What are the methods used for neuronal tracing?

Neuronal tracing is used to map the pathway of neuronal processes. Researchers use neuronal tracing to get a better understanding of connections between specific types of neurons and their functions in the nervous system.  

The four main methods used for neuronal tracing are anterograde tracing, retrograde tracing, viral neuronal tracing, and manual tracing of neuronal imagery. 

1. Anterograde tracing: Anterograde tracing is used to trace axonal transport from the cell body or soma, which is the source, to the synapse, which is the terminal point. In this method, the presynaptic and postsynaptic neurons are labeled, enabling researchers to visualize the biological process of axonal flow. 
2. Retrograde tracing: Retrograde tracing is used to trace neural connections from the synapse, which is their point of termination to the cell body, which is their source. Researchers use this tracing method to map the connections between neurons in a particular structure such as the eye or nose and the target neurons in the brain. This method is also based on the visualization of axonal flow. 
3. Viral neuronal tracing: In Viral neuronal tracing, a virus is used to map neural pathways. The advantage of using viruses is that they act as self-replicating tracers. The downside is they can spread too rapidly, destroying neural tissue.  
4. Manual tracing of neuronal imagery: In this method, neurons are traced manually in 2D or 3D. This neuronal tracing method involves a considerable amount of labor, which is a major limitation.