MH

Lecture on Membrane Potentials and Gated Channels

Membrane Currents and Action Potentials

  • Inward and Outward Currents:

    • Inward currents (e.g., sodium ions) increase the membrane potential toward a positive value.
    • Outward currents (e.g., potassium ions) decrease the membrane potential back toward a resting state.

  • Current Measurement:

    • During depolarization from -65 mV to near 0 mV:
    • Positive current is applied to maintain voltage.
    • A positive current during inward sodium flow and a negative current during outward potassium flow are used to measure differential currents.

Capacitive and Gating Currents

  • Capacitive Current (_C):

    • Represents the charge change across the membrane.
    • Peaks quickly and then decreases as the membrane becomes depolarized.

  • Gating Current (_G):

    • Associated with the opening and closing of voltage-gated channels.
    • Can be defined by changes in membrane voltage which lead to conformational changes in ion channels.

Action Potential Dynamics

  • Voltage Clamp:

    • Used to control membrane potential while observing ionic currents.
    • Allows distinction between inward (sodium) and outward (potassium) currents.

  • Phases of Action Potential:

    • Resting State:
    • Membrane potential is around -60 mV, with voltage-gated sodium channels closed.
    • Depolarization:
    • External current pushes membrane potential to around +40 mV.
    • Sodium channels open due to gating current, allowing Na+ influx.
    • Repolarization:
    • Sodium channels inactivate; potassium channels open, leading to K+ efflux, restoring negative potential.
    • Hyperpolarization:
    • Membrane potential drops below resting potential as K+ continues to exit.

Importance of Ion Conductance

  • Conductances:

    • Conductances for sodium (gNa) and potassium (gK) change during action potentials. Higher gNa during depolarization leads to inward current, while gK contributes to repolarization.

  • Equilibrium Potential:

    • Understanding the equilibrium potential of sodium (E_Na) and the effects of membrane permeability on driving forces.

Experimental Tools and Drugs

  • Neurotoxins and Their Effects:

    • Tetrodotoxin, Saxitoxin: Block sodium channels, preventing action potentials.
    • Tetraethylammonium (TEA): Blocks potassium channels, allowing isolation of sodium current.

  • Calcium Current Analysis:

    • Calcium channels contribute differently to action potentials compared to sodium channels. Their slower kinetics can extend the duration of action potentials.

Propagation of Action Potentials

  • Unidirectional Propagation:

    • Action potentials propagate along axons due to the sequential opening of sodium channels ahead of the depolarized area and inactivation behind it.

  • Myelination:

    • Increases membrane resistance (Rm), thereby reducing capacitance (Cm) and increasing action potential conduction velocity. Myelinated axons exhibit passive and active conduction (saltatory conduction) between nodes of Ranvier.

  • Effects of Demyelination:

    • Leads to failure of action potentials to propagate if there's a significant gap in myelination.

Summary of Properties Affecting Conduction Velocity

  • Diameter of the axon and myelination greatly influence conduction velocity. Larger diameter = lower internal resistance (_i).

  • Length constant identifies how far the current can travel along an axon before decreasing significantly.

  • Membrane time constant (tau) impacts how quickly the membrane can respond to stimuli.

Active vs. Passive Properties of Neurons

  • Passive Electrical Properties: Governed by resistances and capacitances inherent to the neuron's structure.
  • Active Properties: Result from the dynamics of voltage-gated ion channels during action potentials, affecting excitability and the ability to transmit signals efficiently.