Cardiac Electrophysiology

Question 1

Non-pacemaker cells have a resting potential of around ______.

Answer 1

-90mV

Question 2

Pacemaker cells have a resting potential of _____ and are located in the ______ and ______ nodes. Their threshold potential is ______.

Answer 2

Resting is -60mV, located in the SA and AV nodes. Threshold is -40mV.

Question 3

Which node is the primary pacemaker?

Answer 3

SA

Question 4

Describe the three stages of the Phase 4 depolarization of the pacemaker cells.

Answer 4

1. K+ channels close to decrease outward K+ current (IK+) and has the effect of depolarizing the membrane.
2. Funny current (Na+ influx) from opening of monovalent cation channels causes further depolarization.
3. Ca2+ channels activate in the late part of Phase 4.

Once threshold is reached, fast L-type Ca2+ channels open to produce the AP.

Question 5

The SA node depolarizes ______ times per minute.
AV node depolarizes _______ times per minute.
His/purkinje fibers depolarize ______ times per minute.

Answer 5

SA:60-100
AV: 40
His/Purkinje: 20

Question 6

Parasympathetic stimulation of the heart occurs through the _____ _____.

Answer 6

vagus nerve

Question 7

What four effects does parasympathetic stimulation of the heart have?

Answer 7

1. Lowers resting potential.
2. Causes K+ channels to close more slowly - prolonged outward flow of K+ keeps the membrane polarized.
3. Funny current (inward Na+) is decreased.
4. Late Phase 4 calcium influx is decreased.

Question 8

What effect does sympathetic stimulation of the heart have?

Answer 8

Faster HR and stronger contraction. Phase 4 is shortened by reducing the threshold and increasing Calcium and funny current influx.

Question 9

What is a Fast Response myocardial cell?

Answer 9

Non-pacemaker myocardial cells.

Question 10

What are the five phases of action potential generation in a Fast Response cell?

Answer 10

Phase 0: Excitement by adjacent cells, rapid influx of Na+.
Phase 1: Inactivation of Na+ channels and opening of K+ channels.
Phase 2: Plateau phase from inward Ca2+ current through L-type (long-lasting) voltage-gated channels. Em remains close to 0 due to continued K+ efflux.
Phase 3: L-type channels close, outflow of K+ returns Em to resting.
Phase 4: Resting potential is maintained by resting membrane channels that allow for small fluxes of Na+ and K+.

Question 11

What does the absolute refractory period aka the effective refractory period protect against?

Answer 11

Protects against severe tachycardia and failure of the heart to refill.

Question 12

How long does the absolute refractory period aka the effective refractory period last?

Answer 12

100-200ms (beginning of phase 0 to -50mV repolarization)

Question 13

In which part of the heart is the absolute refractory period aka the effective refractory period shortest?

Answer 13

Atrial myocytes

Question 14

When does the relative refractory period of myocytes occur?

Answer 14

From -50mV repolarization to several milliseconds following full repolarization

Question 15

What does the AV node do to electrical conduction velocity?

Answer 15

Slows it to ~1/10th of the original speed

Question 16

State the electrical depolarization pathway of the heart.

Answer 16

SA node, atrial depolarization –> AV node –> Bundle of His –> Right & Left bundle branches –> Purkinje fibers –> cell-cell conduction up to the base and out to the epicardial surface.

Question 17

Describe the repolarization pathway of the ventricles.

Answer 17

The cells that were the last to depolarize are the first to repolarize, beginning with the epicardial myocytes near the base of the ventricles.

Question 18

Electrocardiography measures electrical potentials on the ______ of cells.

Answer 18

outside

Question 19

State the four rules of dipoles.

Answer 19

1. There is no signal when the strip is isoelectric.
2. When a dipole vector indicating depolarization points towards the + electrode, the signal is positive.
3. When a dipole vector indicating repolarization points towards the + electrode, the signal is negative.
4. Electrodes placed perpendicularly to a dipole vector show no signal.

Question 20

Which electrical conduction structure in the heart has the longest action potential duration?

Answer 20

Bundle branches in the interventricular setpum