Data for: A size principle for recruitment of Drosophila leg motor neurons

To move the body, the brain must precisely coordinate patterns of activity
among diverse populations of motor neurons. In many species, including
vertebrates, the motor neurons innervating a given muscle fire in a
specific order that is determined by a gradient of cellular size and
electrical excitability. This hierarchy allows premotor circuits to
recruit motor neurons of increasing force capacity in a task-dependent
manner. However, it remains unclear whether such a size principle also
applies to species with more compact motor systems, such as the fruit
fly, Drosophila melanogaster, which has just 53 motor neurons per
leg. Using in vivo calcium imaging and
electrophysiology, we found that genetically-identified motor neurons
controlling flexion of the fly tibia exhibit a gradient of anatomical,
physiological, and functional properties consistent with the size
principle. Large, fast motor neurons control high force, ballistic
movements while small, slow motor neurons control low force, postural
movements. Intermediate neurons fall between these two extremes. In
behaving flies, motor neurons are recruited in order from slow to fast.
This hierarchical organization suggests that slow and fast motor neurons
control distinct motor regimes. Indeed, we find that optogenetic
manipulation of each motor neuron type has distinct effects on the
behavior of walking flies.