Editing Cerebellum (section)

===== Granular layer =====
[[File:Parallel-fibers.png|right|thumb|Granule cells (GR, bottom), parallel fibers (horizontal lines, top), and Purkinje cells (P, middle) with flattened dendritic trees]]
[[Cerebellar granule cell]]s, in contrast to Purkinje cells, are among the smallest neurons in the brain. They are also the most numerous neurons in the brain: In humans, estimates of their total number average around 50 billion, which means that about 3/4 of the brain's neurons are cerebellar granule cells.<ref name=SOB/> Their cell bodies are packed into a thick layer at the bottom of the cerebellar cortex. A granule cell emits only four to five dendrites, each of which ends in an enlargement called a ''dendritic claw''.<ref name=SOB/> These enlargements are sites of excitatory input from mossy fibers and inhibitory input from [[Golgi cell]]s.<ref name=SOB/>

The thin, [[myelin|unmyelinated]] axons of granule cells rise vertically to the upper (molecular) layer of the cortex, where they split in two, with each branch traveling horizontally to form a '''parallel fiber'''; the splitting of the vertical branch into two horizontal branches gives rise to a distinctive "T" shape. A human parallel fiber runs for an average of 3&nbsp;mm in each direction from the split, for a total length of about 6&nbsp;mm (about 1/10 of the total width of the cortical layer).<ref name=SOB/> As they run along, the parallel fibers pass through the dendritic trees of Purkinje cells, contacting one of every 3–5 that they pass, making a total of 80–100 synaptic connections with Purkinje cell dendritic spines.<ref name=SOB/> Granule cells use [[glutamic acid|glutamate]] as their neurotransmitter, and therefore exert excitatory effects on their targets.<ref name=SOB/>

[[File:Cerebellar glomerulus.tif|thumb|Diagram of the layers of the cerebellar cortex showing a [[Glomerulus (cerebellum)|glomerulus]] in the granular layer.]]
Granule cells receive all of their input from mossy fibers, but outnumber them by 200 to 1 (in humans). Thus, the information in the granule cell population activity state is the same as the information in the mossy fibers, but recoded in a much more expansive way. Because granule cells are so small and so densely packed, it is difficult to record their spike activity in behaving animals, so there is little data to use as a basis for theorizing. The most popular concept of their function was proposed in 1969 by [[David Marr (neuroscientist)|David Marr]], who suggested that they could encode combinations of mossy fiber inputs. The idea is that with each granule cell receiving input from only 4–5 mossy fibers, a granule cell would not respond if only a single one of its inputs were active, but would respond if more than one were active. This combinatorial coding scheme would potentially allow the cerebellum to make much finer distinctions between input patterns than the mossy fibers alone would permit.<ref name=Marr/>