Microtubules in short and in long axons of the same caliber: implications for the maintenance of the neuron.

Microtubules are the morphological manifestation of a defined protein, tubulin, and they sustain axonal transport. Phrenic fibers are seven times longer than abducent fibers; however, in 5-micrometers axons of either nerve the density of the microtubules is the same (approximately 21 microtubules/micrometers2). In addition, no difference was found in microtubule density between the cervical and juxtadiaphragmatic levels of the phrenic axons. These findings are contrasted with predictions based on the perikaryal theory of the origin of axoplasm.