Physiology - Unit 2 Membrane Potentials

Electrical Potential

A force created by separation of positive and negative charges. The greater the charge separation, the higher the voltage (measured in volts). This electrical force drives ions toward each other across a membrane.

Current

The movement of electrical charge. In neurons, current is the flow of ions across the membrane when channels are open.

Electrochemical Gradient

The combined effect of:

• Chemical gradient (ion concentration difference)

• Electrical gradient (charge difference)
  Living cells maintain unequal ion distributions inside vs. outside, creating this gradient.

Membrane Potential

The voltage difference between the inside and outside of a cell at any moment.

Resting Membrane Potential

The membrane potential of a neuron at rest (not excited).

• ECF is defined as 0 mV

• ICF is measured relative to it
  Typical neuronal RMP: –70 mV (range –40 to –90 mV).
  Inside surface is negative, outside is positive, even though both compartments are electrically neutral overall.

Difference in Ion Composition

RMP is determined by:

• Ion concentration differences (mainly Na⁺, K⁺, Cl⁻)

• Selective permeability (membrane is far more permeable to K⁺)

• Na⁺/K⁺ pump, which uses ATP to maintain gradients and counteract ion leak.

Equilibrium Potential (Eₓ)

The membrane voltage at which the electrical force exactly balances the concentration force for a specific ion → no net ion movement.
Greater concentration difference → larger equilibrium potential.

Nernst Equation

Calculates the equilibrium potential for a single ion based on its concentration gradient.

For body temperature (simplified):

Ex = 61/z * log (Xout/Xin)

Examples:

• K⁺: ~ –90 mV (K⁺ wants to leave the cell → inside becomes negative)

• Na⁺: ~ +60 mV (Na⁺ wants to enter → inside becomes positive)

Goldman–Hodgkin–Katz Equation

Calculates the actual resting membrane potential by considering:

• Concentration gradients of multiple ions

• Relative membrane permeability to each ion
  This explains why RMP is closer to K⁺ equilibrium potential (membrane is most permeable to K⁺ at rest).

Depolarization

Membrane potential becomes less negative than RMP (moves toward 0).

Overshoot

Membrane potential becomes positive (above 0 mV).

Repolarization

Membrane potential returns toward RMP after depolarization.

Hyperpolarization

Membrane potential becomes more negative than RMP.