"Electricity is flowing?"
Kana looked at the nerve cell diagram.
Milia nodded. "There's potential difference across cell membrane."
"Like battery?"
"Yes. About -70 mV potential difference."
Rei explained. "Ion concentration difference creates potential."
"Ions like Na⁺ and K⁺?"
"Mainly those. Cl⁻ and Ca²⁺ are also important."
Kana wrote in her notebook. "Why is concentration different?"
"Because ion pumps actively transport them," Milia answered.
"Actively?"
"Using ATP, transport against concentration gradient."
Rei drew a diagram. "Na⁺-K⁺ pump. Na⁺ out, K⁺ in."
"Transports in 3-to-2 ratio. That creates potential difference."
Kana showed interest. "Then what happens?"
"Cell inside becomes negative, outside positive."
Milia continued. "This state is called resting potential."
"Resting? But ions are moving?"
"Dynamic equilibrium. Pump transports, channels leak. Stable overall."
Rei drew another diagram. "Ion channel. Protein traversing membrane."
"Hole?" Kana asked.
"Allows only specific ions. Selection by size and charge."
"How?"
"Channel interior structure matches specific ion."
Milia showed experimental data. "K⁺ channel selects K⁺ smaller than Na⁺."
"Selects smaller? Why doesn't larger Na⁺ pass?"
"Na⁺ is surrounded by water molecules, effectively larger. K⁺ can shed water and pass."
Rei supplemented. "Channel lowers energy barrier for dehydration."
Kana understood. "Selective permeability."
"Yes. This is basis of neural signals."
Milia continued explaining. "When stimulus comes, Na⁺ channels open."
"Na⁺ enters inside?"
"Both concentration gradient and electrical gradient point inward."
Rei drew the movement. "Inside rapidly becomes positive. Depolarization."
"This is action potential."
Kana took notes. "Then what?"
"Soon Na⁺ channels close and K⁺ channels open."
"K⁺ exits?"
"Yes. Inside becomes negative again. Repolarization."
Milia continued. "This potential change spreads to adjacent part."
"Propagation?"
"Action potential travels along nerve fiber. This is neural transmission."
Rei organized. "Ion collisions carry information."
Kana was surprised. "Thinking is also ions?"
"All information processing in brain is based on ion flow."
Milia added. "At synapse, Ca²⁺ is important."
"Calcium?"
"Induces neurotransmitter release."
Rei explained. "When action potential arrives, Ca²⁺ channels open."
"Ca²⁺ influx causes vesicle fusion."
"Vesicle?" Kana asked.
"Bag containing neurotransmitter. Fuses with membrane, releasing contents."
Milia drew a diagram. "Released transmitter binds to receptors on next nerve cell."
"Ion channels open again?"
"Yes. Signal transmits successively."
Kana reviewed her notes. "All ion movement..."
"Life is ion dance," Rei said quietly.
Milia smiled. "Small collisions create thought."
Kana murmured. "In my brain right now..."
"Billions of ion channels opening and closing simultaneously."
Rei continued. "Every time you think, ions flow."
"Amazing..."
Milia opened the window. "Border of chemistry and electricity is life."
Kana closed her notebook. "Small collisions of ions."
"That's source of consciousness," Rei said.
The three fell silent. Invisible ions continuously flow without rest. Carrying thought.