The human brain is a glutton, consuming 20% of our body's energy even though it accounts for only 2% of our mass. New research, however, suggests that little of that energy goes to power the brain's electrical signals. In fact, these impulses travel far more efficiently than previously thought.
In 1939, a pair of British physiologists, Alan Hodgkin and Andrew Huxley, took the first stab at figuring out how neurons transmit electrical signals, known as action potentials. Because most neurons are small--in humans, a cubic millimeter of gray matter can contain 40,000 neurons--the duo turned to squid, which contain a giant axon, the long thin part of a neuron through which action potentials travel. Electrical recordings helped Hodgkin and Huxley to develop a model of how action potentials move through neurons, work for which they won a Nobel Prize.
According to the Hodgkin-Huxley model, the energy required to transmit an action potential in the squid giant axon is three to four times greater than what would be needed if the axon were perfectly efficient. That means the axon is about 25% to 30% efficient, roughly the same as a car engine. This number has been accepted for decades, but it never made much sense to Henrik Alle, a neuroscientist at the Max Planck Institute for Brain Research in Frankfurt, Germany. "One would intuitively think that nature would try to optimize such a really important signal," he says, to make it as energy-efficient as possible.
Alle and his colleagues decided to reexamine the efficiency question using mammalian neurons. The researchers recorded currents running through neurons in the memory and learning centers of rat brains, using a technique unavailable to Hodgkin and Huxley called the patch-clamp method.
After crunching the data, the researchers found that these action potentials travel through rat neurons two to three times more efficiently than the Hodgkin-Huxley model predicts. Rather than being 30% efficient, the process is roughly 70% to 80% efficient, the team reports tomorrow in Science.
Why the large difference? In the Hodgkin-Huxley model, the positive and negative ions that generate the action potentials appear to be fighting one another. Positive sodium ions rush into the cell even as positive potassium ions rush out. It's as if "you're pressing on the gas and hitting the brake at the same time," says Michael Häusser, a neuroscientist at University College London in the United Kingdom who wasn't involved in the research. But Alle and his colleagues found that, in rat neurons, the opening of one ion channel follows the other. Potassium doesn't begin to exit until sodium has almost finished entering. First comes the gas, and then the brake--a much more efficient process.
As to how the brain uses the rest of its energy, Alle says about half of it goes to simply keeping neurons alive. The rest is used for computing. The researchers' findings suggest that more energy is being expended to ferry signals from one neuron to the next than to move electrical signals along the axon.
"This paper lends weight to the idea that nature has worked very hard to make [action-potential signaling] almost as efficient as the theoretical limit," says Häusser. Knowing how much of the brain's energy is spent on different activities may also help scientists better understand how the brain stores information, he says.