Anatomy of the cerebellum major part of the cerebellum by mass, but in other vertebrates, it is typically the spinocerebellum. In amphibians, lampreys, and hagfish the cerebellum is little developed; in the latter two groups it is barely distinguishable from the brain-stem. Although the spinocerebellum is present in these groups, the primary structures are small paired nuclei corresponding to the vestibulocerebellum. The cerebellum follows the general groups-of-three pattern found in anatomy, with three major input and output cerebellar peduncles (fiber bundles). These are the superior (brachium conjunctivum), middle (brachium pontis), and inferior (restiform and juxtarestiform bodies) cerebellar peduncles. There are three sources of input
Anatomy of the cerebellum uniform, with connections organized into a rough, three-dimensional array of perpendicular circuit elements. This organizational uniformity makes the nerve circuitry relatively easy to study. To envision this "perpendicular array", one might imagine a tree-lined street with wires running straight through the branches of one tree to the next. There are three layers to the cerebellar cortex; from outer to inner layer, these are the molecular, Purkinje, and granular layers. The function of the cerebellar cortex is essentially to modulate information flowing through the deep nuclei. The microcircuitry of the cerebellum is schematized in Figure 5. Mossy and climbing fibers carry