Sensory representation in the cerebellum and control circuits of motion

The role of the cerebellum in motion and posture control as deduced from the behaviorial effects of lesions has long been known. Subsequent recording and tracing studies in mammals yielded a cerebellar sensory representation featuring 'animalculi', not unlike those seen in the cerebral cortex. This image of cerebellar representation had to be revised with the demonstration of the 'fractured somatotopy' in somatosensory projections (Welker, 1987). A point in case is the trigeminal representation in the mammalian cerebellum, which still lacks a functional explanation. While a fractured somatotopy--if present--has yet to be demonstrated in birds, the topographical details of the trigeminal representation appear to vary considerably between and within both classes. This undoubtedly reflects the variety of functions trigeminocerebellar links subserve in the two classes, but is also indicative of species-specific cerebellar output variation. In this context the importance of cerebellar afferent topography for cerebellar function is discussed.