How does a neuron-a living cell-generate electrical impulses? To answer this question, we need to examine the nature of a neuron and the fluids in which it floats. A neuron is a balloon-like bag filled with one kind of fluid and surrounded by a slightly different kind of fluid. The axon is a piece of the bag that has been stretched to form a long, hollow tube. The axon tube is so thin that a few dozen axons in a bundle would be about as thick as a human hair.

Floating in the fluids inside and outside the tube are electrically charged particles called ions. Some of these ions, notably sodium and potassium, carry positive charges. Negatively charged ions of chlorine and other elements also are present. When the neuron is at rest (not transmitting information), more negative ions are inside the cell membrane than in the surrounding fluids. The membrane prevents negative and positive ions from flowing into or out of the cell.

The neuron creates electric signals by moving positive and negative ions back and forth through its outer membrane. Embedded in the membrane-the wall of our balloon-are hundreds of thousands of small gates, called ion channels, that open and close to let the ions pass into and out of the cell. Normally, when resting or not sending information, the ion channels are closed and a slight negative charge is present along the membrane of the cell.

Resting potential is the term given to the stable, negative charge of an inactive neuron. That potential, by the way, is about -70 millivolts, so fourteen neurons have the power of a one-volt battery. An electric eel's 8,400 neurons could generate 600 volts.

In reaction to an incoming impulse-perhaps a pinprick or the sight of someone's face-the neuron's voltage threshold increases, and the sodium gates at the base of the axon open briefly. Positively charged sodium ions flow into the neuron, creating a more positively charged neuron. Then potassium channels open, positively charged potassium ions move out through the membrane, and the neuron returns to a negative charge.

The same process occurs as the next group of channels flip open briefly. And so it goes all the way down the axon, just like a long row of cabinet doors opening and closing in sequence. A neuron becomes activated because hundreds of sodium channels open up when its voltage becomes positive.