‘Speed cells’ found in rats’ brains

Specialized nerve cells fire faster when feet are fleet

ON THE MOVE  Speed cells’ activity (colored lines) change with rats’ swiftness (gray lines) as the animals move around an enclosure searching for chocolate crumbs as seen in this graph charting the activity of seven different neurons. 

E. Kropff et al/Nature 2015

From a saunter to a sprint, specialized brain cells keep track of a rat’s swiftness, scientists report 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 Online: 10/6/14). But without information about how fast an animal moves, that calculation was incomplete. “With this discovery, we now have all of the cellular ingredients we need” to explain how internal maps are drawn, Hausser says.

Navigating through the environment is so important and basic that most people take it for granted, says neuroscientist Jeffrey Taube of Dartmouth College. Without a trusty internal map, people would never find their 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 4-meter 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 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 in two different enclosures. The cells could perform their job in darkness, too. These results suggest that the cells are all-purpose speedometers able to encode speed in any given situation.

Speed cells make up about 15 percent of all 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. That distinction shows that the brain possesses dedicated speed sensors, Edvard Moser says.

It’s not yet clear how the cells make speed judgments. Seeing the scenery fly by probably isn’t the most important cue, since the cells can work in darkness, Edvard Moser says. The key signal may instead come from muscles. That means that speed cells might not work for a rat carried in someone’s arms or riding on a fast train.

Now that the cells have been identified, scientists can start teasing apart how speed cells interact with grid cells and other navigators in the brain such as cells that detect borders and which way the head is pointing, Edvard Moser says.

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