conduction system

Specialized Conduction System

  • The cardiac muscle cells undergo a process that generates an action potential.
  • Myocytes (e.g., Purkinje fibers) behave similarly, but with notable differences in membrane potential and action potential upstroke.

Pacemaker Cells

  • Definition: Certain heart cells are capable of self-initiated depolarization, known as pacemaker cells, which possess automaticity.
  • Automaticity: This property allows pacemaker cells to undergo spontaneous depolarization during Phase 4 of the action potential.
  • Triggering Action Potential: When the threshold voltage is reached, an action potential is triggered.
  • Types of Pacemaker Cells:
    • SA Node: The primary natural pacemaker of the heart.
    • AV Node: Another pacemaker object, but it functions mostly as a relay.
  • Automaticity in Other Cells: Atrial and ventricular muscle cells do not normally exhibit automaticity, except under disease conditions such as ischemia.
  • Differences in Action Potential Shapes:
    1. Maximum Negative Voltage:
    • Pacemaker cells: Approximately -60 mV,
    • Ventricular muscle cells: Approximately -90 mV.
    1. Phase 4 Shape:
    • Pacemaker cells show an upward slope due to gradual spontaneous depolarization (caused by the pacemaker current).
    • This current (denoted by I) is primarily carried by Na+ ions.
    1. Phase 0 Upstroke Characteristics:
    • In pacemaker cells, the upstroke is less rapid and of lower amplitude compared to cardiac muscle cells.
    • The upstroke primarily relies on Ca++ influx through slow calcium channels due to inactivated fast sodium channels.

Action Potential of Pacemaker Cells

  • Phase 4: Gradual spontaneous depolarization, characterized by the pacemaker current (I).
  • Membrane Voltage Changes:
    • Threshold voltage for action potential is reached around -40 mV, followed by an upstroke.
  • Repolarization Mechanism:
    • Occurs through the inactivation of Ca++ channels and K+ efflux via potassium channels (Ik and Ikr).

Refractory Periods

  • Overall Length Comparison:
    • Cardiac action potentials are longer than those in nerves and skeletal muscle.
    • This length supports prolonged Ca++ entry and muscle contraction during systole.
  • Physiological Necessity:
    • Allows the ventricles sufficient time to relax and refill before the next contraction.

Types of Refractory Periods:

  1. Absolute Refractory Period (ARP):
    • The period during which the cell is entirely unexcitable to new stimulation.
  2. Effective Refractory Period (ERP):
    • Builds upon ARP, extends into phase 3, though stimulation may produce a localized depolarization (not propagable).
  3. Relative Refractory Period (RRP):
    • Interval where stimulation can trigger an action potential, though the action potential peaks at a slower rate than usual.
  4. Supranormal Period:
    • Follows RRP; a weaker stimulus can successfully trigger an action potential.
  • Comparative Refractoriness:
    • Atrial cells have shorter refractory periods than ventricular muscle cells, allowing more rapid atrial rates during arrhythmias.

Impulse Conduction

  • Impulse Propagation:
    • During depolarization, electrical impulses spread rapidly along cardiac cells via low-resistance gap junctions, enabling action potential spread.
  • Gap Junctions:
    • Special ion channels providing electrical and biochemical coupling between cardiac myocytes, facilitating rapid impulse conduction.
  • Factors Influencing Conduction Velocity:
    • Net inward current (predominantly Na+ channels),
    • Resting potential value, which determines Na+ channel inactivation,
    • Resistance to current flow through gap junctions.

Normal Sequence of Cardiac Depolarization

  • Initiation Site:
    • The heartbeat starts at the SA node, spreading through the atrial muscle.
  • Atrioventricular Node (AV Node) Functionality:
    • No direct electrical connection exists between atria and ventricles.
    • A conduction delay of approximately 0.1 seconds at the AV node allows atria to contract and fully empty before the ventricles are stimulated.
    • Acts as a gatekeeper, controlling conduction rate to the ventricles and preventing rapid ventricular stimulation during fast atrial rhythms.
  • Post-AV Node Conduction:
    • The action potential then spreads to the bundle of His and Purkinje fibers, ensuring rapid and synchronized ventricular contraction, optimizing blood ejection from the heart.