13 and 14 - Electrophysiology of the Heart I and II Flashcards
Describe the concentration gradients for Na+ for a resting cardiac ventricular cell
Higher sodium concentration outside the cell
Movement into the cell
Describe the concentration gradients for K+ for a resting cardiac ventricular cell
Higher potassium concentration inside the cell
Movement out of the cell
Describe the concentration gradients for Ca++ for a resting cardiac ventricular cell
Higher calcium concentration outside the cell
Movement into the cell
Describe the concentration gradients for Cl- for a resting cardiac ventricular cell
Higher chloride concentration outside the cell
Movement into the cell
Describe the electrical gradient of Na+ for a resting cardiac ventricular cell
The ion is positively charged, so it will move into the negatively charged cell
Describe the electrical gradient of K+ for a resting cardiac ventricular cell
The ion is positively charged, so it will move into the negatively charged cell
Describe the electrical gradient of Ca++ for a resting cardiac ventricular cell
The ion is positively charged, so it will move into the negatively charged cell
Describe the electrical gradient of Cl- for a resting cardiac ventricular cell
The ion is negatively charged, so it will move out of the negatively charged cell
Describe the resting cell membrane permeability of Na+
Relatively impermeable in the resting state
Describe the resting cell membrane permeability of K+
The cell is highly permeable to K+
Describe the resting cell membrane permeability of Ca++
Relatively impermeable
Also, there is a calcium pump that pumps Ca+ OUT of the cell
Describe the resting cell membrane permeability of Cl-
Fairly permeable
There are five phases of a cardiac action potential. What are they?
- Phase 4
- Phase 0
- Phase 1
- Phase 2
- Phase 3
What is happening during phase 4 of a cardiac action potential?
The cell is at rest
The membrane potential is around -90 mV
How long will the cell remain at rest?
Until it is stimulated by an external electrical stimulus
What is the resting phase of the action potential associated with?
Diastole of the chambers of the heart
Diastole is the part of the cardiac cycle when the heart refills with blood following systole (contraction)
What happens during phase 0 of a cardiac action potential?
Rapid depolarization
The membrane will go from about -90 mV to about +25 mV
What does the slope of the phase 0 line represent?
The maximum rate of depolarization of the cell
What causes phase 0 to occur?
- The opening of the fast Na+ channels
- This causes a rapid increase in the membrane conductance to Na+
- Thus, there is a rapid influx of Na+ ions into the cell (a “Na+ current”)
What happens during phase 1 of a cardiac action potential?
A brief re-polarization
The membrane will go from about +25 mV to about 0 mV
What causes phase 1 to occur?
The inactivation of the fast Na+ channels
What causes the change in membrane potential during phase 1?
The change in membrane potential is due to K+ channels opening and a net K+ efflux
What happens during phase 2 of a cardiac action potential?
A plateau
The membrane potential remains at approximately 0 mV
What causes the plateau of phase 2?
The balanced influx of Ca++ and Na+ through the open SLOW Ca++ channels and the efflux of K+
What happens at the end of phase 2?
The slow Ca++ channels close, but the K+ channels remain open
What happens during phase 3 of a cardiac action potential?
Re-polarization
The membrane potential returns to about -90 mV
What is occurring with the ion channels during phase 3?
The slow Ca++ channels are closed
The K+ channels are still open
What is the purpose of having the Ca++ channels closed and the K+ channels open?
This ensures a net outward current, corresponding to negative charge in membrane potential, thus allowing more types of K+ channels to open
What causes the actual repolarization?
The net outward, positive current (equal to the loss of positive charges from the cell) causes the cell to repolarize
When do the K+ channels close?
When the membrane potential is restored to about -80 to -85 mV
What is the only phase of the action potential when the FAST sodium channels are open?
Phase 0 (rapid depolarization)
What is the only phase of the action potential when the SLOW sodium/calcium channel is open?
Phase 2 (plateau)
It is a plateau because as K+ is going out, Na+ and Ca+ are coming in
What is conductance again?
The degree to which an object conducts electricity, calculated as the ratio of the current that flows to the potential difference present
When would conductance be high?
When the ion’s channels are open and there is a membrane potential difference present
During phase 4, what is the conductance and permeability of potassium?
Phase 4 - resting cell
Conductance = low (its resting) Permeability = high
During phase 0, what is the conductance and permeability of potassium?
Phase 0 = rapid depolarization
Conductance = low Permeability = low
Recall that when fast Na+ channels open, the K+ channels close
During phase 1, what is the conductance and permeability of potassium?
Phase 1 = brief repolarization
Conductance = increasing Permeability = increasing
During phase 2, what is the conductance and permeability of potassium?
