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., E_{Na}, or reversal potential for current through an ion channel. |

F | Faraday’s constant (9.65 × 10^{4} 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, ∆Q/∆t. 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. |

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 m^{2} ⋅ kg^{–1} ⋅ s^{4} ⋅ A^{2}). |

Hz | Hertz, measure of frequency (expressed in s^{–1}). |

M | Molar measure of concentration of a solution (moles of solute per liter of solution). |

mol | Mole, measure of amount of substance (Sl base unit). |

mol wt | Molecular weight. |

S | Siemens, measure of conductance (expressed in SI base units m^{–2} ⋅ kg^{–1}. s^{3} ⋅ A^{2}). |

V | Volt, measure of electric potential, electromotive force (expressed in SI base units m^{2} ⋅ kg ⋅ s^{–3} ⋅ A^{–1}). One volt is the energy required to move 1 coulomb a distance of 1 meter against a force of 1 newton. Measurements in cells are in the range of millivolts (mV). |

Ω | Ohm, measure of electric resistance (expressed in SI base units m^{2} ⋅ kg ⋅ s^{–3} ⋅ A^{–2}). |