Membrane Potentials

• All cell membranes are electrically polarized – Unequal distribution of charges – Membrane potential (mV) = difference in

charge across the membrane

• Interior of the cell contains negatively charged proteins and phosphate groups– Cannot pass through membrane (fixed anions)

Membrane Potentials

• Tend to attract cations (Na+, K+, Ca2+, etc.) to extracellular surface of membrane– Some cations can enter

through channel proteins in the membrane

When is equilibrium achieved?Let + move into the cell…

When is equilibrium achieved?Not when membrane potential = 0

When is equilibrium achieved?Not when concentration of (+) is equal

When is equilibrium achieved?When the electrical and concentration

gradients balance one another

Cation Distribution

• Cations may become more concentrated inside than outside

• inward flow along electrical gradient countered by outward flow along concentration gradient

Example: Potassium

• Membrane is more permeable to K+ than other ions – more K+ ion channels than any

other type

• K+ enters cell– reaches equilibrium point

between concentration gradient and electrical gradient

– not enough K+ in the cell to balance negative charges

• Cell is more negative inside than outside

Cation Distributions

• For K+

– Concentration gradient draws ion toward ECF– Electrical gradient draws ion toward ICF

• For Na+

– both concentration and electrical gradients draw ion toward ICF

Ion ECF ICF

Na+

145 mM 12 mM

K+

5 mM 150 mM

Typical Concentrations in ECF and ICF

Equilibrium Potential

• Equilibrium (no net movement) will be reached when a particular electrical potential is reached

• Equilibrium potential = electrical potential at which the net flow of ions across the membrane is 0

– balance between EG and CG is achieved

Equilibrium Potential

• Equilibrium potential is calculated for a particular ion using the Nernst Equation

– Ex = 61/z log [Xo]/[Xi]

– Equilibrium potential for K+ = -90 mV

– Equilibrium potential for Na+ = +60 mV

Resting Potentials

• Movements of different ions across the membrane influence each other

• Typical membrane potential for cells (resting potential) = -65 to -85 mV– close to EK, but higher since

some Na+ enters the cell

– K+ tends to leak out of the cell under normal conditions

Resting Potentials

• Concentrations of Na+ and K+ inside the cell are maintained using Na+/K+ pumps

Resting Potentials• NOTE: Resting Potential Equilibrium Potential

for either Na+ nor K+

• Gradients used for coupled transport in most cells

• Nerve and muscle cells = Excitable Membranes – Can rapidly change membrane permeability to Na+ and

K+ by opening gated ion channels

– Membrane potential can undergo rapid changes away from resting level = electrical signal