# Symbols and Units of Measurement

*C*

Capacitance (measured in farads).

*c*

_{m}

Capacitance per unit length of membrane cylinder.

*C*

_{m}

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., g_{Na}.

*g*

_{l}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.

*I*

_{c}

Capacitive current; the current that changes the charge distribution on the lipid bilayer.

*I*

_{i}

Ionic current; the resistive current that flows through ion channels.

*I*

_{l}

Leakage current; the current flowing through a population of resting ion channels.

*I*

_{m}

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*.

*R*

_{in}

Total input resistance of a cell.

*R*

_{m}

Specific resistance of a unit area of membrane (measured in Ω · cm^{2}).

*r*

Resistance of a single ion channel.

*r*

_{a}

Axial resistance of the cytoplasmic core of an axon, per unit length (measured in Ω/cm).

*r*

_{m}

Membrane resistance, per unit length (measured in Ω · cm).

V_{m}

Membrane potential, *V*_{m} = *Q*/*C*_{in} (measured in volts).

*V*

_{r}

Resting membrane potential.

*V*

_{t}

Threshold of membrane potential above which the neuron generates an action potential.

*V*

_{in}

Potential on the inside of the cell membrane.

*V*

_{out}

Potential on the outside of the cell membrane.

*Z*

Valence.

*γ*

Conductance of a single ion channel, e.g., γ_{Na}

*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 = R_{m} · C_{m}

A

Ampere, measure of electric current (SI base unit). One ampere of current represents the movement of 1 coulomb of charge per second.

ÅÅngström, measure of length (10_{−10} m, non-SI unit).

Coulomb, measure of quantity of electricity, electric charge (expressed in SI base units s · A).

FFarad, measure of capacitance (expressed in SI base units m^{2} · kg^{−1} · s^{4} · A^{2}).

Hertz, measure ...