Here, we introduce the reader to the underlying physics of neurons and their signals and to the long-term goal of modeling human memory systems from the bottom up. Neurons are famous for their electricity, as has been known since the days of Luigi Galvani in the late eighteenth century. His publications included the observation that an electrical charge causes the dissected leg of a frog to twitch. Accurate sketches of neurons were originally presented in the late nineteenth and early twentieth centuries by such medical doctors as Johannes Evangelista Purkinje and Santiago Ramón y Cajal, showing that neurons have a treelike structure that is suggestive of electrical circuitry. Eventually, in the mid-twentieth century, much progress took place in showing that neurons are electrically active devices and that, indeed, they form complex circuits.

It is now thought that neural membranes are responsible for the electricity that transverses neurons. A thin membrane serves to separate positive and negative charge to create a steady voltage of a few millivolts. Although this dc voltage is low, it results in a significant electrical field since the membrane is very thin, only a few nanometers thick. This electric field, if it were in air, would create a bolt of electricity. Fortunately, a membrane does not pass electrical charge via spark, otherwise known as dielectric breakdown, thanks to the high dielectric strength of a membrane ...

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