A-alpha-nerve-fiber: number of neurotubules in the stem fibre and in the terminal branches

Summary Cross sections of A--motor stem fibres of the rat accessory nerve and of one of its branches, the N. musculi sternomastoidei were compared with cross sections of terminal branches of the same nerve, with respect to the axonal cross sectional areas and the number of neurotubules. The absolute number of neurotubules in a stem fibre was found to be on average five times that of one of its terminal branches, corresponding to the ratio of their axonal cross-sectional areas. Thus no significant differences could be found in the tubular density of large stem fibres and of small final branches. Taking into account that in the course of the terminal ramification the total axonal cross-sectional area increases (Zenker and Hohberg, 1973), the combined total of the numbers of neurotubules in all terminal branches of a single A--fibre of the nerve innervating the sternomastoid muscle surpasses the amount of tubules in the stem fibre on average about 11 times. These findings are incompatible with the idea of a constant number of neurotubules within a given axon and its branches. In accordance with recent biochemical studies of microtubular protein, our results indicate that neurotubules may be formed in axon branches far from the perikaryon.