Physiology—Membrane Potentials

Vocabulary flashcards covering key terms and definitions related to membrane potentials, ion equilibrium, ion channels, and passive/active electrical properties of cells.

Fick's Law of Diffusion

Describes the passive movement of particles from high to low concentration; flux is proportional to the concentration gradient.

Concentration Gradient

Difference in solute concentration across a membrane that drives diffusion.

Ionic Attraction (Electrical Gradient)

Force created by opposite charges that pulls cations toward negative areas and anions toward positive areas across a membrane.

Nernst Equation

Formula (Eₓ = 60 mV · log([X]o/[X]i)/z at 37 °C) that calculates the equilibrium potential for a single ion.

Equilibrium Potential

Membrane voltage at which the chemical and electrical driving forces for a specific ion are equal and opposite; net ion flux is zero.

ENa

+60 mV, the equilibrium potential for sodium in a typical mammalian cell.

EK

−78 mV, the equilibrium potential for potassium in a typical mammalian cell.

ECl

−64 mV, the equilibrium potential for chloride in a typical mammalian cell.

ECa

+120 mV, the equilibrium potential for calcium in a typical mammalian cell.

Goldman Equation

Predicts membrane potential by weighting the equilibrium potentials of permeant ions according to their relative permeabilities.

Resting Membrane Potential (RMP)

Steady-state membrane voltage (≈ −70 mV) determined mainly by K⁺ permeability and ion gradients.

Leak K⁺ Channel

Potassium channel that is always open, providing baseline K⁺ permeability and helping set the RMP.

Na⁺/K⁺-ATPase Pump

Electrogenic transporter that uses ATP to move 3 Na⁺ out and 2 K⁺ in, maintaining ion gradients and slightly hyperpolarizing the cell.

Hyperkalemia

Elevated extracellular K⁺ that depolarizes RMP, bringing it closer to threshold and potentially causing paralysis.

Hypokalemia

Reduced extracellular K⁺ that hyperpolarizes RMP, making depolarization and muscle contraction more difficult.

Driving Force

Difference between membrane potential and an ion’s equilibrium potential (Vm − E); dictates direction and magnitude of current.

Conductance (g)

Ease with which ions pass through open channels; the inverse of resistance and a determinant of membrane current.

Ohm’s Law (Membrane)

I = g · (Vm − E); current equals conductance times driving force across the membrane.

Voltage-Gated Channel

Ion channel that opens or closes in response to changes in membrane potential.

Activation (Channel)

Voltage- or ligand-dependent opening of a channel’s gate allowing ion flow.

Inactivation (Channel)

Time-dependent closing of a channel despite continued stimulus, halting ion flow.

Ligand-Gated Channel

Channel that opens when an extracellular neurotransmitter binds and closes when the ligand is removed or degraded.

Second Messenger–Regulated Channel

Ion channel modulated by intracellular molecules such as cAMP, cGMP, or phosphorylation (e.g., HCN channels).

Mechanically-Gated Channel

Channel that responds to physical deformation of the membrane, important in touch and baroreception.

Membrane Time Constant (τ)

Product of membrane resistance and capacitance (Rm · Cm); time to charge the membrane to 63 % of its final voltage.

Membrane Capacitance (Cm)

Ability of the lipid bilayer to store charge; increases when the distance between charges is small.

Length Constant (λ)

Distance over which a passive potential decays to 37 % of its original value; proportional to √(Rm/Ra).

Axial Resistance (Ra)

Internal resistance to current flow along the cytoplasm; high Ra shortens the length constant.

Membrane Resistance (Rm)

Resistance to current leak across the membrane; higher Rm lengthens τ and λ.

Potassium Battery Concept

Model describing the cell as having high internal K⁺ and permeability to K⁺, making K⁺ diffusion the primary source of RMP.

Hypoxia Effect on RMP

Reduced oxygen diminishes ATP production, decreasing Na⁺/K⁺-ATPase activity and causing depolarization of RMP.

Ischemia-Induced Depolarization

Insufficient blood flow lowers ATP, inhibiting Na⁺/K⁺-ATPase and leading to partial loss of ion gradients and membrane depolarization.