Cardiac Muscle and Electrical Activity (Section 19.2)

Below are comprehensive, study-ready notes on Cardiac Muscle and Electrical Activity, organized as bullet-point Markdown notes drawn from the provided transcript. All major and minor points are included, with explanations, examples, and real-world relevance where present. LaTeX-formatted equations and numerical references are included in-line where appropriate.

Cardiac Muscle: Structure, Types, and Key Concepts

  • Cardiac muscle shares features with skeletal and smooth muscle but has unique properties, notably autorhythmicity—the ability to initiate an electrical potential at a fixed rate that spreads cell-to-cell to trigger contraction. Unlike smooth or skeletal muscle, cardiac muscle can generate its own rhythm.
  • Heart rate is modulated by the endocrine and nervous systems, even though autorhythmicity is intrinsic.
  • Two major cardiac muscle cell types:
    • Myocardial contractile cells
    • Constitute the bulk (about 99\%) of cells in the atria and ventricles.
    • Function: conduct impulses and generate contractions that pump blood.
    • Myocardial conducting cells
    • Form the conduction system of the heart and constitute about 1\% of cells.
    • Generally smaller than contractile cells; have few myofibrils/filaments for contraction.
    • Function: initiate and propagate action potentials to trigger contractions.
  • Intercalated discs: junctions between cardiac muscle cells that synchronize contraction.
    • Composed of desmosomes, tight junctions, and abundant gap junctions.
    • Allow passage of ions between cells, promoting synchronized contraction.
  • Intercellular connective tissue helps bind cells and support structural integrity against contractile forces.
  • Cardiac muscle metabolism and structure:
    • Cardiac muscle undergoes aerobic respiration; mitochondria are plentiful.
    • Myoglobin, lipids, and glycogen stored in the cytoplasm support energy needs.
    • Sarcomeres and T-tubules present; T-tubules penetrate from the sarcolemma to the cell interior.
    • T-tubules in cardiac muscle are found at the Z discs (not at the A-I junction as in skeletal muscle);
      therefore, there are about one-half as many T-tubules in cardiac muscle as in skeletal muscle.
    • Sarcoplasmic reticulum stores relatively little Ca^{2+}; most Ca^{2+} must enter from outside the cell.
    • Cardiac cells typically have a single central nucleus, though multiple nuclei can occur.
    • Cardiac muscle fibers branch freely, enabling a three-dimensional network.
  • Refractory period and contraction:
    • Cardiac muscle cells have long refractory periods to prevent tetany and ensure the heart can fill and pump properly.
    • Duration and timing of refractory periods are critical for effective cardiac cycling.
  • Regeneration and repair:
    • Damaged cardiac muscle has limited ability to repair through mitosis; stem cells may exist in the heart but repaired cells are often nonfunctional and scar tissue forms after injury.
    • Advances in cardiac regeneration (e.g., stem cells, patches) are areas of active research with potential to improve outcomes after myocardial infarction.
  • Everyday connections:
    • Heart attack (MI) can lead to patches of scar tissue replacing dead cells; ongoing repair mechanisms and the potential for regenerative therapies are clinically important.

