PS231: Neurobiology II

Equilibrium potential

the voltage at which there is no net flow of that ion across the membrane (diffusional force = electric force)

K+ equilibrium potential

-80 mV

Na+ equilibrium potential

62 mV

Ca2+ equilibrium potential

123 mV

Resting membrane potential voltage of a neuron

-65 mV

• This is due to selective permeability, the sodium

Why is the resting membrane potential voltage of a typical neuron approximately -65 mV?

• When enough K+ ions have departed to make the membrane potential ~-65 mV, the electrical attraction pulling K+ in is exactly balanced by the concentration gradient pushing K+ out. This is the K+ equilibrium potential, which is the cell’s resting potential.

• A neuron at rest is highly permeable to potassium (K+ channels are open) and is not permeable to sodium (Na+ channels are closed)

Key points concerning electrical potential across a membrane

• Large changes in membrane potential are caused by minuscule changes in ionic concentration

• The net change in electric charge occurs at the membrane surface

• Equilibrium potentials for individual ions (Eion) can be calculated based on the concentration differences across the membrane using the Nernst equation

• When the membrane is permeable to multiple ions, the membrane potential voltage (Vm) is a function of the relative permeabilities of each permeating ion (calculated using the “Goldman” or “GHK” equation)

What two factors are needed to generate an electrical potential across a membrane?

1. selective permeability of ions

2. an ionic concentration gradient

Active transporters

use energy to pump ions in and out of the cell, establishes concentration gradients

Sodium-potassium ATPase exchange pump

• Each ATP moves 3 Na+ ions out and 2 K+ molecules in against their concentration gradients

• A “resting” neuron has a high internal K+ concentration and low internal Na+ concentration

• This concentration gradient is established and maintained by Na+/K+ pumps that are found along the neural membrane

• These pumps use energy (ATP) to move Na+ ions out of the neuron and K+ ions into the neuron against their concentration gradient

Nernst equation

Calculates the equilibrium potential for a single ion species

GHK equation

Calculates the resting membrane potential by considering multiple ions and their permeabilities (the REAL resting membrane potential of a neuron)

Does the process of establishing and maintaining ionic concentration gradients use energy (active process) or is it a passive process?

• Establishing gradients = active process (requires energy via ATP).

• Maintaining gradients = both active (Na⁺/K⁺ pump) and passive (leak channels, diffusion).