Motor learning without a brain

Do we need a brain for motor learning? No, as we have known since the beginning of the 20th century. The spinal cord integrates sensorimotor information and flexibly adapts movements accordingly. It is therefore capable of learning and has a memory - independently of the brain. Until now, however, it was unclear how this is possible. Scientists from Japan and Belgium have now decoded the exact neuronal circuits that enable brain-independent motor learning. They published their findings in "Science".

The team led by Aya Takeoka from the RIKEN Centre for Brain Science in Hirosawa designed an experiment with mice whose spinal cord had been severed so that it received no input from the brain. Each animal was suspended in a harness so that its hind legs dangled freely. The researchers repeatedly stimulated the rodents' legs electrically, causing them to retract reflexively. Half of the animals received the mild electric shock whenever they let their extremities hang down too far. In the control group, on the other hand, the stimulation was random and not related to the leg position.

After ten minutes, the experts observed a learning effect only in the experimental mice: their legs remained up and avoided any electrical stimulation. The spinal cord is therefore able to associate an external stimulus with the leg position and adjust the motor output accordingly. The next day, Takeoka and her colleagues repeated the test, but swapped the experimental and control mice. The original test animals still kept their legs up: The spinal cord had thus created a memory trace that hindered relearning.

The spinal cord comprises several populations of neurons that are functionally divided into two categories: dorsal (facing the back) neurons, which receive and transmit sensory information, and ventral (facing the abdomen), which modulate motor output. To find out how they are each involved in learning and memory, the group switched off different genes in the experimental mice.

Hope for spinal cord injuries

After the knockout of dorsal cells, the legs of the mice no longer adapted to the electric shocks. This particularly affected those neurones that express the Ptf1a gene. In contrast, the ventral cells of the spinal cord appeared to be decisive for relearning the next day. According to the researchers, the results prove that the spinal circuits also follow the principle of sensorimotor learning. The findings could help to develop therapies that promote motor function after a spinal cord injury.

Spectrum of science

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