How do scientists trace pathways in the brain?

[gib65]

If the brain is essentially a densely compacted bundle of neurons with interconnections that allow electric/chemical signals to be processed along pathways, how do scientists figure out these pathways? That is, what techniques do they use to figure out where a signal travels in the brain? Say you are given a starting point in the brain (like a sensory input point) and you want to know where and how far that signals goes, how would one do that?

 

Gib

[nettron]

AFAIK, there are two primary methods: Anterograde and Retrograde, both involve some type of photographic mapping of axonal pathways rather than injecting and "listening" for a signal. A single nerve bundle can contain millions of axons, so probing to find which axon(s) are carrying the original signal would be a daunting task.