Neurons are secretory cells, but they differ from other secretory cells in that the secretory zone is generally at the end of the axon, far removed from the cell body. The apparatus for protein synthesis is located for the most part in the cell body, with transport of proteins and polypeptides to the axonal ending by axoplasmic flow. Thus, the cell body maintains the functional and anatomic integrity of the axon; if the axon is cut, the part distal to the cut degenerates (Wallerian degeneration).
Orthograde transport occurs along microtubules that run along the length of the axon and requires two molecular motors, dynein and kinesin (Figure 1). Orthograde trans port moves from the cell body toward the axon terminals. It has both fast and slow components, fast axonal transport occurs at about 400 mm/day, and slow axonal transport occurs at 0.5 to 10 mm/day. Retrograde transport, which is in the oppo site direction (from the nerve ending to the cell body), occurs along microtubules at about 200 mm/day. Synaptic vesicles recycle in the membrane, but some used vesicles are carried back to the cell body and deposited in lysosomes. Some mate rials taken up at the ending by endocytosis, including nerve growth factor (NGF) and some viruses, are also transported back to the cell body. A potentially important exception to these principles seems to occur in some dendrites. In them, single strands of mRNA transported from the cell body make contact with appropriate ribosomes, and protein synthesis appears to create local protein domains.
Fig1. Axonal transport along microtubules by dynein and kinesin. Fast (400 mm/day) and slow (0.5–10 mm/day) axonal orthograde transport occurs along microtubules that run along the length of the axon from the cell body to the terminal. Retrograde transport (200 mm/day) occurs from the terminal to the cell body. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology. New York, NY: McGraw-Hill; 2008.)
