Fruit Fly Study Suggests Motor Neurons Help Direct Movement, Not Just Carry It Out

Motor neurons, the nerve cells that control muscles, have long been treated as passive messengers carrying the brain's commands outward. Work in fruit flies suggests they do more than that: some motor neurons send signals back to the brain that help organise the very movement sequences they are executing, researchers report August 24 in Nature Neuroscience.

The team studied feeding in Drosophila melanogaster, the common fruit fly. The sucking motion the flies use to eat requires seven pairs of muscles in the mouth, each controlled by a pair of motor neurons, contracting in a precise order to generate fast rhythmic pressure changes that draw food inward. "It closely resembles the way an infant suckles," said Dong-Gen Luo, a neuroscientist at Peking University in Beijing. His group came to the subject by accident: in 2016 they were studying taste perception when Xiu-Wen Sui, then a PhD student at Peking University, accidentally triggered repetitive sucking.

Sui, Luo and colleagues then recorded individual motor neurons while the flies fed, a tricky task that meant inserting tiny electrodes into fly brains. Some motor neurons, they found, were not just carrying out commands but also signalling back, and those return signals helped determine when the next motor neuron in the sequence became active.

Luo likens it to a chain of falling dominoes. The motor neurons controlling the third pair of muscles receive commands from the brain and do two things at once: they activate their target muscles, and they tell the brain to release the "brakes" on the next pair of motor neurons, which then passes the sequence along to the fifth pair. The paper does not explain how all seven pairs are coordinated. Releasing the brake only when conditions allow could improve the stability and precision of motor control, Luo said, and he suggested the principle could potentially be harnessed to give robots smoother, more adaptive movement.

The finding brings together two previously separate lines of research, said neuroscientist Maarten Zwart of the University of St Andrews in Scotland. Some details of the underlying mechanisms may differ between flies and vertebrates, Zwart said, so there is no straightforward answer as to whether similar principles apply to humans.