Describe how the mechanisms of action potential conduction were discovered.
Sanjay Manohar, Cambridge 2001
Bernstein 1902 - non-specific depolarisation to 0V
1. Cole & Curtis 1939
Wheatstone bridge during AP
Membrane Capacitance same, Resistance fell
=> increased membrane permeability
2. Cole & Curtis 1941
Intracellular recording of AP
Depolarisation crosses zero
-55 to +50 mV
3. Hodgkin & Katz 1949 (Sodium hypothesis)
Replace extracellular Na with glucose
Reduced AP amplitude & rate of rise
4. Keynes & Lewis 1951
Radiolabelled 24-Na
Entry during AP, not while resting
=> Na implicated
5. Hodgkin 1951(Theory)
Positive feedback proposed
Halted by 1) E > ENa, 2) inactivation
6. Hodgkin & Huxley 1952
Voltage clamping: measure current required to hold
membrane voltage at fixed value.
a. Total current vs. voltage
biphasic: inwards then outwards
b. Replace extracellular Na with choline.
Current vs. voltage
both components outwards
=> early component is Na
c. Repeat with E = ENa, so that INa = 0
i.e. if clamping at voltage V, set extracellular [Na]
such that ENa = V (using Nernst)
Current vs. voltage
isolated K and Na currents found
Calculation:
Calculate g = ion current / total potential driving ion
Conductance vs. voltage
fit g(V, t) to first order processes (in time) whose
parameters depend only upon V. n, m, h
Test this by prediction by modelling AP:
at any instant, voltage change can be calculated
voltage change = total current / capacitance
ion current = conductance x potential driving ion
12. Keynes et al
Cs+ blocking K current
TTX blocking Na current
=> confirm H&H current components
Measurement of gating current.
Site directed mutagenesis