Proteins and organelles are transported along axons and dendrites. How do we know this?

Well, in 1948, Paul Weiss tied off a sciatic nerve and noticed that material accumulated on the one side of the axon, the area before the tying off. He concluded that there is a forward movement down the axon of material, which is termed anterograde transport. 

There is also retrograde, or backward, movement of material. Retrograde transport is important in helping the nucleus of the neuron to know what’s happening out in its periphery. Frighteningly, though, some toxins or viruses use retrograde transport as a means of transportation, including the rabies virus.

Both anterograde and retrograde movement occur via fast axonal transport, which is a form of transport that is faster than 400mm per day. Fast axonal transport occurs in a start-and-stop, or saltatory, fashion, because sometimes, organelles dissociate from the track of the axon, and sometimes they collide with other materials.

Other types of proteins move via slow axonal transport, which move less than 400mm per day. These transport mechanisms are more adaptations of certain processes that are associated with organelle movement in secretory cells.