Phase 2 = plateau
Conductance = high Permeability = increasing
During phase 3, what is the conductance and permeability of potassium?
Phase 3 = repolarization
Conductance = high Permeability = high
What is the effective (absolute) refractory period of a cardiac action potential?
A period in which further stimulation will NOT elicit another action potential
The cell is in the inactive state, not the resting state, so Na+ channels cannot open again
Where in the cardiac action potential does the effective (absolute) refractory period begin and end?
Beginning: when the fast Na+ gates open (phase 0)
Ending: about halfway through phase 3 when the sodium channels “reset” to their “ready” position (inactivation gate is open and pore channel is closed)
What is tetany?
A “summation” of action potentials
Why can’t a heart undergo tetany?
Because the relatively long duration of the effective (absolute) refractory period prevents further contraction of the heart muscle until the previous contraction has relaxed
What is a relative refractory period?
The period in the cardiac action potential where are greater than normal stimulus is able to produce an action potential, but the action potential will have abnormal characteristics
Why would an action potential during the relative refractory period be abnormal?
Because not all the Na+ channels have been reset, so the depolarization takes longer
When does the relative refractory period begin and end?
Beginning: when the absolute refractory period ends (inactivation gate is open and the pore channels are closed)
Ending: when a normal action potential can be produced
How can an action potential elicited during the relative refractory period cause ventricular fibrillation?
Only a portion of the ventricle will be depolarized, instead of the entire “syncytium” because some myocytes will still be in the effective (absolute) refractory period
Disorganized depolarization causes cardiac cells to contract before they have completely relaxed, which means they will have ventricular fibrillation
What is an SA nodal pacemaker action potential?
The slow, positive increase in voltage across the cell’s membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential
Why are there different types of action potentials in the heart?
Because there are different types of cells in the heart - the conducting cells have different action potentials than the basic myocytes of the heart
What drives a pacemaker action potential?
Slow calcium channels
How is an action potential reached in a pacemaker cell?
Depolarization occurs at a slow, steady rate until threshold is reached and an action potential is triggered
Are any fast sodium channels involved in a SA nodal pacemaker action potential?
NO
What is the pre-potential of a SA nodal cell?
The pre-potential refers to the positive slope of gradual depolarization in phase 4 of a pacemaker action potential
What is happening with the sodium current during the pre-potential?
As the pre-potential is developing (and leading to an action potential), the sodium current increases
What is the effect of the sodium current increasing during the pre-potential?
There is a buildup of positive charges inside the cell, bringing the membrane potential towards the threshold
What happens to the calcium permeability during the pre-potential?
As the pre-potential increases, the calcium permeability also increases, which brings the membrane potential closer towards the threshold
What happens to the potassium permeability during the pre-potential?
It decreases
Why does the potassium permeability decrease during the pre-potential?
Because the rate at which the potassium is pumped into the cell is the same, but the rate at which it is pumped out of the cell is lower
What is the combined result of the sodium, calcium and potassium permeability during the pre-potential?
This “loads” the cell with positive charges that bring the membrane potential to the threshold so that an action potential can occur
What is a chronotropic effect?
Chronotropic effects are things that change the heart rate
What do chronotropic drugs do?
Chronotropic drugs may change the heart rate by affecting the nerves controlling the heart, or by changing the rhythm produced by the sinoatrial node.
What is the effect of positive and negative chronotropic agents?
Positive chronotropes increase heart rate; negative chronotropes decrease heart rate.
What is an example of a positive chronotropic agent?
Norepinephrine
Where is norepinephrine released from?
Postganglionic sympathetic neurons
What does norepinephrine act on?
Beta-1 SA nodal cell receptors to increase the heart rate
What is the mechanism of norepinephrine?
It increases the permeability and rate of sodium and calcium influx into the cell, thus decreasing the amount of time it takes to reach threshold
What is the effect of norepinephrine on the pre-potential slope?
In the presence of norepinephrine, the pre-potential slope is STEEPER
What is an example of a negative chronotropic agent?
Acetylcholine (ACh)
Where is acetylcholine released from?
CN X (Vagus)
What does acetylcholine act on?
The muscarinic SA nodal cell receptor in order to decrease heart rate
What is the mechanism of acetylcholine?
It increases the potassium permeability, which causes an efflux of potassium from the cell, thus increasing the the amount of time it takes to reach threshold
What is the effect of acetylcholine on the pre-potential slope?
In the presence of acetylcholine, the pre-potential slow is LESS steep
What are ectopic foci?
Areas outside of the SA node that are capable of pacing the heart
What would happen in the absence of ectopic foci?
