Action potentials

Page 1: Action Potential and Neuron Physiology

Resting Membrane Potential

  • Established primarily due to permeability of the membrane to potassium ions (K+).

Initiating Action Potential

  • Location: Action potential begins at the axon hillock.

  • Depolarization:

    • Induced by electrical stimulus.

    • Involves opening of voltage-gated sodium channels.

    • Sodium ions (Na+) flow into the cell, shifting membrane potential from negative to positive.

    • If threshold potential is reached, an action potential is generated.

    • All-or-Nothing Principle: Action potentials only occur if threshold is met; maximum response if threshold reached.

Returning to Resting State

  • Voltage-gated Sodium Channels: Close post-depolarization.

  • Repolarization:

    • Caused by opening of potassium channels.

    • K+ ions move out of the cell, making membrane potential more negative, approaching resting potential.

    • Can overshoot resting potential leading to hyperpolarization.

  • Na+/K+ ATPase: Not involved in repolarization process.

Refractory Period

  • Absolute Refractory Period:

    • Begins after sodium channels close.

    • Channels become inactive, cannot reopen regardless of membrane potential.

  • Relative Refractory Period:

    • Sodium channels recover gradually from inactivation.

    • Neuron can generate action potential but requires a stronger than normal stimulus.

    • Initial required stimulus strength is high, decreases as recovery progresses.

Propagation of Action Potentials

  • Action potentials propagate via local currents.

  • Local currents induce depolarization in adjacent axon regions, causing new action potentials.

  • Recently depolarized areas cannot initiate further depolarization due to the refractory period.

  • Distance of Propagation:

    • Depends on membrane capacitance and resistance.

    • Membrane Capacitance: Ability to store charge; lower capacitance allows longer threshold distance.

Page 2: Myelination and Neuronal Signal Transmission

Myelinated Axons

  • Function: Enhance speed of electrical signal conduction and energy efficiency.

  • Myelin Sheath:

    • Forms an insulating layer around axons.

    • Contains gaps known as Nodes of Ranvier where axonal membrane is exposed.

    • High density of ion channels at nodes enables action potentials only here.

  • Improved Conduction: Myelin sheath increases membrane resistance and reduces capacitance.

  • Saltatory Conduction:

    • Action potentials jump from node to node, rapidly conducting signals down the neuron.

Action Potentials in the Heart

  • General Concept: Action potentials trigger contractions in cardiomyocytes through ion fluxes.

  • Five Phases (0-4) of cardiomyocyte action potential:

    • Phase 4: Resting Phase

      • Resting potential of -90 mV due to K+ leaks through inward rectifier channels.

      • Na+ and Ca2+ channels closed.

    • Phase 0: Depolarization

      • Triggered by action potential from a neighboring cell.

      • Fast Na+ channels open, leading to Na+ influx, raising TMP rapidly toward threshold (-70 mV).

      • Large Na+ current rapidly depolarizes to 0 mV, causing overshoot.

      • L-type Ca2+ channels open at TMP > -40 mV.

    • Phase 1: Early Repolarization

      • TMP becomes slightly positive.

      • Some K+ channels open, K+ leaves the cell, returning TMP to around 0 mV.

    • Phase 2: Plateau Phase

      • L-type Ca2+ channels remain open, allowing a sustained Ca2+ influx.

      • K+ outflow occurs, balancing currents to maintain TMP just below 0 mV.

    • Phase 3: Repolarization

      • Gradual inactivation of Ca2+ channels.

      • K+ outflow exceeds Ca2+ inflow, returning TMP to -90 mV.

      • Ionic gradients restored via Na+-Ca2+ exchanger, Ca2+-ATPase, and Na+-K+-ATPase.