Variance in relative internode length (l/d) in the rat and its presumed significance for the safety factor and neuropathy.

(1) Previous work has shown that the quotient l/d (internode length/fiber diameter) is the product of independent variations in internode length (increasing with fiber elongation) and in axon caliber (whose growth is not as yet well understood). Using these guiding principles, the distribution of l/d in 19 nerves and 7 roots of the rat was determined. (2) Each nerve or root showed a linear decrease of the l/d with fiber caliber, the thin fibers always having relatively longer internodes than the thick ones. Comparing nerves, the highest l/d was found for extremity nerves, particularly those in the hindlimbs. Nerves of the trunk (phrenic, intercostal) had lower l/d with further decreases for the extracranical branches of cranical nerves, the intercranial roots and a minimum for the acoustic nerve. In the facial nerve, the l/d of the intraosseous portions was distinctly lower than that of its branches in the face. Ventral roots showed a cranio-caudal increase in their l/d. For some fiber systems, e.g. cranial nerves, the l/d reflected variance in axon caliber, their internode length being relatively uniform. For others, e.g. roots, the l/d was dominated by their large variation in internode length. (3) Variation of the l/d in fiber populations may correlate with such parameters as internodal conduction time, different sensitivities of thick and thin fibers and, particularly, the safety factor. It is also proposed that the regional variation in the l/d is a critical parameter affecting the regional vulnerability of fiber populations to polyneuropathies or to radiculopathies.