Microtubules and caliber of central and peripheral processes of sensory axons

Abstract

The microtubular content and caliber of sensory axons were studied in the L7 dorsal root, at the distal pole of the L7 spinal ganglion, and in the sural nerve of cats. Calibers of myelinated axons were symmetrical about the ganglion. In contrast, nonmedullated axons were strikingly different; 80% of the population at the root were smaller than 0.4 μm², whilst just across the ganglion the same group was less than 20%. The microtubule densities of myelinated axons of the root were 11.8 and 6.1 microtubules/μm² for 3‐ and 10/μm diameter axons, respectively. Across the ganglion the densities of myelinated axons of equal sizes were 24.2 and 14.4 microtubules/μm², respectively. These values represent an approximate ratio of 1:2 between central and peripheral microtubule densities. Microtubule densities for nonmedullated axons also decreased with the increase in the cross‐sectional area. The densities of root nonmedullated axons ranged between 90 and 10 microtubules/μm²; these were smaller, usually by a factor of three, than the densities of peripheral axons of a similar size (range: 367–44). Contrasting with the differences observed across the ganglion, the microtubular content and caliber of sensory axons seems to be quite uniform along their peripheral course. This is supported by the similar values found in the juxtaganglionic and sural nerves. It is estimated that an axon that contains 90 microtubules/μm² has 26.7 mg of tubulin per ml of axoplasm in its assembled form, and 3.0 mg/ml if it contains 10 microtubules/μm²; these values are the practical limits of assembled tubulin in axoplasms. We conclude that the microtubule density of an axon is related to axonal size and location (central or peripheral), but is independent of axonal length, axoplasmic mass, distance to soma, and neuronal class. We propose that local exogenous factors contribute to the specification of the microtubular content of axons.