All signals that the nervous system send are basically electrical signals, passed from nerve cell to nerve cell through the body, like a fire brigade passing buckets to each other up a ladder. When it reaches a muscle, the electrical signal causes that cells to contract, thus moving the muscles. This is why people may seize up or have a spasm when they get an electric shock—it’s erratically triggering a bunch of muscle cells.
In terms of how it works, it’s all due to chemistry. A neuron detects a charge building up in the cell next to it, so it opens up its membrane a bit, allowing ions (usually of an element like sodium or calcium) to enter the cell. Since ions have an electric charge, they change the overall charge in the cell. This causes the other cell next to it to open its membrane, and it continues down the chain. Soon, the original cell reaches its maximum charge, the cell’s membrane reverses its polarity, and the ions are expelled. The whole process is incredibly rapid, going through the body in a matter of milliseconds.
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