Electrical Activity of the Heart

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Electrical Activity of the Heart
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Outline

• Ionic basis of resting potential
• Ionic basis of the fast response
• Ionic basis of the slow response
• Mechanism of rhythmicity

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Ionic basis of the resting potential

• Potential inside the cardiac cell is -90 mV
  relative to outside.
• Action potentials depolarize the cell and
  overshoot to 20 mV
• Fast response predominant in the atria and
  ventricle
• Slow response found in the SA and AV nodes

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Ionic basis of the resting potential

• The phases of the action potential are associated
  with changes in the permeability of the cell
  membrane to Na, K, and Ca.
• Permeability is controlled by ion channels.

Ion Extracellular Intracellular Potential (mV)
Na 145 10 70
K 4 135 -94
Ca 2 10E-4 132
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Ionic basis of the resting potential

• The resting cell membrane is relatively permeable
  to K via the inwardly rectifying K current.
• The diffusion gradient of K outward is balanced
  by impermeable anions that create an
  electrostatic force.
• The Nerst equation for K predicts a Ek of -94
  which is slightly more negative than the resting
  potential due to slow Na leak
• If left, the leak would eventually depolarize the
  cell so the K/Na/ATPase acts to get rid of Na.

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Ionic basis of the fast response

• Genesis of the upstroke (Phase 0)
• Anything that raises the resting potential beyond
  threshold (-65 mV) will cause an action
  potential.
• Phase 0 due to Na inward.
• m gates open in Na channels as Vm becomes less
  negative.
• Na flows in due to the electric gradient until Vm
  0 then concentration gradient takes over.
• h gates close the channel due to the rising Vm
• h gates remain closed until partially
  repolarization (effective refractory period).

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Ionic basis of the fast response

• Genesis of early repolarization (Phase 1)
• Transient outward current of K causes a brief
  efflux of K because the interior is positive
  relative to exterior.
• Genesis of the plateau (Phase 2)
• Ca and some Na enters through slower activating
  and inactivating channels.
• Ca channels are voltage regulated and activated
  as Vm becomes less negative.

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Ionic basis of the fast response

• Genesis of the plateau (Phase 2)
• Two types of Ca channels L and T type.
• L-type are long lasting and open when Vm -10 mV
  and enhanced by cAMP
• T-type are transient and open when Vm -70 mV but
  inactivate quickly.
• The positive Vm favours the efflux of K but K
  current drops which prevents excessive loss of K
  and loss of the plateau.

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Ionic basis of the fast response

• Genesis of final repolarization (Phase 3)
• Repolarization occurs when K efflux exceeds
  influx of Ca.
• Three K channels with different physiochemical
  properties are responsible for repolarization.

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Ionic basis of the fast response

• Restoration of ionic concentrations
• Ca is pumped out by a Na/Ca exchanger and Na is
  ejected by the Na/K/ATPase pump.
• Small component of Ca/ATPase.

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Ionic basis of the slow response

• Only difference is the loss of phase 0.
• During phase 4, the K channels gradually decrease
  their conductance (close) allowing unopposed Na
  leak inward that depolarizes the cell.
• When the Vm reaches a threshold, the Ca channels
  open to further depolarize.
• The K channels also open to restore polarity.

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Mechanism of Rhythmicity

• The SA node resting potential is only -55mV.
• The cell membrane is naturally leaky to Na and
  Ca.
• The fast Na channels are mostly inactive so only
  the Ca channel can open.
• The slow influx of Na causes the resting
  potential to gradually rise towards threshold.

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