Neuroscience: Action Potentials

Action Potential

• An action potential, a rapid change in membrane potential, is generated at the threshold
• The minimum amount of depolarization needed to trigger an action potential is called the threshold, typically -55mV
• Action potentials are all-or-none events, meaning they either occur fully or not at all
• The amplitude and duration of action potentials are consistent
• Weak stimuli don't produce an action potential

Depolarization

• Depolarization, where the cell becomes more positive, starts when the threshold is reached
• This opens voltage-gated sodium (Na+) channels, allowing more Na+ to flow into the cell
• Increased Na+ influx further depolarizes the cell, opening more Na+ channels creating a positive feedback loop
• This positive feedback loop drives the rising phase of the action potential

Repolarization

• The rising phase of the action potential ends when the positive feedback loop is interrupted:
  • Voltage-gated Na+ channels inactivate after a certain time, closing
  • Voltage-gated potassium (K+) channels, responding slowly to depolarization, start to open
• The slow opening of K+ channels allows K+ to move out of the cell, making the cell more negative (repolarizing)
• The membrane potential becomes more negative than the resting membrane potential, a state called hyperpolarization

Refractory Period

• The action potential is followed by a refractory period, a brief period of decreased excitability
• The refractory period is divided into two phases:
  • Absolute refractory period: the cell cannot be stimulated to fire another action potential, regardless of stimulus strength
  • Relative refractory period: the cell can be stimulated to fire another action potential, but only by a stronger stimulus than usual
• The refractory periods are caused by the inactivation of Na+ channels and increased opening of K+ channels

All-or-None Principle

• Action potential amplitude is independent of the stimulus strength.
• A stronger stimulus does not produce a larger action potential but can trigger more frequent firing.

Action Potential Conduction

• Electrical currents across cell membranes are carried by ions like sodium (Na+) and potassium (K+)

Action Potential Initiation

• Signals from dendrites and the cell body reach the axon hillock
• If these signals cause the membrane potential at the axon hillock to depolarize past the threshold, an action potential is triggered

Nobel Prize in Physiology or Medicine (1963)

• Alan L. Hodgkin and Andrew F. Huxley shared the Nobel Prize in Physiology or Medicine for their quantitative description of the ionic currents involved in the action potential.
• They used voltage-clamp recordings to study the ionic currents in the squid giant axon.

Threshold

• If the stimulus reaching the axon hillock is strong enough, the neuron depolarizes by approximately 15mV and reaches the threshold.