Cardiac Conduction, Action Potentials, and Hemodynamics — Pacemaker and Contractile Cells

Metacognition and Study Approach

• Metacognition: thinking about what you’re thinking about, thinking about what you’re studying, comparing/contrasting, organizing the big picture, and paying attention to how information is presented. This helps you file information into the right mental “folders.”
• File-folder analogy: label slides by their title and make sure you understand what that means before moving on (e.g., pacemaker action potential).
• If you don’t know what terms mean (action potential, depolarization, polarization), seek clarification early; build a solid conceptual framework first.

Pacemaker and Action Potentials: Basic Concepts

• Action potential basics: polarization (resting state), depolarization (activation), repolarization (inactivation and return toward resting potential).
• Resting state: membrane is polarized with a charge difference across the cell membrane (inside negative, outside positive). A charge differential is maintained by ion pumps (e.g., Na⁺/K⁺-ATPase).
• Battery analogy: the cell membrane is like a battery with a negative inside and a positive outside; flow of charges powers cellular activity.
• Depolarization/trigger: once a threshold is reached, voltage changes lead to opening of voltage-gated channels and an action potential.
• Key ions and channels in muscle/nerve cells:
  • Channels open/close to allow ions to cross the membrane (e.g., Na⁺, K⁺, Ca²⁺).
  • Ligand-gated channels (e.g., acetylcholine receptors) can initiate depolarization in muscle cells.
• Pacemaker activity specifics (SA node):
  • The heart has a natural or intrinsic rhythm (SA node), often cited around ~100 beats per minute, with parasympathetic tone reducing actual resting rate to ~75 bpm.
  • The SA node acts as the pacemaker (the “metronome”).
• Pacemaker action potential progression (conceptual):
  • Slow depolarization largely due to a slow Na⁺ current keeps the membrane moving toward threshold.
  • When threshold is reached, voltage-gated channels open to further depolarize.
  • Calcium channels can contribute to depolarization; in some contexts, Ca²⁺ entry helps promote action potential dynamics.
• A note on neurotransmitter action (as described in the transcript): acetylcholine binding to receptors can influence membrane potential by modulating ion flow (e.g., opening a Na⁺ channel in the described scenario), contributing to the pacing mechanism.
• Plateau and repolarization (in general): opening of voltage-gated Ca²⁺ channels can slow repolarization, with K⁺ channels helping bring the membrane back toward the resting potential.

Cardiac Conduction System: Anatomy and Pathways

• Conduction system components:
  • SA node (pacemaker) in the atrium, normally ~75 bpm with parasympathetic braking.
  • AV node at the atrioventricular junction, providing a deliberate delay to allow atrial contraction to complete before ventricular contraction.
  • AV bundle (Bundle of His) conducts signal into the ventricles.
  • Left and right bundle branches.
  • Subendocardial conducting network and Purkinje fibers deliver the impulse to contractile cells.
• Intercellular connections: gap junctions within intercalated discs allow rapid electrical spread between cardiomyocytes.
• Conducting vs. contractile cells: conducting cells are noncontractile but transmit the impulse; contractile cells actually squeeze the heart muscle.
• Intrinsic pacing and autonomic modulation:
  • Sympathetic stimulation speeds the heart (positive chronotropy/positive inotropy).
  • Parasympathetic (vagus) activity slows the heart (negative chronotropy).
• Functional flow in the conduction system:
  • SA node discharges, spreads across atria.
  • AV node pauses conduction (~0.1 s) to allow atrial contraction and complete ventricle filling.
  • Impulse travels through the bundle branches and Purkinje network to rapidly depolarize ventricles.
• The ventricular conduction network coordinates synchronous contraction to optimize blood ejection.

Action Potentials: Pacemaker (Conduction) Cells vs. Contractile Cells

• Pacemaker (conduction) cell APs:
  • Gradual depolarization with a relatively slow upstroke; threshold triggers rapid depolarization via voltage-gated channels.
  • Slow Na⁺/other currents contribute to the gradual rise; Ca²⁺ channels can participate in depolarization dynamics.
  • This pattern underpins the automaticity of the SA node and conduction pathway.
• Contractile (ventricular) cell APs:
  • Fast Na⁺ entry causes a rapid depolarization (phase 0).
  • A plateau phase occurs due to a sustained Ca²⁺ influx (slow Ca²⁺ channels) balancing K⁺ efflux (phase 2).
  • Repolarization follows as K⁺ channels return the membrane toward the resting potential.
• Absolute refractory period: a time interval during which another contraction cannot be initiated, ensuring one beat at a time.

