Propagation of parallel fiber volleys in the cerebellar cortex: a computer simulation

We have studied the dynamic changes occurring during the propagation of active volleys through the parallel fiber (PF) tract in a portion of molecular layer of the cerebellar cortex, by means of a realistic computer simulation of the fibers (random distribution of PF origin; correlation between length and conduction velocity throughout depth). Surface electrical stimulations as well as mossy fiber stimulations of the PFs were mimicked. From a quantitative analysis of the propagation of the different types of volleys it appears that the constraints enforced by the morphological and functional characteristics of the PFs lead to an important spatial and temporal dispersion of the action potentials during their displacement along the folium. In particular, the time relationship between activity in the deep and superficial fibers as well as the number and location of active fibers at a given time (i.e. locus) will considerably vary during propagation. Moreover, in the case of a mossy fiber stimulation, there exists a striking asymmetry of the volleys travelling on each side of the site of stimulation. Therefore any information coded by the conjunctive activation of more than a few parallel fibers will be spatially and temporally dispersed in such a manner that successive postsynaptic Purkinje cells will receive very different combinations of inputs. This seems in favor of the hypothesis that these neurons function as coincidence detectors.