PHYL3001 Lecture Notes - Lecture 4: Channelopathy, Tetrodotoxin, Resting Potential

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LECTURE FOUR: Membrane Mechanisms of the Action Potential
The Action Potential:
Overshoot how far above 0 is the action potential
Shape/characteristics varies depending on type of tissue
Threshold, amplitude, time course and duration depend on
o Gating (opening/closing) and permeability properties of ion
channels
o Intracellular and extracellular concentrations of the ions that pass
through these channels K, Na, Ca, Cl
o Membrane properties including capacitance, resistance and cell
geometry
Membrane Conductance Changes:
Transient increase in Na+ conductance shifts Vm towards ENa
Subsequent increase in K+ conductance shifts Vm back towards EK
This restores original negative resting potential
Depolarizing and repolarizing phases of action potential reflect transient
reversal of the Na/K conductances
Quantification of Individual Conductances:
Ix = Gx (Vm-Ex)
Measure of each ionic current (I) at a known membrane potential (Vm)
allows computation of each conductance
Voltage clamping allows control of voltage and measurement of current
Produces transient inward current (Na) and delayed outward current (K)
Changing Na+ concentration gradient alters early current proving APs
are carried by Na+
Selective blockers of voltage gated ion channels
o Tetrodotoxin (TTX) blocks Na+ channels and reveals K+ current
o Tetraethyl ammonium (TEA) blocks K+ channels revealing Na+
current
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