The precise geometry of large internodes.

The interrelations, in each given internode, between internodal length, axon caliber, axoplasmic volume and several parameters of sheath thickness were studied in a set of 37 large internodes from the sciatic nerve of the German shepherd dog, using multiple electron-microscopic measurements of isolated fibers. Internodal length varied in a nonlinear relation with axon caliber. Nonlinear relations were also found for the number of lamellae in the sheath as related to either axon caliber or to internodal length, particularly for large internodes: the deviations observed for axon caliber (thick fibers had fewer myelin lamellae than would correspond to axon caliber) were the opposite of for internodal length (thick fibers had more lamellae than would correspond to internodal length). A near linear relationship was found if the volume of myelin per internode was related to both the length and the circumference of the axon segment ensheated by it, i.e., axolemmal surface area. The deviations in the proportions of large internodes probably indicate the existence of upper limits of internodal growth conditioned by the enormous metabolic demands placed on the individual Schwann cell by the growth of internodes beyond 1.5--2 mm length or 100 thousand micrometer3 of myelin, respectively. The demands on myelin metabolism from unabated internodal growth and the resultant logistic problems of the Schwann cells are demonstrated from measurement of observed internodes and calculations of extrapolated growth.