Neurofilaments are spaced randomly in the radial dimension of axons

The organization of the cytoskeleton is compared in the large myelinated parasympathetic and somatic motor axons of the avian oculomotor system. Electron microscopic studies demonstrate that neurofilaments are the chief structural elements in these axons, and quantitative analyses of the distribution of neurofilaments in axonal cross-sections found that the average neurofilament packing density is 25% greater in the parasympathetic axons than in the somatic motor axons. In both types of axon the distributions of neurofilaments matched a randomly generated (Poisson) distribution. In axoplasm, a Poisson distribution could arise if the neurofilaments were distributed in the cross-sectional plane by stochastic forces operating randomly and without significant neurofilament-neurofilament interactions. Thus, in these axons, the neurofilaments behave as if they are inert 'molecules' in a dilute solution-subject to non-specific stochastic forces that tend to distribute them at random. We propose that neurofilaments normally are relatively free to move apart from each other and to fill the available space within the axon.