Cardiac Action Potentials (E2)

Question 1

What is different about action potentials in pacemaker cells?

Answer 1

There is no sharp upstroke because there are no voltage gated sodium channels.

Question 2

What causes action potentials in pacemaker cells? Differentiate this from ventricular cells.

Answer 2

Calcium entry; AP is shorter than ventricular AP

Calcium channel stays open for a shorter amount of time than its ventricular counterpart.

Question 3

What causes the pacemaker cell to spontaneously depolarize?

Answer 3

1. Spontaneous closing of K channels during period between APs.
2. Spontaneous opening of Na channels during rest and closure during AP.

Question 4

Describe the Neural Input into cardiac pacemaker cells.

Answer 4

Ne or Epi activate B1 receptors on pacemaker cells, increasing heart rate.
Ach activates muscarinic receptors, decreasing heart rate.

Question 5

What are the four main characteristics of the AV node?

Answer 5

1. Only conducting pathway between atria and ventricles; Av node-Av bundle-Bundle branches
2. Slow conduction velocity (AV delay)
3. Long refractory period
4. Slow spontaneous depolarization to create AP.

Question 6

Why does the AV node have a longer refractory period than the SA node?

Answer 6

1. Prevents ventricles from over stimulation (Sinus tachycardia, A-fib)
2. Prevents backward conduction

Question 7

Why does the AV node spontaneously depolarize?

Answer 7

Acts as pacemaker if the SA node fails or if there is a AV node block. (40-50 bpm)

Question 8

Do the ventricles receive much parasympathetic innervation?

Answer 8

NO