In some sense, this branching technicolor tree marks the boundary between mind and body. Any plan to take a step or scratch your nose starts in the brain, which fires off a signal to your spinal cord through a type of nerve cell called an upper motor neuron. There, the command gets passed to a lower motor neuron, analogous to the fruit fly nerves seen in the image above in red. These cells stretch from your spine to your leg or arm, where they contact muscle fibers directly. When you want to move, these lower motor neurons flood the appropriate muscle fibers with a particular chemical that makes them tighten up.
Disrupting this chain can weaken or break the mind-body link. Amyotrophic lateral sclerosis — or ALS, of viral ice bucket challenge fame — involves damage to both upper and lower neurons, causing poor muscle control and eventually paralysis. By studying animal models like the fruit fly’s simpler brain and body, some researchers aim to pinpoint exactly how motor neurons fail in neurodegenerative disorders like ALS.
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