Cells and cell membranes
All cells are surrounded by a cell membrane. We will neglect all the complexities of the metabolic and structural apparatus found in the interior of the cell and simply consider it as a little bag, formed by the cell membrane, and filled with saline (i.e., water with ions dissolved in it). Likewise, we will assume that the exterior of the cell is a bath of saline. This approximation is drastic but not unreasonable, particularly since we are here only interested in the basis of electrical information processing within the neuron and between neurons.
How can a phospholipid molecule be immersed in water at one of its ends and, at the same time, avoid to be in water at its other end? The answer is, as so often, team work! If enough phospholipid molecules get together, they can bundle up their oily (hydrocarbon) ends together, forming a double-layered sheet with the hydrocarbon ends in the center, and, at the same time, bath their phosphate ends in water on the outside of the sheet. This does not work at the borders of the sheet so it is best to have no ends, i.e., to close the sheet on itself, forming a closed sphere. The result is a certain volume of water (or saline) enclosed by a double layer of phospolipid molecules and -- Voilà ! -- a cell! In fact, such artificial cells can be made from its constituent phospolipid molecules (for references see Scott 1975).
According to our simplification, the inside and the outside of the cell are both solutions of various salts in water. As opposed to the cell membrane, salt water constitutes quite a good conductor because there are free ions that can transport electrical charges. What we have then is two conductors (the inside and the outside of the cell), separated by an insulator (the membrane). This makes it possible to have different amounts of electrical charges inside and outside the cell. I
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