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.