Membrane Potential Calculation Lecture

Jun 4, 2024

Membrane Potential Calculation Lecture

Introduction

  • Discussed four imaginary cells, representing cells in the body.
  • Each cell is permeable to only one specific ion.

Ion Permeability and Concentration Gradients

  • First Cell: Permeable to Potassium (K+), Potassium leaves.
  • Second Cell: Permeable to Sodium (Na+), Sodium enters.
  • Third Cell: Permeable to Chloride (Cl-), Chloride enters.
  • Fourth Cell: Permeable to Calcium (Ca2+), Calcium enters.

Membrane Potentials for Each Ion

  • Potassium (K+): -92 mV
  • Sodium (Na+): +67 mV
  • Chloride (Cl-): -86 mV
  • Calcium (Ca2+): +123 mV
  • These values are calculated using the Nernst equation and are approximations based on typical cellular concentrations.

Real Cell Membrane Potentials

  • Real cells are permeable to multiple ions simultaneously.
  • Need to consider the permeability of each ion to calculate the overall membrane potential.

Example Case 1: Dominated by Potassium

  • Permeabilities:
    • Potassium (K+): 95%
    • Sodium (Na+): 1%
    • Chloride (Cl-): 2%
    • Calcium (Ca2+): 2%
  • Calculations:
    • Potassium: 95% * -92 mV = -87.4 mV
    • Sodium: 1% * +67 mV = +0.7 mV
    • Chloride: 2% * -86 mV = -1.7 mV
    • Calcium: 2% * +123 mV = +2.5 mV
    • Total Membrane Potential: -85.9 mV

Example Case 2: Dominated by Sodium

  • Permeabilities:
    • Potassium (K+): 16%
    • Sodium (Na+): 80%
    • Chloride (Cl-): 2%
    • Calcium (Ca2+): 2%
  • Calculations:
    • Potassium: 16% * -92 mV = -14.7 mV
    • Sodium: 80% * +67 mV = +53.6 mV
    • Chloride: 2% * -86 mV = -1.7 mV
    • Calcium: 2% * +123 mV = +2.5 mV
    • Total Membrane Potential: +39.7 mV

Summary & Key Points

  • Permeability acts like a vote. An ion with higher permeability influences the membrane potential more.
  • Changing the ion permeability can significantly alter the membrane potential.
  • Graph Representation:
    • A graph indicating a change from a highly negative membrane potential to a moderately positive one based on the change in ion permeabilities.

There's a marked shift in membrane potential when dominant ion permeability shifts from Potassium to another ion like Sodium.