It takes just one neuron to make you twitch. Muscles are made up of strands called fibers, and each fiber links up with one, and only one, neuron. When that neuron releases a certain chemical (seen here in red), the muscle fiber contracts, and you move. Your fibers and neurons carry on exclusive one-to-one relationships now. When you were born, they were much less choosy, with a few axons all attaching to the same muscle fiber, as seen above.
As babies use their muscles, they set off a struggle for survival. Axons that are best connected grow stronger while the others wither away in a process called synapse elimination, or synaptic pruning. Neuroscientists have learned a lot about the molecules that play a role in the axons’ battle royale, but the broader question — why the brain attaches so many extra axons to muscle fibers in the first place — remains a mystery.
Thompson, W. J. (1985). Activity and synapse elimination at the neuromuscular junction. Cellular and Molecular Neurobiology, 5(1-2), 167–182. doi: 10.1007/BF00711091
Tomàs, J., Garcia, N., Lanuza, M. A., Santafé, M. M., Tomàs, M., Nadal, L., … Cilleros, V. (2017). Presynaptic Membrane Receptors Modulate ACh Release, Axonal Competition and Synapse Elimination during Neuromuscular Junction Development. Frontiers in Molecular Neuroscience, 10, 132–132. doi: 10.3389/fnmol.2017.00132