The Cerebellum

The cerebellum is located at the back of the skull, below the cerebrum and behind the brain stem. Its two halves are clearly visible from the outside and, like the halves of the cerebrum, are referred to as hemispheres. They are connected by the vermiform body, the cerebellar worm.

When English neurologist Gordon Holmes (1876−1965) examined soldiers with cerebellar injuries in 1917, he realized: “The cerebellum can be seen as an organ that supports movement.” In fact, imaging techniques now confirm that the cerebellum coordinates and modulates movements: whether you are reaching for a coffee cup, playing the piano, or doing a somersault – the neurons in the cerebellum are firing. But Holmes underestimated the cerebrum's little brother.
 

Although it is only about one-sixth the volume of the cerebrum, the cerebellum has five times more neurons. In order to accommodate so much nerve mass in such a small space, the cerebellar cortex, the outer layer of the cerebellum, is heavily folded. The resulting folds are called folia. If you cut one of the cerebellar hemispheres lengthwise, you can see the reason for this name: like the branches of a tree, the white matter of nerve fibers branches out inside. Anatomists refer to it as the “tree of life”: the “arbor vitae”. And the folds of the cerebellar cortex seem to hang like leaves from its branches. The roots of the tree are formed by the cerebellar peduncles, which run from the cerebellum to the brain stem. The cerebellum receives and transmits information via these fibers.

The tree of life is surrounded by the gray matter of the nerve cells. And as can be seen at the microscopic level, this is also structured with extraordinary precision. The same three cell layers are found throughout the cerebellar cortex: the molecular layer, the Purkinje cell layer, and the granular cell layer, viewed from the outside in. The middle layer is particularly striking – it contains the large Purkinje cells, neatly arranged next to each other like a layer of cherries in a cake. They are the central switching points of the cerebellar cortex: With their widely branched dendrite trees, they receive excitatory and inhibitory information from almost all other cortical neurons. There can be up to 200,000 synapses on a single dendrite tree. The Purkinje cells transmit their inhibitory signals to the permanently active cerebellar nuclei, clusters of nerve cells deep in the white matter. From there, the impulses travel via the cerebellar peduncles to other areas of the brain outside the cerebellum.

Anatomists divide the cerebellum functionally into three areas that perform different tasks: the vestibulocerebellum, the spinocerebellum, and the pontocerebellum.

In terms of evolutionary history, the vestibulocerebellum is the oldest: it can even be found in living fossils such as fish-like, eel-shaped lampreys, although these have hardly changed in the last 500 million years. In humans, the vestibulocerebellum essentially consists of the anatomical structures nodulus and flocculus – collectively known as the flocculonodular lobe – and, as the name suggests, it is functionally connected to the vestibular apparatus, i.e., the balance organ of the inner ear. We owe our ability to balance or walk upright to the vestibulocerebellum. It is also involved in controlling eye movements.

The largest part of the cerebellar vermis and a finger-width margin of the adjacent hemispheres together form the spinocerebellum. It ensures that we can stand and walk without having to think about it. The spinocerebellum receives information from the spinal cord about the position of the arms, legs, and trunk, as well as which muscles are tense and which are relaxed. It processes this information and sends it to the brain stem.

The pontocerebellum consists of almost the entire cerebellar hemispheres. It is closely connected to the cerebrum via the pontine nuclei in the brain stem. Whenever we move something voluntarily, the pontocerebellum is involved: its tasks range from precise grasping to coordinating the laryngeal muscles when speaking. The pontocerebellum can be compared to a conductor. Instead of music, it studies movements, tunes them to its musicians – the muscles – and coordinates their interaction. The notes correspond to a movement plan provided by the cerebrum. If something goes wrong, the cerebellum intervenes, for example, if the floor is unexpectedly uneven or the coffee cup is empty and therefore lighter than expected. Cerebellar correction loops are extremely important for successfully completing disrupted movements – or learning them in the first place.

Although brain researchers have a rough idea of the tasks performed by the three areas of the cerebellum, they do not yet understand exactly how the vestibular, spinocerebellar, and pontocerebellar cerebellum perform all these motor tasks.

Recent studies suggest that the cerebellum is not only responsible for motor skills. In 2005, neurologist Catherine Limperopoulos and her colleagues at McGill University in Montréal studied children who were born with cerebellar injuries. In addition to motor problems, the young patients also had difficulties with cognitive processes such as communication, social behavior, and visual perception. Furthermore, imaging techniques show that activity in the cerebellum lights up during a variety of activities: for example, during short-term memory tasks, the control of impulsive behavior, hearing and smelling, pain, hunger, shortness of breath, and much more. Neuroscientists do not yet know what role the cerebellum plays in these various tasks. A common hypothesis is that the cerebellum is responsible for the temporal coordination of the associated neural activity.

Despite the cognitive deficits mentioned above, motor problems are the main issue in cerebellar injuries. These largely correspond to what Holmes observed in his patients and what neurologists refer to as various types of “ataxia”: those affected have problems with balance and coordination; their gait is unsteady and similar to that of a heavily intoxicated person. When these patients reach for something, their hand trembles more as they get closer to the object. Their movements also often overshoot the target. Their speech sounds choppy. Their muscle tone is often reduced, making their bodies appear limp.

The patients' eye movements are also noticeable. Their gaze moves jerkily, their eyes seem to tremble, and they often have to correct their eye position several times in order to fixate on an object.

First published on July 26, 2011
Last updated on August 5, 2025