04-27-2018, 04:58 AM
It seems to me that the probable reason for the microbiome affecting the brain is probably its effect on general health, including the levels of very many chemicals (many of which are as yet undiscovered) in the bloodstream. As far as the brain is concerned, the only things that matter are the chemical composition and flow rate of the blood supply and the cerebrospinal fluid - and of course the sensory data coming in. The latter, of course, includes senses we don't think about and can't consciously perceive. For example, the brain can sense when the gut is getting empty and/or nutrient levels are dropping; this comes to conscious attention as hunger and thirst, but that isn't the raw data.