Cardiac Conduction System: Anatomy and Function

  • The conduction system generates and distributes the electrical impulse that triggers heart contractions.
  • Key components (from initiation to ventricular spread):
    • Sinoatrial (SA) node: the natural pacemaker; located in the superior/posterior wall of the right atrium near the superior vena cava.
    • Internodal pathways: carry impulse from the SA node to the atrioventricular (AV) node; consist of anterior, middle, and posterior bands.
    • Bachmann’s bundle (interatrial band): conducts impulse directly from the right atrium to the left atrium.
    • Atrioventricular (AV) node: located in the inferior portion of the right atrium within the atrioventricular septum; introduces a critical delay to allow atrial contraction to complete before ventricular contraction.
    • Atrioventricular bundle (Bundle of His): travels through the interventricular septum.
    • Right and left bundle branches: descend through the septum; left bundle has two fascicles; right bundle may be connected to the moderator band.
    • Purkinje fibers: subendocardial conducting network that spreads impulse rapidly to ventricular myocardium.
  • Conduction timing and sequence:
    • SA node has the highest inherent rate and initiates impulses that spread through the atria to the AV node.
    • Impulse takes about 50\ \text{ms} to reach the AV node via internodal pathways.
    • AV node delay: approximately 100\ \text{ms}, allowing atrial contraction to complete before ventricular excitation.
    • Impulse travels from AV node to AV bundle and bundle branches in roughly 25\ \text{ms}.
    • Purkinje fibers deliver impulse to ventricular myocardium in about 75\ \text{ms}, causing ventricular contraction to begin at the apex and propagate upward (toward the base) for efficient ejection, similar to squeezing a toothpaste tube from the bottom.
    • Total time from SA node activation to ventricular depolarization: approximately 225\ \text{ms}.
  • Functional implications:
    • The conduction skeleton prevents impulses from spreading directly into ventricular myocytes except via the AV node, ensuring coordinated contraction.
    • The moderator band connects to right papillary muscles; ensures right papillary muscles receive impulse in synchrony with other ventricular regions. There is no corresponding moderator band on the left.
  • Rate hierarchy within the conduction system (intrinsic firing rates, in the absence of autonomic modulation):
    • SA node: ~80-100\ \text{bpm} (pacemaker rhythm).
    • AV node: ~40-60\ \text{bpm} if SA node input is blocked.
    • AV bundle: ~30-40\ \text{bpm}.
    • Bundle branches: ~20-30\ \text{bpm}.
    • Purkinje fibers: ~15-20\ \text{bpm}.
  • Clinical relevance:
    • Extreme SA node stimulation can drive AV node to a maximum rate of about 220\ \text{bpm}, which is often unsustainable for effective pumping.
    • If the SA node is damaged or blocked, other components may assume pacing but at progressively lower intrinsic rates, potentially leading to bradycardia if below ~50 bpm.

Cardiac Action Potentials: Conductive vs Contractile Cells

  • Conductive (pacemaker) cells:
    • Do not have a stable resting potential.
    • Contain a slow Na^+ influx that causes spontaneous depolarization (prepotential) from about -60\ \text{mV} toward threshold, then Ca^{2+} influx causes rapid depolarization to around +15\ \text{mV}.
    • After Ca^{2+} channels close, K^+ efflux repolarizes the cell; once MP returns to about -60\ \text{mV}, K^+ channels close and Na^+ channels reopen, restarting the cycle.
    • This prepotential depolarization leads to autorhythmicity (no stable resting potential).
    • The action potential propagation in these cells triggers contraction in neighboring myocardium.
    • The mechanism explains why the SA node acts as the primary pacemaker (fastest to reach threshold).
  • Contractile (myocardial) cells:
    • Typically have a stable resting membrane potential: atrial myocytes around -80\ \text{mV}; ventricular myocytes around -90\ \text{mV}.
    • Upon stimulation, rapid depolarization occurs as voltage-gated Na^+ channels open, producing a rapid upstroke to about +30\ \text{mV} (fast depolarization; duration ~3-5\ \text{ms}).
    • Plateau phase: Ca^{2+} influx balances K^+ efflux, keeping membrane potential near zero for about 175\ \text{ms}; the plateau prolongs action potentials and contributes to long refractory periods.
    • Repolarization: Ca^{2+} channels close, K^+ channels open, membrane potential returns toward resting values (repolarization lasts ~75\ \text{ms}).
    • Total action potential duration in cardiac muscle: typically 250-300\ \text{ms}.
  • Key contrasts:
    • Conductive cells have no stable resting potential and exhibit prepotential depolarization; contractile cells have a stable resting potential and a distinct fast upstroke, plateau, and repolarization.
    • Plateau in contractile cells is essential for the long refractory period, preventing premature contractions and enabling effective pumping.
  • Calcium’s dual role in contraction:
    • Entry of Ca^{2+} through slow Ca^{2+} channels during the plateau is critical for the prolonged plateau and the absolute refractory period.
    • Ca^{2+} binds troponin in the troponin-tropomyosin complex to remove inhibition of actin-myosin cross-bridge formation, enabling contraction.
    • About 20\% of the Ca^{2+} required for contraction enters the cell during the plateau; the remaining Ca^{2+} is released from the sarcoplasmic reticulum (SR).

