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N5315 Advanced Pathophysiology
Cardiac Action Potentials, Contraction and EKG
Conduction System
Electrical impulses are usually generated from the SA node, located in the right atrium. The SA
node generates impulses of around 60-100 action potentials per minute, which translates to 60-
100 heartbeats per minute. The impulse generated from the SA node is transmitted to the AV
node. The atrial node is located in the right atrium wall just above the tricuspid valve. It mediates
how fast impulses are transmitted to the ventricles. If the SA node fails, then the AV node will
generate an impulse. It can generate 40-60 action potentials per minute which translates to 40-60
beats per minute. The AV node will pass the impulse down the conduction system to the bundle
of His. If the AV node fails, the bundle of His (right and left branches) will generate an action
potential at less than 40 beats per minute. The impulse will then travel to the purkinje fibers
which travel up the wall of the ventricles and cause contraction of the ventricles.
Cardiac Action Potentials and Contraction
To understand cardiac potentials, it is important for you to review the action potential lecture
from earlier in the class. I recommend you review that first and then come back and review how
action potentials occur in the heart. Action potentials differ between those generated in the
bundle of His and purkinje fibers from those generated in the SA or AV nodes. Remember the
purpose of an action potential is to transmit an impulse and in the case of the heart to cause
cardiac contraction.
Ventricular action potentials are generated by the bundle of His or the purkinje fibers. They
occur in five phases labeled phase zero through phase four. These cardiac cells start with a
resting membrane potential of approximately -85mV. The phases below are slightly different
than what is in your textbook and I want you to know what is in this lecture.
• Phase 0 represents a rapid depolarization of the cell. Na+ influx occurs as a result of the
voltage gated Na+ channels opening.
• Phase 1 represents initial repolarization of the cells. The voltage gated Na+ channels are
closed and the voltage gated K+ channels begin to open and K+ begins to leave the cell,
slowly.
• Phase 2 is known as a plateau phase. The voltage gated Ca+ channels open which allows
for the influx of Ca+ into the cell. This influx balances out the potassium efflux thus
causing a temporary plateau in the repolarization. The influx of Ca+ triggers the release of
more calcium from the sarcoplasmic reticulum and thus causes myocardial contraction.
• Phase 3 results in a rapid repolarization with massive K+ efflux. The voltage gated K+
channels open and the voltage gated Ca+ channels close which causes the rapid
repolarization.
• Phase 4 represents the resting membrane potential of -85mV. There is high potassium
permeability from the potassium channels.
N5315 Advanced Pathophysiology
Cardiac Action Potentials, Contraction and EKG
Conduction System
Electrical impulses are usually generated from the SA node, located in the right atrium. The SA
node generates impulses of around 60-100 action potentials per minute, which translates to 60-
100 heartbeats per minute. The impulse generated from the SA node is transmitted to the AV
node. The atrial node is located in the right atrium wall just above the tricuspid valve. It mediates
how fast impulses are transmitted to the ventricles. If the SA node fails, then the AV node will
generate an impulse. It can generate 40-60 action potentials per minute which translates to 40-60
beats per minute. The AV node will pass the impulse down the conduction system to the bundle
of His. If the AV node fails, the bundle of His (right and left branches) will generate an action
potential at less than 40 beats per minute. The impulse will then travel to the purkinje fibers
which travel up the wall of the ventricles and cause contraction of the ventricles.
Cardiac Action Potentials and Contraction
To understand cardiac potentials, it is important for you to review the action potential lecture
from earlier in the class. I recommend you review that first and then come back and review how
action potentials occur in the heart. Action potentials differ between those generated in the
bundle of His and purkinje fibers from those generated in the SA or AV nodes. Remember the
purpose of an action potential is to transmit an impulse and in the case of the heart to cause
cardiac contraction.
Ventricular action potentials are generated by the bundle of His or the purkinje fibers. They
occur in five phases labeled phase zero through phase four. These cardiac cells start with a
resting membrane potential of approximately -85mV. The phases below are slightly different
than what is in your textbook and I want you to know what is in this lecture.
• Phase 0 represents a rapid depolarization of the cell. Na+ influx occurs as a result of the
voltage gated Na+ channels opening.
• Phase 1 represents initial repolarization of the cells. The voltage gated Na+ channels are
closed and the voltage gated K+ channels begin to open and K+ begins to leave the cell,
slowly.
• Phase 2 is known as a plateau phase. The voltage gated Ca+ channels open which allows
for the influx of Ca+ into the cell. This influx balances out the potassium efflux thus
causing a temporary plateau in the repolarization. The influx of Ca+ triggers the release of
more calcium from the sarcoplasmic reticulum and thus causes myocardial contraction.
• Phase 3 results in a rapid repolarization with massive K+ efflux. The voltage gated K+
channels open and the voltage gated Ca+ channels close which causes the rapid
repolarization.
• Phase 4 represents the resting membrane potential of -85mV. There is high potassium
permeability from the potassium channels.
