CSB332H1 Lecture 3: Section 1- L3
14 May 2018
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Lecture 3: Nernst Equation
Question
• Why are some IV curves non-linear?
ð IV curves that are non-linear represent ion channels that rectify:
o Opening and permeability of those ions channels are voltage dependent; therefore,
causing ions to move through their pore more rapidly in one direction than the other
§ Non-linear relationship = voltage-gated channel
Nernst Equation:
• Allows the calculation of the potential required to balance the concentration gradient
o Calculate the equilibrium potential
o
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&' (𝑙𝑛 ["]-
["].
§ R = thermodynamic gas constant
§ T = absolute temperature
§ Z = valence of the ion (varies depending on ion)
§ F = Faraday (# coulombs of charge in 1 more of a monovalent ion
Equilibrium Potential of Potassium (K):
• For K, at 20°C RT/ZF = 25.26 mV
o EK = 25mV ln(3/90)
§ EK = –87.6 mV (equilibrium potential for K, is -87.6mV)
ð K+ is the major ion underlying the resting membrane potential (-65 mV)
o Why is it not -87.6 mV? This is because are other ions involved
Electrophysiology Understanding…
• Voltage across a neuron’s cell membrane is ALWAYS measured from the inside of the neuron
relative to the outside of the neuron
o When neuron is not firing – maintains a negative resting membrane potential (Vm)
compared with its extracellular space: -65mV
• Knowing the concentration gradient for an ion allows us to calculate the electrical gradient
that results in NO net flow across the cell membrane
o Electrical gradient is measured in volts – and is known as “equilibrium potential”,
(E(ION)) or the “reverse potential” for a specific ion
§ Reverse potential is the membrane potential at which there is NO net passive
movement for that ion – concentration gradient = electrical gradient
Valence (Z) of the ion
(charge of the ion)
