Does each of the different nerve endings (i.e. those in the tip of my index finger - separate ones for heat/cold, pressure etc) have its own exclusive neural connection to the brain, or is there some kind of multiplexing going on?
[IANA Neuroanatomist. Just someone who remembers an intro course in undergrad]
It depends. There is some processing that goes on in the spinal cord and other places, so there’s in general no one-to-one mapping, but I think some receptors to reach all the way to the brain [wherever you want to arbitrarily draw the line between ‘brain’ and ‘receptors’].
For instance, in the eye, there are a lot of neurons in back of the eye, immediately behind the actual rod and cone receptors, that do things like check for movement by comparing the output of adjacent receptors, and try to decide if it’s horizontal or vertical movement (by comparing the output of adjacent movement-detector neurons). This is all done before signals are sent along the optic nerve to the vision areas in back of the skull.
Likewise the spinal cord does some aggregating and processing of touch receptors (including triggering reflex actions).
As a former neuroscience graduate student, I will just let you know that this may be the most complex question that can be asked and is not possible to fully answer with today’s science. What you call “multi-plexing” is definetley true but that is only the broadest idea. To begin to understand this question requires years of academic work, probably a Ph.D., and that would give a person only a partial undertsanding of one sense like the eye.
The best that I can give you is that different senses do generally have a somewhat dedicated pathway to the brain. The eye for example, has a dedicated optic nerve that leads directly into the visual cortex of the brain. The ear sends inputs from the acoustic nerve to the cochlear nucleus and into the auditory cotrex of the brain. Plenty of work has been done on smell but it is less clear. After, senses pass through their primary input region, the information is processed in other areas of the brain and this is still poorly understood.
To fully understand any of these, we can elaborate on a specific sense but the brain is pretty complicated. Multiple courses in neuroscience and neuroanatomy are required to understand what is known and what we know is fairly little.
About ten years ago I did some work for a neurobiologist. While it was mostly “design a machine that does X” I did manage to learn a little bit while I was there. My memory might be a bit faulty, but as I recall the nuerons in your finger are going to start firing, then further up in the nerves those cells won’t fire until they get a bunch of signals from the ones in your finger. Then once they do start firing, they keep firing for a bit even after the ones in your finger have stopped. There’s also a weird integration thing that goes on, so if you get stimulated with hot cold hot cold hot cold (very precisely, so the hot bits are all the same temp and the cold bits are all the same temp) what you feel is hot, cold, hotter, cold but not as cold, even hotter, and even less cold (you feel like the overall temp goes up when it doesn’t). This all happens down in the nerves, before it ever gets to your brain.
I’m not sure exactly why, but things can also get confused when the signals are in the same nerve bundle. This apparently is also why when you have a sore throat your ears itch (the signals from your throat and your ears end up in the same nerve bundle). My EE mind wants to say there is crosstalk between the nerve fibers but I’m not sure if that is an accurate description of what is going on or not.
It’s interesting stuff. Unfortunately I was funded by a grant and couldn’t make a permenant job out of it.