G2i: Slow Action Potential

  • Site: SA Node, AV Node, ventricular conduction system
  • Duration: 150 milliseconds
  • RMP: -60mV
    • Less negative because their membranes are naturally leaky to Na+/Ca2+
    • At -55mV Fast Na+ channel n gates already closed ∴inactive
    • ∴only slow type Ca2+ channel can cause an AP
    • ∴rate of depol. is slow & Ph 0 upstroke is slow with less amplitude → hence name ‘Slow AP’

Features

• Ph 0: Depolarisation

  • Membrane depolarised to -40mV
  • Slow L-Type Ca2+ channel open
  • Inward Ca2+ movement (slow ∴slow upstroke)
  • Transient ↓K+ permeability further contributes to depolarisation

• Ph 3: Repolarisation

  • Delayed rectifier K+ channel opens
  • ↑K permeability repolarises cell
  • Ca2+ channel closing & inactive
  • Ph 3 ends when membrane potential reaches -65mV

• Ph 4: Spontaneous Slow Decay

  • Mechanism unclear
  • Multiple ionic currents involved
    • ↓K+ permeability
    • Slow inward Na+ movement KAfunny current’
    • ↑Ca2+ permeability through T-type Ca2+ channel (T = transient)
  • Then as begins to approach threshold, the L-type Ca2+ channel opens

NB: CCB do not block the T-type Ca2+ channels

Regulation of Slow AP

  • Pacemaker potential seen in cells of cardiac excitatory system
  • Denervated heart → SA Node fires at rate 100bpm
  • At rest, vagal tone predominates → ∴AP generated ~70bpm
  • This high intrinsic rhythmicity suppresses automaticity at other loci
  • “Characteristics of PM potential are predominantly under control of ANS”
  • ANS influences rate of PM firing
  • Alters
    1. Slope of Ph 4
    2. Threshold trigger of Ph 0
    3. Degree of hyperpolarisation at end of Ph 3

Parasympathetic

  • ↑K+ out & ↓Na+ & Ca2+ in
  • Decrease slope Ph 4 ∴more time taken to reach threshold = ↓discharge rate
  • More negative Ph 4 membrane potential

Sympathetic

  • ↓K out & ↑Na+ & Ca2+ in
  • Increase slope Ph 4 → decrease time to reach threshold → faster discharge rate

Notes on Spontaneous Decay

  • Not really a ‘RMP’ in PM cell
  • Constantly decaying ‘MP’
  • Multiple ionic currents responsible influenced by cAMP
  • ∴SYMP = ↑cAMP = ↑Ca2+ influx = steeper Ph 4 & rate of d/c
  • Other factors all affect time to threshold (Rate of Decay)
    • O2
    • Thyroid hormones (↑Ca2+ & ↑[Adv rec])
    • Temp
    • K