1D: Cardiac Electrophysiology Flashcards
What is the basis of the electrical activity of excitable cells?
Electrical potential differences across biological membranes are the basis of the electrical activity of excitable cells
What is the pathway of the SA node?
On the right side: SA NODE–>Internodal tract–>AV NODE–>AV bundle–>R/L Bundle Branches
On the left side : SA NODE–> Interatrial tracts
Describe excitable cells
- Neurons
- Muscle cells (skeletal, cardiac & smooth)
- Cells comprising the cardiac conduction system
What does the appearance of the cardiac AP depend on?
Depends upon WHERE it is measured \
What are the 2 types of Cardiac AP’s?
1) Non-pacemaker or “fast response”
2) Pacemaker “slow response”
What are the Cardiac AP phases ?
O: Upstroke
1: Early phase repolarization
2: Plateau
3: Repolarization
4: Diastole
What is the Non-pacemaker or fast response?
- occur in the atria ventricles & Purkinje fibers
- Undergo “rapid” depolarization
What is the pacemaker or “slow response”
- Occurs in the SA node & AV node
- Undergo “slow” depolarization
What occurs in phase O (rapid depolarization) ?
- Upstroke of the AP
- “Fast” Na+ channels open
- Several types of K+ channels close
What is Phase 1 ?
Early/Initial Repolarization
What occurs in Phase 1 (early/initial Repolarization)
- Transient outward current as K+ channels open
- “Fast” Na+ channels are closed
What is phase 2?
plateau phase
What occurs in Phase 2 (plateau)
- Long lasting (L -type ) Ca++ channels open leading to inward Calcium movement
- Efflux of K+ thru several types of K+ channels
How does skeletal and cardiac contraction differ in their requirement for Ca++ influx during an AP?
Whereas exviation-contraction coupling in skeletal muscle does NOT require influx thru L-type Ca++ channels,
cardiac contraction has an absolute REQUIREMENT for Ca++ influx thru these channels during the AP .
Describe the amount of Ca++ entering the cardiac muscle cell during an AP
Amount is SMALL and does NOT promote actin-myosin interaction
What does the influx of calcium during an AP serve to trigger?
- Induces calcium release form the Sarcoplasmic Reticulum. (**Calcium-induced calcium release = CICR) Which then promotes actin-myosin interaction and hence contraction;
- This occurs via ryanodine receptors (RyR2)
What can alter calcium conductance?
-Neurotransmitters and drugs can alter conductance.
-NE increases
whereas Ach, B-blockers & Ca++ channel blockers decrease it!!!
What is phase 3?
Later or Final Repolarization
What occurs in Phase 3 (late or final Repolarization?)
- Continual efflux of K+ thru several types of K+ channels
- L-type Ca++ channels eventually close
What is phase 4?
RMP Resting Membrane Potential
What occurs in phase 4 (RMP) ?
- K+ channels remain open
- Ca+ extrusion mechanisms become highly active
What are the main calcium extrusion mechanisms ?
1) SR Ca++ pumps (SERCA)
2) Sarcolemmal Ca++ pumps
3) 3 Na+/1Ca++ exchangers (NCX)
How do the electrical and mechanical events in cardiac muscle differ from the skeletal muscle?
The events in cardiac muscle overlap considerably in time, as \opposed to the skeletal muscle where the electrical even is over before the mechanical event begins
True or False. It is impossible to produce the summation and tetanus found in skeletal muscle during high frequency stimulation in cardiac muscles?
True
What would tetany of the heart lead to ?
Death
-The prolonged refractory periods (in part I) allow the ventricles to relax and fill with blood before the next contraction.
What are refractory periods?
Once a fast response AP has been initiated, the depolarized cell is not longer excitable until the cell is partially repolarized.
What is the effective or (absolute) refractory period?
a 2nd AP absolutely cannot be initiated, no mater how large the stimulus.
What is the relative refractory period?
