Comparative Neurobiology of the Elasmobranch Cerebellum: Theme and Variations on a Sensorimotor Interface

The organization of the vertebrate cerebellum has been thoroughly studied over the past century, but the function of this structure remains poorly understood. In elasmobranch fishes, the cerebellum displays tremendous variation in size and development although the basic and conservative nature of cerebellar circuitry as seen in other vertebrate taxa is largely retained. Large and morphologically complex cerebelli have evolved independently in both sharks and batoids, and the relative development of this structure in both taxa parallels those of birds and mammals. There are relatively few studies of the physiological role of the cerebellum in generating or shaping behaviors, however, and a convincing explanation of cerebellar hypertrophy in elasmobranchs is lacking. The purpose of this article is to review the current understanding of the structure of the cerebellum in elasmobranch fishes, the physiological responses of cerebellar neurons and the possible role of the cerebellum in behavior. I will also provide a number of hypotheses for future research directions, based upon models that have been suggested by different investigators. These hypotheses include models of cerebellar function as a sensory coincidence detector, a dynamic state estimator and/or a direct modulator of motor programs. Hypotheses concerning the possible organization of cerebellar microcomplexes, the evolution of afferent and efferent cerebellar connections paralleling those observed in mammals and the role of the cerebellum in learning are also suggested.