Electrocardiography (ECG) and Cardiac Sounds: Clinical Correlates

• ECG waveform basics:
  • P wave: atrial depolarization (onset of atrial contraction).
  • QRS complex: ventricular depolarization (and atrial repolarization occurs within the QRS but is not visible).
  • T wave: ventricular repolarization.
• Heart sounds and valves:
  • First heart sound (S1): closure of the AV valves at the start of ventricular systole.
  • Second heart sound (S2): closure of the semilunar valves at the start of ventricular diastole.
• Pressure-volume link: dicrotic notch in the aortic pressure waveform corresponds to closure of the aortic semilunar valve.
• The echocardiograph is a separate modality, while ECG measures electrical activity; the transcript notes the potential diagnostic value of ECG patterns in heart dysfunction.
• Ischemia indicators (conceptual): elevated ST segments on ECG can indicate ischemia or other pathology.

The Cardiac Cycle: Phases, Volumes, and Valve Dynamics

• Core terms:
  • Systole: period of contraction.
  • Diastole: period of relaxation.
  • Atrial systole vs ventricular systole; atrial diastole vs ventricular diastole.
• Ventricular filling: occurs during diastole; about 80% of ventricular filling is passive (gravity-driven), with the remaining ~20% contributed by atrial contraction (atrial kick).
• End-diastolic volume (EDV): the maximum ventricular volume at the end of diastole.
• End-systolic volume (ESV): the residual ventricular volume after systole.
• Stroke volume (SV): amount ejected per beat, SV = EDV − ESV.
• Isovolumetric phases:
  • Isovolumetric contraction: ventricles contract with all valves closed, causing pressure to rise without changing volume.
  • Isovolumetric relaxation: ventricles relax with all valves closed, volume remains constant while pressure falls.
• Afterload: back pressure exerted by the aorta and arterial tree; higher afterload makes ejection more difficult and reduces SV.
• Preload: the initial stretch of the ventricular muscle prior to contraction, largely determined by venous return.
• Atrial kick importance: the ~20% contribution to LV filling is critical for optimal ventricular performance.
• Elastic recoil of the aorta: after ventricular contraction, the elastic arterial walls recoil to maintain arterial pressure and continue driving blood forward.
• Pressure-volume relationships:
  • Ventricular pressure must exceed afterload (aortic pressure) to open the aortic valve and eject blood.
  • When ventricular pressure exceeds atrial pressure, AV valves close (S1).
  • The subsequent rise in ventricular pressure opens semilunar valves (when ventricular pressure > aortic pressure).
• End-diastolic volume strategies for optimizing SV:
  • Higher preload (more venous return) increases EDV and SV (up to physiological limits).
  • Higher afterload or reduced contractility decreases SV.
• Practical relationships:
  • Cardiac output (CO) = SV × heart rate (HR).
  • SV = EDV − ESV; EF (ejection fraction) = SV/EDV.
  • Normal CO ≈ 5 L/min; maximal CO can reach ~35 L/min (cardiac reserve).
  • Venous return, skeletal muscle pump, and respiratory pump enhance preload and SV.

Regulation of Cardiac Output: Mechanisms and Factors

• Cardiac output is modulated by two main levers:
  • Stroke volume (SV) changes: preload, afterload, and contractility.
  • Heart rate (HR) changes: autonomic input (sympathetic increases, parasympathetic decreases).
• Preload and venous return:
  • Preload increases with greater venous return (more blood in the atria and ventricle at end diastole).
  • Exercise, upright posture, and breathing patterns increase venous return via the skeletal muscle pump and respiratory pump.
  • Higher preload generally increases SV (to a limit).
• Afterload and contractility:
  • Increased afterload (higher aortic pressure) lowers SV.
  • Increased contractility raises SV by ejecting more of the EDV.
• Intrinsic and extrinsic regulators of contractility:
  • Positive inotropes (e.g., norepinephrine, epinephrine, calcium, thyroid hormone) increase contractility.
  • Negative inotropes decrease contractility; calcium channel blockers are clinically used to reduce contractility in certain patients.
• Neurohumoral signaling (norepinephrine/epinephrine):
  • Bind to cell-surface receptors (often G protein-coupled receptors) → second messenger cascades (e.g., ↑ cAMP).
  • Result: increased calcium influx and/or calcium handling efficiency, enhancing contractility and speed of conduction.
  • Increases in intracellular calcium augment cross-bridge formation and force of contraction.
• Calcium and excitation-contraction coupling:
  • In cardiac muscle, Ca²⁺ entry from the extracellular space via L-type calcium channels triggers calcium release from the sarcoplasmic reticulum, increasing cytosolic Ca²⁺ and promoting contraction.
  • The amount of available Ca²⁺ is a key determinant of contractile strength.
• Metabolic and hormonal context:
  • Hormones and calcium handling are influenced by neurohumoral state, exercise, and metabolic demands.

