Lecture Notes: Membrane Potential and Ion Permeability

Key Concepts

• Potassium Concentration Difference

  • Inside cell: ~150 millimoles per liter
  • Outside cell: ~5 millimoles per liter
  • Potassium bound to anions (negative charge)

• Movement of Potassium

  • Potassium leaves the cell due to concentration gradient
  • Leaves behind anions (negative charge inside cell)
  • Negative charge inside cell attracts potassium back
  • Equilibrium potential for potassium: -92 millivolts

• Thought Experiment: Positive Charge Injection

  • Inject positive charge into cell
  • New membrane potential: -46 millivolts
  • Potassium leaves cell due to weaker attraction
  • Anions increase negative charge, returning membrane potential back to -92 millivolts

Factors Affecting Membrane Potential

• Concentration Gradient

  • Desire for potassium to leave cell

• Permeability

  • Only permeable to potassium
  • Means for potassium to leave the cell
  • If both concentration gradient and permeability exist, membrane potential is -92 millivolts

Membrane Potential Formula

• Formula: Vm = 61.5 * log([K outside]/[K inside])

  • Vm: Membrane potential
  • [K outside]: Concentration of potassium outside the cell
  • [K inside]: Concentration of potassium inside the cell

• For potassium, Vm = -92 millivolts

• Other Ions and their Equilibrium Potentials

  • Sodium: +67 millivolts
  • Chloride: -86 millivolts
  • Calcium: +123 millivolts
  • For calcium (2+ charge), use 30.75 instead of 61.5 in the formula*

Summary

• Membrane potential depends on the concentration gradient and permeability of ions, especially potassium.
• The membrane potential for potassium is usually around -92 millivolts.
• The concentration and movement of other ions also affect membrane potential. Sodium, chloride, and calcium contribute differently compared to potassium.