Are there any neurotoxins or substances like them which do imbalance to hormones like serotonin, dopamine, etc. and/or damage existing neural cells on a large scale and for a long term, cause mental disorders? If so, what is their working mechanism? i.e. how do they attack on our brain's parts wh...

I am concerned by the fact that babies cant walk because the muscles in their limbs arent developed and tuned to give directional control, it takes years before babies gain mobility and dexterity. So technically the neural nets in the motor cortex requires years of training before that capability...