The Cerebellar Cortex

The cerebellar cortex consists of three layers: the molecular layer, the granule cell layer and the Purkinje cell layer in the middle. The input to the cerebellum comes via the mossy fibers and via the climbing fibers. Both of these are glutamatergic.

# The mossy fibers come from various sources including the pons and the spinal cord (medulla spinalis). The terminals of the mossy fibers are exceptionally large. They have multiple release sites that face granule cell dendrites. The granule cell dendrites also receive inhibitory input from Golgi cell terminals. Because the mossy fiber terminals are large (like a small cell body) are surrounded by multiple granule cell dendrites, it follows the the complexes of synapses is large and characteristic. The entire complex is often called a glomerolus.

# The granule cells are the most abundant cell type in the CNS. There are more granule cells than all other CNS cells combined. These cells give rise to the parallel fibers which are also glutamatergic. These fibers synapse onto all the cell types in the molecular layer. The synapses onto Purkinje cells are onto spines while the synapses onto the other cell types (basket cells, stellate cells and golgi cells) are typically not on spines, but directly onto dendritic shafts.

# The climbing fibers are thousand times less abundant than the parallel fibers so they are harder to identify. Typically, they synapse onto double spines on somewhat more proximal parts of the Purkinje cell dendrites as indicated.

Figure 1. The Purkinje cell axons carry all of the output from the cerebellum. It is interesting to note that this is inhibitory (GABAergic). Thus, cerebellum inhibits activity in other places in the CNS. NB: Do not confuse the cerebellar Purkinje cells with the Purkinje fibers in the heart (Figure source: Niels Chr. Danbolt, University of Oslo, Norway. Reproduction is permitted provided the author is acknowledged).