Cerebellum-Like Systems in Actinopterygian Fishes with a Special Focus on the Diversity of Cerebellum-Like System in the Mesencephalon

Abstract

In addition to the cerebellum, most vertebrates possess cerebellum-like structures with components similar to the molecular layer, granule cells, and Purkinje cells. Most of such cerebellum-like structures situate in the medulla oblongata. In actinopterygian fishes, however, an additional cerebellum-like system is present in the mesencephalon. The torus longitudinal is the granule layer-like structure that sends parallel fiber-like axons to the most superficial layer of the optic tectum, or the stratum marginale. Pyramidal cells or type I cells of the optic tectum, which send ascending dendrites to the stratum marginale, are Purkinje cell-like neurons. This chapter introduces morphology, neural circuitry, and functions of the cerebellum-like systems, with a special focus on the mesencephalic cerebellum-like system in actinopterygian fishes. A recent study suggests that the mesencephalic cerebellum-like system is involved in the visual attention shift, although further studies are necessary to understand fully the significance of this peculiar cerebellum-like system.