Resting membrane potential

Resting Membrane Potential

Introduction to Concepts

  • Resting membrane potential is a key physiological state of a cell.

  • Four interconnected concepts that will be discussed:

    1. Electrochemical gradients

    2. Equilibrium potential

    3. Resting membrane potential

    4. Membrane permeability

  • Objective: Understand and link the above concepts.

Electrical Disequilibrium of Body Fluids

  • Electrochemical disequilibrium: Refers to the unbalanced distribution of ions between the intracellular fluid (ICF) and extracellular fluid (ECF).

    • Inside of the cell is more negative compared to the outside (approximately -70 millivolts).

  • **Terminology:

    • Intracellular fluid (ICF): interior of the cell, with a resting membrane potential of -70mV.

    • Extracellular fluid (ECF): outside the cell, generally more positive.**

  • Electrical neutrality of the body: Overall equality of positive and negative ions but uneven distribution leads to electrical disequilibrium.

Ions and Their Significance

  • Cations: Positively charged ions (e.g., sodium (Na extsuperscript{+}), potassium (K extsuperscript{+}), magnesium (Mg extsuperscript{2+})).

    • Mnemonic to remember:

    • Cation = + (positive)

  • Anions: Negatively charged ions (e.g., chloride (Cl extsuperscript{-})).

    • Mnemonic to remember:

    • Anion = - (negative)

  • Distribution of cations and anions creates electrical disequilibrium: more cations outside the cell, less inside.

Ion Movement and Electrochemical Gradients

  • Electrochemical gradients: Two forces are at play that influence the movement of ions:

    1. Concentration gradient: Movement from high concentration to low concentration (e.g., Na extsuperscript{+}, K extsuperscript{+}).

    2. Electrical gradient: Positive ions are attracted to negatively charged environments and repelled by positive ones.

  • Example to illustrate:

    • Imagine two compartments separated by a membrane that allows only water. Each compartment has equal electrical charge initially.

    • When a potassium leak channel is introduced, here is what happens:

    1. Potassium moves from high concentration to low concentration (from compartment B to A).

    2. This creates differences in charge (positive in A, negative in B).

    3. The electrical gradient will pull potassium back into B, leading to no net movement when concentration = electrical force.

Equilibrium Potential

  • Electrochemical equilibrium: Balance point where concentration gradient movement equals electrical gradient movement.

    • No net movement of ions occurs at this point.

  • Equilibrium potential for potassium: Approximately -90 millivolts.

    • At this potential:

    • Potassium ions have no net movement due to equal opposing forces of concentration and electrical gradient.

Nernst Equation

  • Nernst equation: Formula to calculate equilibrium potential but is not required for direct calculations for this course.

  • Importance: Changing concentration gradients affects equilibrium potential.

Role of Sodium and Potassium in Resting Membrane Potential

  • Concentration and equilibrium potential:

    • Equilibrium potential for potassium: -90mV

    • Equilibrium potential for sodium: +60mV

    • Resting membrane potential: -70mV

  • Conceptual understanding using analogy:

    • Think of potassium and sodium as two opposing forces trying to reach their equilibrium.

    • Potassium pulls towards -90mV, sodium towards +60mV.

  • Membrane permeability: Factors how strongly an ion influences resting potential. The closer the equilibrium potential to resting potential, the more influence it has.

  • Permeability of Potassium vs Sodium:

    • The membrane is approximately 40 times more permeable to potassium than to sodium due to greater presence of potassium channels.

Factors Affecting Membrane Potential

  • More potassium channels than sodium channels lead to a higher permeability for potassium.

  • Permeability Definition: Ability of a membrane to allow specific molecules to pass through.

    • Example: Membrane permeability to water vs. glucose.

  • Changing the number of channels alters membrane potential:

    • Increase in sodium channels: Increases permeability, allowing sodium to flow into the cell, making it more positive (depolarization).

    • Increase in potassium channels: Increases permeability, allowing potassium to exit the cell, making it more negative (hyperpolarization).

Key Terms

  • Depolarization: Cell becomes more positive, above -70mV.

  • Hyperpolarization: Cell becomes more negative, below -70mV.

  • Repolarization: Movement back to resting potential (-70mV).

Examples and Applications

  • Recognizing concepts in hypothetical examples:

    • Alien cell with resting potential at -20mV:

    • Ion A equilibrium potential = +20mV, Ion B = -40mV.

    • Determine which ion influences the resting potential more by proximity to resting potential:

      • Ion B has smaller difference, hence stronger pull.

  • Understanding the role of the sodium-potassium ATPase pump:

    • Pumps three sodium ions out and two potassium ions in.

    • Net movement results in maintaining a negative intracellular fluid compared to extracellular fluid, preventing disturbances to resting potential.

Conclusion

  • The resting membrane potential is crucial in understanding cell physiology, with implications for health and well-being.

  • Overall determinants:

    • Concentration of sodium and potassium in ICF and ECF.

    • Membrane permeability to these ions.

    • Adjustments in permeability can lead to significant physiological changes.