Student or Learner
In the name of the Merciful Allah,
Hello, when I say: It takes thousands upon millions of neurons flashing their .....in waves back and forth through the incredibly neural networks in your brain to produce thoughts, feelings, and sensations", do upon here indicates repetition or/and addition, or what? Many thanks ahead.
When I hear phrases of that kind, e.g. "thousands upon thousands of XYZs", it suggests to me "thousands of XYZs metaphorically piled on top of other thousands".
(Of course, with some kinds of XYZ, the resulting image may be surprising.)
Not a professional ESL teacher.