Walking around in the real world, as opposed to the uncluttered hallways of your school, requires flexible and adaptive fine-tuning of the basic alternating stepping pattern of our two legs. For animals to walk, central pattern generators (CPGs) must be modulated to allow different stride lengths in each leg or different swing heights to step over obstacles. These local motor circuits in the central nervous system get proprioceptive information, or ascending inputs, from your leg’s mechanosensors to ensure the accurate timing of each joint movement. It also uses information from your brain, or descending inputs, to adjust the order, timing, or amplitude of individual leg movements. Just think of Michael Jackson’s moonwalk! His convincing impression of walking forward, while actually moving backward, apparently took many hundred hours of practice, and shows that the basic kinematic pattern of leg movements can be highly modified!
While many studies have focused on the proprioceptive input into CPGs, few have tried to decipher the nature and identity of the descending commands. A new study in Science used the Drosophila fruit fly model to identify specific neurons, whose activation could change the fly’s walking direction. To do this, the researchers introduced a heat-sensitive channel called TrpA1 into specific neuron types; then they raised the temperature and noted which flies walked backwards! One type of fly in particular was able to reverse both the order and timing of its individual leg movements, so it was called the “moonwalker” line.
When control flies reach a “dead end,” they walk backwards for several millimeters. When the researchers blocked synaptic transmission in the moonwalker fly, they saw that the flies would often stall at a dead end before squeezing around to walk forward again in the opposite direction, essentially making a U-turn! This showed that activity of moonwalker neurons is essential for backwards, but not forward, locomotion.
So what neuron is responsible for backward movement? By comparing the neurons that were activated in control and moonwalking flies, the researchers found that sustained backward walking actually requires the coactivation of two neurons: MDN, or moonwalker descending neuron, and MAN, or moonwalker ascending neuron. Upon further analysis using functional assays, the researchers found that activation of the descending neuron MDN triggered backward walking, while the ascending neuron MAN worked to inhibit forward walking and allow persistent backward walking.
These results may seem trivial, but the identification of MDN and MAN provides a first glimpse into the signals that pass between the brain and local motor circuits, directing an animal toward or away from certain targets. Perhaps Micheal Jackson had exceptional control over this circuit!
Bidaye S.S., Machacek C., Wu Y. & Dickson B.J. (2014). Neuronal Control of Drosophila Walking Direction, Science, 344 (6179) 97-101. DOI: 10.1126/science.1249964