DSA Cardiac Electrical Properties

Question 1

how can the heart modulate pacemaker activity

Answer 1

Slower depolarization requires more time to reach threshold

starting from a more negative Vm requires more time to reach threshold

positive shift in threshold
-reaching a more positive threshold requires more time

Question 2

what are the affects of Acetylcholine on the SA and AV nodes

parasympathetic - vagus

Answer 2

SA node
decreases funny current in the SA node, reducing the steepness of the phase 4 depolarization

opens GIRK channels, increasing relative K conductance and making the maxiumum diastolic potential of SA nodeal cells more negative

reduces ca current in the SA node thereby reudcing the steepness of the phase 4 depolarization and moving threshold to more positive values

LOWER HEART RATE

AV node–> slow conduction velocity by inhibition of Ca current that also makes the threshold more positive for AV nodal cells
-making it more difficult to depolarize its neighbors

Question 3

what are the effects of catecholamines
on Nodal cells (2)

sympathetic

Answer 3

increase heart rate***

NOdal cells
1) increase funny currrent in nodal cells, increasing steepness of the phase 4 depolarization

2) increase ca current in all myocardial cells –> steepens the phase 4 depolarization and makes threshold more negative

Question 4

what are the effects of catecholamines on atria and ventricles

sympathetic

Answer 4

atrial and ventricular cells:
increase strength of contraction
***

1) increase in Ca influx leads to a greater local increase in Ca and Ca Ca induced Ca release from SR
2) increase the sensitivity of the SR Ca release channel to cytoplasmic Ca
3) enhance Ca pumping into the SR by stimulation of the SERCA (increasing stores for later use)
4) increased Ca current presents more Ca to SERCA so that SR Ca stores incrrease over time