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Cardiac Conduction
na
28
Pharmacology
01/01/2012

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Cards

Term
What is the absolute refractory period?
Definition
Inexcitable period where Na channels are open/inactive. ~250 ms, longer than skeletal muscle.
Term
What are the areas of auto-rhythmic cells and their order of depolarization?
Definition
  • Sinoatrial node - in right atria. Called the heart's pacemaker, sense sinus rhythm.
  • Atrioventricular node - Spreads to AV node via internodal pathway. Immediately above tricuspid valve. Delays the signal slightly so atria can finish contraction.
  • AV bundle/Bundle of His - in the septum, connects atria and ventricles
  • Right and Left bundle branches - split in septum going towards apex. Depolarizes from left to right.
  • Purkinje fibers - branches out into ventricular muscle, papillary muscle
  • total time: 0.22 s
Term
What is resting membrane potential?
Definition
The interior membrane is negatively charged relative to the outside due to potassium ion permeabilty. RMP varies from -40mV to -90mV
Term
What is the flow of conduction in an SA nodal automatic cell?
Definition
If (funny channel) opens, and Na/K influx. Depolarization from -45 to -20. At -20, Ca channels open at rapid depolarization occurs. At +10, Ik opens and potassium efflux, returning to -45.
Term
What happens if you increase membrane permeability to potassium, And/Or decrease sodium permeability? Also happens when closing the funny channel.
Definition
Depresses phase4 slope, RMP becomes more negative, heart rate slows.
Term
What happens when membrane permeability to potassium increases due to Ik openings?
Definition
Phase 3 and 4 severely depressed, hyperpolarization becomes more negative.
Term
What happens when you decrease calcium permeability in phase 0?
Definition
Slope of phase 0 curve shifts right without affecting phase 3 and 4.
Term
How does the action potential of Purkinje fibers work?
Definition
Phase 4 slow depolarization due to funny channel or potassium channels, then upstroke in phase 0 due to SA nodal signaling BEFORE reaching threshold potential. Without SA node, fires once reaching -45, too slow for life.
Term
How does the action potential of ventricular muscle cells work?
Definition

NON-Automatic:

- Phase 4 (no-slope) is repolarization with K efflux.

- Phase 0 - depolarization with Na then Ca influx --> rapid upstroke.

- Phase 1 - Cl- influx, repolarization begins.

- Phase 2 - main contraction. Ca binds to troponin, CICR.

- Phase 3 - K efflux and hyperpolarization returning to phase 4

Term
What is the late sodium channel?
Definition
In phase 2/contraction of a muscle cell, there is a late sodium channel that opens after Ca channels open. Opening prolongs repolarization.
Term
When does abnormal sodium influx occur?
Definition
Normally, Na can leave the cell due to a Na/K exchanger which requires ATP. In hypoxia, there is no ATP, so Na enters the cell from sodium and sodium/H exchangers. All the Na then exchanges for a massive amount of Ca, leading to arrhythmia.
Term
What determines velocity of conduction/speed of transmission?
Definition
- Size of the fibers - small fibers are slower
- Presence of gap junctions - fewer are slower
- Level of Resting Membrane Potential (Potassium dependent) - Increase = more negative = increase velocity.
-Membrane responsiveness - sodium dependent, whether sodium channels reactivate or not.
Term
How does sodium influence membrane responsiveness and conduction?
Definition
The more negative the RMP, the more Na channels can open. More Na - greater rate of depolarization/phase 0/velocity. AKA more rapid upstroke.
Term
How do the activation/inactivation gates work in sodium channels?
Definition
Sodium gates start with Activation gate close and inactivation open. In phase 0, Activation opens and Na flows. As RMP becomes positive, inactivation closes for duration of refractory period.
Term
What are the refractory periods?
Definition
Absolute refractory period - no sodium channels have been reactivated, will not respond. During phase 2.
Effective refractory period - Some Na channels reactivated, still not enough. Beginning of phase 3.
Relative refractory period - a slower depolarization can occur in the end of phase 3, looks like a bell curve.
Term

 

How do you prolong refractory periods?

Definition
  • Open the late sodium channel at the end of phase 2. Cell stays positive longer but does NOT prolong the action potential
  • Decrease K efflux in phase 3, cell stays positive longer, prolongs the AP.
  • Delay reactivation of phase 0 sodium channel. AP is normal, ERP is longer as sodium channels cannot be stimulated.
Term
What happens if you influence the reactivation of phase 0/slow Na channels on ERP?
Definition
Repolarization does not slow down, but ERP is prolonged.
Term
What are the nervous ennervations of the heart?
Definition
the PNS (vagus nerve) ennervates the SA and AV node only, while the sympathetic cardiac nerve/SNS ennervates these plus ventricular muscle.
Term
How does the SNS affect the funny channel?
Definition
Activation of the B1 receptor forms cAMP. cAMP binds directly to If, opening the channels and increasing the rate of phase 4 depolarization and therefore heart rate.
Term
What is the intrinsic conduction system?
Definition
Allows for proper sequence of activation and depolarization of atria and ventricles, no contraction.
Term
What is a latent pacemaker?
Definition
Everything besides the SA node, has the ability to become the pacemaker if the SA node is disabled. Conducts at a slower rate.
Term
How long does it take the wave of depolarization to reach the AV node?
Definition
0.03 seconds
Term
How long does it take the wave of depolarization to make it through the AV node and bundle of His?
Definition
0.16 seconds. Point of slowest impulse. Main area of delay is the AV node.
Term
Why does the impulse slow down in the AV node?
Definition
Smaller fibers (large fibers conduct faster), less gap junctions, a more positive RMP (more negative = faster conduction), and slower/less sodium influx (More sodium = faster conduction)
Term
At rest, what are the normal concentrations of ions inside and outside of the cell?
Definition
K is higher inside of the cell, Na and Ca are higher outside of the cell. At rest, only permeable to potassium.
Term
When a drug form of potassium is given, what happens to the cell?
Definition
Increased levels of extracellular K result in K influx, RMP becomes more positive. Can lead to arrhythmia.
Term
What are the phases of an SA nodal cell depolarization?
Definition

- Phase 4 - Potassium efflux. As it begins to depolarize and become more positive, K efflux slows, increase in Na influx. Funny channel opens - K/Na influx.

- Phase 0 - Reaches threshold, becomes permeable to calcium. Depolarizes into positive numbers.

- Phase 3 - K efflux and hyperpolarization returning to normal and phase 4. Funny channel reactivates.

Term
What is cardiac excitability?
Definition
How far below threshold potential phase 4 is. If RMP is more negative, decreased cardiac excitability.