C
Capacitance (measured in farads).
cm Capacitance per unit length of membrane cylinder.
Cm Membrane capacitance: either total input capacitance of a cell (measured in units of F) or capacitance of a unit area of membrane (F · cm−2), depending on context.
E Equilibrium (or Nernst) potential of an ion species, e.g., ENa, or reversal potential for current through an ion channel.
F Faraday's constant (9.65 × 104 coulombs per mole).
G Conductance (measured in siemens).
g Conductance of a population of ion channels to one or more ion species, e.g., gNa.
gl Resting (leakage) conductance; total conductance of a population of resting (leakage) ion channels.
I Current (measured in amperes). The flow of charge per unit time, ΔQ/Δt. Ohm's law, I = V · G, states that current flowing through a conductor (G) is directly proportional to the potential difference (V) imposed across it.
Ic Capacitive current; the current that changes the charge distribution on the lipid bilayer.
Ii Ionic current; the resistive current that flows through ion channels.
Il Leakage current; the current flowing through a population of resting ion channels.
Im Total current crossing the cell membrane.
i Current flowing through a single ion channel.
Q Excess positive or negative charge on each side of a capacitor (measured in coulombs).
R Gas constant (1.99 cal · °K−1 · mol−1).
R Resistance (measured in ohms). The reciprocal of conductance, l/G.
Rin Total input resistance of a cell.
Rm Specific resistance of a unit area of membrane (measured in Ω · cm2).
r Resistance of a single ion channel.
ra Axial resistance of the cytoplasmic core of an axon, per unit length (measured in Ω/cm).
rm Membrane resistance, per unit length (measured in Ω · cm).
V
m Membrane potential, Vm = Q/Cin (measured in volts).
Vr Resting membrane potential.
Vt Threshold of membrane potential above which the neuron generates an action potential.
Vin Potential on the inside of the cell membrane.
Vout Potential on the outside of the cell membrane.
Z Valence.
γ Conductance of a single ion channel, e.g., γNa
λ Cell membrane length constant (typical values 0.1–1.0 mm). 
τ Cell membrane time constant; the product of resistance and capacitance of the membrane (typical values 1–20 ms). τ = Rm · Cm
A
Ampere, measure of electric current (SI base unit). One ampere of current represents the movement of 1 coulomb of charge per second.
Å
Ångström, measure of length (10−10 m, non-SI unit).
C
Coulomb, measure of quantity of electricity, electric charge (expressed in SI base units s · A).
F
Farad, measure of capacitance (expressed in SI base units m2 · kg−1 · s4 · A2).
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