13. APs and Conduction Flashcards
Describe the conduction system of the heart
SA node (60 ms) → AV node (100 ms) → Bundle of His (10 ms) → R/L Bundle Branches → Purkinje Fibers
what is the purpose of slow conduction through the AV node
slow conduction to allow time for ventricles to fill
increasing conduction velocity → decreased CO and SV
what physiologically makes the SA node the pacemaker of the cell
fastest rate of phase 4 depolarization, shortest AP duration and refractory period)
which parts have the greatest conduction velocity
larger diameter fibers are faster
purkinje > atrial and ventricular muscles > AV node
what is the purpose of overdrive suppression
to prevent the latent pacemakers (AV node, bundle etc) from driving HR
APs of ventricles, atria, and purkinjes: phase 0
- what is it
- what ions are responsible
- why doesnt the membrane potential reach the ion potential
- membrane potential at peak
- upstroke, rapid depolarization
- increase in Na conductance → inward Na current
- inactivation gate closing prevents membrane potential from reaching Na potential
- peak of upstroke, membrane potential = 20 mv
APs of ventricles, atria, and purkinjes: phase 1
- what is it
- what ions are responsible
- initial repolarization
- inactivation gates of Na channels close, decreasing inward Na current
- outward K current
APs of ventricles, atria, and purkinjes: phase 2
- what is it
- what ions are responsible
- plateau, long period of relatively stable depolarized membrane potential
- increased Ca conductance → inward Ca current
- outward K current balances Ca current
APs of ventricles, atria, and purkinjes: phase 3
- what is it
- what ions are responsible
- repolarization
- decrease in Ca conductance → decrease inward Ca current
- increase in K conductance → increase outward K current
APs of ventricles, atria, and purkinjes: phase 4
- what is it
- what ions are responsible
- resting membrane potential (-85 mv)
- outward K current balanced by inward Na and C currents
which ion current is known as the inward rectifier
I K1
close in response to depolarization, slow to close and remain open for a determined amount of time, then opens to help with phase 3
APs of SA Node: phase 0
- what is it
- what ions are responsible
- upstroke
- increased Ca conductance → inward Ca current
APs of SA Node: phase 3
- what is it
- what ions are responsible
- repolarization
- increased in K conductance → outward K current
APs of SA Node: phase 4
- what is it
- what ions are responsible
- spontaneous depolarization or pacemaker potential
- funny Na channels → inward Na current
describe absolute refractory period
- no APs can be generated
- most of Na channels are closed
describe relative refractory period
- APs can be generated but requires greater stimulation
- Na channels start to be available to carry inward currents
- second AP will have abnormal configuration, shortened plateau phase, and will be weaker
describe supranormal period
- cell is more excitable than normal and easier to generate an AP, may have abnormal conduction and AP will be weaker
- requires less inward current to depolarize cell
- Na channels are recovered (inactivations gates open again)
what is a +/- chronotropic effect?
- change in HR, slope of depolarization in phase 4 of SA node
+ : increased HR by increasing phase 4 depolarization, increase If (S)
- : decreased HR by decreasing phase 4 depolarization, decrease If (PS)
what is a +/- dromotropic effect?
- change in conduction velocity, slope of phase 0
+ : increased velocity, increased inward Ca current (S)
- : decreased velocity, decrease inward Ca current, increased outward K current (PS)
what effect would a +/- dromotropic effect have on the PR interval
+ : decreased PR interval
- : increased PR interval
what is an ionotropic effect? lusitropic?
Ionotropic- effects strength of muscular contraction
Lusitropic- effects rate of muscular relaxation
what effect does the parasympathetic system have on HR
- chronotropic effect: slower opening of funny Na channels during phase 4
- dromotropic effect: reduced inward Ca current, increase outward K current via special K-ach
PS: vagus nerve, ach → muscarinic (M2/M3)
innervates SA and AV node
no ionotropic or lusitropic influence
what effect does the sympathetic system have on HR
+ chronotropic effect: more rapid opening of funny Na channels during phase 4
(beta agonist meds increase rate of phase 4 depolarization by increasing If, beta antagonists decrease)
+ dromotropic effect: increased inward Ca current
S: NE → B1
innervates SA node, AV node, ventricular myocytes (not atrial)
positive ionotropic or lusitropic influence
what is the key difference in phase 4 of AP found in SA/AV node vs myocytes
- rmp gradually depolarizes until it reaches threshold
- d/t funny Na channels that open upon complete repolarization of membrane
what is the key difference in phase 0 of AP found in SA/AV node vs myocytes
- opening of voltage gated Ca channels rather than voltage gates Na channels
- closure of voltage gated K channels (rectifies)
what is the key difference in phase 3 of AP found in SA/AV node vs myocytes
- reversal of phase 0
- closure of voltage gated Ca channels and opening of voltage gated K channels (both rectifiers and traditional)
which phase of the AP corresponds with the QRS complex?
T wave?
ST segment?
QRS: phase 0; rapid depolarization, influx of Na
T: phase 1/3, repolarization, efflux of K
ST: phase 2, plateau, influx of Ca = efflux of K
