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Chapter III Book Notes: Voltage-Dependent
Membrane Permeability
9/29/16 3:47 PM
Ionic Currents across Nerve Cell Membranes
Voltage clamp technique allows experimenters to control membrane
potential (= difference in electrical potential between inside and outside of
the cell) and measure the underlying permeability changes
H&H first goal was to determine whether neuronal membranes have voltage
gated permeabilities
• When they hyperpolarized Vm (membrane potential) from -65mV to
-130mV à redistribution of charge

•

•

o Capacitative current is almost instantaneous
o Little membrane current flows
When they depolarize the Vm from -65mV to 0m, there is a
capacitative current followed by rapidly rising inward ionic current
(cations entering cell)
This proves that Vm is voltage dependent

Two Types of Voltage Dependent Ionic Current
No significant ionic currents flow at membrane potentials that are more
negative than RMP
Currents flow and change at magnitude when the Vm is more positive than
RMP
Early inward current has a U shaped dependence of membrane potential
à increasing over a range of depolarization’s up to approx
...

TEST: removing external Na+ makes ENa+ more negative which
should reverse the electrochemical gradient for Na+ and cause the
current to come outward

Late outward current increases monotonically with positive membrane
potentials
• Due to the K+ ions exiting the neuron
• Amount of K+ efflux correlates with the magnitude of the late
outward current
Summary: Going from RMP to a positive potential produces (1) an early
(transient) influx of Na+ and (2) a delayed (sustained) efflux of K+
Tetrodotoxin (TTX) à blocks Na+ current without affecting K+
Tetraethylammonium ions à block K+ current without affect Na+
Two Voltage-Dependent Membrane Conductance’s
H&H next wanted to describe the Na+ and K+ permeability changes
mathematically à assumed ion currents are due to a change in membrane
conductance (opposite of membrane resistance)
• Membrane conductance is closely related to membrane permeability
but not identical
• Iion = gion (Vm – Eion)
• I = ionic current
• g = conductance of membrane
• Vm = membrane potential
• E ion = equilibrium potential
• (Vm – Eion) = driving force acting on the ion

H&H set the Vm on the clamp, set the concentrations of ions on each side of
the neuron, the current can be recorded à so now they can find g of Na+
and K+ and drew 2 conclusions
• (1) Na+ and K+ conductances change over time
o K+ needs time to activate, Na+ activates quickly
o The more rapid activation of the Na+ conductance allows Na+
to open first
o Depolarization also causes the conductance to decrease as
time goes on because Na+ channels inactivate using ball
and chain method
o Although both Na+ and K+ channels use time dependent
activation only Na+ uses ball and chain
• (2) Both Na+ and K+ conductances are voltage dependent – they
both increase progressively as the neuron is depolarized
Summary: ionic currents that flow when the neuronal membrane is
depolarized are because of (1) activation of Na+ channels (2) activation of
K+ channels (3) deactivation of Na+ channels
Reconstructing an Action Potential
Na+ flows in, membrane potential rises, axon experiences refractory
period
• rate of depolarization falls because the electrochemical driving force
on the Na+ decreases because the Na+ conductance deactivates
Then, K+ repolarizes because voltage dependent K+ conductance is
activated by depolarization
• K+ conductance becomes higher than it is at RMP so there is a
hyperpolarization which causes the K+ conductance to turn off
and the membrane goes back to RMP
AP’s are all or none responses because of the regenerative cycle above
Long Distance Signaling Means by Action Potentials
Current conduction in the absence of Action Potentials is called passive
current flow

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