1. Resting potential

2. Action potential Excite State : 10mV Resting state : -90mV
(Polarization)
depolarization
repolarization

3. Action potential

4. Action potential

5. Voltage clamp

6. Voltage clamp

7. Time Course of Action Potential
t =0: 20mV pulse
t =0.4ms: depolarization increase of Na+ current (passive, negative)
t =0.6ms: voltage rise sharply
t =0.8ms: increase of K+ current (passive, positive)
t =1ms: dip in Na+ current (peak of action potential)
t =1ms ~3.2ms: voltage decrease (small excessive K+ current )

8. Response to step depolarization
Iion= INa+ IK
A:Initial response from Na+
B:Replacement of extracellular fluid with choline
Effect only on INa
INaA /INaB = K(independent of time)

9. Conductance for K

10. Curve fitting 1 Estimation of n(v), n(v)  (v), (v)
for each applied
voltage v

11. Curve fitting 2 Estimation of n(v), n(v) as a function of voltage v
n(v)=f(v)
n(v)=fn(v)
(v)= f(v)
(v)= f(v)

12. Conductance for Na

13. Estimated parameters

14. Simulation

15. Change of Conductance  Voltage clamp  Na+ Channel  K+ Channel
: 2ms pulse
: channel selection
- by single ion
- by selective channel block
 Na+ Channel
: rapid channel open
: slow channel closure
 K+ Channel
: slow activation
: remain open for positive potential

16.  Onset of Action Potential  End of Action Potential
Ratio of Conductance
 Resting state
- gK  100 • gNa
- much greater leakage current
 Onset of Action Potential
fold increase of gNa
- positive feedback
 End of Action Potential
- negative potential
- closure of K+ channel
 Ratio of conductance
- gNa /gK (middle curve)

17.  Equivalent membrane conductance
Equivalent Circuit
 Axon
: axoplasm(conductor)
: surrounded by insulating membrane(cable in sea water)
: all of actions are in membrane(variable,nonlinear, imperfect insulator)
 Equivalent membrane conductance
: gNa --- Na+ ion
: gK --- K+ ion
: gCl --- Cl- ion
(except Na+ & K+)
그림 3-7

18. Equations for model  I = IC + INa + IK + ICl
= C(dV/dt)+ gNa(V-115)+ gK(V+12)+ gCl(V-10.6)
 3 activity coefficients: m, n, h
- gNa = 120m3h,
- gK = 36n4
- gCl = 0.3
dm (25-V)
dt e 0.1(25-V) - 1
= (1-m) - 4m e-V/18
dh (25-V)
dt e 0.1(30-V) + 1
= 0.07e-V/20(1-h) -
dn (10-V)
dt e 0.1(10-V) - 1
= (1-n) n e-V/80

19. Calculated Results  Peak Ion Currents  Peak net Currents
- peak of lNa
: -803 uA/cm2(at t=1.8ms)
- peak of lK
: 837 uA/cm2(at t=1.8ms)
- peak lCl
: 29 uA/cm2 (at t=0.9ms)
 Peak net Currents
- paek of lnet
: -315 uA/cm2(at t=0.71ms)
( lNa=-417, lK=83, lCl=19 uA/cm2 )
그림3-6

20. Voltage gated ion channel

21. Voltage gated ion channel

22. Voltage gated sodium channel

23. Bert Sakmann
The Nobel Prize in Physiology or Medicine 1991

26. Propagation of action potential

27. Propagation of action potential

28. Axoplasm Rapid voltage attenuation 0.3um unmyelinated axon (smallest)
V = Vine-x/0 : 0 = length constant
fall to 37% of amplitude
0.3um unmyelinated axon (smallest)
0 = 0.15mm
1.3um unmyelinated neuron (largest)
0 = 0.33mm
Length constant
k•(axon diameter)1/2
may be viable for small organism
length constant of 1mm  10 times increase of diameter (13um; unfeasible)
1.3um of copper wire
0 = 2800mm (8500 times more conductive)

29. Sensory vs Motor Neuron

30. Synapse

31. Synapse

32. Postsynaptic potential

33. Take home message Characteristics of action potential
Propagation mechanism