The axon tree of rat motor fibres: morphometry and fine structure

A quantitative light and electron microscopic study of the axon tree of rat motor fibres was undertaken to supply data relevant to the understanding of the spread of excitation. Continuous proximodistal foreshortening of internode length and dwindling of fibre calibre were statistically verified. Internodes had progressively foreshortened geometric proportions (length/diameter quotient). The postbranching internodes were significantly shorter than would correspond to the general trend, but no such foreshortening was evident for the prebranching internodes. Relative sheath thickness (g-ratio, i.e. quotient axon diameter/fibre diameter) of filial fibres did not differ from the stem. Increase in axon area in filial fibres versus stem fibre averaged 1.3. The total proximodistal increase in axoplasmic area from the stem fibre to the distal branches was less than two since increases at branch points were compensated by the proximodistal dwindling of fibres. Nodal membrane area of branch points was greater than in nonbranching fibres, and there were larger than normal microvillar spaces. The majority of branches were approximately symmetric, but asymmetric branchings were also found. The thinner branches of these had wider nodal gaps than the thick branches. The observed changes were interpreted as adaptations to facilitate uniform spread of excitation along the axon tree; asymmetric branchings, however, may permit routing of impulses.