Spark by Timothy J. Jorgensen

Author:Timothy J. Jorgensen
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2021-08-03T00:00:00+00:00

I said earlier that an electric current is the rate of flow of electric charge past some particular point. Notice I did not specifically say “negative charge”; I just said “charge.” Yes, I know; when we discussed the electron, my explanation left the impression that all electric currents are due to the movement of negatively charged electrons. I did that because for most electronic circuits that is exactly the case. But that’s not so for electric currents in neurons. The electric current across the cell membranes of neurons is due to the flow of positive charge, not negative charge. If you feel I’ve deceived you, please accept my apology. I hope you aren’t now skeptical of me. But I thought telling the half-truth was justified in order to illustrate a larger point: electrons represent the most fundamental unit of electrical charge. Now that we all appreciate that point, let’s look at a situation where, remarkably, you can have an electric current without moving electrons.

As we discussed in chapter 9, the importance of the electron largely has to do with its chemistry. Electrons make up the “glue” that holds atoms together to form molecules. In the case of metallic elements, the atomic nuclei have only a loose grip on the electrons in their outer orbitals. This permits metals to act in a special way as far as electricity is concerned. The fact that the positively charged nuclei of metals don’t have a solid grip on their outer negative electrons means that the atoms of metals readily ionize into positive ions when external forces dislodge one of their outer electrons. These charged atoms (i.e., ions) have a net positive charge because their vagabond outer electrons have gone missing, taking their negative charge along with them. What this means, in practical terms, is that for a solid piece of metal, like a copper wire, only the electrons are available to take a journey; the positively charged atomic nuclei must stay put. That’s why, for electrical circuits made from metals, the movement of electrons, and electrons only, is producing the current. They move in synchrony among the immobilized metal ions, hopping from nucleus to nucleus, as they travel within the wire.

But for other materials, such as salts—compounds that spontaneously form ions when dissolved in water—and ions that are dispersed within a gas, electrons are only a part of the story. In such cases, all the ions, both positively and negatively charged, are free to move within the liquids and gases, and thus participate in generating the electric current, moving along the lines of force of an electric field toward the attracting electrode. However, the large size of most ions retards their movement. As a result, any race between ions and electrons in either a liquid or a gas is always won by the electrons. Their extremely small size allows electrons to travel amazingly fast. Ions, in contrast, are relative slowpokes.

You might wonder why neurons put their money on the slowpoke. Basically, it comes down to a matter of control.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.