Understanding Membrane Potential Dynamics

Sep 8, 2024

Membrane Potential and Ion Movement

Definition

  • Membrane Potential: The difference in electrical charge between the inside and outside of a neuron.
  • Measurement:
    • Uses two electrodes: a reference electrode in extracellular solution and a recording electrode inside the neuron.

Changes in Membrane Potential

  • Depolarization:
    • Membrane potential moves toward zero.
    • Membrane becomes less polarized (smaller charge difference).
    • Described as a decrease in membrane potential.
    • Example: Neuron's membrane potential moves from rest (~ -65 mV) toward zero.
  • Hyperpolarization:
    • Membrane potential moves away from zero.
    • Membrane becomes more polarized (greater charge difference).
    • Described as an increase in membrane potential.

Resting Membrane Potential

  • Ion Distribution:
    • Ions are not equally distributed across the membrane.
    • Sodium, calcium, chloride: concentrated outside the cell.
    • Potassium, negatively charged molecules (amino acids, proteins): concentrated inside the cell.
  • Result: Negative resting membrane potential due to ion distribution.

Electrochemical Gradients

  • Drive ion flow in different directions:
    • Sodium: Inward flow when membrane is permeable.
    • Potassium: Outward flow when membrane is permeable.

Equilibrium Potential

  • Definition: The membrane potential at which electrical and concentration gradients for an ion balance out.
  • Sodium Example:
    • Sodium channels open, sodium enters the cell due to concentration and electrical gradients.
    • When membrane potential becomes positive, electrical gradient weakens.
    • Equilibrium is reached when driving strengths of gradients balance.
    • Sodium's equilibrium potential: approximately +60 mV.

Nernst Equation

  • Used to calculate the equilibrium potential of an ion.
  • Key Values:
    • Universal gas constant, temperature of mammalian cells.
    • Z: Charge of ion.
    • Intracellular and extracellular concentrations.
  • Sodium Calculation:
    • Z = 1, extracellular = 145 mM, intracellular = 15 mM.
    • Equilibrium potential = +60 mV.

Predicting Ion Movement

  • Compare ion's equilibrium potential to neuron's membrane potential.
  • Examples:
    • Sodium: Equilibrium potential is +60 mV; will enter the cell at -70 mV to reach equilibrium.
    • Chloride: Equilibrium potential is -65 mV; will leave the cell at -70 mV to make potential more positive.