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Flashcards in Cardiac Electrophysiology Deck (10)
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1
Q

Differentiate between the electrocardiogram and cardiac action potential.

A
  • Cardiac Action Potential: 3 types
    • Ventricular
    • Atrial
    • Nodal
    • These action potentials occur in distinct regions of the heart
  • ECG
    • The ECG is an extracellular recording of instantaneous, heart electrical activity
      • Measures potentials from the ENTIRE heart via electrodes on the surface of the body
      • Recorded ECG potential is a result of an instantaneous vector sum of the changing polarizations of all cells in the ENTIRE heart
  • Differences:
    • Previous recordings were intercellular
      • Measured potentials from an electrode inside a single cell
    • ECG is an extracellular recording from many cells
2
Q

Identify the electrical phases (0 through 4) of ventricular and nodal action potentials.

A
  • Cardiac action potentials are divided into phases: 0-4
    • Ventricular and atrial action potentials have all 5 phases
    • Nodal action potential lacks phases 1 and 2
  • Phase 4:
    • Resting potential in ventricular and atrial cells
    • Pacemaker potential in nodal cells
  • Phase 0:
    • Rapid depolarization at the start of the action potential
    • Present in ventricular, atrial and nodal cells
    • Responsible ion is different in nodal cells
  • Phase 1:
    • Brief, partial repolarization of Ventricular and Atrial action potentials
    • NOT present in nodal action potentials
  • Phase 2:
    • Plateau of the ventricular action potential
    • Abbreviated in the atrial action potential
    • NOT present in the nodal action potential
  • Phase 3:
    • Repolarization that returns membrane potential to resting/pacemaker level
    • Present in ventricular, atrial and nodal cells
3
Q

Ventricular AP: Phase 4

A
  • Resting diastolic membrane potential
  • produced mainly by a non-gated, inwardly rectifying K+ channel that behaves as if it was voltage-gated
  • Stabilizes resting Em near EK
  • The inward rectifying iK1 potassium current through the channel -Is primarily responsible for the resting potential
  • After depolarization Mg2+ and polyamines partially block the channel and decrease K+ permeability
    • Easier for other currents (Na and Ca) to depolarize
4
Q

Ventricular AP: Phase 0

A
  • Initial action potential depolarization
  • Produced by Voltage activated Na+ channels: “fast sodium channels”
  • When activated by depolarization they open and close increasing and then decreasing the Na+ permeability automatically.
5
Q

Ventricular AP: Phase 1

A

-Brief, partial repolarization
-90% of the fast Voltage-Gated Na+ channels opened during phase 0 close.
-10% of the Na+ channels remain open until repolarization is well underway. Then they finally close as well
-Another Voltage-Gated K+ channel ito1 Briefly opens
-Channel is only open over a limited range of voltage and time
K+ leaves the cell and repolarizes Em To ~ 0 mV, the plateau voltage of Phase 2

6
Q

Ventricular AP: Phase 2

A
  • Plateau
  • Near 0 mV the ito1 channel closes
  • The iK1 channel continues to rectify
  • This decreases the hyperpolarizing Effect of K+ and makes it easier for The cell to remain depolarized on The plateau
  • While the iK1 channel is rectifying, a
  • Voltage-gated Ca++ channel opens: slow or L-type Ca++ channel
    • similar to fast Na+ channel only it opens and closes much more slowly
    • When open, this channel increases the Ca++ permeability
    • Inward Ca++ current helps maintain the plateau voltage and initiates contraction in the ventricular cell
7
Q

Ventricular AP: Phase 3

A
  • Repolarization
  • During phase 2: remaining Ca++ channels close and iKr and iKs delayed K+ rectifying channels fully open
  • The iK1 channel that was plugged in on depolarization is unplugged, Finishing repolarization
  • Once back at the resting level, the iKr and iKs close and the membrane is ready for another action potential
8
Q

Differences in Nodal Action Potentials

A
  • The duration of the nodal action potential is much longer
    • Phase 2, the plateau is absent in nodal potentials
  • Phase 4, or resting potential is constant in the ventricular action potential.
  • Phase 4 (pacemaker potential) in the nodal potential is NOT constant: depolarizes toward threshold.
    • All ion potentials are changing spontaneously
      • No stimulation or depolarization required
    • iK channel closes, decreasing hyperpolarizing K+ currents
      • Allows Na+ and Ca2+ to move in and depolarize
    • Na+ Funny channels: open rather than close on hyperpolarization
      • inward Na++ flow thru these channels depolarizes
    • Another voltage gated Ca2+ channel opens (iCaT)
      • Ca2+ completes the depolarization to threashold
9
Q

Parasympathetic Effects (Vagus)

A
  • Vagal stimulation releases ACh to modify channel permeabilities
  • Changes slow the heart rate
  • Release of ACh
    • Depolarizes the threshold
    • During the pacemaker potential
      • Decreases Na+ permeability
      • Decreases Ca++ permeability
      • Increases K+ permeability
      • All of these permeability changes promote hyperpolarization causing it to take longer to reach threshold, thus slowing the heart rate
10
Q

Sympathetic Effects

A
  • Sympathetic stimulation releases norepinephrine
  • Norepinephrine ~ works opposite of ACh
  • Release of norepinephrine during the pacemaker potential
    • Increases Na+ & Ca++ permeabilities
    • Decreases K+ permeability
  • All of these permeability changes promote depolarization
  • Allows membrane to reach threshold faster
    • Decreases time between action potentials
    • Increases heart rate