1. The Resting Potential: A Battery at Rest (~ -70 mV)

Think of a neuron at rest like a tiny, charged battery. The cell membrane acts as the barrier, separating different electrical charges. In this "static" state, the neuron is polarized.

Visualizing the Setup: We need to establish our baseline environment. We use specific color coding and icons that will remain consistent throughout this sequence.

• The Inside is Negative: The interior cytoplasm (shaded orange) has a net negative charge, marked by numerous small negative (-) signs.
• The Outside is Positive: The extracellular environment (shaded blue) has a net positive charge, marked by positive (+) signs.
• The Gates are Closed: Large blue sphere proteins represent the essential ion channels (specifically for Sodium, or $Na^+$). In this resting image, they are tightly shut. The scene is calm, representing a cell waiting for a signal.

2. Depolarization: The Trigger and the "Spike" (~ -55 mV to +40 mV)

The action potential begins when the neuron receives a stimulus. If that stimulus is strong enough (reaching a "threshold"), it triggers the rapid firing phase. The membrane permeability dramatically changes, and the "battery" flips its polarity.

Visualizing the Action: In this image, the calm scene established in Image 0 is disrupted.

• The Stimulus Arrives: A small, glowing yellow 'stimulus' icon hits the membrane surface.
• The Gates Open Wide: Crucially, the blue sphere protein channels (the $Na^+$ gates) that were closed in Image 0 are now wide open.
• The Red Rush ($Na^+$ Influx): The stimulus triggers a massive, energetic rush of small, bright red spheres (representing positive $Na^+$ ions) into the cell.
• The Charge Flips: This flood of positive charge completely overwhelms the internal negative environment (the orange cytoplasm). The interior is momentarily positive, represented by the bright red positive (+) signs replacing the starting negative signs from Image 0.

This rapid flip from negative to positive is the "spike" of the action potential.

3. Repolarization: The Reset Phase (Returning to -70 mV)

The positive spike of Depolarization (Image 1) is incredibly brief. Almost immediately, the neuron moves to reset itself and restore the original negative resting potential established in Image 0. This is the Repolarization phase.

Visualizing the Correction: The energetic mood of Image 1 is now replaced by directional flow, counteracting the previous step.