Week 3 - Cardiac Conduction and Electrophysiology

Electrical Conduction System of the Heart

Electrical Conduction of the Heart
  •  Sinoatrial (SA) node
    • Located in the right atrium near where blood enters via the inferior vena cava
    • "Pacemaker of the heart" - depolarizes regularly, which leads to action potentials
      • Action potentials from the cells of the SA node result in a wave of depolarization across the atria
      • Depolarization results in the simultaneous contraction of the two atria
      • Depolarizations travel through atria to the atrioventricular node
  • Atrioventricular (AV) node
    • Located at the base of the right atrium
    • Receives action potentials from the atria
      • These action potentials are delayed for about 0.1 seconds after being received
      • After monmentarily delaying the propogation of action potentials, they are passed to the bundle of Hib fibers
  • Bundle of His
    • Located in the interventricular septum
    • Only path of electrical connection between atria and ventricles
    • Receives action potential from AV node and the propogates it down through the interventricular septum to the left and right bundle branches
  • Left and right bundle branches
    • Radiate from the apex of the heart towards the left and right ventricles, respectively
    • Receive action potential propogated by the bundle of His and and then in turn connect to the Purkinje fibers.
  • Purkinje fibers
    • Located of each of the ventricles
    • Receive input from the left and right bundle branches
    • Purkinje fibers cause the cells of the ventricles to rapidly depolarize
      • This rapid depolarization results in the simultaneous contraction of the ventricles
      • During contraction, ventricles contract from the apex towards the top of the heart to ensure blood is pumped out of the heart as efficiently as possible
Basic Electrophysiology of the Heart
  • Resting Membrane Potential (RMP)
    • Voltage across a membrane in the absence of a stimulus
    • Determined by relative permeabilities of ions and concentration gradient across the membrane
      • Na+/K+-ATPase maintains RMP by establishing a high intracellular concentration of K+ and a high extracellular concentration of Na+
      • Because the membrane is more permeable to K+ than to Na+ RMP is closer to the equilibrium potential of K+ than that of Na+
  • Action Potential (AP) - Rapid change from RMP used to transmit information along a membrane.
    • Occurs via a rapid depolarization of the membrane, followed by depolarization and reinstatement of RMP.
    • Mechanism by which AP is fired is specific to the type of cell
  • Cardiac Muscle Cell AP
    • AP propagation is characterized by a rapid depolarization followed by a plateau phase where the membrane
      potential levels of near 0 mV. After the plateau phase, repolarization occurs.
    • AP begins with rapid opening of voltage-gated Na+ channels and the inactivation of leaky K+ channels.
    • Shortly after opening voltage-gated channels close; however, repolarization does not occur immediately
      • Leaky K+ channels remain closed momentarily
      • Ca+2 channels open allowing an influx of Ca+2
    • Myocardial cells contain L-type Ca+2 channels, these voltage-gated channels open much more slowly than Na+ channels as a result of depolarization
    • Ca+2 channels balance out K+ efflux via milady open leaky channels resulting in the plateau phase
    • Eventually L-type Ca+2 channels close and voltage-gated K+ open resulting in rapid repolaization, a period of hyperpolarization, and eventually a return to RMP
  • Cardiac Autorhythmic Cell AP
    • Nodal cells have a slow depolarization that occurs until threshold is reached. Once threshold is reached, an AP is fired but its amplitude is smaller than the one associated with the depolarization of a cardiac muscle cell.
    • Nodal cells contain F-type channels, which are nonspecific voltage-gated channels that open due to a negative
      membrane potential. F-type channels primarily allow an influx of Na+ when opened
    • Cardiac nodal cells also contain T-type Ca+2 channels that periodically open momentarily, providing bursts of depolarization, which can help the cell reach threshold.
    • The combination of the nonspecific F-type channels and the closing of K+ channels results in a steady depolarization when membrane potential is negative, these channels paired with the effects of T-type Ca+2 channels allow recurring depolarization following repolarization and therefore facilitate the pacemaker potential
    • Once threshold is reached, an AP is fired; however, the AP is the result of an influx of Ca+2 through L-type channels rather than Na+ resulting in a slower depolarization
    • Repolarization is achieved as normal via the opening of K+ channels
    • This type of pacemaker ability is present in all of the heart's conducting system; however, it happens most rapidly in the SA node, which is why the SA node sets the heart rate
Electrocardiogram (ECG/EKG)
  • The EKG is a tool that is used to monitor the electrical activity of the heart
  • Measures dipoles that result from the simultaneous depolarization and repolarization of many myocardial cells, which manifests itself as current that can be detected by electrodes on the surface of the skin
  • P wave - results from atrial depolarization
  • QRS wave - results from ventricular depolarization. In addition, atrial repolarization occurs during this time; however its obscured due to the much larger signal caused by ventricular depolarization
  • T wave - results from ventricular repolarization