Figure 1.

The axon is a long and slender protrusion from the neuronal cell body that consists of a system of longitudinally aligned microtubules. Microtubules are composed of heterodimers of α- and β-tubulin, shown in green and blue, that form 13 laterally joined protofilaments, each up to 100 μm long. Axons can extend several centimeters in length and their microtubules never run continuously from the cell body to the distal end. Instead, they form overlapping segments with 10–50 microtubules in any given cross section. Neuronal microtubules are stabilized and cross-linked by tau proteins, shown in red, which bind to microtubules with their three or four binding repeats, shown in yellow. Several components of the axon including neurofilaments, other crosslinking proteins, and cytoskeletal organelles are not displayed.