The cerebellum is a region of the brain that is important for making smooth, coordinated movements. However, exactly how the cerebellum performs these functions remains unknown. One of the striking features of the cerebellum is its precise neuronal circuitry, such as one of the main circuit elements: the parallel fibers, which consist of billions of neural axons that run in parallel and activate Purkinje cells.
Purkinje cells are some of the largest neurons in the human brain, and are responsible for the motor coordination in the cerebellar cortex.
The “beam” hypothesis proposes that when a group of parallel fibers are excited they activate a beam of Purkinje cells. The competing “radial” hypothesis postulates that parallel fibers only activate Purkinje cells locally.