L22 Cardiac Action Potentials (see pics too)

Question 1

explain the ionic basis of the cardiac AP recorded from myocytes and from the SA node

Answer 1

Myocytes

• Na+ influx to initiate depolarization
• Ca2+ influx to give a long depolarization i

Question 2

explain how the wave of depolarization spreads throughout the heart

Answer 2

• depolarize atria
• depolarize septum from L to R
• depolarize anteroseptal region of myocardium toward the apex
• depolarize bulk of ventricular myocardium - from endo to epicardium
• depolarize posterior portion of base of L ventricle
• ventricles are now depolarized

Question 3

explain the significance of the delay at the AV node

Answer 3

-allows time for the atria to contract before ventricles contract = more blood can be pushed into ventricles

atrial tachyarrhythmias will not be transmitted to the ventricles - safeguard because AV nodes have high electrical resistance - it slows down signal from SA node

Question 4

explain why the SA node is the pacemaker of the heart

Answer 4

its the fastest pacemaker

Question 5

describe how the parasympathetic nervous system decreases heart rate

Answer 5

ACh causes a slower rate of rise of the pacemaker potential = decrease HR

slower depolarization takes much more time to reach threshold

also sets Vm more hyper polarized

Question 6

describe how the sympathetic nervous system increases heart rate

Answer 6

NE increases the rate of depolarization in phase 4 of SA node AP