I've suffered muscle twitching in multiple areas and I am also currenly experiencing it down my arms during certain neck positions.
From what I know, twitches can be caused by many things, including spinal nerve compression, peripheral neuropathy, metabolic diseases, and anxiety (among other things). If you currenlty have problems in your spine, such as arthritis that is causing nerve compression, that could be a possible reason.
I have to say that before I had spins surgery (for lumbar stenosis), I had terrible nerve pain and symptoms all over, including my feet and legs. When I had severe sciatica, it was so strange. Just as you said, I could actually see and feel exactly where my twitchign was occuring. It was happening down the exact dermatome where my nerve compression was coming from. So it is a fact that nerve compression can cause it.
Currently, I think my cervical spine is starting in with something similar that I had in my lumbar spine. If my head tilts back a bit too much, I get pain. And If I cough or something, OMG....I get massive twitching down both backs of my arms. Once I tilt my head forward, it stops. I've also had and still get weird buzzing sensations all over the place as well. Again, sometime after surgery when I had severe case of sciatica, if I bent my head forward at all, I would get this buzzing sensation in the bottom of my big toe area. My PT said that is because the dura around the spine is stretching and pushing into nerves. My twitching always happens when I have either some kind of lumbar compression or moving my spine in a way that actually causes the twitchign. LIke if I tilt my head forward now when I cough, I don't get the twitches. But when my head is too straight or bent back while I do it, both arms twitch for a while... plus my arms feel a deep achy pain and weakness for a few minutes. Just something you might want to have checked out.
Since you are getting symptoms in your spine area, I would probably look into that first to see if you have any spinal nerve compression that might be causing it. Like I said, I know for sure, from experience and professional advice, that compression can cause twitching symptoms. Perhaps, if you haven't already, you might want to get some imaging done on your spine.