Answer: At about 50 billion in number, cerebellar granule cells make up more than half of all neurons in the brain.
The class of cells called “granule cells” are generally the smallest neurons in the brain. They often have diameters as small as 10 microns. Because of their small size, they can be densely packed close to one another.
Granule cells in the cerebellum only have a handful of dendrites where they receive synaptic input. They get excitatory inputs from the mossy fibers. They also get inhibitory inputs from Golgi cells. They send excitatory glutamatergic axonal projections up to the cerebellar cortex, where they split into two branches, forming the parallel fibers. These cells therefore have a distinctive T shape.
It has been difficult to determine their function during behavior. Because they are so densely packed, it becomes tricky to identify which cells are firing and which ones are not.