CSB332H1 Lecture Notes - Lecture 4: Voltage Clamp, Current Clamp, Choline
14 May 2018
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Lecture 4(b): Resting Membrane vs Action Potentials
Voltage Clamp – Membrane Responses:
Which Ions are on the Move in an AP?
• Depolarizing the membrane and clamping the voltage at
-9mV and recording the change in current flowing through
the membrane
o Two different components:
a) Early inward-current representative of Na+
b) Late outward-current representative of K+
Dependence of Action Potentials on [Na+]OUTSIDE:
• If we change the concentration of extracellular Na+ is it possible
to change the action potential generated?
o Is there a dependency on the extracellular [Na+]?
Experiment:
• Maintain a current clamp on the membrane and keep it in a depolarized
state by injecting positive ions
o Test this out with variable extracellular concentration of Na+
ð As the extracellular concentration of Na+ is decreased, the AP peak generated
also decreased
o Therefore, extracellular Na+ must be responsible for generation the
peak of the AP
Question – How did they figure out which ion is the inward versus outward flow?
a) Ion substitution experiment
o Replace Na+ ions with choline
§ Choline is a replacement for Na+ however is incapable of moving through
Na+ channels
• Once replaced and current was recorded, the early current was absent
b) Pharmacologically:
o TTX => functions to block voltage-gated Na+ channels
§ Voltage clamp mode was applied to maintain a
depolarized membrane
• Only outward current is visible; therefore,
Na+ is responsible for generating the inward
current
o TTE => functions to block voltage-gated K+ channels
§ Voltage clamp mode was applied to maintain a
depolarized membrane
• Only inward current is visible; therefore,
K+ is responsible for generating the outward
current
Document Summary
Which ions are on the move in an ap: depolarizing the membrane and clamping the voltage at. 9mv and recording the change in current flowing through the membrane: two different components, early inward-current representative of na, late outward-current representative of k+ Experiment: maintain a current clamp on the membrane and keep it in a depolarized state by injecting positive ions, test this out with variable extracellular concentration of na+ As the extracellular concentration of na+ is decreased, the ap peak generated also decreased: therefore, extracellular na+ must be responsible for generation the peak of the ap. Question how did they figure out which ion is the inward versus outward flow: ion substitution experiment, replace na+ ions with choline. Choline is a replacement for na+ however is incapable of moving through. Na+ channels: once replaced and current was recorded, the early current was absent, pharmacologically, ttx => functions to block voltage-gated na+ channels.
