The cerebellum contains ~80% of the brain’s neurons despite being ~10% of its volume, largely due to the massive parallel fiber–Purkinje cell array. Its architecture is remarkably uniform: granule cells (excitatory) → parallel fibers → Purkinje cells (inhibitory) → deep cerebellar nuclei → thalamus.
The climbing fiber input from the Inferior Olive is thought to carry a teaching/error signal — the basis of Marr-Albus-Ito theories of cerebellar motor learning.