6.021/Notes/2006-12-04

Ion channels

• Two state gate model
  • compare with Hodgkin-Huxley model
  • Only one time constant (simple exponential) in two state model
  • Can't explain inactivation
• Use 4 gates, 3 activating, 1 inactivating
  • All independent
  • channel is open iff all gates open
  • Pr[channel open] = Pr[gate1 open]*Pr[gate2 open]*Pr[gate3 open]*Pr[gate4 open] = [math]\displaystyle{ m^3h }[/math]
  • [math]\displaystyle{ G_{Na} = N\gamma m^3h }[/math], [math]\displaystyle{ \overline{G_{Na}}=N\gamma }[/math]
  • Exactly Hodgkin-Huxley!
• Activation is independent of inactivation
  • Also pharmacologically independent
  • Pronase only inhibits inactivation of sodium channel
• What HH model cannot explain
  • Tail currents
  • Current that occurs at end of step should be 3 times faster than [math]\displaystyle{ \tau_m }[/math] but the actual data doesn't agree