Practical Takeaways and Real-World Relevance

• The interplay of preload, afterload, and contractility determines stroke volume and cardiac output, affecting performance in exercise and daily life.
• The autonomic nervous system dynamically tunes heart rate and contractility to meet physiologic demand: sympathetic activation accelerates HR and increases contractility; parasympathetic activity slows HR.
• Clinical correlations (conceptual):
  • Ischemia can alter the ECG pattern and cardiac function; ST segment changes reflect myocardial oxygen supply/demand imbalance.
  • Abnormal CO/EF can indicate heart failure risk or vascular issues; maintaining proper preload (venous return) and afterload (vascular health) supports adequate cardiac output.
  • Medications that alter calcium handling or autonomic signaling (e.g., calcium channel blockers, adrenergic agents) can change SV and CO.

Formulas and Key Equations

• Stroke volume
  $SV = EDV - ESV$
• Cardiac output
  $CO = SV imes HR$
• Ejection fraction
  EF = rac{SV}{EDV} imes 100\, ext{%}
• Relationship of preload, afterload, and contractility to SV (conceptual directions):
  • Preload ↑ → SV ↑ (directly proportional)
  • Contractility ↑ → SV ↑ (intrinsic)
  • Afterload ↑ → SV ↓ (inversely proportional)
• Cardiac cycle relationships (basics):
  • EDV: volume in ventricle at end of diastole.
  • ESV: volume in ventricle at end of systole.
  • Isovolumetric phases: contraction or relaxation with no volume change due to closed valves.

Terminology to Practice

• Systole vs diastole (atria vs ventricles):
  • Atrial systole and ventricular diastole coordinate to fill ventricles before contraction.
  • Ventricular systole and atrial diastole coordinate to eject blood and then relax.
• Key anatomical landmarks: SA node, AV node, AV bundle, bundle branches, Purkinje fibers.
• Vessel and valve dynamics: AV valves close at the start of isovolumetric contraction (S1); semilunar valves close at the start of isovolumetric relaxation (S2).
• Diffusion concept mentioned: diffusion rates depend on gradient magnitude and concentrations; relevant to ion movement and extracellular ion balance (e.g., Ca²⁺, Na⁺, K⁺).
• Ischemia and diagnostic patterns: ST segment changes on ECG can indicate ischemia; clinicians use ECG and imaging to assess cardiac function without invasive procedures.

Quick Reference: Numerical Benchmarks Mentioned in the Lecture

• Intrinsic SA node rate (typical): ~100 beats per minute; parasympathetic input can reduce to ~75 bpm.
• Resting cardiac output estimate: ≈ 5 L/min in a typical adult at rest.
• Maximal cardiac output (cardiac reserve): can reach ~35 L/min in highly trained individuals.
• Venous return and atrial kick: ~80% of ventricular filling is passive (gravity-driven) with ~20% contributed by atrial contraction (atrial kick).
• Equations:
  $SV = EDV - ESV$
  $CO = SV \times HR$
  $EF = \frac{SV}{EDV} \times 100\%$
• Notable qualitative figures: 75 bpm (SA node with parasympathetic tone), EDV vs ESV, and the 20% atrial kick as a clinically significant component of ventricular filling.

Study Prompts (Metacognitive Check)

• Can you explain how the AV node delay helps ventricles fill before contraction?
• How would an increase in venous return influence EDV and SV according to the Frank-Starling mechanism?
• If afterload increases (e.g., stiff arteries), how does that affect SV and CO, and why?
• What role does calcium play in both the electrical (conductive) and mechanical (contractile) aspects of the heart?
• How do sympathetic and parasympathetic inputs differently affect HR, contractility, preload, and afterload?