Cardiac Muscle Cells and Action Potential

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Questions and Answers

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?

  • 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?

  • Tight junctions
  • Gap junctions (correct)
  • Desmosomes
  • Adherens junctions

Which feature is NOT characteristic of myocardial contractile cells?

<p>Multiple nuclei (D)</p> Signup and view all the answers

What specialized channels are found in the sarcolemma of cardiac muscle cells that are not present in skeletal muscle?

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

Which statement about the contraction mechanism of cardiac muscle is FALSE?

<p>Cardiac fibers are anchored at their ends. (B)</p> Signup and view all the answers

How do auto-rhythmic cells differ from contractile cells in the heart?

<p>They initiate and coordinate the heartbeat. (B)</p> Signup and view all the answers

What physiological property primarily enables the electrical activity to be spread throughout the cardiac muscle?

<p>Presence of intercalated discs (A)</p> Signup and view all the answers

What is the primary function of the working myocardium?

<p>To produce contractions and generate force (A)</p> Signup and view all the answers

What type of cardiac cell primarily lacks contractile fibers?

<p>Auto-rhythmic cells (D)</p> Signup and view all the answers

What is the primary difference in the refractory periods of skeletal and cardiac muscle?

<p>Cardiac muscle has a much longer refractory period than skeletal muscle (A)</p> Signup and view all the answers

Which component is responsible for initiating action potentials in the heart?

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

What role does Ca2+ play during cardiac muscle contraction?

<p>Ca2+ binds to troponin to expose myosin binding sites (B)</p> Signup and view all the answers

Which structure is NOT part of the intrinsic conduction system of the heart?

<p>Ventricular muscle fibers (D)</p> Signup and view all the answers

What is the consequence of cardiac muscle's inability to undergo tetanus?

<p>It prevents the disruption of regular heart rhythm (C)</p> Signup and view all the answers

How does the action potential spread in cardiac contractile cells?

<p>It propagates through GAP junctions to adjacent myocytes (A)</p> Signup and view all the answers

Which mechanism is involved in pumping Ca2+ back into the sarcoplasmic reticulum after contraction?

<p>ATP dependent pumps and Na+/Ca2+ exchanger (D)</p> Signup and view all the answers

What effect does the presence of gap junctions have on action potentials in cardiac muscle?

<p>They enable rapid and synchronized contraction across muscle fibers (D)</p> Signup and view all the answers

Which of the following best describes the contraction phase of cardiac muscle compared to skeletal muscle?

<p>Contraction phase in cardiac muscle is longer due to slower calcium handling (A)</p> Signup and view all the answers

What is the primary function of the AV node in the conduction system of the heart?

<p>To prevent simultaneous contraction of atria and ventricles (C)</p> Signup and view all the answers

Why does the SA node function as the pacemaker of the heart?

<p>It has the quickest recovery of sodium channels (D)</p> Signup and view all the answers

What is the role of the fibrous skeleton of the heart?

<p>To separate electrical conduction between atria and ventricles (D)</p> Signup and view all the answers

What occurs during the absolute refractory period of a myocyte?

<p>The myocyte is unexcitable due to inactivated sodium channels (D)</p> Signup and view all the answers

What effect does the delay in conduction at the AV node have on the heart's function?

<p>It permits complete atrial contraction before ventricular depolarization (C)</p> Signup and view all the answers

What characterizes the conduction speed through the Purkinje fibers?

<p>It rapidly carries the signal throughout the ventricles (C)</p> Signup and view all the answers

What prevents excessive atrial contraction rates from leading to fast ventricular rates?

<p>The small diameter of AV node cells (D)</p> Signup and view all the answers

Which structure in the heart is responsible for creating the fastest electrical impulse?

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

What physiological mechanism ensures the ventricles remain relaxed while the atria contract?

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

What effect does sympathetic stimulation have on the heart rate when analyzed at the SA node?

<p>Increases heart rate by decreasing vagal influence (D)</p> Signup and view all the answers

Which neurotransmitter is associated with the parasympathetic influence on heart function?

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

What describes the dromotropic effect in relation to the autonomic nervous system's influence on the heart?

<p>Rate of impulse conduction through the AV node (D)</p> Signup and view all the answers

What characterizes the relative refractory period in myocardial action potential generation?

<p>Stimulation can produce a weak action potential (C)</p> Signup and view all the answers

In the context of cardiac function, a positive chronotropic effect is primarily achieved by which of the following mechanisms?

<p>Increase in norepinephrine activity (B)</p> Signup and view all the answers

Which receptor type does noradrenaline primarily bind to in the context of sympathetic stimulation of the heart?

<p>Beta-1 receptors (A)</p> Signup and view all the answers

What physiological effect does increased vagal tone have on heart function?

<p>Decreased heart rate (C)</p> Signup and view all the answers

Which of the following best defines the term 'inotropic effect' in relation to cardiac function?

<p>The force of contraction of the myocardium (C)</p> Signup and view all the answers

How does parasympathetic stimulation affect the ionic conductance in heart nodal tissues?

<p>Increases K+ conductance and decreases Ca2+ conductance (D)</p> Signup and view all the answers

What is the primary role of the autonomic nervous system in relation to heart rhythm?

<p>Modulating cardiac activity and adjusting heart rate (C)</p> Signup and view all the answers

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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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