A 2nd AP may be evoked ONLY when the stimulus is sufficiently strong (supra-threshold)
What is the SA node? (Sinoatrial)
- Is the pacemaker of the heart
- Has a unstable RMP
- Exhibits phase 4 depolarization or Automaticity ( the ability to intiate its own AP)
What is a slow response cardiac AP in phase O?
And what is it caused by?
- The upstroke of the AP
- Caused by an increase in Ca++ conductance
- Result of an inward Na+ current
In slow response cardiac action potential what is phase 3? And what is it caused by?
- is Repolarization
- Caused by an increase in K+ conductance; This increase results in an outward K+ current that causes Repolarization of the membrane potential
What occurs in slow response cardiac AP, phase 4? and what is it caused by?
Is slow depolarization
- accounts for the pacemaker activity of the SA node (automaticity)
- caused by an increase in Na+ conductance, which results in an inward current called (I. f)
What is (I. f ) turned on by?
The Repolarization of the membrane potential during the preceding AP
Describe the cardiac conduction system of the SA Node
SA node of the right: SA Node–> internodal tracts–> AV node–>AV bundle–>R/L Bundle Branches–>Purkinje fibers
SA on the left: SA–> Interatrial tracts
What is HR?
The number of times the SA node discharges per minute
What is the Conduction velocity (CV)
reflects the time required for excitation to spread throughout cardiac tissue
Where is CV (conduction velocity) the fastest? Where is it the slowest?
FASTEST In the Purkinje system
SLOWEST in the AV node (seen as the PR internal on the ECG)
-This allows time for ventricular filling before ventricular contraction.
What happens if conduction velocity through the AV node is increased ?
Ventricular filling may be compromised
1) What are Chronotropic effects?
a) What does a negative and b)postive Chronotropic effect do?
1) Produce changes in HR.
a) Negative- decreases HR by decreasing the firing rate of the SA node
b) Positive- increases the HR by increasing the firing rate of the SA node
***Changes in the HR are reflected on the ECG by changes in the R-R intervals
What are Dromotopic effects?
-produces changes in conduction velocity, primarily in the AV node
What does a negative and positive dromotropic effect do??
Negative dromotroptic effect- decreases conduction velocity thru the AV node slowing the conduction of AP from the atria to the ventricles & increasing the PR interval
Positive dromotropic effect- increases conduction velocity thru the AV node, speeding the conduction of APs from the atria to the ventricles & decreases the PR interval
The SA node, atria, & AV node have ____________ vegal innervation, but the ventricles do not.
Parasympathetic
The neurotransmitter for parasympathetic is _____________ which acts at muscarinic receptors
acetylcholine (ACh)
The SA node, AV node, atria and ventricles have _____________ innervation. The neurotransmitter is _________________ which acts at Beta-1 receptors.
a) sympathetic innervation
b) Norepinephrine (NE)
The parasympathetic effects on HR- What occurs during Negative chronotropic effect?
- Decreasing HR by decreasing the rate of phase 4 depolarization
- Fewer APs occur per unit time b/c the threshold potenital is reached more slowly and less frequently.
- The mechanism of the negative chronotropic effect is decreased (I.f) the inward Na+ current is responsible for phase 4 depolarization in the SA node
The parasympathetic effects on HR- What occurs during Negative Dromotropic effect?
- Decreases conduction velocity thru the AV node.
- APs are conducted more slowly from the atria to the ventricles
- Increases the PR interval ****
- The mechanism of the negative dromotropic effect is decreased inward Ca++ current and increased outward K+ current.
The sympathetic effects on HR- What occurs during Positive chronotropic effect?
-Increases HR by increasing the rate of phase 4 depolarization
- More AP occur per unit time because the threshold potential is reached more quickly and therefore more frequently
- The mechanism of the positive chronotropic effect is increased (I. f), the inward Na+ current that is responsible for phase 4 depolarization in the SA node.
The sympathetic effects on HR- What occurs during Positive dromotropic effect?
- Increases conduction velocity thru the AV node
- AP are conducted more rapidly from the atria to the ventricles & ventricular filing may be compromised.
- Decreased the PR interval
- The mechanism of the positive dromotropic effect is increased inward Ca++ current
