Previous page 3.2.3.2.2.2.3. Slow-response cardiac action potential   332 / 795  Next page 3.2.3.2.2.2.4. ECG Return 3.2.3.2.2.2.3. Slow-response cardiac action potential
Notes
      3. Old stuff
          3.2. Old physio stuff (around 2005)
              3.2.3. Physiology
                  3.2.3.2. Cardiovascular
                      3.2.3.2.2. Heart
                          3.2.3.2.2.2. Electrical activity
                              3.2.3.2.2.2.3. Slow-response cardiac action potential
 3.2.3.2.2.2.3.1. Pacemaker potential 

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 (if)
  2. Calcium current (iCa)
  3. Outward K+ current (iK)

See fig 2-24 at [BL8:p31]

Inward current (if)

  • 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 (iCa)

  • 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 if and iK

Outward K+ current (iK)

  • 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:

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

 

Acetylcholine released from vagus

--> Increase K+ current

--> Hyperpolarisation

--> Slower rate

NB:

  • Also depresses if and iCa.

 

Custom fields
1 :20050604
3 :20050815
Previous page 3.2.3.2.2.2.3. Slow-response cardiac action potential Next page 3.2.3.2.2.2.4. ECG Return 3.2.3.2.2.2.3. Slow-response cardiac action potential
Table of contents  | Bibliography  | Index
Revised at