a mouse is running on a treadmill embedded in a virtual reality corridor. In its mind, it found itself gliding down a tunnel with a peculiar pattern of light ahead. Through training, the rat learned that if it stopped in front of a light and stayed there for 1.5 seconds, it would receive a small glass of water as a reward. It can then dash to another set of lights for another reward.
This structure is the basis for the research published in July In Mobile reporting by neuroscientists Elie Adam, Taylor Johns and Mriganka Sur of the Massachusetts Institute of Technology. It explores a simple question: How does the brain—in mice, humans, and other mammals—work fast enough to stop us even a dime? New work reveals that the brain is not wired to transmit the sharp “stop” command in the most direct or intuitive way. Instead, it uses a more complex signal system based on computational principles. This arrangement may sound overly complicated, but it’s a surprisingly clever way to control behaviors that need more precision than the brain’s commands.
The control of the simple mechanisms of walking or running is pretty easy to describe: The midbrain motor region (MLR) of the brain sends signals to nerve cells in the spinal cord, which send signals to neurons. excitatory or inhibitory impulses to motor neurons that innervate the muscles of the legs: Stop . To go. Stop. To go. Each signal is a spike of electrical activity produced by groups of activated neurons.
However, the story becomes more complicated when goals are given, such as when a tennis player wants to run to a precise spot on the court or a thirsty mouse closes its eyes to receive a reward. attractive in the distance. Biologists have long understood that targets form in the cerebral cortex. How does the brain translate a target (stop running there so you get a reward) into a precisely timed signal to tell the MLR to hit the brake?
“Humans and mammals have an extraordinary ability when it comes to sensory motor control,” says Sridevi Sarma, a neuroscientist at Johns Hopkins University. “For decades, people have been studying what it is about the brain that makes us so agile, active, and strong.”
Fastest and fastest
To understand the answer, the researchers monitored neural activity in the brains of mice and calculated how long it took the animals to decelerate from full speed to a complete stop. They expect to see an increased inhibitory signal in the MLR, causing the pins to stop almost instantaneously, like a power switch turning off a light bulb.
But discrepancies in the data quickly undermined that theory. They observed a “stop” signal passed into the MLR while the rat slowed down, but it didn’t spike in intensity fast enough to explain how quickly the animal stopped.
“If you just take the stop signals and feed them into the MLR, the animal will stop, but the math tells us that stopping won’t be fast enough,” says Adam.
“The cortex doesn’t provide a switch,” Sur said. “We think that’s what the cortex does, going from 0 to 1 with a fast signal. It doesn’t do that, that’s the puzzle.”
So the researchers knew that there must be an additional signaling system at work.
To find it, they reviewed the anatomy of the mouse brain. Between the cortex where the targets originate and the MLR that controls locomotion lies another region, the hypothalamic nucleus (STN). It is known that the STN connects to the MLR by two pathways: One that sends an excitatory signal and the other that sends an inhibitory signal. The researchers realized that the MLR responds to the interaction between the two signals rather than relying on the strength of either signal.