Ion Movement, Membrane Potentials, and Their Roles

  • Conductive cell cycles (autorhythmicity):
    • Prepotential: slow Na^+ influx raises membrane potential toward threshold.
    • Threshold reached, rapid depolarization via Ca^{2+} influx to about +15\ \text{mV}.
    • Repolarization via K^+ efflux returns potential to ~-60\ \text{mV}, where the cycle restarts.
  • Contractile cell cycles:
    • Rapid depolarization to ~+30\ \text{mV} (Na^+ channels open).
    • Plateau (~+0\ to\ +20\ \text{mV}) due to continued Ca^{2+} entry and limited K^+ exit.
    • Repolarization as Ca^{2+} channels close and K^+ exits, returning to resting levels.
  • Overall: action potentials in cardiac tissue coordinate strongly with calcium dynamics, linking electrical events to mechanical contraction.

Electrocardiography (ECG/EKG) and the Cardiac Cycle

  • ECG basics:
    • ECG records the aggregated electrical activity of the heart via leads; standard ECG uses 3, 5, or 12 leads. A 12-lead ECG uses 10 skin electrodes.
    • Holter monitor: portable device for continuous ECG recording (often 24 hours).
    • The normal ECG shows five prominent points: P wave, QRS complex, and T wave.
  • Wave meanings:
    • P wave: atrial depolarization; atrial contraction begins about 25\ \text{ms} after the start of the P wave.
    • QRS complex: ventricular depolarization; ventricles begin contracting as the QRS reaches its peak (R peak).
    • T wave: ventricular repolarization; atrial repolarization occurs during the QRS complex and is typically masked by the larger ventricular QRS signal.
  • Segments and intervals:
    • PR segment: from end of P wave to start of QRS complex.
    • PR interval: from beginning of P wave to beginning of QRS complex; measures conduction time from atrial depolarization to the start of ventricular depolarization.
    • ST segment and QT interval are also notable components (referenced in figures).
    • Delays in conduction (e.g., SA to AV node) lengthen the PR interval.
  • Correlation with the cardiac cycle (ECG–mechanical events):
    • The ECG tracing segments/intervals map to specific electrical/mechanical events in the cardiac cycle (see correlating figures in the source material).
  • Ectopic focus and arrhythmias:
    • Ectopic focus or ectopic pacemaker: when another region initiates impulses outside the SA node, potentially causing premature contractions.
    • Triggers: ischemia, certain drugs (caffeine, digitalis, acetylcholine), autonomic nervous system effects, or various diseases.
    • Most ectopic activity is transient/non-life-threatening; chronic ectopy can lead to arrhythmias or fibrillation.
  • Common ECG abnormalities and clinical insights:
    • Enlarged P waves can indicate atrial enlargement; enlarged Q waves can signal a myocardial infarction (MI).
    • Suppressed or inverted Q waves can indicate enlarged ventricles.
    • ST-segment elevation often signals acute MI; ST depression can indicate hypoxia.
  • Interpretive caution:
    • ECG interpretation requires substantial training; it provides electrical information but not direct pumping effectiveness. Additional tests (e.g., echocardiography or nuclear imaging) may be needed to assess pumping function.
  • Abnormalities shown in educational figures include:
    • Second-degree block: some P waves are not followed by QRS complexes.
    • Atrial fibrillation: irregular rhythm with abnormal P waves and increased interval variability.
    • Ventricular tachycardia: abnormal QRS morphology.
    • Ventricular fibrillation: lack of organized electrical activity.
    • Third-degree (complete) block: no correlation between atrial (P) and ventricular (QRS) activity.
  • Defibrillation and external devices:
    • Ventricular fibrillation is a medical emergency; defibrillation with external paddles can restore normal sinus rhythm by briefly stopping the heart so the SA node can take over.
    • External automated defibrillators (AEDs) are widely installed in public spaces to improve survival from sudden cardiac events.
  • Blocks in conduction pathways:
    • SA nodal blocks, AV nodal blocks, infra-Hisian blocks (bundle of His), bundle branch blocks (left or right), and hemiblocks (partial fascicular blocks).
    • Clinically common blocks include AV nodal block and infra-Hisian blocks.
  • Pacemakers:
    • If arrhythmias persist, a pacemaker can be implanted to deliver electrical impulses to ensure contractions and blood flow.
    • Pacemakers can be temporary on-demand or continuous, and some include defibrillator capabilities.