Without ectopic foci, any damage to the SA node would result in the heart not being able to pace itself
What do ectopic foci do in the event of a damaged SA node?
They are able to take over when there is damage to the pacemaker cells
What is the sequence of depolarization of the heart?
- SA node depolarization
- Atrial syncytium depolarization
- AV node depolarization
- Septum depolarization
- Apex depolarization
- Free wall depolarization
- Base of left ventricle depolarization
First, the depolarization is initiated at the _________
Sino-atrial node (SA node) - the primary pacemaker for the heart
How fast does the sino-atrial node beat?
Variable, but the intrinsic rate is 60 bpm
Where does the SA node depolarization travel to first?
The atrial syncytium
How does the depolarization travel in the atrium of the heart?
The signal is propagated cell to cell
What would happen if this cell to cell communication was interrupted?
It would result in arrhythmias
How does the heart attempt to spread the depolarization as quick as possible?
There are numerous tracts within the atria, some depolarize faster than others
Where does the “fast tract” of depolarization go first?
Right first, then left
What do the tracts of the depolarization allow for?
Quick spread of depolarization through syncytium - and subsequent contraction)
Where does the depolarization go next?
The atrio-ventricular node
Why does the AV node delay the impulse?
Two reasons:
- There are fewer gap junctions there
- It allows the atria to contract into the relaxed ventricles
The AV node is considered the _________ pacemaker of the heart
Secondary
How fast does the AV node beat?
It has an intrinsic rate of 40 bpm
Where does the depolarization go after the AV node?
The bundle of His
What is the bundle of His?
A collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the atria and the ventricles
Under normal conditions, can the body use more than just the bundle of His to communicate, or is that the only option?
NO - normally, bundle of His is the only way
Where dos the signal after it reaches the bundle of His?
The bundle fibers
What are the two bundle branches?
1 - Right bundle branch
2 - Left bundle branch
The left bundle branch has three branches. What are they?
1 - Septal branch
2 - Left anterior branch
3 - Right anterior branch
After the depolatization passes through the bundle branches, where does it go?
The Purkinje fibers
Do the Purkinje fibers transmit the depolarization fast or slow?
Very fast transmission
What is the role of the Purkinje fibers?
Synchronous contraction of the ventricles
Is there any pacemaker capacity in the Purkinje fibers?
Yes - it is the tertiary pacemaker
15-40 bpm
How deep do the Purkinje fibers penetrate?
Only to the subendocardium (so, not quite to the endocardium, which would be the innermost lining of the atrium and ventricles
After the depolarization of the Purkinje fibers, where does the signal transmit next?
The ventricular muscles
How does the depolarization occur? From the inside out or from the outside in?
From the endocardium to the epicardium, so from the inside out
Is the cell-to-cell communication fast or slow in the ventricles?
Slow
What would happen if there was a blockage in the fibers preceding the myocardium (the right and left bundle branches)?
It would cause more of the depolarization to be cell-to-cell (so SLOW) and you would be able to pick up on this on an ECG
There would be prolonged ventricular depolarization
What is the function of the SA node?
- Pacemaker of the cell
- Initiates depolarization of the atria
What is the function of the AV node?
- Delays impulse from atria to ventricles
- Allows for complete ventricular filling
What is the function of the bundle of His?
- Electrical highway that impulse takes to the ventricles
- Carries depolarization to the bottom of the ventricles before branching to individual cells
What is the function of the Purkinje fibers?
- Terminal branches of right and left bundle branches
- Serve as fast conductors of depolarization wave to myocardial cells
Describe the average vector for atrial depolarization
- Down and to the left
- The magnitude (length of arrow) is proportional to the voltage of the potential
What is the average duration of atrial depolarization?
0.06 to 0.10 seconds to depolarize
Very fast
A slower duration can indicate a mitral stenosis or a mitral regurgitation
Describe the average vector for ventricular depolarization
- Down and to the left
- The magnitude (length of arrrow) is proportional to the voltage and will be larger than the atria or individual vector segments
What is the average duration of ventricular depolarization?
0.080 seconds
4 equal 0.020 second segments
What is the mechanism by which hyperkalemia increases the chance of a fatal dysrythmia?
- Slows the heart rate
- Can block the AV node conduction long-term
- Can dilate the heart
- Resting membrane potential becomes higher than usual***
Why would a calcium channel blocker (rather than a fast channel blocker) be used to reduce the conduction velocity through the AV node?
- They work by delaying calcium entry
- The AV node is a slow channel, so by blocking the calcium channel, you are blocking the force of contraction of the heart
- If you blocked the sodium channels, you would reduce the rate of depolarization and the action potential and therefore
