Pacemaker potential

[Ref: BL8:p29-32]

... slow spontaneous depolarisation during Phase 4, until a threshold potential is reached and AP trigged.

aka prepotential

In the SA node, the diastolic depolarisation is mediated by 3 currents:

3 currents

1. Inward current (i_f)
2. Calcium current (i_Ca)
3. Outward K+ current (i_K)

See fig 2-24 at [BL8:p31]

Inward current (i_f)

• Mostly Na+
• Different from the voltage-gated sodium channel
• ?? Hyperpolarisation cytonucleotide-gated channel
  * Supposedly a type of K+ channel but Na+ passes through as well
• Activated during repolarisation when Vm becomes more negative than -50mV
• The more negative Vm gets, the greater the activation

Calcium current (i_Ca)

• Activated near the end of phase 4 when Vm depolarise to about -55mV
• Influx of Ca2+
• T-type Ca2+ channel involved
• L-type Ca2+ channel probably not involved
• Calcium current is not involved in automaticity in Purkinje fibres
  --> Only i_f and i_K

Outward K+ current (i_K)

• Opposes the other 2 currents
• Repolarise the cell after AP, and continues beyond maximum repolarisation
• Diminishes throughout phase 4
  --> Vm gradually becomes less negative in phase 4

Effects of ANS on packmaker potential

Catecholamine

--> Increase inward current and (??) calcium current

--> Increased slope in Phase 4

--> Faster rate

NB:

• i_K also increases, but less so
  --> Overall increase in slope

 

Acetylcholine released from vagus

--> Increase K+ current

--> Hyperpolarisation

--> Slower rate

NB:

• Also depresses i_f and i_Ca.