Electrical Activity of the Electrical Activity of the HeartHeart

OutlineOutline

Ionic basis of resting potentialIonic basis of resting potential Ionic basis of the fast responseIonic basis of the fast response Ionic basis of the slow responseIonic basis of the slow responseMechanism of rhythmicityMechanism of rhythmicity

Ionic basis of the resting potentialIonic basis of the resting potential

Potential inside the cardiac cell is -90 mV Potential inside the cardiac cell is -90 mV relative to outside.relative to outside.

Action potentials depolarize the cell and Action potentials depolarize the cell and overshoot to +20 mVovershoot to +20 mV

Fast response predominant in the atria and Fast response predominant in the atria and ventricleventricle

Slow response found in the SA and AV nodesSlow response found in the SA and AV nodes

Ionic basis of the resting potentialIonic basis of the resting potential

The phases of the action potential are associated with The phases of the action potential are associated with changes in the permeability of the cell membrane to changes in the permeability of the cell membrane to Na, K, and Ca.Na, K, and Ca.

Permeability is controlled by ion channels.Permeability is controlled by ion channels.

IonIon [Extracellular][Extracellular] [Intracellular][Intracellular] Potential Potential (mV)(mV)

NaNa 145145 1010 7070

KK 44 135135 -94-94

CaCa 22 10E-410E-4 132132

Ionic basis of the resting potentialIonic basis of the resting potential

The resting cell membrane is relatively permeable to The resting cell membrane is relatively permeable to K via the inwardly rectifying K current.K via the inwardly rectifying K current.

The diffusion gradient of K outward is balanced by The diffusion gradient of K outward is balanced by impermeable anions that create an electrostatic force. impermeable anions that create an electrostatic force.

The Nerst equation for K predicts a Ek of -94 which The Nerst equation for K predicts a Ek of -94 which is slightly more negative than the resting potential is slightly more negative than the resting potential due to slow Na leakdue to slow Na leak

If left, the leak would eventually depolarize the cell If left, the leak would eventually depolarize the cell so the K/Na/ATPase acts to get rid of Na.so the K/Na/ATPase acts to get rid of Na.

Ionic basis of the fast response Ionic basis of the fast response Genesis of the upstroke (Phase 0)Genesis of the upstroke (Phase 0)

Anything that raises the resting potential beyond threshold Anything that raises the resting potential beyond threshold (-65 mV) will cause an action potential.(-65 mV) will cause an action potential.

Phase 0 due to Na inward.Phase 0 due to Na inward.m gates open in Na channels as Vm becomes less negative.m gates open in Na channels as Vm becomes less negative.Na flows in due to the electric gradient until Vm = 0 then Na flows in due to the electric gradient until Vm = 0 then

concentration gradient takes over.concentration gradient takes over.h gates close the channel due to the rising Vmh gates close the channel due to the rising Vmh gates remain closed until partially repolarization h gates remain closed until partially repolarization

(effective refractory period).(effective refractory period).

Ionic basis of the fast responseIonic basis of the fast response

Genesis of early repolarization (Phase 1)Genesis of early repolarization (Phase 1)Transient outward current of K causes a brief Transient outward current of K causes a brief

efflux of K because the interior is positive relative efflux of K because the interior is positive relative to exterior.to exterior.

Genesis of the plateau (Phase 2)Genesis of the plateau (Phase 2)Ca and some Na enters through slower activating Ca and some Na enters through slower activating

and inactivating channels.and inactivating channels.Ca channels are voltage regulated and activated as Ca channels are voltage regulated and activated as

Vm becomes less negative.Vm becomes less negative.

Ionic basis of the fast responseIonic basis of the fast response

Genesis of the plateau (Phase 2)Genesis of the plateau (Phase 2)Two types of Ca channels; L and T type.Two types of Ca channels; L and T type.L-type are long lasting and open when Vm -10 mV L-type are long lasting and open when Vm -10 mV

and enhanced by cAMPand enhanced by cAMPT-type are transient and open when Vm -70 mV T-type are transient and open when Vm -70 mV

but inactivate quickly.but inactivate quickly.The positive Vm favours the efflux of K but K The positive Vm favours the efflux of K but K

current drops which prevents excessive loss of K current drops which prevents excessive loss of K and loss of the plateau.and loss of the plateau.

Ionic basis of the fast responseIonic basis of the fast response

Genesis of final repolarization (Phase 3)Genesis of final repolarization (Phase 3)Repolarization occurs when K efflux exceeds Repolarization occurs when K efflux exceeds

influx of Ca.influx of Ca.Three K channels with different physiochemical Three K channels with different physiochemical

properties are responsible for repolarization.properties are responsible for repolarization.

Ionic basis of the fast responseIonic basis of the fast response

Restoration of ionic concentrationsRestoration of ionic concentrationsCa is pumped out by a Na/Ca exchanger and Na is Ca is pumped out by a Na/Ca exchanger and Na is

ejected by the Na/K/ATPase pump.ejected by the Na/K/ATPase pump.Small component of Ca/ATPase.Small component of Ca/ATPase.

Ionic basis of the slow responseIonic basis of the slow response

Only difference is the loss of phase 0.Only difference is the loss of phase 0.During phase 4, the K channels gradually decrease During phase 4, the K channels gradually decrease

their conductance (close) allowing unopposed Na their conductance (close) allowing unopposed Na leak inward that depolarizes the cell.leak inward that depolarizes the cell.

When the Vm reaches a threshold, the Ca channels When the Vm reaches a threshold, the Ca channels open to further depolarize.open to further depolarize.

The K channels also open to restore polarity.The K channels also open to restore polarity.

Mechanism of RhythmicityMechanism of Rhythmicity

The SA node resting potential is only -The SA node resting potential is only -55mV.55mV.

The cell membrane is naturally leaky to Na The cell membrane is naturally leaky to Na and Ca.and Ca.

The fast Na channels are mostly inactive so The fast Na channels are mostly inactive so only the Ca channel can open.only the Ca channel can open.

The slow influx of Na causes the resting The slow influx of Na causes the resting potential to gradually rise towards potential to gradually rise towards threshold.threshold.

Questions??Questions??