Exploring a critical parameter of timing in the mouse cerebellar microcircuitry: the parallel fiber diameter.

Since the conduction velocity of the parallel fibers is a critical parameter for a theory of timing in the cerebellar cortex, we set out to quantify the diameter of these axons on an ultrastructural level. The overall mean of the fiber diameter was 0.18 microm. Our results confirm that the parallel fibers of the upper molecular layer are significantly thinner than those of the lower layers. Nevertheless, the difference of about 0.02 microm determined by this study is surprisingly small. In addition, the distribution of the fiber diameters of the upper layers differed slightly, but significantly from a normal distribution, partly on account of a positive skew and a positive kurtosis excess. In summary, the results show that there are fewer differences between the parallel fibers of different levels of the molecular layer than previously assumed and that these differences do not contradict a theory of timing in the cerebellar cortex.