BIOL325 - Resting Membrane Potential (RMP)

Charge

Electrical property carried by ions such as Na+, K+, and Cl−.

Current (I)

Flow of electrical charge; in cells, movement of ions.

Voltage (V)

Electrical potential difference between two points.

Resistance (R)

Opposition to ion current; higher permeability means lower resistance.

Ohm’s law

Current = voltage ÷ resistance (I = V/R).

Separation of charges

Unequal charge distribution immediately across membrane produces voltage.

Membrane potential

Voltage of cell interior relative to extracellular fluid.

Resting membrane potential (RMP)

Membrane voltage at rest; approximately −70 mV in course examples.

Chemical gradient

Force driving an ion from higher to lower concentration.

Electrical gradient

Force attracting opposite charges and repelling like charges.

Electrochemical gradient

Combined chemical and electrical forces on an ion.

Electrochemical equilibrium

Electrical and chemical forces balance; no net passive flux.

Equilibrium potential (Eion)

Voltage that exactly balances an ion’s concentration gradient.

Nernst equation

Eion = (61 mV/z) log10([ion]outside/[ion]inside) at 37°C.

What determines an ion’s equilibrium potential?

Its concentration ratio, charge (valence), and temperature.

Does permeability change Eion?

No; permeability affects influence on membrane voltage, not Eion.

EK+ in course examples

Approximately −90 mV.

ENa+ in course examples

Approximately +60 mV.

Leakage channels

Channels allowing ions to cross membranes passively at rest.

Why does K+ dominate RMP?

Resting membrane is about 50–100 times more permeable to K+ than Na+.

Na+/K+ ATPase

Uses ATP to move 3 Na+ out and 2 K+ in.

Primary role of Na+/K+ ATPase in RMP

Maintains Na+ and K+ concentration gradients.

Electrogenic effect of Na+/K+ pump

3 positive ions out for 2 in; contributes about 10–20% of RMP.

Intracellular nonpenetrating anions (A−)

Negatively charged proteins and phosphates trapped inside cells.

Why is RMP not equal to EK+?

Small Na+ leak inward makes RMP less negative than EK+.

Na+ passive movement at rest

Na+ tends to enter through leak channels.

K+ passive movement at rest

K+ tends to leave through leak channels.

Steady state

Active pumping balances passive leakage, keeping concentrations stable.

What happens if K+ permeability increases?

Membrane voltage moves toward EK+ and becomes more negative.

What happens if Na+ permeability increases?

Membrane voltage moves toward ENa+ and becomes less negative.

What happens if extracellular K+ rises?

EK+ becomes less negative; RMP generally depolarizes.

What happens if extracellular Na+ rises?

ENa+ becomes more positive; Na+ driving force increases.

Cl− and RMP in the lecture model

Cl− distribution passively follows membrane voltage at rest.

What happens if Na+/K+ ATPase stops long term?

Ion gradients gradually dissipate and normal signaling fails.