Written by Travis M. Moore
Last edited 11-Jun-2020
The previous lesson covered how an action potential is generated in a neuron. But where is all this depolarizing and repolarizing happening exactly? How does the action potential reach the axon terminals at the other end of the neuron? We will answer these questions here, as well as touch on what activity we are measuring when recording electrical potentials from the scalp with electrodes.
The action potential is initially generated at a point in the neuron called the axon hillock (blue area in Figure 1). When a neuron "fires" (depolarizes) it starts at the hillock. All the Na+ and K+ movement first happens here. But then what? The axon hillock is a long way away from the axon terminals, which connect to the next neuron in the chain (the postsynaptic neuron). What happens is that the neuronal signal starts at the axon hillock, and then travels down the length of the axon to the terminals. Let's think about how this happens.
We know that current (the flow of electricity) in the body involves the transfer of ions. Just like outside the body, electricity needs some kind of wire along which it can travel in order to get from one place to another. If you've ever seen power lines running across poles on the side of the road then you have already seen evidence that electricity must be transported. When your power goes out and you hear someone say "a tree fell on the lines", it means the tree physically broke the connection (i.e., the wires) and now electricity cannot reach your home. Similarly, think of an axon as a wire that conducts electricity from the axon hillock to the axon terminals. If the axon breaks, the neuron can no longer communicate with the postsynaptic neuron and the neural signal cannot continue.
We have already discussed that current in the body takes the form of moving ions across gradients. Well, it's no different in the axon itself than at the axon hillock. There are Na+ and K+ channels along the axon (Figure 2), so the process of depolarization can occur down its entire length. To understand how this happens, we need to learn the term dipole. If we break the word down, di- is a Greek prefix meaning "two" and "pole" refers to opposite ends of an object or axis. The earth has two poles, one in each hemisphere (one has Santa the other has penguins). The term pole can also have the connotation of two ends that carry opposite electrical charges. Thus, a dipole refers to a positive charge on one end and a negative charge on the opposite end. Like a battery. Or ions in a gradient separated by a cell membrane. Sound familiar?
That means a dipole is created when the neuron is at rest, and an opposite dipole once Na+ enters the cell to make the net charge positive (i.e., depolarization). In other words, the resting potential can be considered a dipole where the outside of the cell membrane is positive and the inside negative. When a neuron depolarizes, the dipole reverses; the dipole is more negative outside the cell membrane and more positive inside. Figure 3 shows the dipole at the axon hillock during peak depolarization.
Notice that the depolarized part of the neuron only affects a small length of the axon. Depolarization is a relatively localized phenomenon, meaning it happens in a restricted area. Remember that the sodium channels that open initially do so because of the signal from a sensory cell (like an inner hair cell). The Na+ released when the hair cell depolarizes is only enough to reach a limited number of sodium channels in the auditory neuron. So only a small length of the axon depolarizes at a time. Another way of putting it is the dipole created by the initial depolarization is pretty small. So now what? How does the signal keep moving down the axon?
Take a look at Figure 4. Look how the charges are aligned at the front of the depolarized area. Notice anything? There is positive charge on the inside of the flipped dipole and there is negative charge inside the rest of the axon (i.e., it's at rest). We know that opposite charges attract, so that positivity in the dipole moves forward.
Notice what happened in the initial area that was depolarized. It repolarized, making the inside of the cell negative again. Once the ion pumps have restored the Na+ and K+ gradient, the neuron can fire again. A simple way to think about the dipole traveling down the axon is to imagine a row of dominoes (Figure 5). As soon as the positive charge from depolarization reaches the negative charge in the axon ahead, it is pulled forward. It keeps getting pulled forward until the dipole reaches the axon terminals (i.e., the end of the line). Figure 6 shows what's happening at the ion channels as the dipole moves down the axon. Figure 7 gives the stage of the action potential as it moves down the axon.
Figure 8 shows an animation of what this traveling dipole looks like. The figure includes a measure of the voltage, and what the ion channels are doing at each stage of the action potential. Note that the voltage waveform is backwards to the one we looked at previously. That's because normally when the action potential is plotted time starts at 0 ms along the left edge of the plot, like most plots. However, the animation shows the voltage in real time, where the rightmost edge is now the 0 point. Figure 9 below shows the whole processes sped up so you can appreciate the elegance of this process. The formal way to refer to the dipole traveling down the axon is propagation or transduction.
Now you know what is going on in the animation at the beginning of the biological signals lesson. The animation is repeated below so you can take a second to appreciate how much is really going on when the action potential travels down the axon. In fact, take a closer look at the image and you can see the dipole moving with the blob of light. Didn't notice that before, did you?
For more information about dipoles, watch the following 6-minute video. Don't worry if you don't come away being able to do vector addition - for our purposes that is overkill. But the principle is important, and will show up later in this module.