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Flashcards in Normal Cardiac Electrophysiology Deck (45)
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How does hypercalcemia decreases muscle weakness

Muscles get less excitable


What happens when there are more Ca ions outside the cell

There are more divalent cations outside in the cell that changes the preception of resting potential as seen by the Na channels. Ca ions do not affect the resting potential, they only increase the threshold potential to more positive - the field effect


Path of wave of excitation in the heart

1. Starts at SA Node

2. Travels along the atrial myocytes

3. Goes to the AV Node

4. Bundle of his (above the median septum)

5. Purkinje fibers

6. Ventricular myocytes



Velocities are high in bundle of his and the purkinje node, very slow in the AV node


What are the different kinds of action potential that drive the rythm of the heart

In the SA and AV node it is calcium dependent action potential (maybe thats why they look different)

In the atrial myocytes, bundle of His, Purkinje fibers and ventricular myocytes it is Na drived action potential (maybe thats why they all are so rapid and look the same)


What kind of channels are in the SA and AV nodes

Calcium L channel


Define the different segments of EKG

1. P Wave: atrial depolarization

2. PR interval: P wave plus PR segment, AV node depolarization

3. QRS segment: ventricular depolarization

4. ST segment: phase 2 of ventricular action, action potential doesnt change

5. T wave: ventricular repolarization, K current

QT interval = QRS + ST + T wave


 What determines each of these segments

Voltage gated Na channel and Ca channel determines the PR segment

ST segment depends on a balance between Ca current and K current

T wave is by K current

QT segment is the entire time of ventricular depolarization and then complete repolarization


What are the different phases of the action potential in a cardiomyocyte that he kept talking about


Why are the Na channels on SA node called funny

They open at a membrane potential of -50mV and close when the potential is more positive to -50 mV. So they are on during phase 4 and turn off during phase 0 


Explain what is happening in each of these phases

In phase 4 funny Na channels open at -50mV which causes the phase 4 depolarization trend. Na channels are open during phase 4 and closes during phase 3.

Phase 0 marks the opening of T type Ca channels which open initially and causes the opening of the L type Ca channels. Both of these channels drive the phase 0 depolarization current and as a result Ca flows into the SA node cell.

Phase 3 is marked by the opening of delayed rectifier K channels that open and causes repolarization of the cells. The membrane potential moves to -50 mV and the Na channels open again.

Important to note that towards the end of phase 3 Na funny channels start to open but their current is opposed by the K current which allows the potential to drop down to -50mV


What is the same and different about SA node and AV node

SA and AV node has the same ionic current and they dont have any resting membrane potential but the AV node has a lower intrinsic firing rate than the SA node. This means that if the SA node is damaged due to hypoxia or injury the AV node takes over the beating frequency of the heart.


Why does the AV node have a lower firing rate than the SA node

The slope of the phase 4 is less steep than the one in SA node


  What is unique about funny Na channels and what is the significance of it

Funny Na channels have only one gate which is called the activation gate. What makes them "funny" is that they open during repolarization (slowly) which causes the I-Na to increase. During phase 3 I-Na increase steadily and I-K decreases steadily giving us the shape of the graph of the SA node


What is the definition of threshold

The balance between I-depolarizing and I repolarizing, when depolarizing is more there is a generation of action potential as the threshold is reached


What causes the depolarization of the SA node cells? Is it Na or Ca

Na does not cause the depolarization, it is Ca involved in phase 4 which causes depolarization of the SA node cells. So the threshold in this case depends on the balance between I-Ca and I-K, but how quickly the threshold is reached depends on the funny Na current



Define the structure of the Ca L channel

It is very similar to the Na channel, it has 2 gates one activation and the other inactivation gate. The activation gate opens and closes quickly whereas the Inactivation gate opens and closes slowly


What is different between the Ca channel and the Na channel

There is Calcium window that is present in the Ca channel as it is the area of overlap in the probability of gate open curve


What is determined the by the magnitude of Ca current in the SA and AV node

1. Threshold potential

2. Amplitude of action potential

3. Conduction velocity

4. Rate of rise of the action potential


What happens to the PR segment when there is a larger Ca current generated

The PR segment is the flat space where the AV node is depolarizing. Due to higher Ca current the conductance velocity will be higher and the PR segment will decrease


Phases of ventricular potential

Phase 0: resting membrane potential at about -70 mV. This is more positive than E-K so there has to be leakage of K out of the cell and some Na into the cell. K moves out through the inward rectified K channel and Na moves in through leak channels

Phase 0: upstoke due to Na current

Phase 1: transient repolarization due to K current into the cell

Phase 2: This is the platuea phase due to the balance between Ca moving out of the cell and K moving in

Phase 3: Eventually Ca current ends and there is repolarization only due to K current


Inward rectifier K channel and Delayed rectifier K channel

These 2 are completely different K channels, only the inward rectifier K channel is open during resting membrane potential


What will happen if the Na current is reduced in a cardiomyocyte

There will be less positive membrane potential and as a result fewer Ca gates will open, the function of Na current is to open the Ca volatage gated channels so that Ca can move out of the cell


How does phase 2 end in a cardiomyocyte

There is finally time for the Ca inactivation gate to close that will stop Ca current, driving repolarization


What is the purpose of this slide

There are several K currents, the delayed rectifier is all that is important for our purposes


What are the effects of autonomic nervous system on the AV and SA node that regulate the rythym of the heart

Symapthetic nervous system firing:

1. Increases I-funny Na current

2. Increases Ca current

3. Increases K current

All of this is accomplished through adrenaline acting on the beta 1 receptors. Heart rate is primarily increased by the increasing I-funny. Increase in I-Ca increases the conduction velocity as well and decreases the PR interval

Parasympathetic nerve firing:

1. Decreases I-funny Na current

2. Decreases Ca current

3. Decreases K current

4. Very high firing of parasympathetic activates a different channel called I-KAch which increase K current

All of this is achieved through acetylcholine


What is the mechanism of increase of funny Na current

Sympathetic nerve firing shifts the activation curve for funny Na current to a more positive potential, which will cause the funny Na current gated channels to open sooner and cause depolarization at a faster rate. This is done by increasing the conductance of Na. It is important to know how the curve looks like


What will be the effects on the graph for increasing funny Na current

1. More positive MDP

2. Steeper phase 4


How does the graph look like when there is parasympathetic nerve firing

1. Phase 4 will be less steep than usual

2. At very high rates of parasympathetic nerve firing MDV will become more negative


Relative importance of parasympathetic and sympathetic nerve stimulation to the heart

Parasympathetic (blocking acetylcholine) has a much more profound effect