Resting Membrane Potential

Resting Membrane Potential

  • Definition: The resting membrane potential is the membrane potential of a neuron at rest.

  • Representation:

    • Graphical representation of membrane potentials is done on a graph with:
    • X-axis labeled as membrane potential (in millivolts)
    • Y-axis labeled as time (in milliseconds)
    • Typical values on the membrane potential axis include:
    • Positive: 15, 30, 45
    • Negative: 0, -15, -30, -45, -70, -85, -90, -100

  • Voltage:

    • Defined as a measure of potential energy or the ability of ions to move due to a separation of charge.
    • Without a separation of charge, ions perceive no motive to move.

Measurement of Membrane Potential

  • Techniques:

    • Use of a voltmeter (Clamp technique) to measure voltage in millivolts:
    • One electrode measures ion concentration just outside the membrane.
    • Another electrode, a thin micropipette, records fluid concentration from the intracellular environment by piercing the membrane.

  • Neuronal Structure:

    • Dendrites, soma, axon hillock, and axon terminals are parts of a neuron.
    • A phospholipid bilayer containing protein channels (transmembrane proteins) regulates ion movement across the membrane which affects resistance.

Ion Concentration Differences

  • Intracellular ion concentrations (inside the cell):

    • Potassium ( ext{K}^+): 140 millimolar
    • Sodium ( ext{Na}^+): 15 millimolar
    • Chloride ( ext{Cl}^-): 4 to 30 millimolar
    • Calcium ( ext{Ca}^{2+}): 0.0001 millimolar

  • Extracellular ion concentrations (outside the cell):

    • Sodium: 140 millimolar
    • Chloride: approximately 110 millimolar
    • Calcium: 1 to 2 millimolar
    • Potassium: 5 millimolar

  • The movement of ions occurs down their concentration gradient (from high to low) until an electrical gradient begins to repel them:

    • Example: The movement of positive ions (like K ext{+} or Na ext{+}) can be repelled by the positive charge accumulating outside the cell.

Potassium Leak Channels

  • Characteristics:

    • Non-gated channels that are always open.
    • Allow ext{K}^+ ions to move down their concentration gradient from inside to outside the cell.

  • Mechanism of action:

    • As ext{K}^+ ions leave, the inside of the cell becomes more negative (efflux of K ext{+}).
    • This continues until the electrical gradient inhibits further potassium movement; this is where equilibrium occurs.
    • ext{Equilibrium Potential for K ext{+}}: approximately -90 millivolts (common in muscle cells).

Sodium Leak Channels

  • Characteristics:

    • Fewer in number compared to potassium leak channels.

  • Movement:

    • Sodium ions, given a pathway through these channels, move from the exterior (high concentration) into the cell (low concentration).
    • Increases the positivity inside the cell, leading to depolarization.

  • Effects:

    • As sodium enters, the resting membrane potential shifts (e.g., may move from -90 millivolts toward -45 millivolts).

Chloride Ion Movement

  • Concentration: Higher outside the cell (approximately 110 millimolar).

  • Movement upon channel activation:

    • Chloride will tend to move into the cell down its concentration gradient.
    • When chloride moves in, it tends to make the inside of the cell more negative, but eventually repelled by a negative electrical gradient when the cell is already negative.

Combined Effects of Ion Channels on Resting Membrane Potential

  • Resting Membrane Potential:

    • Settles at approximately -70 millivolts due to the cumulative effects of potassium, sodium, and chloride channels.

  • Gated Channels:

    • Gated channels can be opened to influence the membrane potential further, allowing more sodium or potassium to enter or leave as needed.
    • The resting membrane potential can change, causing depolarization, repolarization, and hyperpolarization depending on the activity of these channels.
    • Depolarization: Movement toward zero, making the inside less negative.
    • Repolarization: Return to resting membrane potential.
    • Hyperpolarization: Moving beyond resting potential to a more negative value.

  • Key Points:

    • The anatomy and ion channel behavior directly influence the resting membrane potential.
    • Understanding these principles prepares us for exploring graded potentials and action potentials in future studies.