Bioelectricity and nerve signals
Living cells use electrical signals: nerves and muscles fire by letting charged atoms rush across their membranes.
Bioelectricity is electricity generated and used by living things. Every nerve and muscle cell keeps a small voltage across its outer membrane, typically about −70 millivolts at rest, by holding different amounts of charged atoms (ions) inside and out: more sodium outside, more potassium inside.
A nerve signal, called an action potential, is a brief reversal of that voltage. When a cell is nudged past a threshold, voltage-gated sodium channels in its membrane spring open and sodium rushes in, making the inside positive; that opens channels further along, so the pulse travels down the fibre. Potassium channels then let positive charge back out and reset the cell. In most neurons the whole spike lasts about a thousandth of a second, and signals travel at between 1 and more than 100 metres per second. Heart muscle cells fire longer pulses lasting a few hundred milliseconds, which an artificial pacemaker can trigger with small, regular electrical pulses.
The idea goes back to the Italian physician Luigi Galvani, who found in 1780 that a dead frog's leg muscles twitched when touched by an electric spark, and published his theory of "animal electricity" in 1791. The fact that external electricity can make muscles contract, even after death, later fuelled some gruesome public demonstrations.
Some poisons work by jamming these channels. Grayanotoxins in "mad honey", for example, stop sodium channels closing properly, disrupting nerve and heart signals.
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