The Action Potential

Movement of ions

Movement of Na+

  • Flows inwardly through sodium-selective channels because concentration and electrical gradients are inward
  • Driving force for Na+ influx = Vm - ENa
    • When negative inward movement of Na+ occurs - membrane potential driven towards ENa

Movement of K+

  • Flows outwardly through potassium selective channels because concentration gradient is outward and has an energy which exceeds the (inward) chemical gradient
  • Driving force for K+ influx = Vm - EK+
    • When positive, outward movement of K+ occurs - membrane potential driven towards EK+

Ion channels

  • Protein complexes that speed the rapid flow of selected ions
  • Closed state = no ion flux, open state = conducts selected ions
  • Types of ion channels:
    • Voltage-gated - responsible for action potentials
    • Ligand-gated
    • Mechanical, thermal etc. - respond to physical stimuli

Action potential

  • Brief electrical signals in which the polarity of the nerve cell membrane is momentarily reversed
  • Constant magnitude and velocity along axon, allowing signaling over long distances
  • ‘All or nothing’ - only generated if the threshold potential is reached
notion image

Depolarization

  • The membrane potential becomes less negative (or even positive) - upstroke
  • Mediated by the opening of voltage-activated Na+ channels
    • Positive feedback - opening of a few channels causes further channels to open → further depolarization

Repolarization

  • The membrane potential is returning back to resting value - downstroke
  • Closure of Na+ and opening of K+ voltage gated channels
    • Negative feedback - outward movement of K+ causes repolarization (turns off stimulus for opening)

Hyperpolarization

  • The membrane potential becomes more negative - undershoot
  • Voltage-gated K+ channels remain open after the resting potential has been reached

Refractory period

  • Closed state (non-conducting) → depolarization → open state (conducting) → maintained depolarization → inactivated state (non-conducting) → repolarization → back to closed state – ready for next action potential

Absolute refractory period

  • No stimulus, however strong, can elicit a second action potential
  • All Na+ channels inactivated

Relative refractory period

  • A stronger than normal stimulus may elicit a second action potential
  • Mix of inactivated and closed channels, plus membrane is hyperpolarised

Conduction along the axon

Passive conduction

  • Nerve cell membrane is not a perfect insulator - passive signals do not spread far from their site of origin
  • Insulation by myelin and increasing axon diameter will increase the conduction velocity

Saltatory conduction

  • Action potential jumps from one node of Ranvier to the next