Electrical Activity of the
Heart
Outline
Ionic basis of resting potential
 Ionic basis of the fast response
 Ionic basis of the slow response
 Mechanism of rhythmicity

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
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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]
Na
145
[Intracellular] Potential
(mV)
10
70
K
4
135
-94
Ca
2
10E-4
132
Ionic basis of the resting potential
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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.
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
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Genesis of early repolarization (Phase 1)
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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
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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
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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
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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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