionic conductance in heart nodal tissues?
How does parasympathetic stimulation affect the ionic conductance in heart nodal tissues?
What is the primary role of the autonomic nervous system in relation to heart rhythm?
What is the primary role of the autonomic nervous system in relation to heart rhythm?
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Study Notes
Cardiac Cells
- 2 types of cardiac muscle cells: contractile cells (working myocardium) and autorhythmic cells (conducting system).
- Contractile cells: responsible for contraction, make up 99% of the heart muscle
- Autorhythmic cells: initiate and distribute electrical activity, control the heartbeat, do not contribute to contraction
Myocardial Contractile Cells
- Striated, short, branched with a single central nucleus
- Connected by gap junctions and intercalated discs
- Specialized sarcolemmas with voltage-gated Ca2+ channels
The Action Potential in Skeletal vs Cardiac Muscle Contractile Cells
- Cardiac muscle cell action potential wider and the contraction phase longer than in skeletal muscle
Cardiac Muscle Action Potential and Myocardial Contraction
- Current spreads from autorhythmic cells to contractile cells through gap junctions
- Ca2+ release from SR triggered by Ca2+ influx into the cell, leading to muscle contraction
- Ca2+ pumped back into SR and out of the cell using ATP-dependent pumps and Na+/Ca2+ exchanger
The Refractory Period in Cardiac Muscle
- Long refractory period prevents summation and tetanus in the myocardium
Autorhythmic Cardiac Cells
- Initiate action potentials in the heart
- Found throughout the conducting system, including the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His and Purkinje fibers
Pacemaker and Action Potentials
- SA node has the shortest cycle of repolarization and depolarization
- SA node initiates the heartbeat by firing most frequently
Pathways for Flow of Electrical Signals in the Heart
- SA node generates the signal, spreads through internodal pathways and atrial muscle
- AV node and bundle convey the signal to the ventricles
- Purkinje fibers rapidly carry the signal throughout the ventricles
How is Electrical Activity Precisely Controlled?
- Fibrous skeleton electrically separates atria from ventricles, preventing simultaneous contraction
Facilitating Order (2): Timing of Electrical Events
- SA node is the pacemaker of the heart, firing most frequently and initiating the heartbeat
Delay in Conduction Between Atria and Ventricles at the AV Node
- Slow conduction at the AV node prevents excessively fast ventricular rates
Refractory Periods of the Myocyte
- Prolonged plateau in cardiac muscle action potential results in an absolute refractory period which prevents tetany
External Factors Affecting Electrical Activity in the Heart
- Central control of cardiac function through the autonomic nervous system
- Autonomic nervous system modulates heart activity, not generating it
Influence of Autonomic Nervous System (ANS) on the Heart
- At rest, vagal influences dominate, resulting in vagal tone
- Sympathetic stimulation (noradrenaline) increases heart rate (positive chronotropic effect)
- Parasympathetic stimulation (acetylcholine) decreases heart rate (negative chronotropic effect)
- Autonomic nervous system modulates the frequency of depolarization of pacemaker cells
- Sympathetic stimulation increases rate of depolarization
- Parasympathetic stimulation decreases rate of depolarization
- ANS also influences:
- Inotropic effect (force of contraction)
- Dromotropic effect (rate of impulse conduction)
- Bathmotropic effect (rate of attainment of threshold/automatism)
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