19.2 Cardiac Muscle and Electrical Activity

Cardiac muscle tissue is the powerhouse of the heart, enabling its continuous pumping action. Its unique structure, including intercalated discs and branching cells, allows for coordinated contractions. High mitochondrial content fuels the heart's relentless energy demands.

The heart's electrical system orchestrates its rhythm through specialized structures like the SA node and conduction pathways. Understanding cardiac electrical activity is crucial for interpreting ECGs and diagnosing abnormalities that can disrupt the heart's vital function.

Cardiac Muscle Tissue

Structural features of cardiac muscle

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• Intercalated discs connect adjacent cardiomyocytes allowing rapid transmission of electrical impulses and coordinated contraction
  • Contain gap junctions that enable electrical coupling between cells
  • Desmosomes provide mechanical strength to withstand the force of contraction
  • Fascia adherens anchor actin filaments to the cell membrane for efficient force transmission
• Autorhythmicity enables the heart to generate its own electrical impulses without external stimulation
  • Pacemaker cells (SA and AV nodes) possess unstable resting membrane potentials that spontaneously depolarize
  • Allows the heart to maintain a regular rhythm even in the absence of neural input
• Branching structure of cardiomyocytes facilitates efficient contraction and pumping of blood
  • Shorter and wider than skeletal muscle cells (ventricular cardiomyocytes ~100 μm long and 10-25 μm wide)
  • Branching morphology increases the surface area for intercellular connections and force generation
• High mitochondrial content supports the heart's continuous energy demands
  • Mitochondria occupy ~30-40% of cardiomyocyte volume
  • Produce ATP through oxidative phosphorylation to fuel the constant contraction and relaxation cycle
• The myocardium, composed of cardiac muscle tissue, forms the middle and thickest layer of the heart wall
  • Contains sarcomeres, the basic functional units of muscle contraction, arranged in a highly organized structure

Electrical Activity of the Heart

Components of cardiac conduction system

• Sinoatrial (SA) node acts as the primary pacemaker initiating electrical impulses
  • Located in the right atrium near the opening of the superior vena cava
  • Exhibits the highest intrinsic rate of spontaneous depolarization (~60-100 bpm)
• Atrioventricular (AV) node receives impulses from the SA node and delays their transmission to the ventricles
  • Positioned in the interatrial septum near the tricuspid valve
  • Allows time for atrial contraction to complete before ventricular contraction begins
• Bundle of His rapidly conducts impulses from the AV node to the ventricles
  • Divides into left and right bundle branches within the interventricular septum
  • Ensures synchronized activation of the ventricular myocardium
• Purkinje fibers are the terminal branches of the bundle branches
  • Spread electrical impulses throughout the ventricular walls
  • Enable rapid and coordinated ventricular contraction from apex to base

Ion movements in cardiac cells

• Resting membrane potential of cardiomyocytes is maintained at around -90 mV
  • Primarily determined by the concentration gradient and permeability of K+ ions
  • High intracellular K+ concentration and open K+ leak channels contribute to the negative potential
• Depolarization occurs when voltage-gated Na+ channels open allowing rapid influx of Na+ ions
  • Raises the membrane potential towards the Na+ equilibrium potential (+30 mV)
  • Reaches a threshold potential triggering an action potential
• Plateau phase is characterized by a slow influx of Ca2+ ions through L-type calcium channels
  • Prolongs the action potential duration (200-400 ms) compared to skeletal muscle
  • Sustains contraction by maintaining a depolarized state
• Repolarization restores the negative resting membrane potential
  • Efflux of K+ ions through delayed rectifier K+ channels
  • Closure of Ca2+ channels and activation of Na+/K+ ATPase pump
• The refractory period, during which the cardiac muscle cannot be re-excited, prevents tetanic contractions and ensures coordinated pumping

ECG interpretation and cardiac cycle

• P wave represents atrial depolarization preceding atrial contraction
  • Typically <0.12 s in duration and <2.5 mm in amplitude
  • Absence may indicate atrial fibrillation or sinoatrial block
• QRS complex corresponds to ventricular depolarization and contraction (systole)
  • Normally <0.12 s in duration and 5-30 mm in amplitude
  • Widened QRS suggests bundle branch block or ventricular hypertrophy
• T wave reflects ventricular repolarization during relaxation (diastole)
  • Follows the QRS complex and is typically upright in most leads
  • Inverted or flattened T waves may signify myocardial ischemia or electrolyte imbalances
• PR interval measures the time from the start of atrial depolarization to ventricular depolarization
  • Normally 0.12-0.20 s in duration
  • Prolonged PR interval indicates an AV conduction delay (1st-degree AV block)
• QT interval encompasses the duration of ventricular depolarization and repolarization
  • Varies with heart rate and is typically corrected (QTc) using Bazett's formula: $QTc = QT / \sqrt{RR}$
  • Prolonged QTc (>450 ms in men, >460 ms in women) increases the risk of ventricular arrhythmias
• The cardiac cycle, consisting of systole and diastole, is regulated by the Frank-Starling law, which relates ventricular stretch to contractile force

Cardiac conduction abnormalities

• Sinus bradycardia is a slow heart rate (<60 bpm) originating from the SA node
  • May result from increased vagal tone, medications (beta-blockers), or hypothyroidism
  • Generally benign but can cause symptoms like fatigue or dizziness if severe
• Sinus tachycardia refers to a rapid heart rate (>100 bpm) initiated by the SA node
  • Occurs in response to physiological stressors (exercise, fever) or pathological conditions (anemia, hyperthyroidism)
  • Rarely requires treatment unless underlying cause is identified
• Atrioventricular block involves impaired conduction between the atria and ventricles
  • First-degree: Prolonged PR interval (>0.20 s) but all impulses are conducted
  • Second-degree: Intermittent conduction with some P waves not followed by QRS complexes (Mobitz I or II)
  • Third-degree: Complete dissociation between atrial and ventricular activity, requiring pacemaker implantation
• Bundle branch block refers to impaired conduction in either the left or right bundle branch
  • Leads to abnormal ventricular activation and a widened QRS complex (>0.12 s)
  • Left bundle branch block (LBBB) may indicate underlying heart disease or cardiomyopathy
• Atrial fibrillation is characterized by rapid, irregular, and disorganized atrial activation
  • Multiple ectopic foci in the atria fire at rates up to 300-600 bpm
  • Increases the risk of thromboembolic events (stroke) due to ineffective atrial contraction and stasis of blood

Historical context

• Willem Einthoven developed the first practical electrocardiogram (ECG) in the early 20th century, revolutionizing the study of cardiac electrical activity and laying the foundation for modern cardiology