Slow going - active and passive transport in axons

Material can be moved throughout a cell in a variety of ways. The simplest manner is diffusion that relies upon a concentration gradient, but needs no external energy source. While efficient for short distances, this technique is not adequate for nerve cells where the axonal processes are quite long and material is synthesized only in the cell body. In this case an energy dependent system is used to actively supply material independent of concentration. In axons, diffusion has been augmented by slow transport which moves cytoskeletal proteins and soluble enzymes at rates of approximately 1 μm/min. Little is known about the mechanisms of slow transport and although culture systems provide ready access to individual axons, the distances involved are often small enough that diffusion contributes a significant amount to the total supply of material. By using a larger isolated system that allows these competing transport effects to be separated we have been able to characterize them individually, and investigate their relative contributions along the axon.