Fast-response cardiac action potential
[Ref: BL5:chp3]
Resting membrane potential
Resting membrane potential (RMP) of cardiac muscle
--> -90mV (on the inside)
Phases of fast-response cardiac action potential
Occurs in myocytes and conducting fibres
- Phase 0 - Rapid depolarisation
- Phase 1 - Early, brief partial repolarisation
- Phase 2 - Plateau
- Phase 3 - Repolarisation
- Phase 4 - RMP
Total duration: 250ms
Phase 0 - Rapid depolarisation
Transmembrane potential (Vm) reachs threshold potential
--> m gates in voltage-dependent sodium channels open
--> Inward rush of Na+
--> Rapid depolarisation to about +20mV on the interior
--> h gates start to close
Duration: 1-2ms
NB:
- The greater the extracellular [Na+], the greater the amplitude of the spike
- Influx of Na+ is fast initially, but slows down as Vm becomes positive
- Quantity of Na+ moved in the influx is too small to change the intracellular [Na+]
- Threshold potential = -65mV
* [BL8:p13]
Phase 1 - Partial repolarisation
The interior of cell membrane becomes positive
--> Brief efflux of K+ (transient outward current, ito)
--> Brief, limited repolarisation
NB:
- Transient outward current is due to electrochemical gradient of K+
- Phase 1 is more prominent in Purkinje fibres and in epicardial fibres, than in endocardial fibres
Phase 2 - Plateau
Ca2+ enter via Ca2+ channels (L-type)
--> Influx of positive charges (Ca2+) is balanced by efflux of positive charges (K+)
NB:
- K+ conductance is greatly decreased after phase 0 (especially during phase 2)
--> thus preventing excessive K+ loss when both electrical and concentration gradient favours K+ efflux
- Inwardly rectifying K+ currents (iK1 currents) is reduced when Vm is positive.
Phase 3 - Repolarisation
- Delayed rectifier K+ current (activated near end of phase 0) finally start to become significant
- Ca2+ current decreases (Ca2+ channels are inactivated during phase 2)
--> Vm becomes less positive
--> Further increases iK1 currents
--> Accelerates repolarisation
Phase 4 - RMP
- Resting cell membrane relatively permeable to K+ but not to Na+ or Ca2+
- Inwardly rectifying K+ channel is open
--> OUTWARD K+ current (iK1)
Restoration of electrolytes
- Na-K ATPase pump restores the small changes in [Na+] and [K+]
- Most of Ca2+ that enter the cell are pumped out by Na/Ca exchanger (3 Na+ for 1 Ca2+)
- A small fraction is pumped out by Ca2+ ATPase pump
Other notes