Cardiac Muscle Metabolism and Energy

  • Normal metabolism is predominantly aerobic.
  • Oxygen delivery and storage:
    • Oxygen is carried to the heart by the lungs and bound to hemoglobin in erythrocytes.
    • Myocardial myoglobin stores are present to help buffer oxygen availability.
  • Substrates for ATP production:
    • Fatty acids and glucose are oxidized in mitochondria to generate ATP.
    • Lipid droplets and glycogen are stored in the cytoplasm as nutrient reserves.
  • Practical note: under peak demand, the heart relies on both circulating oxygen and myoglobin stores to meet metabolic needs; dysfunction can compromise energy supply and contractility.

Key Quantitative Details to Remember

  • Cell-type composition:
    • Contractile cells: ext{about }99\%
    • Conducting cells: ext{about }1\%
  • Conduction timing (typical):
    • SA to AV node: 50\ \text{ms}
    • AV node delay: \approx 100\ \text{ms}
    • AV node to bundle: \approx 25\ \text{ms}
    • Bundle to Purkinje network: \approx 75\ \text{ms}
    • Total SA-to-ventricular depolarization: \approx 225\ \text{ms}
  • Max intrinsic heart rates (without autonomic input):
    • SA node: 80-100\ \text{bpm}; bradycardia defined as often < 50\ \text{bpm} for many individuals.
    • AV node: 40-60\ \text{bpm} (if SA node blocked).
    • AV bundle: 30-40\ \text{bpm}.
    • Bundle branches: 20-30\ \text{bpm}.
    • Purkinje fibers: 15-20\ \text{bpm}.
  • Action potential durations:
    • Contractile cells: total duration 250-300\ \text{ms}; plateau ~175\ \text{ms}; repolarization ~75\ \text{ms}.
    • Absolute refractory period: about 200\ \text{ms}; relative refractory period: about 50\ \text{ms}; total refractory period ≈ 250\ \text{ms}.
  • Resting membrane potentials:
    • Atrial resting: V_{rest} \approx -80\ \text{mV}
    • Ventricular resting: V_{rest} \approx -90\ \text{mV}
  • Calcium dynamics:
    • Influx through slow Ca^{2+} channels contributes to the plateau and long refractory period.
    • Approximately 20\% of Ca^{2+ required for contraction enters during the plateau; remainder released from SR.

Practical and Clinical Implications

  • The long refractory period ensures the heart contracts effectively and avoids tetany, which would be incompatible with life.
  • ECG interpretation requires clinical training; changes in waves/intervals provide insights into atrial and ventricular function, conduction delays, ischemia, or infarction.
  • Ectopic foci can cause premature contractions; chronic ectopy can lead to arrhythmias or fibrillation if not managed.
  • Defibrillation and AEDs are critical life-saving tools in ventricular fibrillation and certain arrhythmic scenarios.
  • Pacemakers and implanted cardioverter-defibrillators (ICDs) are used to maintain rhythm and prevent sudden cardiac death when intrinsic conduction is compromised.
  • Cardiac metabolism emphasizes the heart’s reliance on oxygen delivery; impairments can severely impact function, underscoring the importance of coronary health and metabolic regulation.