Cardiac Electrophysiology II Flashcards
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
- need to get rid of ca2+ from E-c coupling
- After depolarization Na+ out/ Ca2+ in
- Net Outward current
- contributes to Phase 1 - early repolarization
Describe the L-type calcium channel:
Threshold
Inactivation
Size of AP
Threshold: low
inactivation: slow (L = long lasting)
Size of AP: L = large
Describe the T-type calcium channel:
Threshold
Inactivation
Size of AP
Threshold: high
inactivation: fast (T=transient)
Size of AP: T = tiny
What are the three mechanisms that can slow the SA node pacemaker?
- Decreased rate of diastolic depolarization
- Diastolic hyperpolarization
- Increased threshold
???? does anyone have notes for this?
What mechanism does the vagus nerve use to slow the heart rate?
- Release of ACh at SA node
- binding of ACh to M2 receptors
- activation of Gi
- Inhibition of adenylyl cyclase
- Decreased cAMP
What mechanism do sympathetic motor neurons use to increase heart rate?
- Release of norepinephrine
- norepinephrine binds to B1 receptors
(metabotropic) - Actviation of Gs
- activation of adenylyl cyclase
- 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:
- dec inward I f
- Dec inward I ca T
- 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
(50)
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?
60 beats/min
What is the effect of inhibiting acetylcholinesterase on the heart?
-Bradycardia - dec HR
Ex: Sarin
- 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