So this is why ronnie coleman arms shrunk down to 17 inches.
lets say you have 100k muscle fibers in your triceps.
lets say you have disc degeneration in the c5/c6 that "blocks" 30% of the fibers in your triceps... so when you go to train, instead of having all 100k of those fibers firing during the workout/sets, now you only have 70% firing and being stimulated... the other 30% won't fire, and if they don't fire, they'll atrophy.
that's a very "dumbed down" explanation, but yea, something like that.
look at Paul Orndorph's arm (the pro wrestler from the 80's).
look at the right side, especially the lower body, of Jay Cutler.
etc etc...