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LECTURE: Electrophysiology of the Heart

Overview

Myocytes (+ connective tissue) => myofibrils => 2 atria + 2 ventricles
Myocytes: made up of actin-myosin complexes with troponin-tropomyosin complex; semipermeable membrane
Myofibrils: contract (shortens) & expand (lengthens)

Electrical Potential

Semipermeable membrane => different intra- & extra-cellular ion concentrations
Intracellular: greater [K]
Extracellular: greater [Na] & [Ca]
More positive extracellular charge => relatively negative intracellular charge

Difference in electrical charge = electrical potential

Semipermeable membrane allows for passive transport of ions

Ions to move down their concentration gradient and therefore, requires no ATP
K moves out of cell, Na moves in cell

Na-K ATPase in membrane:

Maintains electrical potential (-70 to -90 mV in resting myocyte)
Active transport: moves ions against their concentration gradient and therefore, requires ATP
3 Na move out of cell, 2 Na move in cell

Myocyte Action Potential & Contraction

Net Na influx (phase 4) => threshold reached => fast-Na channels open (rapid depolarization, phase 0) => Cl influx slows Na influx => fast-Na channels close (max positive charge, phase 1) => slow-Na & Ca channels open (phase 2, plateau phase; Na & Ca influx) => Ca binds troponin/tropomyosin complex => actin & myosin move along each other => contraction => K channels open (K efflux, rapid repolarization, phase 3) => resting potential

Note: these are not the action potential phases that occur at the sinus node

Phase 4 differs along conduction system due to different threshold potentials for myocytes

From lowest to highest threshold potential: SA nodal cells (maintain normal pacemaking function of heart) => AV nodal cells => bundle cells => Purkinje cells => ventricular myocytes
Lowest threshold potential => fastest intrinsic rate
Reason for slowing intrinsic rate along conduction system

Sum of electrical discharges => large electrical current => mean electrical axis

Abbreviations: K, potassium; Na, sodium; Ca, calcium; Cl, cholride

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