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.