Cerebellum

The cerebellum is located in the neck of the brain, attached to the pons where all the communication in and out of the cerebellum goes through. The cerebellum has, similar to cerebral cortex, two hemispheres and a folded cortical surface, which in cerebellum has parallel groves. The cerebellar cortex includes three distinct layers, the molecular layer, the Purkinje layer, the granular layer and underlying white matter. The cerebellum is rich in white matter, which holds the white matter nuclei (dentate, globose, emboliform, and fastigial) which acts as relay for the connections to and from the cerebellar cortex.

The three layers include a large number of specialized neurons, making the cerebellum the most neuronal dense region of the brain. In human, it is estimated to include 80% of all the neurons in the brain, but it only represents 10% of the brain volume.

• Molecular layer, includes interneurons such as stellate and basket cells that receives input from the paralell fibers from granular cells and provides input to the Purkinje cells. Stellate cells are mainly found closer to the pial surface while basket cells are branching out and create synapses to the Purkinje cell body. The molecular layer includes projections from granular cells and Bergmann glia as well as the large dendritic tree of Purkinje cells.
• Purkinje layer includes the Purkinje cells, Bergmann glia (the cerebellum specific glia cell) and interneurons such as Golgi and Candelabrum cells in the vicinity.
• Granular layer holds the very abundant number of granular cells. Granular cells are interneurons with a small cell body but long projections into the molecular layer, called parallel fibers, which sends input to the Purkinje dendrites as well as interneurons of the molecular layer. The main input to granular cells is located at the synaptic glomeruli, densely packed synaptic compartments where mossy fibers from outside the cerebellum provide input.