cardiac muscle 2024 (21)

Cardiac Muscle Overview

• Cardiac muscle is essential for the functioning of the heart.

Important Readings

• Silverthorn 8e – Chapter 12 (Cardiac Muscle) Section 12.4.

• Chapter 14 – Section 4.3 "Cardiac Muscle and the Heart", Pages 408-409.

• Beginning at subsection "Cardiac Muscles Contract Without Innervation" on page 446.

Goals of Study

• Roles of three types of cardiac muscle cells in generating electrical activity.

• Similarities and differences between cardiac action potentials and neuronal action potentials; implications on tetanus.

• Explanation of Excitation-Contraction Coupling in cardiac contractile cells.

Outline of Topics

• Types of cardiac muscle cells

• Electrical conduction in the heart

• Action potentials in pacemaker cells vs. contractile cells

• Prolonged action potentials in contractile cells

• Mechanism of Excitation-Contraction Coupling.

Cardiac Muscle Cells Structure

• Cardiac muscle cells (myocytes) are approximately 0.1 mm in diameter.

• Contains a nucleus and intercalated discs that link the cells.

Intercalated Discs

• Interconnected Structure:

  • They form functional syncytia, allowing for synchronized contractions.

• Types of Membrane Junctions in Intercalated Discs:

  • Desmosomes: Provide structural support during contractions.

  • Gap Junctions: Allow electrical signals to pass quickly between cells.

Types of Cardiac Muscle Cells

1. Myocardial Autorhythmic Cells

   • Initiate and maintain electrical activity.

   • Do not contract.

2. Conducting Cells

   • Conduct electrical signals throughout the heart.

   • Also do not contract.

3. Myocardial Contractile Cells

   • Comprise 99% of cardiac muscle cells.

   • Responsible for contractions and the mechanical pumping of the heart.

   • Electrically joined by gap junctions.

Electrical Conduction in the Heart

• Origin of Cardiac Impulse: Begins at the SA node.

• Action Potential Spread:

  • Spreads through left and right atria.

  • Passes from atria to ventricles via the AV node (only electrical contact point).

  • Delays briefly at AV node for complete ventricular filling.

• Impulse Pathway:

  • Travels rapidly down the interventricular septum (bundle of His).

  • Disperses throughout myocardium via Purkinje fibers.

  • Activated via cell-to-cell spread through gap junctions.

Action Potential in Pacemaker Cells

• Intrinsic Conduction System:

  • Autorhythmic cells initiate action potentials.

  • No stable resting membrane potential; rather, a pacemaker potential that drift towards threshold.

• Membrane repolarizes to -60 mV.

• Ion Currents in Pacemakers:

  • IF: Sodium (Na+) current.

  • ICaT: Fast calcium (Ca2+) current.

  • ICaL: Slow Ca2+ current.

  • Rising phase due to calcium influx through L-type Ca2+ channels.

Action Potential of Contractile Cells

• Characterized by:

  • Rapid depolarization.

  • Rapid partial early repolarization.

  • Prolonged slow repolarization (plateau phase).

  • Rapid final repolarization.

• Important for maintaining contraction duration.

Duration of Action Potentials in Cardiac Muscle

• Prolonged Action Potential:

  • Caused primarily by slow calcium channel activation (L-type).

  • Ensures adequate blood ejection and prevents tetanus.

Refractory Periods Comparison

• Skeletal Muscle:

  • Short refractory period; allows summation and tetanus.

• Cardiac Muscle:

  • Long refractory period close to entire muscle twitch duration; prevents tetanus.

Avoiding Tetanus in Cardiac Cells

• Long action potentials lead to an extended refractory period, preventing tetanus and ensuring rhythmic heartbeats.

Excitation-Contraction Coupling in Cardiac Muscle

• Mechanism Explained:

  • Ca2+ entry through T tubules triggers massive Ca2+ release from the sarcoplasmic reticulum (SR).

  • This leads to cross-bridge cycling and muscle contraction.