"Are nerves like electrical wires?"
Toma asked while looking at a nerve cell diagram.
Rei denied. "No. Electrons don't flow."
"Then what?"
"Ions. Sodium and potassium."
Kana showed interest. "Ions carry signals?"
"More precisely, ion flow changes potential," Rei explained.
Toma wrote in his notebook. "Membrane potential?"
"There's a potential difference across the cell membrane. Usually about -70 mV."
"Negative?"
"Inside the cell is more negative than outside. More potassium ions, fewer sodium ions."
Kana asked. "Why the difference?"
"Sodium-potassium pump," Rei drew a diagram. "Uses ATP for active transport."
"Three sodium out, two potassium in. Creates concentration gradient."
Toma understood. "Making a battery with energy?"
"You could say that. This gradient is the basis of signals."
Rei continued. "When stimulation comes, sodium channels open."
"Channel?"
"Tunnel ions can pass through. Made of protein."
Kana looked at the model. "Hole?"
"Selective hole. Sodium channel only lets sodium through."
"How does it select?" Toma asked.
"Size and charge," Rei explained. "Internal channel structure fits specific ions."
"When sodium channel opens, sodium flows in."
"Flows in?"
"From outside to inside. Both concentration gradient and potential gradient are driving forces."
Kana calculated. "High concentration outside, negative potential inside. So positive sodium enters?"
"Perfect," Rei acknowledged. "This influx makes membrane potential positive."
"Called depolarization."
Toma was surprised. "From -70 mV to where?"
"About +30 mV. One hundred millivolt change."
"Big!"
"This is the peak of action potential."
Kana asked. "What if it stayed open?"
"It doesn't," Rei explained. "Sodium channel inactivates."
"Inactivates?"
"Automatically closes after opening. Image two gates."
"One is voltage-dependent. The other is time-dependent."
Toma was confused. "Voltage-dependent?"
Rei drew a diagram. "When membrane potential changes, channel shape changes."
"Protein structural change. Has sensor part that detects electric field."
Kana understood. "Door operated by electricity."
"Yes. When potential exceeds threshold, it opens suddenly."
"Then, potassium channel opens," Rei continued.
"Potassium from inside to outside?" Toma guessed.
"Correct. High concentration inside, so goes out."
"This returns membrane potential. Repolarization."
Kana confirmed timing. "Sodium channel opens first, potassium channel later?"
"Right. Time lag. That's why potential goes up then down."
Toma got excited. "This is the nerve signal!"
"Action potential. Follows all-or-none law."
"All-or-none?" Kana asked.
"If threshold is exceeded, always fires. If not, nothing happens."
Rei summarized. "Stimulus→depolarization→sodium influx→potential rise→potassium efflux→repolarization"
"Then, sodium-potassium pump restores concentration gradient."
Kana asked. "How fast is this signal?"
"Depends on nerve type. With myelin sheath, over 100 meters per second."
Toma was surprised. "Fast!"
"Saltatory conduction. Action potential occurs only at nodes of Ranvier."
Kana stared at the ion channel diagram. "Tiny doors opening and closing."
"That opening and closing creates thought," Rei said quietly.
"Huh?"
"Brain nerve cells, one hundred billion. Each transmits signals through ion channels."
Toma murmured. "Thinking is also ion flow?"
"You could say that. Consciousness also arises from channel opening and closing."
Kana was moved. "Signals when ion channels open. That's my thoughts."
"Life's electrical signals," Rei acknowledged.
The three fell silent. Invisible ion flow creates the mind.