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Humanity seems to underestimate the mental activity needed for limb control because so much of it is subconscious.

I believe that. It takes, what four or five years for a kid to learn how to catch a ball?

A good part of that is that the kid's nervous system is continually being detuned due to growth. Consider that dog learns how to catch a ball reliably once it has reached full size.

Interestingly the myelin sheath around your peripheral nerves thins as you grow. I have always assumed this is to reduce capacitance so that signal propagation time between (say) your fingertips and brain remains roughly constant as your arm gets longer. I don't know if anyone has ever studied this though.

You may be right that thinning during maturation could serve a purpose, but if so you have the effect backwards: thinner myelin should mean higher capacitance (C ~ A/d) and slower conduction velocity. One would expect, if anything, that the myelin would need to get thicker as the limb lengthens. Perhaps nerve fiber diameter also increases with maturation---which also results in faster conduction velocity---and the myelin sheath thins to compensate.

Sorry that was a brain fart -- I meant to type thicken.

Myelination continues Into adulthood btw -- you can see a change in the distribution of white matter due to learning a new language for example.

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