The role of the cerebellum
The cerebellum (or little brain) plays an extremely important role in our lives. It alone contains more than half of the neurons of the entire brain [54], with more than 50 billion neurons [159]!
1. Functions of the cerebellum:
The cerebellum plays a vital role in voluntary motor control, posture, maintaining balance, the coordination of complex movements, their learning, and many other roles recently highlighted, notably vegetative and cognitive-emotional ones [36, 41].
No matter how simple the movements to be performed are, the cerebellum is always there to control and coordinate them. Without the cerebellum, it would be impossible for us to walk or even to stand upright. We would find it impossible to perform the simplest gestures of life that require a certain degree of precision, and without the cerebellum, we would be unable to learn to perform more or less complex tasks.
2. Cerebellar Cortex:
The cerebellar cortex is different from its cerebral counterpart; while the latter consists of various cellular layers whose number varies by cortical region up to 6, thus delimiting the Brodmann areas, the cerebellar cortex contains three cellular layers everywhere [75]; therefore, there are no Brodmann areas in the cerebellum.
If the cerebellar cortex is unfolded, it turns out that its surface area represents about 75% of that of the cerebral cortex [4].
3. Purkinje Cells:
Out of the 50 billion cerebellar cells, only the Purkinje cells (which number approximately 15 million [3]) project outside the cerebellum [32] (representing 0.03% of all cerebellar neurons).
Each Purkinje cell can have up to 300,000 synapses with other neurons [57]; this demonstrates the degree of integration that takes place in the cerebellum [82].
Patients with cerebellar pathology show clinical symptoms similar to those of a drunk person. In fact, alcohol is highly toxic to Purkinje cells, which are the most precious cells of the cerebellum.
4. Cerebellar Pathways:
To perform these diverse functions, the cerebellum must receive all kinds of information from almost all parts of the CNS. Thus, it receives proprioceptive information directly from the spinal cord, as well as visual and vestibular signals to ensure body balance. It also receives tracts from most of the basal ganglia as well as from the brainstem nuclei.
Through the pons, the cerebellum receives collateral relays from the fibers of the pyramidal tract [41], which allows it to constantly be aware of the movements to be performed and to monitor their execution in real-time through a set of sensory feedbacks, notably visual and proprioceptive ones.
The cerebellum thus has all the information regarding the circumstances of the movement's execution. From there, it ensures the control of these movements through control loops that end on the contralateral primary motor cortex to join the pyramidal tract [41]. Since this tract also crosses the midline, it follows that the cerebellar hemispheres have ipsilateral control of motor function [5].