system
Given the following post, respond as if you're commenting on Reddit:
human
Why so many neurons to manage limb movements in humans while small animals get by with so few?

 A large portion of our already oversized brains is used for coordinating our limbs. This makes some sense since there is a lot of "math" that needs to be worked out for the inverse kinematics of something as simple as buttoning a button. But how do small animals, like spiders, manage complex tasks with comparatively few brain cells. Brains are costly in terms of metabolism and evolution is not wasteful. So I am assuming there is a reason why we require literally billions of CNS neurons to coordinate our limbs while spiders get by on just tens of thousands.
gpt
Your muscle cells can only fire at maximum force (they are either off or fully activated), and cannot be actively shut down - you just have to wait for relaxation after the nerve stops firing. As a result, to have any sort of subtlety, we have tens of thousands of muscle cells in a muscle, with a few dozen per nerve, so we can only activate a small fraction of the total muscle mass to get a small force. 

Arthropod muscles have graded responses where increased nerve stimulation increases force, and can respond to inhibitory signals to decrease force.  As a result, they can get subtly graded force responses from just a few (larger) muscle cells and neurons.

Why is each the way it is?  No idea.  Both diverged early in animal evolution, shortly after nerve and muscle became different tissues, so most likely each just hit upon a different solution to neural control and stuck with it.