Contrary to popular belief, the brain is not a computer. However, brains do, in their own way, compute. They integrate informational inputs to generate outputs, including behaviors, thoughts, and feelings.

To process vast amounts of data, the brain uses a kind of digital code. Its cells produce discrete bursts of electric current, known as action potentials, that function as the zeros and ones of the nervous system. This code is assumed to be a vital aspect of computation in animals—that is, in most animals. The tiny roundworm C. eleganshas long been considered a curious exception; until now, action potentials had never been observed in the organism.

But in a recent study, Rockefeller scientist Cori Bargmann and her colleagues, discovered, among other things, a C. elegans olfactory neuron that produces action potentials. The finding, published in Cell, overturns decades of dogma and could help scientists understand the fundamental principles of brain computation.

Trial by fire

Neurons communicate with one another by exchanging chemical messages. Each message alters the state of the receiving cell; and as a neuron collects more and more chemical input, it approaches a threshold of activation.

An action potential occurs when the cell reaches this threshold, at which point the neuron is said to "fire" or "spike" as an electrical impulse ripples through its extremity. In producing this spike, the cell translates analog chemical messages into digital electric code.

"The C. elegans has just 302 neurons, so it's one of the few animals where you can look at each individual neuron," says Liu, a research assistant professor in Bargmann's lab who set out to measure how all of these neurons respond to stimulation.

Almost immediately, Liu was met with a surprise. While stimulating AWA, a neuron that processes smell signals, he observed that the cell's electrical voltage rose very rapidly before dramatically plummeting. Though unexpected, this dynamic was also very familiar: it looked like an action potential.

A neuron with potential

Additional experiments confirmed that AWA neurons indeed spike. The researchers suspect that other C. eleganscells also produce action potentials; yet they note that this is not the norm for this animal's neurons. In fact, their experiments revealed that even AWA fires rather infrequently.

Typically, the neuron responds to odors in a more subtle, graded manner. Liu observed action potentials only during experiments in which the stimulus grew stronger over time, suggesting that in nature, AWA fires when the animal is approaching the source of an important smell.

"There's a huge field of people working on the coding and computational principles of nervous systems," says Kidd. "And our work with C. elegans is likely to uncover principles that were unfamiliar to scientists who have been working in these areas for a long time."

"Computation in the brain is a deep and important problem," says Bargmann. "With this study, we've shown that C. elegans can help solve this puzzle—and in fact, we've already exposed a whole new piece of it."