AXONAL TRANSPORT IN THE CNS AND
PNS
Intracellular organelles and molecules are transported both
away from the cell body down the axon (anterograde transport) and toward the
cell body from the axon (retrograde transport).
I. Fast anterograde
transport moves vesicles, organelles, membrane proteins, neurotransmitter
elements, and smooth endoplasmic reticulum components at a rate of 100-400
mm/day in a stop-start fashion, using kinesin as a transport mechanism. 
II.
Fast retrograde transport returns endosomes, damaged organelles, growth
factors and trophic factors, and some viruses and toxins at a rate of 200-270
mm/ day, using dynein as a transport mechanism. Fast anterograde and retrograde
transport mechanisms have been exploited in experimental neuroanatomical
studies using labelled compounds (e.g., horseradish peroxide, fluorogold) for
retrograde tract tracing, and radiolabeled proteins for anterograde tract
tracing.
III. Slow anterograde transport carries microtubules,
neurofilaments, and some cytoskeletal proteins at 0.2-2.5 mm/ day (slow
component a), and other enzymes and proteins at 5.0-6.0 mm/day (slow component
b). This slow transport process is the rate-limiting factor governing axonal
recovery after injury or insult; recovery usually proceeds (if it occurs at all)
at approximately 1 mm/day.
