The extrapyramidal system

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 over 50 billion neurons ^[159].

Functions of the cerebellum :

The cerebellum plays a vital role in voluntary motor function, posture, the maintenance of balance, the coordination of complex movements, motor learning, and many other roles recently highlighted, including autonomic and cognitive-emotional functions ^{[36, 41]}.

No matter how simple the movements to be performed, the cerebellum is always there to control and coordinate them. Without the cerebellum, it would be impossible for us to walk or even stand upright. It would be impossible to perform the simplest daily activities that require any degree of precision, and without the cerebellum, we would be unable to learn how to carry out even the simplest tasks.

Cerebellar cortex :

The cerebellar cortex differs from its cerebral counterpart; while the latter consists of various cell layers whose number varies by cortical region (reaching up to six and defining the Brodmann areas), the cerebellar cortex contains three cell layers throughout ^[75]; therefore, there are no Brodmann areas in the cerebellum.

Even though the cerebellum looks small, it is packed with incredibly tight folds. If you were to unfold the cerebellar cortex, its surface area would actually cover about 75% of that of the much larger cerebral cortex ^[4].

Purkinje cells :

Of all 50 billion cerebellar cells, only the Purkinje cells (numbering approximately 15 million ^[3]) project outside the cerebellum ^[32] (accounting for only 0.03% of all cerebellar neurons).

Each Purkinje cell can have up to 300,000 synapses with other neurons ^[57], demonstrating the high degree of integration that occurs within the cerebellum ^[82].

Patients with cerebellar damage often show signs similar to alcohol intoxication. This is because alcohol is highly toxic to Purkinje cells - the most critical neurons in the cerebellum for coordinating movement.

Cerebellar pathways :

To work its magic, the cerebellum acts like a high-speed processor, constantly gathering data from almost every corner of the nervous system. It monitors your intentions from the brain and combines them with proprioceptive information from the spinal cord to track your body's position. At the same time, it processes visual and vestibular signals to maintain your balance. By integrating all of this with input from the basal ganglia and brainstem, the cerebellum ensures your every move is smooth and perfectly coordinated.

Through the pons, the cerebellum receives collateral relays from the fibers of the pyramidal tract ^[41], allowing it to be constantly aware of intended movements and to monitor their execution in real-time through a set of sensory feedback mechanisms, particularly visual and proprioceptive ones.

The cerebellum thus possesses all information regarding the circumstances of movement execution. From there, it ensures control of these movements through relay control loops that terminate in the contralateral primary motor cortex to join the pyramidal tract ^[41]. Since this tract also crosses the midline, the cerebellar hemispheres ultimately exert ipsilateral control over motor function ^[5].