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30 terms

Chapter 20 Physiology (cardiac conduction system)

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What is the cardiac conduction system?
*Group of specialized, highly exciteable cells that give the heart its rhythmicity
What are the parts of the cardiac conduction system?
*SA Node
*AV Node
*AV Bundle (Bundle of His)
*Purkinje Fibers
What is the function of the cardiac conduction system?
*To spontaneously generate and conduct action potentials

*These action potentials in turn trigger action potentials in cardiac muscle, allowing the heart to contract
What is the resting Vm of the cardiac conduction system?
-60 mV
What is threshold for the cardiac conduction system?
-40 mV
What occurs during the prepotential phase of action potential generation in the cardiac conduction system?
1. The HCN channels open

-HCN channels are permeable to both K⁺ and Na⁺
-More Na⁺ enters than K⁺ leaves, so the net result is depolarization

2. Threshold is reached, HCN channels close, Voltage Gated Ca²⁺ channels open

-This marks the end of the prepotential phase
What occurs during the depolarization phase of action potential generation in the cardiac conduction system?
1. Threshold has been reached (-40 mV)

2. HCN channels close

3. An action potential (called a PACEMAKER POTENTIAL) has been generated

4. Voltage Gated Ca²⁺channels open

-Ca²⁺enters, causing depolarization
What occurs during the repolarization phase of action potential generation in the cardiac conduction system?
1. Voltage Gated Ca²⁺channel closes

2. Voltage Gated K⁺ channel opens

-K⁺leaves the cell, causing repolarization
By what type of conduction are action potentials transmitted in the cardiac conduction system?
*Via continuous conduction

*These action potentials spread both to the rest of the cardiac conduction system and cardiac muscle cells of the atria and ventricles
Where is the SA Node located?
*In the wall of the right atrium, medial to the opening of the superior vena cava
Which part of the cardiac conduction system is also known as the "pacemaker of the heart"
SA Node
How many action potentials does the SA Node generate per minute?
~ 70 to 80 action potentials per minute
Where does the SA Node spread action potentials to?
*To the cardiac muscle of the right and left atria via the internodal pathways

*To the AV Node via the internodal pathways
Where is the AV Node located?
*Near the interventricular septum in the lower portion of the right atrium
From where does the AV Node receive action potentials?
SA Node
To where does the AV Node send action potentials?
AV Bundle (Bundle of His)
How long is the action potential delayed at the AV Node?
~110 msec
Why is the action potential delayed at the AV Node?
*Allows the atria to complete its contraction before the ventricle begins to contract

*Without this delay, the atria and ventricles would contract at the same time. Since the ventricle is bigger and stronger than the atrium, contracting at the same time would cause the blood to be forced backward from the ventricle into the atrium
Where is the AV Bundle (Bundle of His) located?
*In the interventricular septum, near the atria

*Divides into right and left bundle branches (one per ventricle)
From where does the AV Bundle (Bundle of His) receive action potentials?
*From the AV Node
To where does the AV Bundle (Bundle of His) send action potentials?
*To the Purkinje fibers
Where are Purkinje fibers located?
*Branch superiorly into ventricular myocardium
From where do Purkinje fibers receive action potentials?
*From the AV Bundle (Bundle of His)
To where do Purkinje fibers send action potentials?
*To the cardiac muscle of the ventricles
What is an ectopic pacemaker?
Term used to describe some part other than the SA Node generating the heart's rhythm
What effect does an ectopic pacemaker have on heart rate?
*Can cause the heart rate to slow down or speed up, depending on what area of the heart is acting as the pacemaker
What is the most common ectopic pacemaker?
AV Node
How does the AV Node acting as the pacemaker effect heart rate?
*Slows the heart rate to ~ 40 - 60 beats/min
How does the Purkinje fibers acting as the pacemaker effect heart rate?
*Slows the heart rate to ~ 25 - 45 beats/min
Why does the entire heart still receive action potentials with an ectopic pacemaker?
*Since the myocardium is a syncitium, all parts of it will receive action potentials, regardless of where in the syncitium those action potentials are generated

*In the case of an ectopic pacemaker, the action potentials do reach all of the myocardium but not efficiently, which is why heart rate is effected.