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

1. Phase 0 - Rapid depolarisation
2. Phase 1 - Early, brief partial repolarisation
3. Phase 2 - Plateau
4. Phase 3 - Repolarisation
5. 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, i_to)

--> 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 (i_K1 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 i_K1 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 (i_K1)

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