4-6. Climbing Fibers

Climbing fibers are a unique structure of the cerebellum with no homolog elsewhere in the CNS (Color Plate IX A–B). The major transmitter of climbing fibers is glutamate. Each Purkinje cell is innervated by one climbing fiber. This is a consequence of the postnatal elimination of multiple innervation, which, after birth in rats and mice, attains its maximum in one week and fades out in two weeks via its interaction with developing parallel fiber-Purkinje cell synapses (Mariani and Changeux, 1981; Hashimoto and Kano, 2003; Scelfo and Strata, 2005; Hashimoto et al., 2009). Each climbing fiber forms numerous synaptic contacts with the dendrites of a single Purkinje cell [~1,300 in proximal dendrites of rat Purkinje cells (Strata, 2002), but a much larger number,~26,000, is derived from the density ratio of climbing fiber to parallel fiber synapses (Nieto-Bona et al., 1997)].