Cardiac Pacemaker Mechanisms

Question 1

What is the hierarchy of pacemaker activity? (list it)

Answer 1

SA node > latent atrial pacemakers > AV nodal/His bundle (junctional) > bundle branches > Purkinje fibers

Question 2

The pacemaker activity of the SA node comes from ________.

Answer 2

diastolic depolarization

Question 3

What are the 4 main properties that underlie the SA node’s pacemaker activity?

Answer 3

1) T-type calcium current
2) Hyperpolarization-activated inward current (aka, funny current, If)
3) Deactivation of potassium current (Ik) = inward rectification
4) Inward Na/Ca exchange current activated by intracellular SR calcium release

Question 4

How does inspiration affect the cardiac cycle?

Answer 4

It causes a decrease in cycle length (increase in HR) by inhibiting parasympathetic nerve activity. Stretch receptors in the lungs feed back and decrease vagal tone (which increases HR).

Question 5

How does expiration affect the cardiac cycle?

Answer 5

It causes an increase in cycle length (decrease in HR) by stimulating parasympathetic nerve activity.

Question 6

What is one of the major benefits of exercise?

Answer 6

It increases vagal tone, which is anti-arrhythmic.

Question 7

Do parasympathetics directly control contractility?

Answer 7

no; sympathetics do

Question 8

What causes sinus arrhythmias?

Answer 8

primarily respiratory changes in parasympathetic (vagal) nerve activity to the SA node (not sympathetic; all through vagal modulation)

Question 9

What are the mechanisms responsible for changes in heart rate?

Answer 9

• change in slope of diastolic depolarization
• change in maximum diastolic potential
• change in threshold (by phosphorylating channel proteins)
• pacemaker shifts (changes in site)

Question 10

Do all pacemakers have all 4 pacemaker mechanisms?

Answer 10

no, but all 4 are important to the SA node

Question 11

What is the difference between the T-type and L-type calcium channels?

Answer 11

T-type responsible for diastolic depolarization of SA node and L-type responsible for plateau of cardiac action potential (both are slow)

Question 12

Explain what the funny current is.

Answer 12

It is known as the pacemaker channel, and it is called “funny” because it is activated by hyperpolarizaton instead of depolarization. When the cell repolarizes, it turns on this channel, leaks some Na+ in, and contributes to the diastolic depolarization.

Question 13

What can change the slope of the diastolic depolarization?

Answer 13

• Ex: ACh decreases the slope, causing the cell to take a longer period of time to reach threshold
• Ex: Increase in funny current increases the slope, causing the cell to take less time to reach threshold (results in more APs)

Question 14

When a pacemaker is stimulated/driven at a frequency higher than its intrinsic frequency, stopping the stimulation results in a temporary suppression of pacemaker activity (takes a few seconds before another pacemaker kicks in).

Answer 14

overdrive suppression

Question 15

What are some of the clinical applications of overdrive suppression?

Answer 15

• SA or AV nodal block
• stopping artificial pacemakers
• sick sinus syndrome

Question 16

Vagal nerve stimulation (ACh) primarily does what?

Answer 16

inhibits pacemaker cells within the SA node, atria, and AV nodal regions

Question 17

Vagal nerve stimulation (ACh) increases permeability to which ion?

Answer 17

K+; this increases the maximum diastolic potential, making it more negative and, thus, harder for the pacemaker cell to reach potential

Question 18

What is the most likely reason we need ectopic pacemaker activity?

Answer 18

it may help with anticipation of depolarization (all pacemakers gradually drift toward threshold; only takes a little bit to depolarize a pacemaker cell the rest of the way when the wave comes from the SA node through the His-Purkinje system)

Question 19

Sympathetic nerves stimulate which cardiac pacemakers?

Answer 19

all!

Question 20

How does NE affect pacemaker activity?

Answer 20

it increases cAMP-dependent slow inward L-type Ca2+ current, increasing conduction through the AV node; it also increases the funny current, causing an increase in the slope of diastolic depolarization (net effect=increased HR!)