Those cerebellar afferent fibers that originate in the olivary nucleus become climbing fibers (labeled at left). All other cerebellar afferents are mossy fibers (labeled bottom right). Via granule cells, mossy fibers slightly depolarize many Purkinje cells. Climbing fibers intensely activate only one Purkinje cell. Afferents fibers send collateral branches to cerebellar nuclei before proceeding on to the cerebellar cortex. Thus, cerebellar afferents activate cerebellar nuclei and then Purkinje cells which, in turn, selectively inhibit neurons of cerebellar nuclei.