G2i: Fast Action Potential

  • Site:
    • Normal atrial myocardial fibres
    • Normal ventricle myocardial fibres
    • Purkinje fibres
  • Duration: 250 msec
  • RMP: -90mV

Features:

• Ph 4: RMP – TRUE RMP

  • Maintained by constant leak through INWARD RECTIFIER K+ CHANNELS
  • Na+ & Ca2+ channels are closed

• Ph 0: Rapid Depolarisation

  • A nearby AP causes the MP to rise above RMP
  • Fast Na+ channels start to open one by one, Na+ leaks in
  • At -65mV THRESHOLD is reached
  • All Fast Na+ channel open → massive Na+ influx
  • Depolarises MP to a peak +30mV
  • At -55mV h-gates of Na+ channels close
  • But at -40mV Slow L-type Ca2+ channel open & Ca2+ small but steady influx

• Ph 1: Early Repolarisation

  • Some K+ channel open briefly
  • Outward flow K
  • Returns MP to 0mV

• Ph 2: Plateau

  • Ca2+ channel still open
  • Small constant influx of Ca2+
  • K+ leaks out through DELAYED RECTIFIER K CHANNELS
  • Two counter-currents electrically balanced
  • ∴Plateau maintained

• Ph 3: Repolarisation

  • Ca2+ channel gradually inactivated (open for 100 msec)
  • Persistent K+ outflow
  • Brings MP back towards 90mV
  • Ion gradients restored by:
    • Na/Ca exchanger
    • Ca/ATPase
    • Na/K/ATPase
Repolarisation

REGULATION: Myocardial myocytes are stimulated by AP generated by PM cells

Refractory Period: the time from Ph 0 until the next possible depolarisation of a myocyte

  • Cardiac myocytes have different degrees of refraction → reflect the number of Na+ channels which have recovered from their inactive state
  • Cardiac myocytes have longer RF due to Ph 2
    • Protects ventricles emptying & allows filling prior to next contraction
    • Prevents tetany

Absolute Refractory Period: no stimulus can excite the cardiac myocytes → h-gates of Na+ channel closed

Relative Refractory Period: a supramaximal stimulus will depolarise the cell & cause an AP (not all Na+ channels have recovered yet)

  • Because → RECALL → depol. requires mass opening of Na+ channels
Regulation