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
