The anatomy of the cerebellum can be viewed at three levels. At the level of gross anatomy, the cerebellum consists of a tightly folded and crumpled layer of cortex, with white matter underneath, several deep nuclei embedded in the white matter, and a fluid-filled ventricle in the middle. At the intermediate level, the cerebellum and its auxiliary structures can be broken down into several hundred or thousand independently functioning modules or compartments known as microzones. At the microscopic level, each module consists of the same small set of neuronal elements, laid out with a highly stereotyped geometry.
Gross anatomy
The cerebellum is located at the base of the brain, with the large mass of the cerebral cortex above it and the portion of the brainstem called the pons in front of it. It is separated from the overlying cerebrum by a layer of tough dura mater; all of its connections with other parts of the brain travel through the pons. Anatomists classify the cerebellum as part of the metencephalon, which also includes the pons; the metencephalon in turn is the upper part of the rhombencephalon or "hindbrain". Like the cerebral cortex, the cerebellum is divided into two hemispheres; it also contains a narrow midline zone called the vermis. A set of large folds are conventionally used to divide the overall structure into ten smaller lobules.
Because of its large number of tiny granule cells, the cerebellum contains more neurons than the rest of the brain put together, but it only takes up 10% of total brain volume. The cerebellum receives nearly 200 million input fibers; in contrast, the optic nerve is composed of a mere one million fibers.
The unusual surface appearance of the cerebellum conceals the fact that the bulk of the structure is made up of a very tightly folded layer of gray matter, the cerebellar cortex. It has been estimated that if the human cerebellar cortex could be completely unfolded it would give rise to a layer of neural tissue about 1 meter long and 10 centimeters wide—a total surface area of 500-1000 square cm, all packed within a volume of 100-150 cubic cm. Underneath the gray matter of the cortex lies white matter, made up largely of myelinated nerve fibers running to and from the cortex. Embedded within the white matter—which is sometimes called the arbor vitae (Tree of Life) in the cerebellum because of its branched, tree-like appearance—are four deep cerebellar nuclei.
The cerebellum can be divided according to three different criteria: gross anatomical, phylogenetical, and functional.
Gross anatomical divisions
On gross inspection, three lobes can be distinguished in the cerebellum: the flocculonodular lobe, the anterior lobe (rostral to the "primary fissure"), and the posterior lobe (dorsal to the "primary fissure"). The latter two can be further divided in a midline cerebellar vermis and lateral cerebellar hemispheres.
Phylogenetic and functional divisions
The cerebellum can also be divided in three parts based on both phylogenetic criteria (the evolutionary age of each part) and on functional criteria (the incoming and outgoing connections each part has and the role played in normal cerebellar function). From the phylogenetically oldest to the newest, the three parts are:
Much of what is understood about the functions of the cerebellum stems from careful documentation of the effects of focal lesions in human patients who have suffered from injury or disease or through animal lesion research.
Cellular anatomy
As explained in more detail in the Function section, the cerebellum differs from most other brain areas in that the flow of neural signals through it is almost entirely unidirectional: there are virtually no backward connections between its neuronal elements. Thus the most logical way to describe the cellular structure is to begin with the inputs and follow the sequence of connections through to the outputs.
Deep nuclei
The four deep nuclei of the cerebellum are the dentate, emboliform, globose, and fastigii nuclei and they act as the main centers of communication, sending and receiving information to and from specific parts of the brain. In addition, these nuclei receive both inhibitory and excitatory signals from other parts of the brain which in turn affect the nuclei's outgoing signals.(The globose and the emboliform nuclei make up the interposed nucleus).
Cortical layers
The cytoarchitecture (cellular organization) of the cerebellum is highly 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.