|Connections between the Cortex and Cerebellum|
The main function of the cerebellum is to compare motor commands with actual events in the musculature and visual and auditory space, and provide the motor cortex with information of any deviation from the intended movement.
The cerebellum receives copies of the messages (motor commands) sent down the corticospinal tract, along with a plethor a sensory information relayed by the cerebral cortex.
These cortical inputs reach the cerebellum via nuclei in the brainstem .
After processing of the sensori-motor information by the cerebellar cortex, error signals are relayed back to the motor areas of the cortex via the dentate nucleus, whose neurones project to the red nucleus (midbrain) and the thalamus.
Note that the Purkinje cells of the Flocculo-nodular lobe project to the vestibular nuclei of the brainstem.
There are two main pathways into the cerebellum :
The terminals of the ponto-cerebellar axons are known as 'mossy fibres' because of the ultrastructure of their terminals.
Mossy fibres synapse on granule cells, the origin the parallel fibres that distribute information from many areas of the cortex throughout the cerebellar cortex. These inputs are concerned with motor commands signals and somatosensory, visual and auditory information about the position of parts of the body. The terminals of the spino-cerebellar pathway also have mossy fibre morphology
The second input to the cerebellum arises in the inferior olive (in the medulla) and these fibres climb around the Purkinje cells and are known as 'climbing fibres'.
|The Cerebellar Cortex|
The cerebellar cortex has three layers -
The mossy fibres of the ponto-cerebellar neurones make contact with the granule cells.
The axons of Granule cells are unmyelinated and run towards the surface of the molecular layer where they bifurcate, sending collaterals in opposite directions to make contact with Purkinje cells.
Climbing fibres from the inferior olive are the other input to the Purkinje cells
Purkinje cells have axons that terminate on neurones in the deep cerebellar nuclei; these neurone also receive axon collaterals of the mossy and climbing fibres that provide inputs to these Purkinje cells.
Most neurones in the deep cerebellar nuclei send their information to the red nucleus of the midbrain and onward to the thalamus and cortex
The sifting of feedback signals to extract information about errors is done by Purkinje cells, the principal cell of each cerebellar module.
The communication of error signals to the cortex depends on the output of Purkinje cells, through the deep cerebellar nuclei to the cortex (with synapses in the red nucleus and the thalamus).
The Cerebellar Cortex is where comparisons are made between the command signal and the feedback signals from proprioceptors, touch, vision and hearing.