Flashcards in Cardiac Electrophysiology II Deck (61)
What are the phases of SA node action potentials?
Phase 0: Rapid depolarization
Phase 3: Slower repolarization
Phase 4: Slow, ramping depolarization
What causes Phase 0 - rapid depolarization in the SA node?
Inward ca2+ current
-L - type channels
What causes phase 3 in the SA node?
-inactivation of L-type channels
-Delayed outward rectifier -iK
What interaction between three currents causes pacemaker activity?
-interaction btwn T-type ca2+ channels, iK and If
What is If? (funny current)
-net inward current
-activates in response to hyperpolarization
-has both inward Na, and outward K+ components
What causes Phase 4 of the SA node?
- slow, ramping depolarization
-activation of If
What is the exchange rate of the Na+/ca2+ exchanger?
3Na+ / 1 ca2+
How does the Na+/ca2+ exchanger contribute to the current of the heart?
-At rest- Na+ in / Ca2+ out
-need to get rid of ca2+ from E-c coupling
-net inward current
-After depolarization Na+ out/ Ca2+ in
-Net Outward current
-contributes to Phase 1 - early repolarization
Describe the L-type calcium channel:
Size of AP
inactivation: slow (L = long lasting)
Size of AP: L = large
Describe the T-type calcium channel:
Size of AP
inactivation: fast (T=transient)
Size of AP: T = tiny
What are the three mechanisms that can slow the SA node pacemaker?
1. Decreased rate of diastolic depolarization
2. Diastolic hyperpolarization
3. Increased threshold
???? does anyone have notes for this?
What mechanism does the vagus nerve use to slow the heart rate?
1. Release of ACh at SA node
2. binding of ACh to M2 receptors
3. activation of Gi
4. Inhibition of adenylyl cyclase
5. Decreased cAMP
What mechanism do sympathetic motor neurons use to increase heart rate?
1. Release of norepinephrine
2. norepinephrine binds to B1 receptors
3. Actviation of Gs
4. activation of adenylyl cyclase
5. Increased cAMP
How does sympathetic stimulation affect the cardiac currents?
-Increases Phase 4 steepness:
1. Increasing inward If
2. Increasing inward I caT
-Also decreases the threshold of I caL
What specifically is unaffected by sympathetic stimulation of the heart?
-Maximum diastolic potential (lowest potential)
How does parasympathetic stimulation affect the cardiac currents?
Reduces phase 4 steepness by:
1. dec inward I f
2. Dec inward I ca T
3. inc outward I k Ach
What is atropine's effect on the heart?
-Blocks parasympathetic stimulation(M2 receptors)
-Doubles resting heart rate (120)
What is propanolol's effect on the heart?
-Blocks sympathetic stimulation (B1 receptors)
-reduces resting heart rate, but not by much
What is the effect of combined atropine and propanolol on the heart?
-Increased heart rate
-shows us that parasympathetic effects on the heart are of much greater proportion
What is the basal heart rate of a healthy individual?
What is the effect of inhibiting acetylcholinesterase on the heart?
-Bradycardia - dec HR
- can cause asphyxation, muscle spasms, death
What is the effect of some antidepressants on the heart?
-They block uptake of NE
-Tachycardia - increased heart rate
What phases are the atrial muscle action potential composed of?
Phases 0, 1, 2, 3 , 4
What is phase 0 of atrial muscle action potential, and what contributes to this phase?
- Rapid depolarization - overshoots to 0mV
-due mostly to I Na
What is phase 1 of atrial muscle action potential, and what contributes to this phase?
-small, limited repolarization
-activation of I TO (K+ outward)
-Decreased I Ca and I Na
What is phase 2 of atrial muscle action potential, and what contributes to this phase?
-Short plateau ~200 ms
-Prolonged I ca plus a I kur (ultrarapid K+ out)
BALANCE of the two currents
What is phase 3 of atrial muscle action potential, and what contributes to this phase?
-Inactivation of Ica
What is phase 4 of atrial muscle action potential, and what contributes to this phase?
- Resting value of Vm due to inc iK1
(inward rectifying K+ channel)
-no depolarizing ramp
Does the atria have an intrinsic pacemaker?
NO - there is no If
What are the consequences of the plateau phase - phase 2?
ca2+ entry --- contraction
refractory period promotes filling
Does the SA node have intrinisc pacemaker activity?
What is the intrinsic rate of action potential firing in the AV node?
(notes also have 40-55/min)
What is the action potential of the AV node most similar to?
How long is the delay stage between atria and ventricles? What is the reason for this?
-This is due to slow conduction of the AV node
-High internal resistance
-Small diameter of AV nodal cells
-No gap junctions
What is responsible for the upstroke in the AV node?
Is the AV action potential large of small?
Small - only reaches ~0mV
How does the upstroke caused by calcium differ from upstrokes caused by sodium?
Calcium upstrokes are much slower and lower
How does parasympathetic inhibition affect the AV node?
- Decreases firing rate
-Decreased conduction velocity
How does sympathetic stimulation affect the AV node?
-Increased firing rate
-Increased conduction velocity
Where is the highest conduction velocity in the heart?
Do purkinje fibers have a pacemaker current?
What is the intrinsic firing rate of the purkinje fibers?
(notes say 25-40, guyton hall says 15-40)
How is coordinated contraction achieved?
1. Wave of electrical excitation
2. contraction coupled to electrical excitation
3. The heart consists of two electrical syncytia
What are gap junctions?
intracellular channels that permit the passage of ions
- consist of 4 or 6 copies of connexins
-provide the basis for electrical coupling
What is the threshold for the initiation of a propagated cardiac AP?
Why don't depolarizations below threshold stimulate action potentials?
The inactivation gates of Na+ channels are closed
Do ventricular muscle cells have intrinsic pacemaker activity?
How would you describe the activation of ventricles?
Orderly and spatiotemporal
- allows a very efficient ejection of blood
What is the order in which ventricular muscle cells are activated?
1. Ventricular apex (bottom)
-Endocardium to epicardium (in to out)
2. Rapid conduction via Purkinje Fibers
3. Ventricular base (top)
Endocardium to epicardium
Which cells of the ventricle have the slowest action potential?
- M cells (midmyocardial cells)
What are the three layers of the ventricles?
What are the 5 steps of electrical activation of the heart?
1. Depolarize the Atria
2. Depolarize septum from left to right
3. Depolarize anteroseptal region of the myocardium towards the apex
4. depolarize bulk of ventricular myocardium, from endocardium to pericardium
5. depolarize posterior portion of base of the LV
What two regions of the heart do not have pacemaker abilities?
Atrial and ventricular myocardium
Which cells of the heart have the fasted intrinsic pacemaker rate?
Name in order the 7 structures that electrical activation of the heart occurs in?
1. SA node
2. Atrial myocardium
3. AV node
4. AV bundle
5. Bundle branches
6. Purkinje network
7. Ventricular myocardium
What is the rate of action potential firing in the SA node?
60-100/min (This means normal basal HR is 60)
What is the intrinsic rate of action potential firing for the AV bundle and the bundle branches?
What is the threshold of the SA node?
(due to ramping depolarization - leaking in of Na+)
List in order from highest rate of action potential firing to slowest:
purkinje, SA node, AV node, AV bundle and branches
1. SA node
2. AV node
3. Purkinje Fibers, AV bundle and branches
What is the Normal resting potential at the SA node?