Variability and temperature dependence of the velocity of retrograde particle transport in myelinated axons

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Abstract
The velocity of the retrograde transport of optically detectable particles was studied in isolated myelinated axons from Xenopus laevis at room temperature and with varying temperature. At room temperature (22–23.1 °C) the mean particle velocity was significantly different in some axons of the same animal. Results pooled from a number of axons did not indicate any significant difference in the average particle velocity in different animals. At room temperature, even though the mean particle velocity might differ between axons, the coefficient of variation of particle velocity in separate fibers tended to remain constant.The temperature dependence of mean particle velocity (V) could be adequately described by the relationship V = 0.26 exp 0.073T – 0.33, where V is in micrometres per second and T is degrees Celsius. Throughout the temperature range at which transport took place (approximately 5–35 °C) the coefficient of variation of particle velocity remained approximately constant at 0.40. All particles ceased their motion abruptly, and to some extent reversibly, at 37 °C. On morphological evidence, none of the effects of temperature appeared to be related to microtubule depolymerization.The results indicate that retrogradely transported particles with a wide range of velocities respond homogeneously to temperature changes. This supports the view that a common mechanism may underlie the motion of all the particles. No evidence was obtained that the differences between axons at room temperature could be attributed to the presence in separate axons of dissimilar proportions of faster and slower particles. Nor was the velocity–temperature characteristic in the region of room temperature sufficiently steep to explain the differences between axons. It is suggested that axons may differ in general mechanical or chemical properties so that the motion of all particles is affected uniformly.