Slow-response cardiac action potential

Compared with fast-response, slow response action potential...

Difference in the phases

• Phase 0 much less steep
• Phase 1 absent
• Phase 2 short and not flat
• Phase 3 not very distinct from phase 2

In nodal tissues,

• Pacemaker potential present in phase 4

Difference in ion channels

• Voltage-gated sodium channel not involved
  * No sharp spikes
  * Not affected by tetrodotoxin
• In nodal tissues,
  Very few i_K1 type of K+ channel
  --> Vm much less negative (-60mV) in phase 4
• Phase 2 is produced by Ca2+ current (L-type)

Difference in Vm

• RMP is about -60mV (compared to -90mV)
• Threshold potential = -40mV (rather than -65mV)

• Relative refractory period during the slow-response often extends well beyond phase 3
  --> postrepolarisation refractoriness
• Recovery of full excitability is much slower than the fast response

Other notes

• Duration of AP = 150ms (compared to 250ms)
• Amplitude of AP = Smaller
• Conduction slow (0.05m/s)