While Galvani was later proven wrong in the details, he wasn’t totally off. Virtually every cell on every branch of the tree of life expends a hefty chunk of its energy budget — in some cells, more than half — on maintaining a voltage across its membrane. The voltage difference that results, called the membrane potential, stores potential energy that can be released later. It’s like the pressure behind a dam: Gravity tugs water downhill, and dams store energy by holding water at the top of a hill. Like gravity, the electrochemical force tugs charges “downhill” — positive charges stream toward negative charges and vice versa in electrical currents. Blocking that flow, for example with a cell membrane, stores up electrical potential energy.
The electric currents that pour from our wall sockets are streams of electrons. In cells, “it’s quite similar, but not exactly the same,” said Elias Barriga, who studies tissue biophysics at the Dresden University of Technology. “We are fueled by ions.”
Ions are atoms or molecules that carry charge because of extra or missing electrons, which give them negative or positive charges, respectively. They can enter and exit cells only through specialized protein channels and pumps. Just as hydroelectric plants can use surplus energy to pump water back up into the reservoir for later use, cells use their chemical energy to pump ions “uphill” against the electric flow. By controlling the natural current and letting positive or negative charge build up on either side of their membranes, cells maintain their membrane potential. And if this energy is used or leaks away, cells can replenish it by expending more of their chemical energy.
Elias Barriga has shown that frog embryos generate electrical fields to guide cell migration. The study of bioelectricity, formerly stranded in biology’s backwaters, is “coming back like crazy,” he said. Courtesy of Elias Barriga
“You generate a potential: what’s inside versus what’s outside, a different concentration of ions,” Barriga said. “That is the source of bioelectricity.”
Neurons make use of this biological electricity to share information. By releasing messenger molecules called neurotransmitters that open and close ion channels, neurons can nudge their neighbors’ membrane potentials up or down. If these chemical nudges push a neuron’s membrane potential beyond a threshold, the cell “spikes” — voltage-sensitive ion channels throw open the floodgates for positive sodium ions, which rush into the cell and cause a rapid voltage swing that ripples along the neuron’s length. When that signal reaches the interface between neurons, voltage-sensitive channels open wide, triggering the release of neurotransmitters to more neurons downstream.
Muscle contraction also kicks off with a voltage spike; neurons send electrical signals streaming into muscle fibers, triggering contractions. This is why Galvani’s electrified frog legs twitched, and why a jolt of electricity can jump-start a stopped heart. (Specialized cells in the heart use electricity to set the pace of its regular contractions.) While all tissues maintain membrane potentials, researchers don’t really know what they do. Compared to electrophysiology, which often focuses on electricity in the brain and heart, the field of bioelectricity — a grab-bag term for electrical activity everywhere else in organisms — has lagged behind, Barriga said.
“I think that at some point it got stuck,” he said. “But now I can tell you that that is coming back like crazy.”
A Shocking Discovery
The epithelial tissues that make up skin and line organs, blood vessels, and body cavities quietly burn about 25% of their available energy to maintain membrane voltages between minus 30 and minus 50 millivolts. But researchers interested in these tissues typically study mechanical forces, chemical signaling, and gene expression — not currents and voltage, Rosenblatt said.
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