CPR Cardiac Action Potentials & Conduction Flashcards
Describe how an action potential spreads in heart
SA node AV node Bundle of His R/L bundle branches Purkinje fibers
What maintains the order of stimulus in the heart?
overdrive suppression of the SA node
Why is the SA node in “control” of pacemaking properties?
the SA node generates action potentials most frequently (followed by AV node & then the Purkinje fibers)
Where is the delay in the action potential?
@ the AV node
Which cells have the highest velocity?
Purkinje cells because have the largest diameter
Describe the order of contraction of the heart
RA contracts before LA
endocardium contracts before epicardium
RV epicardium contracts before LV epicardium
What is unique about the SA node membrane potential?
resting Vm is more positive compared to resting Vm of ventricles & atria
What is phase 4 assoc w/ in the SA node?
open of funny Na channels to cause rise in membrane potential in SA node
What is responsible for phase 2?
combo of inward Ca2+ current, outward K+ current & inward rectifiers (K1)
inward Ca2+ current via L-type Ca2+ channels
What is responsible for phase 1?
transient outward K+ current via K+ channels
What is responsible for transition from phase 2 to phase 3?
voltage gated K+ channels (outward K+ current!)
Does the AP or contraction occur first?
the AP occurs before the contraction
What channels are always open?
K+ leak channels are open all the time
would have increased current outward during phase 2
What provides the rhythmicity of contractions?
SA & AV nodes are primary source
What occurs in phase 4 in SA & AV nodes?
resting Vm gradually depolarizes until reaches threshold & then fires (more slowly in AV node)
due to funny voltage-gated Na+ channels that open upon COMPLETE repolarization of membrane
What occurs in phase 0 in SA node?
opening of voltage gated Ca+ channels (not Na+ channels) & closure of voltage gated K+ channels
What occurs in phase 3 in SA node?
reversal of phase 0 w/ closure of voltage gated Ca+ channels & opening of voltage gated K+ channels
How are the SA & AV nodes controllers of the contracting rhythm of heart?
will naturally depolarize to reach threshold w/o electrical stimulation
stimulation of AV node by SA node will trigger AV node to generate AP before it can make its own
Why are refractory periods longer in cardiac cells?
help to prevent arrhythmias
What is the supranormal period?
cell is more excitable than normal & easier to generate AP (may have abnormal conduction)
How does an AP compare if stimulated in RRP or SNP?
conduction of AP will be weaker
Describe parasympathetic innervation of heart
carried by vagus N to SA & AV nodes
ACh binds to muscarinic receptor (M2/M3)
Parasympathetic effects on AP
negative chronotropic (slower opening of funny Na+ & hyperpolarization of SA node by increasing K+ outflow via special K+ channels)
negative dromotropic (reduced Ca+ inward current & hypopolarizes SA node b/c decreases outward + current)
Describe sympathetic innervation of heart
to SA node, AV node & ventricular myocytes
Norepi binds to B1 adrenergic receptor
Sympathetic effects on AP
positive chronotropic effects (more rapid opening of Na+ channels & hypopolarizes SA node)
positive dromotropic effects (increased inward Ca2+ current & hyperpolarization of SA node)
positive inotropic influence
Sympathetic v Parasympathetic on HR
Sympathetic: hypopolarize & increase inward Na+
Parasympathetic: hyperpolarize & decrease inward Na+
