Electrophysiology of heart

Question 1

1. Describe the concentration gradients of the following for a resting cardiac ventricular cell

Answer 1

EC: Na, Ca, Cl
IC: K

Question 2

2. Describe the electrical gradients of the following for a resting cardiac ventricular cell

Answer 2

Na inwards
K inward
Ca inward
Cl outward

Question 3

3. Describe the resting cell membrane permeabilities of:

Answer 3

Na impermeable
K hi permeable
Ca impermeable
Cl kinda permeable

Question 4

cAP Phase 4

Answer 4

Resting membrane potectial
Stays here until stimulated by stimulus
Associated w/ diastole

Question 5

cAP Phase 0

Answer 5

Rapid depolarization - 90 to +25

Due to opening of fast Na channels, increase membrane conductance and rapid influx of Na

Question 6

cAP Phase 1

Answer 6

Brief small repolarization from +25 to 0
Inactivation of Na channels
MB potential due to K opening so efflux

Question 7

cAP Phase 2

Answer 7

Plateau phase stays at 0
due to balance of influx of Ca and efflux of K
at end of this phase Ca channels CLOSE and K’s remain open

Question 8

cAP Phase 3

Answer 8

Re polarization to - 90
Ca still closed and K still open
more K channels open
Outward + movement K channels close when potential is at -80 to -85

Question 9

What is the effective refractory period; aka: absolute refractory period?

Answer 9

When no further stimulation will not elicit another AP

Cell is still in the inactive state. Na channels cannot reopen yet

Question 10

9. Where in the cardiac AP does the effective refractory period begin and end?

Answer 10

At phase 0 when FAST Na gates open M activation gates opened

Ends when H and M gates reset

Question 11

10. Why can’t a heart undergo tetany?

Answer 11

The long duration of the ERP prevents further contraction until it is relaxed.
Non true tetany……

Question 12

What is the relative refractory period?

Answer 12

Greater than norm stimulate WILL contract AP.

Not all Na reset yet

Question 13

How can an action potential elicited during the relative refractory period cause ventricular fibrillation (R on T phenomenon)?

Answer 13

Can produce a depolarization during repolarization phase.

Cardiac cells begin to contract before they have completely relaxed

Question 14

SA nodal pace maker AP

Answer 14

Almost bell shaped
Driven by slow Ca channels
P4 nodal has + slope, slow depolarization
prepotential
Depolarization occurs at steady rate until threshold is attained.

Question 15

In the SA nodal pacemaker cell, explain the potassium current (, the sodium current and the calcium current and how these account for the pre-potential.

Answer 15

Na current increases gradually as prepotential develops. Brings MB potential closer to threshold
Ca permeability increases, same
K permeability decreases, pumped at same rate and leaks out at slower rate.
Brings MB potential to threshold

Question 16

+ chronotropic agent added

Answer 16

Sympathetic Norepinephrine, Beta 1 SA nodal receptor.
Increases HR
Increases permeability and rate of influx of Na and Ca. Faster to potential. STEEPER SLOPE

Question 17

• chronotropic agent added

Answer 17

ACh, from CN X, acts on muscarinic SA nodal receptor.
Decreases HR
Increases K permeability efflux up.
Less steep slope

Question 18

Explain the role of ectopic foci as “fail-safe” mechanisms.

Answer 18

Outside of SA node can pace heart.
Can take over pacing heart in event of damage to SA node.
SA node, junctional area, bundle of his, perkinje fibers and ventricle

Question 19

Atrial depolarization

Answer 19

tracts faster spread supplies atria.

Delay in AV node for ventricles to begin later and maximizes its contraction

Question 20

HIS bundle depolarization

Answer 20

Only electrical  connect btw atria & ventricles.
can pace independently
Atria paced SA node
Ventricles ectopic foci
if here cause of 1/3 block

Question 21

Ventricular depolarization

Answer 21

To right and left ventricles further to purkinje fibers.

Question 22

Purkinje fibers

Answer 22

large cells specialized for fast conduction
only penetrates subendocardium
Wave from endocardium to epicardium (inside to outside)

Question 23

Describe the average vector for atrial depolarization and the duration of atrial depolarization.

Answer 23

Average vector down and left. Magnitude proportional to voltage of potential
Avg duration .06 to .110 to depolarize. Longer duration can indicate mitral stenosis and/or mitral regurgitation

Question 24

Describe the average vector for ventricular depolarization and the duration of ventricular depolarization.

Answer 24

Average vector down to left. Mag is prop to voltage.

Avg duration 0.08

Question 25

22. Explain why the calcium channel blocker (rather than the fast channel blocker) would be used to reduce the conduction velocity through the AV node.

Answer 25

a. The AV node is a slow channel, so by blocking the calcium channel you are blocking the force of contraction of the heart.