Scientists had studied the gross structure of the cerebellum for almost 300 years, recognizing various sub-structures, before Cajal identified a variety of cell types present in the cerebellum; the mossy and climbing fibers were unknown before, and their structure and placement suggested intriguing connections between the cerebellum and other parts of the brain.
In the 1960s, Sir John Eccles used electro-physiological analyses to reveal elegant feedforward circuitry. Incoming mossy fibers deliver sensory signals from pre-cerebellar nuclei and the spinal cord to granule cells (g), the most numerous neuron type in the brain. Granule cell axons bifurcate to form the densely arrayed parallel fibers, which contact the (relatively) giant Purkinje cells (a).
Meanwhile, climbing fibers transmit signals from the inferior olivary nucleus to Purkinje cells. Each parallel fiber makes a single weak synapse onto a Purkinje cell, whereas the parallel fiber connection is very strong, comprising hundreds of distinct synaptic contacts.
Purkinje cells transmit inhibitory signals to the deep nuclei, outside the cerebellar cortex, which will relay information to the cerebral cortex. The cerebellum coordinates and fine-tunes movements, although recent research has revealed roles for the cerebellum in spatial cognition and language.
