| 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, ∆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. |
| 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). |
| Vm | 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 |