Action Potential Stages to Know for Neuroscience

Action potentials are essential for neuron communication, allowing signals to travel along the axon. Understanding the stages—resting state, depolarization, rising phase, peak, repolarization, hyperpolarization, and refractory period—helps us grasp how neurons transmit information in the nervous system.

1. Resting state

   • The neuron is at a stable, negative charge, typically around -70 mV.
   • Sodium (Na+) channels are closed, while potassium (K+) channels are partially open.
   • The sodium-potassium pump actively maintains the concentration gradient, pumping 3 Na+ out for every 2 K+ in.
   • The membrane is polarized, creating a potential difference across the membrane.
   • This state is crucial for the neuron to be ready to respond to stimuli.

2. Depolarization

   • A stimulus causes the membrane potential to become less negative, moving towards zero.
   • Voltage-gated sodium channels open, allowing Na+ to rush into the cell.
   • The influx of positive charge further depolarizes the membrane, creating a positive feedback loop.
   • This phase is critical for initiating the action potential.
   • The threshold potential (around -55 mV) must be reached for the action potential to occur.

3. Rising phase

   • The rapid influx of Na+ continues, causing the membrane potential to rise sharply.
   • The membrane becomes positively charged, reaching values around +30 mV.
   • This phase is characterized by the opening of more voltage-gated sodium channels.
   • The rapid change in voltage is essential for the propagation of the action potential along the axon.
   • The rising phase is a key indicator of neuronal excitability.

4. Peak

   • The membrane potential reaches its maximum positive value, around +30 mV.
   • Voltage-gated sodium channels begin to inactivate, stopping the influx of Na+.
   • Voltage-gated potassium channels start to open in response to the depolarization.
   • This phase marks the transition from depolarization to repolarization.
   • The peak is crucial for the timing of subsequent phases of the action potential.

5. Repolarization

   • K+ ions flow out of the neuron as voltage-gated potassium channels open.
   • The membrane potential begins to return to its resting state, becoming more negative.
   • The efflux of K+ helps restore the negative charge inside the neuron.
   • This phase is essential for resetting the neuron's membrane potential.
   • Repolarization prepares the neuron for the next action potential.

6. Hyperpolarization (undershoot)

   • The membrane potential temporarily becomes more negative than the resting state, often around -80 mV.
   • This occurs due to the continued efflux of K+ ions after repolarization.
   • Hyperpolarization makes it more difficult for the neuron to reach the threshold for another action potential.
   • This phase contributes to the refractory period, ensuring unidirectional propagation of the action potential.
   • It serves as a protective mechanism to prevent excessive firing of the neuron.

7. Refractory period

   • The period following an action potential during which the neuron is less excitable.
   • It consists of two phases: the absolute refractory period (no action potential can occur) and the relative refractory period (a stronger stimulus is required).
   • The inactivation of sodium channels during the absolute refractory period prevents immediate re-firing.
   • This period is crucial for maintaining the directionality of action potentials along the axon.
   • The refractory period ensures that action potentials are discrete events, allowing for proper signaling in the nervous system.