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