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The Brain Uses Calculus to Control Fast Movements - Quanta Magazine

The Brain Uses Calculus to Control Fast Movements - Quanta Magazine

The Brain Uses Calculus to Control Fast Movements - Quanta Magazine
Nov 28, 2022 1 min, 38 secs

Control over the simple mechanics of walking or running is fairly easy to describe: The mesencephalic locomotor region (MLR) of the brain sends signals to neurons in the spinal cord, which send inhibitory or excitatory impulses to motor neurons governing muscles in the leg: Stop.

How does the brain translate a goal (stop running there so you get a reward) into a precisely timed signal that tells the MLR to hit the brakes.

They expected to see an inhibitory signal surge into the MLR, triggering the legs to stop almost instantaneously, like an electrical switch turning off a lightbulb.

They observed a “stop” signal flowing into the MLR while the mouse slowed, but it wasn’t spiking in intensity fast enough to explain how quickly the animal halted.

“If you just take stop signals and feed them into the MLR, the animal will stop, but the mathematics tell us that the stop won’t be fast enough,” said Adam.

“We thought that’s what the cortex would do, go from 0 to 1 with a fast signal.

It was already known that the STN connects to the MLR by two pathways: One sends excitatory signals and the other sends inhibitory signals.

As the sprinting mouse prepares to stop, the MLR receives an inhibitory signal from the STN.

Almost immediately afterward, it also receives an excitatory signal.

Each signal comes on slowly — but the switch between them is fast, and that’s what the MLR pays attention to: It registers the difference between the two signals.

The greater the difference, the faster the change in the inhibitory signal and the more rapidly the MLR commands the legs to stop.

If stopping depended only on how much of a stop signal the MLR received, then it could be thought of as a form of integration; the quantity of the signal would be what mattered.

“There’s an excitatory signal and an inhibitory signal and the two are being compared instantaneously,” Sur said.

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