Relations between axon length and axon caliber. "Is maximum conduction velocity the factor controlling the evolution of nerve structure"?

A search was made for any existent relationship between the length of a nerve fiber and the caliber of its axon. This was done in the hope of defining morphological parameters useful for assessing conduction time. Four fiber populations were examined: (1) phrenic fibers in rat and rabbit during different phases of body growth; (2) phrenic fibers of mature animals of greatly different body size including mouse and cow; (3) rat intercostal nerves which vary in length by a factor exceeding 5 due to the funnel-shape of the thorax; and (4) ventral root fibers of the cow. In all of these fiber populations, there was no evidence for a direct relationship between the length of a fiber and its caliber. Rather, a tendency was noted for fiber caliber to approach certain ceilings independent of length. These data, seen in conjunction with other information on fiber structure, cast serious doubt on the widely accepted concept that maximum conduction velocity is the factor controlling nerve structure. A much more likely factor controlling the structure of myelinated nerve fibers is the capacity to modulate information by frequency coding of impulses.