Don Arnold- a molecular and computational biologist at USC College-and colleagues have now solved the mystery for key proteins in the brain.

"There's no little man sitting there, putting the protein in the right place. Proteins have to have in them encoded information that tells them where to go in the cell," Nature quoted Arnold as saying.

Neurons have separate structures for receiving signals (dendrites) and for sending them (axons).

The electrical properties of both types of neurons depend on different proteins.

But the proteins travel in bubbles, or vesicles, powered by motors known as kinesins that travel along tiny molecular paths.

Even though the paths point to both axons and dendrites, dendritic proteins end up in dendrites, and axonal proteins go to the axons.

The researchers discovered a crude but effective sorting mechanism, in which firstly kinesins blindly carry both types of proteins towards the axon.

But, dendritic proteins enable the vesicles transporting them to bind to a second motor, known as myosin, that walks them back into the dendrite.

The filter ensures that only axonal proteins make it into the axon, while the others are caught by the second motor and diverted to the dendrite.

"This mechanism fishes these things out of the axon," said Arnold.

Once in the dendrite, the proteins either land in a place where they can do their electrical work or they move back towards the axon, only to be fished out again.

Arnold said that the process looks inefficient, "but it is very effective."

The discovery could allow finer control over neurons for basic research or for treatment of neurological disorders.

Also, scientists could target only dendrites or axons in a neuron for studying its outgoing or incoming impulses.

Apart from these applications, the study contributes a lot to the understanding of the brain and of protein transport in general.

"It's a very basic question, something people have been wondering about for a long time," said Arnold.