The cerebellum gets involved in eye movements in a number of ways. The vestibulo-ocular reflex is probably the best studied, but each of the degrees of oculomotor freedom has a pathway through the cerebellum, and the cerebellum is vital for keeping the eyes on target.

Lesions of the superior colliculus do not eliminate eye movements, after a week-long recovery period they seem mostly quite normal with a few exceptions. However lesions in the vermis and flocculus of the cerebellum can result in very severe symptoms including hyper and hypometria and an inability to locate the target and keep the eyes on target.

The two main areas of the cerebellum involved in eye movements are the vermis and the flocculus, portions of the "medio-posterior cerebellum" including lobules VIc and VII. These areas connect with the oculomotor control system in the brainstem, including specifically the pause neurons in the nucleus prepositus raphe. The fastigial oculomotor region (FOR) in the caudal fastigial nucleus fine tunes the accuracy of horizontal saccades and contributes to smooth pursuit movements.

Systemic Pathways

Each degree of oculomotor freedom has a pathway through the cerebellum.

In addition to these pathways, there are additional inputs from the vestibular nuclei related to the VOR.

The two main areas of the cerebellum involved in eye movements are the vermis and the flocculus, portions of the "medio-posterior cerebellum" including lobules VIc and VII. These areas connect with the oculomotor control system in the brainstem, including specifically the pause neurons in the nucleus raphe prepositus.

Architecture of the Cerebellum

The cerebellar circuits related to eye movements are organized like control systems in the engineering sense. There are two main inputs to the cerebellum, the mossy fibers and the climbing fibers. The output from the cerebellum comes from the Purkinje cells. Inside the cerebellum is a very simple arrangement of neurons with behaviors that are mostly pretty well understood by now, although much is still being learned.

The figure shows the overall internal architecture of the cerebellum. Mossy fibers enter from the precerebellar nuclei, and branch to connect with granule cells and Golgi cells in the specialized "dendritic claw" synapses, and with the deep cerebellar nuclei (both connections are excitatory). Each mossy fiber connects with about 50 granule cells. In turn, the deep cerebellar nuclei are also systemic outputs (they receive inhibitory inputs from the Purkinje cells) and project to premotor areas (mostly in the brainstem). The climbing fibers originate from the inferior olive, and branch to connect with the deep cerebellar nuclei and the dendrites of Purkinje cells. The cerebellar cortex therefore engages two other large structures, the deep cerebellar nuclei and the inferior olive.

Much is still being learned about the cerebellum, even though it is one of the best studied areas in the brain. For example, it was previously thought that Golgi cells performed a smooth integration along the dendrites, but now it is clear they have voltage dependent calcium conductances and exhibit dendritic mini-spikes. Similarly it was once thought that Golgi cells were the only neurons that inhibit granule cells, but now it has been discovered that Purkinje cells directly inhibit granule cells in some specialized areas of the cerebellum, including those related to eye movements and vestibular function (Guo et al 2018). Golgi cells do not synapse directly onto Purkinje cells, that we know of - only onto granule cells via the dendritic claw synapses associated with the mossy fiber glomeruli.