Dr. Andrea H. Brand CC BY-NC
While most brain cells stay closer to home, motor neurons must send axons out into the body to find one muscular mate. Studies of fruit fly larvae, which have simple enough bodies that researchers can easily track all their muscle groups, have found that a given motor neuron — such as the one above — always reaches the same muscle. From left to right, the cell’s axon pushes out into the fly’s body, feeling its way toward one particular target muscle as it grows.
How each neuron finds its partner is still a mystery, but researchers have found that chemical attractants and repellents solve part of the puzzle. These messenger molecules float along the route like traffic officers, bumping into the growing axon and directing it right or left. When the axon grows close, molecules wafting from the target muscle tell it that it’s found the right partner. Too many neuron-muscle pairs exist for each to have their own handshake molecules, however. Researchers suspect other guiding mechanisms remain to be discovered.
Bonanomi, D., & Pfaff, S. L. (2010). Motor Axon Pathfinding. Cold Spring Harbor Perspectives in Biology, 2(3), a001735. doi: 10.1101/cshperspect.a001735
Ruiz‐Cañada, C., & Budnik, V. (2006). Introduction on The Use of The Drosophila Embryonic/Larval Neuromuscular Junction as A Model System to Study Synapse Development and Function, and A Brief Summary of Pathfinding and Target Recognition. In The Fly Neuromuscular Junction: Structure and Function Second Edition: Vol. 75. International Review of Neurobiology (pp. 1–31). doi: 10.1016/S0074-7742(06)75001-2