The cell at rest

Cell Membrane and Resting Potential

  • Resting State of the Cell:

    • The cell has a negative charge inside when at rest.

    • This is represented by the presence of negatively charged ions and an overall negative membrane potential.

  • Cell Membrane Structure:

    • Comprised of a bilipid membrane that separates the inside and outside of the cell.

    • Important for maintaining ionic gradients.

Electrode Measurements

  • Recording Technique:

    • Insertion of electrodes into the axon allows for measurement of membrane potential via voltmeters.

  • Key Characteristics:

    • Negative inside (intracellular) compared to the positive outside (extracellular).

Ionic Composition Inside and Outside the Cell

  • Ions Present:

    • Sodium ( ext{Na}^+), Chloride ( ext{Cl}^-), Potassium ( ext{K}^+).

    • Negatively charged anions that cannot leave the cell are often ignored in potential calculations.

  • Focus Ions:

    • Sodium and Potassium play critical roles in determining the membrane and action potentials.

Electrostatic Pressure and Diffusion

  • Sodium Ions ( ext{Na}^+):

    • Electrostatic Pressure (EP): Tries to pull Na inside since the inside is negative and opposites attract.

    • Diffusion: Due to higher concentration of Na outside, diffusion also drives Na into the cell.

    • Outcome: Both forces push Na into the cell, increasing the chances sodium enters.

  • Potassium Ions ( ext{K}^+):

    • Electrostatic Pressure: Wants K to stay inside (as it’s negative inside), yet also encourages its entry to neutralize charge.

    • Diffusion: Opposes this, as K concentration is higher inside, thereby pushing it out of the cell.

    • Outcome: Conflicting forces at play for K with EP wanting it retained while diffusion drives it out.

Forces at Play and Their Direction

  • Comparison of Sodium and Potassium:

    • Sodium has a higher net driving force towards the inside compared to Potassium due to the synergy of both EP and diffusion pushing it in the same direction.

    • Since both forces for Na align, it experiences greater pressure to enter vs. K, which faces conflicting forces.

  • Analogy for Clarity:

    • Airplane Scenario:

    • Engines (representing a strong driving force) and wind (secondary influence) together determine movement.

    • If both engines and wind push the plane the same way, it moves more swiftly compared to scenarios where forces conflict.

Summary of Driving Forces

  • Net Driving Forces:

    • Sodium ions experience a stronger overall push to enter the cell given that both EP and diffusion support this.

    • Potassium ions face opposing forces, hindering its net entry despite EP's desire for retention.