Yes you are right. It is the dentin.
Enamel has got no nerves.
So when the dentist use his/her instrument, initially there is no pain (if the enamel is intact).
The next layer is dentin. As you said, dentin has got dentinal tubules containing dentinal fluid.
Whenever there is any stimulus which has not yet reached the pulp, but may have reached the dentin, ...
I think the encoding is quite well understood, and in fact there are artifical cochleas available for deaf people: https://en.wikipedia.org/wiki/Cochlear_implant
For retina replacements, I think the hurdle is establishing the connections from any device to the nerve. There are roughly 120 million rods and 6 million cones in the human eye. (Per Wikipedia: ...
Generally textbooks take the following pedagogical flow in basic neurophysiology:
Ions flow, so voltage changes propagate
This is the "electrotonic part". The key concept is that if you add some ions or change the voltage of one part of a neuron, adjacent areas will also change in voltage as current flows 'passively'. The further away you go, the ...
It's difficult to say a number because memory and activity are intermixed. Activity induces long-term modifications in the brain wiring which can be considered as memory, but at the same time, those connections play functional roles.
For example, in the retina, the codification of the visual stimuli that are going to be transmitted to the brain is done by a ...
In general I would say a lot is known:
Seeing starts with the photo-receptors. See "signal transduction" section of the Wiki:
Signal transduction works though rhodopsin and subsequent closing and opening of ion channels.
Then the signal is transferred from the optic nerve to the brain.
Hearing works though hair cells and mechanotransduction.
Not all hormones enter the blood - just as a quick addendum to the thorough reply above, and as per a comment above stating not all hormones from neuroendocrine cells go into circulation - seems to be true: many of the neuroendocrine cells release hormones directly into the organ where those cells appear - as per the gut, heart, lungs - here's a nice visual ...