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Rat brains have a speedometer: specialized nerve cells fire faster when feet are fleet.

From a saunter to a sprint, specialized brain cells keep track of a rat's swiftness, scientists report online July 15 in Nature. These "speed cells" may be a missing piece in understanding how animals and people navigate the world, says neuroscientist Michael Hausser of University College London.

Scientists have previously uncovered cadres of brain cells that help an animal constantly calculate its location in space, work that led to Nobel Prizes last year (SN: 11/1/14, p. 15). But without information about how fast an animal moves, that calculation was incomplete. "With this discovery," Hausser says, "we now have all of the cellular ingredients we need" to explain how internal maps are drawn.

Navigating in the environment is so basic that most people take it for granted, says Dartmouth College neuroscientist Jeffrey Taube. Without an internal map, people couldn't find a parked car; animals would suffer more dire consequences. "If a small animal turns the wrong way or goes the wrong direction, it's going to be dead meat very quickly," he says.

Speed cells were spotted in the brains of rats as they walked or ran along a track in a bottomless car. Like Fred Flintstone, the rats "drove" the car by moving their paws, but the researchers set the pace. As the rats moved, electrodes picked up signals from nerve cells, or neurons, in the rats' entorhinal cortex, a brain area known to be important for navigation.

When the rats shifted into overdrive, some of these neurons did too, Edvard Moser of the Norwegian University of Science and Technology in Trondheim and colleagues found. In one experiment, researchers toggled the rats between four speeds on the track, from a slow walk at 7 centimeters per second to a jog at 28 centimeters per second. Speed cells kept pace, firing off more signals at faster speeds and fewer at slower speeds.

The speed cells performed just as well off the track, keeping track of swiftness as rats searched for chocolate crumbs. The cells did their job in darkness, too, suggesting that they are all-purpose speedometers for any situation.

Speed cells make up about 15 percent of cells in the entorhinal cortex, which also harbors map-making grid cells. Speed cells seem to be distinct from grid cells, whose discovery led to Nobels for Moser and his wife, May-Britt Moser, coauthor of the new study.

It's not clear how the cells make speed judgments. Seeing scenery fly by probably isn't the most important cue, since the cells can work in darkness. The key signal may come from muscles and so might not work for a rat riding on a fast train.

Caption: On the move Speed cells' activity (colored lines) changes with rats' swiftness (gray lines) as they move around an enclosure in search of chocolate crumbs. This graph charts the activity of five different neurons.


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Title Annotation:BODY & BRAIN
Author:Sanders, Laura
Publication:Science News
Geographic Code:4EUUK
Date:Aug 8, 2015
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