The lobules and fissures of the cerebellum are more easily understood if it is imagined that the surface of the cerebellum has been flattened as shown opposite. Using this representation, many of the areas of the cerebellum can be quickly and easily drawn schematically, and their relationship to other cerebellar structures understood.
The medial vermal cerebellum has been subdivided into lobes running down the middle. These are, from dorsal to ventral, the lingula, culmen, declive, folium, tuber, pyramid, uvula, and nodule. The various lobular subdivisions and the main lobes and fissures are also distinguished using this view.
Most of the cerebellar cortex is buried in the folia, and only about 15% is visible. In section, the cerebellar cortex is seen to be uniformly structured throughout, with three clearly defined layers that contain five different types of neurons. The cerebellar cortical layers are, from the surface inwards, the molecular, Purkinje cell (sometimes called piriform), and the granular layers. The medullary layer lies beneath the granular layer.
The molecular layer is relatively sparsely populated with two types of nerve cells: basket cells and outer stellate cells. The axons and dendrites of the outer stellate cells do not leave the molecular layer, and neither do the dendrites of basket cells. These processes run roughly horizontally in the layer, transverse to the long axis of the depth of the folia or infolding. The basket cell bodies are close to those of the Purkinje cells in the next layer, and project fibers that form basket shapes around the cell bodies of the Purkinje cells. Below this layer is the relatively narrow Purkinje cell layer. The Purkinje cells are large Golgi type I neurons; their cell bodies lie in rows along the folia, and their axons project to the intracerebellar nuclei. Some of these Purkinje axons in the archicerebellum project to the brainstem vestibular nuclei. Purkinje dendrites proliferate densely, transverse to the plane of the folia. Immediately below is the relatively wide granular layer, whose cells are very tightly packed and send axons up into the molecular layer, where they branch in Tshapes and run as parallel fibers along the horizontal axis of the folia. Each Purkinje dendritic tree may form synapses with up to half a million parallel fibers that have projected up from the granular layer. Also in the granular layer is a relatively small population of inhibitory Golgi neurons, which project their dendritic trees up into the molecular layer. One Golgi cell may synapse with a row of ten to twelve Purkinje cells, and it appears that Golgi cells do not overlap with respect to the innervation of the Purkinje cells.
There are two main types of afferent input to the cerebellum, and both are excitatory. Each Purkinje cell is supplied by one climbing fiber from the contralateral inferior olive . The phylogenetically more ancient archicerebellum and paleocerebellum are served by the correspondingly older accessory olivary nuclear cells. The neocerebellum is supplied with fibers by the newer inferior olive. The second afferent input is through the mossy fibers from many different sources, including the pontine nuclei. These fibers diverge extensively, and one mossy fiber may serve several folia. The mossy fiber axons form multiple rosettes, which synapse with several granular cell dendrites. Inhibitory Golgi axons synapse in these rosettes. It follows, therefore, that since mossy fiber rosettes synapse with granular fibers, which in turn synapse with Purkinje cells, that one mossy fiber can indirectly affect electrical activity in very many Purkinje cells.