Fig. 9.20A ). The cerebellar lobes are separated by two major fissures, the primary fissure and the posterolateral fissure, and each lobe is made up of one or more lobules. Each lobule of the cerebellar cortex is composed of a series of transverse folds called folia.
The cerebellar cortex has also been divided into longitudinal compartments (see
Fig. 9.20B and C). Initially, the cerebellar cortex was divided into three such compartments: the vermis, which lies on the midline; the paravermis, which lies adjacent to both sides of the vermis; and the lateral hemispheres. These regions have now been subdivided into many further compartments on the basis of myeloarchitectonics (patterns of axonal bundles in the white matter) and the expression patterns of specific molecules, such as aldolase C. Although the functional significance of these compartments is not fully known, the topography of cerebellar afferent fibers, particularly the olivocerebellar system, is precisely aligned with them, and the receptive field properties of cerebellar Purkinje cells also tend to follow this organizational scheme.
There are two major classes of cerebellar afferent systems: mossy fibers and olivocerebellar fibers. Mossy fibers are named for their distinctive appearance in the cerebellar cortex: as a mossy fiber courses through the granule layer, on occasion it swells and sends out a bunch of short twisted branchlets. These entities are called rosettes and are points of synaptic contact between these fibers and neurons in the granule cell layer. Mossy fibers arise from many sources, including the spinal cord (the spinocerebellar pathways), dorsal column nuclei, trigeminal nucleus, nuclei in the reticular formation, primary vestibular afferent fibers, vestibular nuclei, cerebellar nuclei, and the basilar pontine nuclei. The details of specific mossy fiber projection patterns are beyond the scope of this chapter; however, several general points are worth noting: 1.
Mossy fibers are excitatory.
2.
They convey exteroceptive and proprioceptive information from the body and head and form at least two somatotopic maps of the body across the cerebellar cortex. However, like those of the motor cortex, these maps are fractured in the sense that contiguous body regions are not necessarily represented on contiguous areas of the cerebellar cortex; rather, the maps are complicated mosaics.
3.
Mossy fibers conveying vestibular information are restricted to the flocculonodular lobe and regions of the vermis. As a result, the flocculonodular lobe and regions of the vermis are sometimes referred to as the vestibulocerebellum. However, these same regions also receive a variety of other information (e.g., visual, neck, oculomotor), and so their function is not exclusively vestibular.
4.
The largest sources of mossy fibers are the basilar pontine nuclei, which serve to relay information from areas throughout much of the cerebral cortex.
5.