Cardiac Eletrical Activity Flashcards
What are the Phases of the Fast Cardiac Action Potential? Which cell types experience these?
Normal atrial/ventricular myocytes; Purkinje fibers
Phase 0: Rapid Upstroke
Phase I: Early partial repolarization (Only fast reponse)
Phase II: Plateau of 0.1 - 0.2 seconds
Phase III: Membrane repolarization Slower than Phase 0
Phase IV: Resting state polarization
Which cells of the heart conduct slow potentials? What is the main difference from Fast to Slow potentials?
SA/AV Nodes
Slow Lack Phase I
What phase does cardiac muscle relaxation occur?
Phase IV
What major electrochemical changes occur for each phase in the Fast cardiac potential?
Phase 0: Opening of Fast Na channels resulting upstroke
Phase I: K+ efflux causes partial repolarization
Phase II: Ca channels open; Net flow of Ca in and K out
Phase III: Efflux f K predominate; Ca channels close
Phase IV: K efflux through channels predominates over influx
Which channels is responsible for the effective refractory period?
Na Channels
Describe L-type Calcium Channels.
- Predominant type in Myocytes
- Once open, inactivate slowly
- Activates during AP upstroke when Vm is about -20 mV
Describe T-type Ca channels in the heart
- Less abundant in heart
- Activated at membrane potentials more negative than -70 mV
- Inactivates more quickly
- opening of Channels results in increase in Ca conductance and calcium current soon after action potential upstroke
What are the inward rectifying channels, and what are the types?
Potassium channels
- iKS - Slowly activating
- iKr rapidly activating
Which channel is responsible for restoring resting membrane potential in cardiac myocytes?
NA/K ATPase
What is Postrepolarization refractoriness?
- Slow response action potential relative refractory period extends well beyond phase III;
- After cell has completely repolarized, it may be difficult to evoke propagated response for some time
How does decreased cycle length affect action potentials?
- Changes in cycle are important for terminating arrhythmia s
- As cycle length diminishes, duration of AP decreases
- K rectifier channels activate slowly and inactivates very slowly
- AP occur earlier in activation period
- As cycle length decreases, increase in K+ current shortens plateau
What is movement types toward a positive/negative pole?
Positive Deflection: Toward + Pole
Negative deflection: Toward (-) deflection
Where is the SA node located?
Les posteriorly in groove at junction btwn SVC + RA
What are the two principles cells of the SA Node?
1.
Small, round cells that have few organelles and myofibrils. Probably pacemaker cells. 2.
Slender, elongated cells that are intermediate in appearance between round and “ordinary” atrial myocardial cells. Conduct impulses within node and to the nodal margins.
Describe the affect of tetrodotoxin. Does it affect cardiac APs?
- Tetrodoxon blocks fast Na+ Channels
- no effect on SA Nodes Slow Potential because upstroke is not due to inward Na via fast channels
What distinguishes pacemaker cell Action potential from other cardiomyocytes?
- Phase IV in nonautomatic Cells remains the same
- pacemaker fibers is characterized by slow diastolic depolarization is phase IV
