Chapter 19 - 19.4-19.7

19.4a Microscopic Structure of Cardiac Muscle

• Myocardium Composition

  • Composed of cardiac muscle tissue.

  • Cardiac muscle cells are short and branched, housing one or two central nuclei.

  • Supported by areolar connective tissue known as endomysium.

• Key Structures

  • Sarcolemma: Plasma membrane that invaginates to form T-tubules extending into the sarcoplasmic reticulum.

  • Sarcoplasmic Reticulum: Surrounds bundles of myofilaments, with myofilaments arranged in sarcomeres.

  • The organization of these structures gives rise to the striated appearance of cardiac muscle under a microscope.

• Functional Properties:

  • Optimal length for contraction is achieved when the heart fills with blood, allowing for greater contractile force with more blood volume.

19.4b Intercellular Structures

• Cell Connections

  • Sarcolemma folded at connections between cells enhances structural stability of myocardium.

  • Intercalated Discs: Structures connecting cardiac muscle cells.

    • Desmosomes: Mechanically join cells with protein filaments providing strength.

    • Gap Junctions: Join cells electrically, allowing ion flow and enabling each heart chamber to act as a functional unit, known as a functional syncytium.

19.4c Metabolism of Cardiac Muscle

• Cardiac muscle has a high energy demand necessitating extensive blood supply and numerous mitochondria.

• Has Myoglobin and Creatine Kinase for energy stores.

• Can utilize diverse fuel sources including:

  • Fatty acids

  • Glucose

  • Lactic acid

  • Amino acids

  • Ketone bodies

• Primarily relies on aerobic metabolism, making it susceptible to failure under ischemic conditions where oxygen supply is low. Interference with blood flow can lead to cell death.

19.5a The Heart’s Conduction System

• Function of Conduction System

  • Initiates and conducts electrical events ensuring proper timing of heart contractions.

  • Comprised of specialized cardiac muscle cells that carry action potentials without contracting.

  • Influenced by autonomic nervous system activity.

19.5b Conduction Pathways

• Sinoatrial (SA) Node

  • Initiates heartbeat and acts as the pacemaker located in the posterior wall of the right atrium.

• Atrioventricular (AV) Node

  • Located in the floor of right atrium near the right AV valve, critical for coordinating contractions.

• AV Bundle (Bundle of His)

  • Extends from the AV node through the interventricular septum, bifurcating into left and right bundles, ultimately connecting to Purkinje fibers.

• Purkinje Fibers

  • Extend from the bundles at the heart’s apex, coursing through the ventricular walls, ensuring coordinated contraction.

19.5c Innervation of the Heart

• Autonomic Regulation

  • Cardiac center located in the medulla oblongata includes both cardioacceleratory and cardioinhibitory centers.

  • Receives signals from baroreceptors and chemoreceptors and sends responses through sympathetic and parasympathetic pathways, modifying heart rate and force of contraction.

• Parasympathetic Influence

  • Decreases heart rate, operates via vagus nerves, specifically the right vagus influencing the SA node and the left vagus influencing the AV node.

• Sympathetic Influence

  • Increases heart rate and myocardial contraction force, originating from the cardioacceleratory center and extending from spinal cord segments, enhancing coronary vessel dilation.

19.6 Stimulation of the Heart

• Contraction Events

  • Heart contractions involve two critical phases: the conduction system initiates an action potential, and cardiac muscle cells subsequently react by contracting.

19.6a SA Nodal Cells

• Role of SA Node

  • Initiates heartbeat, spontaneously depolarizing to generate action potentials with a resting membrane potential (RMP) around -60mV, without a stable RMP.

  • Notable for having pacemaker potentials and various membrane proteins including Na+/K+ pumps and specific voltage-gated channels.

19.6b Electrical Events at SA Node

• Stepwise Initiation of Action Potential:

  1. Reaching Threshold

     • Slow voltage-gated Na+ channels open, Na+ influx raises membrane potential to -40mV.

  2. Depolarization

     • Fast voltage-gated Ca2+ channels open causing further depolarization above 0mV.

  3. Repolarization

     • Calcium channels close while voltage-gated K+ channels open, returning membrane potential back to -60mV, ready to repeat.

19.6c Action Potential Spread

• Spread of Action Potential:

  1. Starts at SA node, spreads through atria, leading to simultaneous contraction.

  2. Pauses at the AV node to allow ventricular filling.

  3. Proceeds through the AV bundle to Purkinje fibers for ventricular contraction.

19.6d Specialized Ventricular Features

• Purkinje Fiber Function

  • Larger diameter allows for rapid action potential transmission ensuring synchronized ventricular contraction, which begins at the heart’s apex and ensures efficient blood ejection.

Clinical View: Ectopic Pacemaker

• Description

  • Ectopic pacemaker refers to any pacemaking activity initiated by cells other than the SA node, often slower than SA node activity.

  • In case of SA node failure, the AV node can act as a default pacemaker, maintaining life but at a reduced rate.

19.7 Cardiac Muscle Cells at Rest

• Cardiac muscle cells have Na+/K+ pumps and specific voltage-gated channels, maintaining a resting membrane potential of -90mV when at rest, crucial for action potential generation.

19.7b Electrical and Mechanical Events

• Phases of Cardiac Action Potential:

  1. Depolarization: Na+ influx leads to rapid depolarization.

  2. Plateau: K+ efflux while Ca2+ influx maintains depolarization stimulating Sarcoplasmic Reticulum for calcium release.

  3. Repolarization: Ca2+ channels close, membrane potential returns to -90mV.

• Mechanical Events: Ca2+ triggers crossbridge cycling crucial for muscle contraction and relaxation phases through regulated calcium levels.

19.7c Refractory Period

• Significance of Refractory Period

  • Cardiac muscle’s long refractory period prevents sustained contraction (tetany), allowing adequate time for relaxation between successive stimuli.

19.7d Electrocardiogram (ECG)

• ECG Functionality

  • Records electrical signals of cardiac muscle, serving as a crucial diagnostic tool.

• Waves and Segments:

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization and simultaneous atrial repolarization.

  • T wave: Ventricular repolarization.

• P-Q and S-T Segments:

  • Associated with atrial and ventricular plateau phases respectively during contraction.

19.7d Intervals Analysis

• Clinical Insights:

  • P-R interval illustrates time for atrial to ventricular conduction, while Q-T interval reflects time of ventricular action potentials, both vital for assessing heart rhythm anomalies.

Clinical View: Cardiac Arrhythmia

• Arrhythmia Types:

  • Heart blocks correlated with conduction impairments, leading to symptoms like fainting.

  • PACs and PVCs: Can arise due to stress or stimulants; not always detrimental unless frequent.

  • Atrial and ventricular fibrillation are severe conditions leading to ineffective pumping, often requiring emergency interventions such as defibrillation.