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Elmoselhi - Electrophysio
Cardiac Electrophysiology
20
Physiology
Professional
09/24/2008

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Cards

Term
What are the four phases of the ventricular action potential?
Definition

Phase 0: depolarization

 

Phase 1 = partial repolarization

 

Phase 2 = plateau

 

Phase 3 = repolarization

 

Phase 4 = Resting membrane potential

Term
What occurs during phase 0 of the ventricular AP?
Definition
Phase 0:
Fast voltage-gated Na+ channels open --> Na+ influx
Term
What occurs during phase 1 of the ventricular AP?
Definition
Phase 1:
Partial repolarization due to closing of Na+ channels and opening of K+ channels
Term
What occurs during phase 2 of the ventricular AP?
Definition

Plateau phase

 

Slow voltage-gated Ca++ channels (L-type) open -> Ca++ influx

 

Voltage-gated K+ channels close -> reduce K+ efflux, however ungated K+ channels still open

 

Inward Ca++ current is balanced by outward K+

 

current Ca++ influx -> contraction + release Ca++ from SR

Term
What occurs during phase 3 of the ventricular AP?
Definition

Repolarization

 

Ca++ channels close -> stop Ca++ influx

 

Voltage-gated K+ open -> K+ efflux

Term
What occurs during phase 4 of the ventricular AP?
Definition
Phase 4: Stable resting membrane potential due to high K+ permeability
Term
What are the different phases of the SA node AP?
Definition

Phase 0 = Upstroke phase

 

Phase 1 & 2 = NOT present

 

Phase 3 = repolarization

 

Phase 4 = spontaneous depolarization or pacemaker potential

Term
What occurs during phase 0 of the SA node AP?
Definition

Depolarization spike

 

Phase 0: Slow voltage-gated Ca++ channels (T-type) open -> Ca++ influx

Term
What occurs during phase 3 of the SA node AP?
Definition

Repolarization

 

Phase 3: Voltage-gated K+ open -> K+ efflux

Term
What occurs during phase 4 of the SA node AP?
Definition

Spontaneous depolarization

 

"Specific" Na+ channels open -> inward Na+ current If -> increase excitability -> once reach threshold action potential is generated

 

Other mechanisms:

Decrease K+ conductance

 

Rate of phase 4 depolarization set the rate of the heart

Term
Explain transmission of the cardiac impulse from the SA node to the atria
Definition

SA node is pacemaker

 

Rapid velocity in most of the artia 0.3 m/sec

 

More rapid in specific area (e.g. internodal pathways) 1 m/sec

Term
Explain transmission of the cardiac impulse from the atria to the ventricles
Definition

From Atria to ventricles

 

AV nodal and bundle delay = 0.13 second

 

Very rapid transmission in purkinje fibers (1.5 - 4 m/sec)

 

Direction of cardiac impulse transmission

*AV node -> AV bundle -> right and left bundle branches -> purkinje fibers -> ventricular fibers (from endocardium to the epicardium)

 

Signal conduction in AV bundle is one-way direction

Term
Explain the AV nodal and bundle delay
Definition

In transmission of the cardiac impulse from the SA node all the way to the ventricles there is an AV nodal and bundle delay of 0.13 second

 

Significance: allows ventricular filling, the atria contract before the ventricles so the ventricles will be full when they contract. doesn't make sense to contract atria and ventricles at the same exact time

 

Mechanism: small number of gap junctions

 

Fibrous tissues between atria and ventricle acts as insulator - slows down transmission

Term
Explain the differing velocities of cardiac impulse transmission through the heart
Definition

DON'T HAVE TO MEMORIZE #'s

 

From SA to AV node = 0.03 sec

 

AV node and bundle delay = 0.13 sec

 

From AV bundle to end of purkinje fibers = 0.03 sec

 

From endocardium (end of purkinje fibers) to epicardium (ventricular muscle fibers) = 0.03 sec

 

* Understand that the time of cardiac transmission is pretty constant except for the AV node and bundle delay

 

Fastest conducting fiber = Purkinje fiber (1.5 - 4 m/sec) - ventricles are bigger so need to conduct faster to keep up

 

Slowest conducting fiber = AV node (0.01 - 0.05 m/sec) - slow to cause the AV nodal and bundal delay

Term
What are the intrinsic firing rates of various parts of the heart?
Definition

Normal pacemaker and intrinsic firing rate of various part of the conductive system:

 

SA node - 100-110 per minute

 

AV node - 40-60 per minute

 

Bundle of His - 40 per minute

 

Purkinje fibers - 15-20 per minute

 

SA node is the normal pacemaker of the heart because it has the highest intrinsic firing rate

Term
Explain the normal sinus rhythm
Definition

1- Action potential originate in SA node

 

2- Regular SA node impulse at 60-100 impulses/min

 

3- Correct sequence and timing for the heart activation

Term
What are the effects of parasympathetic stimulation of the heart?
Definition

1) decrease frequency of SA action potential -> decrease heart rate (negative chronotropic effect)

 

2) decrease AV conduction velocity (negative dromotropic effect)

Term
What are the mechanisms of parasympathetic effects of the heart?
Definition

Ach released at vagal nerve ending -> stimulation of muscarinic (M2) receptors

 

1- decrease Na+ channels If current via Gi pathway

 

2- increase membrane permeability for K+ which leads to K+ efflux -> hyperpolarization of sinus nodal fibers -> more difficult to fire action potential

Term
What are the effects of sympathetic stimulation on the heart?
Definition

1) increase frequency of SA action potential -> increase heart rate (positive chronotropic effect)

 

2) increase AV conduction velocity (positive dromotropic effect)

 

3) increase strength of cardiac contractility (positive inotropic)

Term
What are the mechanisms of sympathetic stimulation on the heart?
Definition

Norepinephrine at sympathetic nerve ending -> stimulation of beta 1 receptors

 

increase Na+ channels If current (also increase Ca2+ influx was proposed) via Gs protein pathway -> more positive resting membrane potential -> easy to fire action potential

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