Module 4 - Introduction to resting membrane potential (1)

Resting Membrane Potential (RMP)

Overview of Potentials

  • Three Types of Potentials: RMP, action potentials, and graded potentials.

  • Resting Membrane Potential (RMP):

    • Voltage of the neuron/cell at rest.

    • Changes occur due to the opening of ion channels.

  • Excitation vs. Inhibition:

    • Excitation: Transient depolarization from RMP.

    • Inhibition: Transient hyperpolarization from RMP.

  • Polarized RMPs:

    • Neurons, muscles, and glial cells typically have RMPs ranging from -30 to -90 mV.

    • Non-excitable cells (e.g., epithelial cells, red blood cells) have less polarized RMPs, typically -8 to -30 mV.

Ion Concentration Differences

  • Living Cells:

    • Ion pumps and exchangers maintain specific ion concentrations:

      • K+ is more abundant inside than outside.

      • Na+ is more abundant outside than inside.

      • Ca2+ concentration is very low inside (toxic; can precipitate proteins).

    • [Cl-] gradient varies in neurons during development.

Ion Concentrations in Sea Animals

  • Comparison with Mammals:

    • Sea water animals have over double the ion concentrations compared to mammals.

    • Similar ion ratios across the membrane, with higher K+ inside and higher Na+ outside.

Energies Affecting Ion Movement

  • Movement through Conductors:

    • Like electrons, ions can move through a conductor (solution, ion channels) based on electrical potential energy (voltage).

    • Ions in solution are also influenced by random kinetic energy, promoting diffusion along concentration gradients.

  • Total Energy for Ion Movement:

    • Derived from voltage (membrane voltage, Vm) and the concentration gradient of the specific ion

    • Electrical driving force vs chemical driving force

Thought Experiments on Ion Movement

  • Kinetic Energy Influence:

    • When an ion is positively charged and more concentrated inside, it diffuses along its gradient, while voltage influences its direction based on the membrane potential state (negative inside compared to outside).

  • Manipulating Voltage:

    • Questions to consider:

      • How to enhance ion flow to the outside?

      • How to reverse ion flow to the inside?

      • How to reach equilibrium where net ion flow is zero?

Concentration Gradient and Voltage Impact

  • No Initial Voltage:

    • If no voltage exists initially, ions will flow according to their concentration gradient (no electrical driving force)

    • As positive charges flow, membrane voltage will change.

    • Ions will stop flowing when a new equilibrium is established, reflecting battery behavior.

Nernst Equation for Ion Concentration Gradient

  • Calculating Voltage:

    • The Nernst equation determines the voltage (equilibrium potential) where ion movement halts, influenced by concentration gradients.

    • The equilibrium potential is where the net movement of ions across the membrane is zero, balancing the chemical concentration gradient with the electrical gradient.

    • Key Variables: R (gas constant), T (temperature in K), z (valence), F (Faraday constant).

    • For monovalent ions at 20°C, RT/zF ~ 25 mV. [ E_{ion} = \frac{RT}{zF} \ln \frac{[Ion]{out}}{[Ion]{in}} ]

Squid Example of RMP

  • K+ Flow in Squid:

    • When K+ gradient is maintained, flow continues until RMP equals -74.9 mV (EK).

      • Calculation example shows K+ equilibrium at approximately -74.9 mV.

  • Na+ Flow in Squid:

    • Na+ continues to flow until RMP equals +54.4 mV (ENa).

      • Example: Na+ equilibrium calculated to be approximately +54.4 mV.

Cell Circuit Diagram Understanding

  • Understanding Voltage:

    • Cell membrane acts as a capacitor; different ion gradients function as "batteries."

    • Rapidly flowing ions across the membrane better control Vm.

Ionic Competition influencing Membrane Voltage (Vm)

  • Influence of Ionic Batteries:

    • Ions with higher conductance can effectively pull Vm towards their equilibrium potentials.

    • Most neurons and muscle cells possess RMP close to EK due to high permeability to K+ at rest (via K+ leak channels).

    • Na+ channels counteract K+ flow, leading to more depolarized RMPs.