L5 Cardiac RMP and AP

Question 0

Relationship between EC [K] and RMP

Answer 0

More K depolarizes the RMP and makes APs easier to happen. (Hypokalemia doesn’t really change the RMP because as K decreases, it stops itself from leaving the cell via inward rectification!)

Question 1

What are the electrical steps in conductance in the heart? On an EKG?

Answer 1

Conductance:

1) SA node
2) AV node
3) Bundle of His
4) Septal bundle system
5) Purkinje fibers

EKG:
P = atrial activation (not seen in AP)
QRS = ventricular activation; upstroke of AP
T = ventricular recovery (phase 2-4 of AP)
PR segment = AV nodal conduction time, 120-200 msec
QRS = 70-100 msec
ST = plateau indicating inward anomalous rectification
QT = 250-430 msec

Question 2

Inward Anomalous Rectification

Answer 2

Stops K from leaving the cell when the ECM [K] gets low, or during repolarization. This helps with energy conservation (bc Na/K pump would have to use ATP to get it back in).
Responsible for the ST plateau

Question 3

Why does the actual RMP differ from the Nernst eqn for K+?

Answer 3

The nernst eqn estimates the potential of a membrane if it were to be permeable to just one ion. In the case of heart cells, K is the most permeable and crucial ion.
The RMP differs from what the Nernst eqn would estimate bc there are other ions present in the cell that throw it off.

Question 4

Can the RMP and ∆ in voltage be detected on EKG?

Answer 4

No, because the EKG detects relative changes in voltage between two areas.

Question 5

Fast response channels

Answer 5

Respond to Na first, then respond to Ca for contraction and K for repolarization

Question 6

Slow response channel

Answer 6

Do not have Na channels. Respond to Ca influx

Question 7

Relationship between phases of AP and EKG

Answer 7

Phase 0 = upstroke, QRS
Phase 1 = K (Ito) open and allow brief repol.
Phase 2 = ST plateau
Phase 3/4 = repolarization; T wave

P wave not seen

Question 8

Fast - slow response Transformation

Answer 8

If something blocks Na channels (like Tetradotoxin), then the cell can use the slow Ca channel for its slow depolarization.

The SLOWER conduction can lead to arrhythmias caused by re-entry excitation

Question 9

When are slow response channels activated on an EKG?

Answer 9

AV and SA node use slow response - seen in p wave area but not really seen

Question 10

Does the EKG record electrical activity of tissues with slow response AP?

Answer 10

No! But why…..

not drastic enough of a voltage change?