Cerebellar granule cell: ascending axon and parallel fiber.

Morphometric data have indicated that most (> 95%) of the granule cell axonal synapses are located along the parallel fibers. The ascending axons of granule cells, however, exert powerful excitatory effects on the activities of Purkinje cells. To resolve this apparent conflict, we propose that some of the parallel fiber synapses overlying a functionally homogeneous granule cell patch can discharge in close synchrony with the ascending axon synapses from that same granule cell patch, thereby augmenting the excitatory action from ascending axons. Moreover, we also propose that age may be an important factor in the ascending axon synapses. We examined the synaptic varicosities along the ascending axons and parallel fibers in the anterior lobe of the male NIA C57BL/6j mouse. Between 2.5 and 10 months, the fraction of synaptic varicosities anatomically associated with the ascending axons increased from 2.6% to 6.6% whereas the fraction of synaptic varicosities that can discharge in synchrony with ascending axon synapses (including some parallel fiber synapses) increased from 17.3% to 39% (assuming a granule cell patch of 300 x 300 microm). The results of the present study may be relevant to understanding the role of the ascending axon vs. the parallel fibers. Differential age-related synaptic changes in these two portions of the granule cell axons may constitute an important age-dependent synaptic remodeling in the granule cell-Purkinje cell synaptic system.