“Professor?” one of the students interrupts.

“Yes? You have a question?”

“Since the neuron is continually letting in sodium and letting out potassium, wouldn’t it eventually fill up with sodium and exhaust its supply of potassium?”

Mindstein nods in agreement. “Yes, that is another problem for our neuron. Eventually it has to get the sodium out and the potassium back in. That job is done by many sodium/potassium pump proteins that expel three sodium ions and then bring in two potassium ions from outside the cell.”

“So the neuron reacts by first admitting a number of sodium ions through a sodium channel, wiping out its electrical charge, then it allows potassium to leave, thereby regenerating its negative charge. Finally it pumps out sodium and pumps potassium back in, restoring the resting balance of sodium and potassium ions.”