Lecture 09: Mechanisms Underlying Normal Heart Rate Changes (Hayward) Flashcards
sinus tachycardia
normal ECG with fast HR (>150bpm in medium dog)
RR interval
R peak to R peak. Used to determine HR
beta-1 adrenergic receptors on SA node bind what?
Norepinephrine from sympathetic postganglionics
sinus bradycardia
normal ECG with slow HR (<60 bpm)
2 possible causes of sinus bradycardia
Decreased sympathetic or increased parasym. input to SA node
Why does HR normally increase with inspiration?
increased pulmonary stretch afferent input during inspiration which inhibits parasympathetic output
Which ANS dominates HR?
Parasympathetic
Firing rates of pacemaker cells determined by 3 things:
1) rate of phase 4 spontaneous depolarization
2) maximum negative diastolic potential (max. neg. MP reached following repolarization)
3) threshold potential
Most important modulator of SA node
ANS
NE binding to beta-1 receptors –> probability of pacemaker Na and Ca channels to open via 2nd messenger system
increases
increased intracellular cAMP —> Pacemaker Na and Ca channels
make them more likely to open
how does beta-1 stimulation increase cAMP?
By activation of G protein
Increased pacemaker depolarization –> phase 4
makes phase 4 slope steeper
Increased pacemaker depolarization –> time required to reach AP threshold
decreases
activation of muscarinic receptors is linked with what protein?
inhibitory intracellular G protein
Inhibitory intracellular G protein 2 actions:
1) Decreases slops of phase 4 or spontaneous depolarization
2) increases time to threshold potential by increasing K+ conductance
Muscarinic receptor binding –> probabilty Na and Ca channels open?
Decrease
Muscarinic receptor binding –> probability K channels open
increase
Which part of ANS might be linked to 2nd degree AV block?
parasympathetic
Ach –> potential?
Decreases potential
HR is normally determined by:
SA node intrinsic rate
Funny Na+ channels are active in slow-type AP during which phase?
phase 4
