Electrocardiography Flashcards
Pacemaker potential
1) Phase 4 depolarization: membrane potential slowly rises to potential
2) Depolarization: action potential begins when threshold is reached
3) Repolarization to resting potential
phase 4 depolarization – ionic basis
[membrane potential slowly rises to potential]
1) decrease in outward flux of K+, I(k)
2) I(f), funny current: inward slow flux of Na+, thru nonspecific channels
3) gradual influx of Ca++ as threshold is approached I(Ca)
Depolarization (pacemaker potential) – ionic basis
Opening of voltage-gated L-type Ca++ channels
Repolarization (pacemaker potential) – ionic basis
Opening of voltage-gated K+ channels
hierarchy of pacemakers
1) SA node (60-100 bpm)
2) AV node (40-50 bpm)
3) His-Purkinje Fibers (20 bpm)
parasympathetic control of heart rate
Vagus nerve releases ACh to SA node.
Voltage gated K+ channels close more slowly. Hyperpolarize resting potential.
Decreased funny current.
Reduce slope of phase 4 depolarization to slow heart rate.
sympathetic control of heart rate
Cardiac plexus releases norepi to SA/AV nodes and myocardium.
Increases I(f) and I(Ca) in all myocardial cells.
Alters thrshold toward more negative voltage.
Increases heart rate and increases force of contraction (positive ionotropic effect).
non-pacemaker cell action potentials
resting potential: -90 mV
0) rapid depolarization
1) slight repolarization
2) plateau phase
3) repolarization
4) maintenance of resting potential
phase 0 of non-pacemaker cell AP
Rapid influx of Na+ through voltage gated channels.
Rapid depolarization.
phase 1 of non-pacemaker cell AP
Inactivation of Na+ channels.
Opening of outward K+ channel.
Starts depolarization.
phase 2 of non-pacemaker cell AP
Plateau phase.
Inward flux of Ca++ (L-type voltage gated channels).
Slow outward flux of K+.
Balance each other, stay close to 0 mV.
phase 3 of non-pacemaker cell AP
Repolarization.
Ca++ channels close.
Potassium efflux.
Membrane returns to resting potential.
phase 4 of non-pacemaker cell AP
Small fluxes of Na+ and K+ maintain resting potential.
Effective Refractory Period (ERP)
Phase 0-3 of non-pacemaker cell AP.
New AP cannot be elicited.
Allows cell to pump out blood and refill before the next beat.
Sequence of heart activation
1) SA node fires AP
2) Signal travels thru atrial muscle via gap junctions
3) Signal reaches AV node (tiny fibers –> slow velocity –> delays signal)
4) Signal enters bundle of His, then R/L bundle branches, then Purkinje fibers
5) Signal spreads cell to cell via gap junctions
septum –> apex –> base
endocardium –> epicardium