<P> The first indications of compartmental structure came from studies of the receptive fields of cells in various parts of the cerebellar cortex . Each body part maps to specific points in the cerebellum, but there are numerous repetitions of the basic map, forming an arrangement that has been called "fractured somatotopy". A clearer indication of compartmentalization is obtained by immunostaining the cerebellum for certain types of protein . The best - known of these markers are called "zebrins", because staining for them gives rise to a complex pattern reminiscent of the stripes on a zebra . The stripes generated by zebrins and other compartmentalization markers are oriented perpendicular to the cerebellar folds--that is, they are narrow in the mediolateral direction, but much more extended in the longitudinal direction . Different markers generate different sets of stripes, the widths and lengths vary as a function of location, but they all have the same general shape . </P> <P> Oscarsson in the late 1970s proposed that these cortical zones can be partitioned into smaller units called microzones . A microzone is defined as a group of Purkinje cells all having the same somatotopic receptive field . Microzones were found to contain on the order of 1000 Purkinje cells each, arranged in a long, narrow strip, oriented perpendicular to the cortical folds . Thus, as the adjoining diagram illustrates, Purkinje cell dendrites are flattened in the same direction as the microzones extend, while parallel fibers cross them at right angles . </P> <P> It is not only receptive fields that define the microzone structure: The climbing fiber input from the inferior olivary nucleus is equally important . The branches of a climbing fiber (usually numbering about 10) usually activate Purkinje cells belonging to the same microzone . Moreover, olivary neurons that send climbing fibers to the same microzone tend to be coupled by gap junctions, which synchronize their activity, causing Purkinje cells within a microzone to show correlated complex spike activity on a millisecond time scale . Also, the Purkinje cells belonging to a microzone all send their axons to the same small cluster of output cells within the deep cerebellar nuclei . Finally, the axons of basket cells are much longer in the longitudinal direction than in the mediolateral direction, causing them to be confined largely to a single microzone . The consequence of all this structure is that cellular interactions within a microzone are much stronger than interactions between different microzones . </P> <P> In 2005, Richard Apps and Martin Garwicz summarized evidence that microzones themselves form part of a larger entity they call a multizonal microcomplex . Such a microcomplex includes several spatially separated cortical microzones, all of which project to the same group of deep cerebellar neurons, plus a group of coupled olivary neurons that project to all of the included microzones as well as to the deep nuclear area . </P>

What is the largest part of the mature brain