A neuron that’s at rest—not currently firing a neural impulse or message—is actually electrically charged. The inside of the cell is really a semiliquid (jelly-like) solution in which there are charged particles, or ions. A semiliquid solution also surrounds the outside of the cell and contains ions, too. Although both positive and negative ions are located inside and outside of the cell, the relative charge of ions inside the cell is mostly negative, and the relative charge of ions outside the cell is mostly positive due to both diffusion, the process of ions moving from areas of high concentration to areas of low concentration, and electrostatic pressure, the relative balance of electrical charges when the ions are at rest. The cell membrane itself is semipermeable. This means some substances that are outside the cell can enter through tiny protein openings, or channels, in the membrane, while other substances in the cell can go outside. Many of these channels are gated—they open or close based on the electrical potential of the membrane—more about that in a minute. Inside the cell is a concentration of both smaller positively charged potassium ions and larger negatively charged protein ions. The negatively charged protein ions, however, are so big that they can’t get out, which leaves the inside of the cell primarily negative when at rest. Outside the cell are lots of positively charged sodium ions and negatively charged chloride ions, but they are unable to enter the cell membrane when the cell is at rest because the ion channels that would allow them in are closed. But because the outside sodium ions are positive and the inside ions are negative, and because opposite electrical charges attract each other, the sodium ions will cluster around the membrane. This difference in charges creates an electrical potential.

Think of the ions inside the cell as a baseball game inside a stadium (the cell walls). The sodium ions outside the cell are all the fans in the area, and they want to get inside to see the game. When the cell is resting (the electrical potential is in a state called the resting potential, because the cell is at rest), the fans are stuck outside. The sodium ions cannot enter when the cell is at rest, because even though the cell membrane has all these channels, the particular channels for the sodium ions aren’t open yet. But when the cell receives a strong enough stimulation from another cell (meaning that the dendrites are activated), the cell membrane opens up those particular channels, one after the other, all down its surface, allowing the sodium ions (the “fans”) to rush into the cell. That causes the inside of the cell to become mostly positive and the outside of the cell to become mostly negative, because many of the positive sodium ions are now inside the cell—at the point where the first ion channel opened. This electrical charge reversal will start at the part of the axon closest to the soma, the axon hillock, and then proceed down the axon in a kind of chain reaction. (Picture a long hallway with many doors in which the first door opens, then the second, and so on all the way down the hall.) This electrical charge reversal is known as the action potential because the electrical potential is now in action rather than at rest. Each action potential sequence takes about one-thousandth of a second (See Figure 2.2.) Now the action potential is traveling down the axon. When it gets to the end of the axon, something else happens: The message will get transmitted to another cell (that step will be discussed momentarily). Meanwhile, what is happening to the parts of the cell that the action potential has already left behind? How does the cell get the “fans” back outside? Remember, the action potential means that the cell is now positive inside and negative outside at the point where the channel opened. Several things happen to return the cell to its resting state. First, the sodium ion channels close immediately after the action potential has passed, allowing no more “fans” (sodium ions) to enter. The cell membrane also literally pumps the positive sodium ions back outside the cell, kicking the “fans” out until the next action potential opens the ion channels again. This pumping process is a little slow, so another type of ion gets into the act. Small, positively charged potassium ions inside the neuron move rapidly out of the cell after the action potential passes, helping to more quickly restore the inside of the cell to a negative charge. Now the cell becomes negative inside and positive outside, and the neuron is capable of “firing off” another message. Once the sodium pumps finish pumping out the sodium ions, the neuron can be said to have returned to its full resting potential, poised and ready to do it all again.

Figure 2.2

The Neural Impulse Action Potential

To sum all that up, when the cell is stimulated, the first ion channel opens and the electrical charge at that ion channel is reversed. Then the next ion channel opens and that charge is reversed, but in the meantime the first ion channel has been closed and the charge is returning to what it was when it was at rest. The action potential is the sequence of ion channels opening all down the length of the cell.

 So if the stimulus that originally causes the neuron to fire is very strong, will the neuron fire more strongly than it would if the stimulus were weak?

Neurons actually have a threshold for firing, and all it takes is a stimulus that is just strong enough to get past that threshold to make the neuron fire. Here’s a simple version of how this works: Each neuron is receiving many signals from other neurons. Some of these signals are meant to cause the neuron to fire, whereas others are meant to prevent the neuron from firing. The neuron constantly adds together the effects of the “fire” messages and subtracts the “don’t fire” messages, and if the “fire” messages are great enough, the threshold is reached and the neuron fires. When a neuron does fire, it does so in an all-or-none fashion. Neurons are either firing at full strength or not firing at all—there’s no such thing as “partial” firing of a neuron. It would be like turning on a light switch—it’s either on or it’s off.

So what’s the difference between strong stimulation and weak stimulation? A strong message will cause the neuron to fire repeatedly (as if someone flicked the light switch on and off as quickly as possible), and it will also cause more neurons to fire (as if there were a lot of lights going on and off instead of just one).

 Now that we know how the message travels within the axon of the cell, what is that “something else” that happens when the action potential reaches the end of the axon?