Cerebellum and Its Function

Overview

Even though it can seem like a very complex and enigmatic formation at first glance, the cerebellum is a very precise and clearly organized organ. It is positioned in the back skull pit. Its key functions include movement coordination, balance, standing, and walking.

Development of Cerebellum

Developmentally (embryologically), the cerebellum represents one of the older brain segments. More precisely, it is a derivative of the upper rhombencephalon, called metencephalon, and rhombencephalon itself is the most caudal part of the neural tube (the core of the central nervous system) from which the brain develops.

Neural tube below the rhombic brain forms the basis for the spinal cord, while the remaining encephalic vesicles are divided into:

• myelencephalon,
• mesencephalon
• and the two lateral vesicles that
  form the basis of the hemispheres of the brain.

The cavities inside these vesicles/bubbles gradually differentiate into the brainstem system. Brain chambers, by their structure and name, follow the final division of the bubbles so that they form a fourth ventriculus quatrus in the rhombic brain. The so-called cerebral aqueduct forms in the mesencephalon.

The cerebral cortex is formed by the migration of primitive nerve cells and their redistribution in several stages. Finally, the outer layer of the three-layer cerebral cortex is formed: the molecular layer (stratum molecular), the Purkinje cell layer, and the inner granular layer (stratum granulosum internum) which also contains two specific cell groups.

Interestingly, the cerebellum contains as much as 50% of all the neurons located throughout the brain.

Function of Cerebellum

As already mentioned, the cerebellum contains around 50 % of all neurons in our brain. It has several functions. The most important ones include balance, motoric activities, walking, standing, and coordination of voluntary movements. It also coordinates muscular activity and speech.

It also coordinates eye movements, thus heavily impacting our vision. Cerebellum also takes part in activities such as riding a bicycle, dancing, various sports, and playing a musical instrument. 

Most importantly, the cerebellum is responsible for receiving signals from other parts of the brain, the spinal cord, and senses. Therefore, damage to this part of our brain often leads to tremors, speech problems, lack of balance, lack of movement coordination, and slow movements.

Poor muscle control, irregular eye movements, and poor mobility are results of various cerebellum damages and disorders. Those can be caused by a stroke, inborn anomalies, toxins, or cancer.

Position, Volume, Origin, and Arterial Supply

The cerebellum is an integral part of the central nervous system. It is located in the posterior cranial pit and is covered with a duplicate dura mater, which is scientifically called tentorium cerebelli.

Although it only takes up a tenth of the volume of brain tissue, it contains as many as half of the whole number of brain neurons. It is anatomically roughly divided into two hemispheres, vermis, and the flocculonodular lobe.

The cerebellum covers the fourth cerebral chamber and has a close anatomical relationship with the brainstem and the temporal bone pyramids. When it comes to the arterial supply, it sources from the posterior cerebral circulation and the venous drainage is connected to the surrounding sinuses of the dura.

Developmentally, it originates from the posterior brainstem. The cerebral cortex is three-layered and contains the Purkinje cells, which are among the largest human brain cells.

Anatomy

Alike the classic representation about the brain we usually have, the cerebellum consists of two hemispheres between which a very specific worm-like central part called vermis is squeezed. This is the key feature of the cerebellum. The hemispheres and the central vermis form a single continuous tissue structure.

The boundaries between the two parts are visible just like two shallow paravermal grooves between their touch surfaces on the upper surface of the cerebellum. This surface is also called “tentorial surface” because it lies just below the doubling point of the dura mater or the hard brain envelope and it separates the posterior or occipital lobes of the cerebellum in the form of a tent.

Tentorium is also an important landmark for all formations in the cranial cavity, as all formations located above this segment are called "supratentorial", including the whole telencephalon and the first four brain nerves. Everything located below it is marked as 'infratentorial' formation, including the cerebellum, brainstem and the remaining eight brain nerves.

These two shallow paravermal grooves deepen significantly toward the inferior or “suboccipital” surface of the cerebellum. The deepening of the above-mentioned grooves forms a furrow between the hemispheres and the vermis of the cerebellum.

In this part, the vermis is positioned deeper than the surface of the hemispheres, so this recess is called the "posterior incision of the cerebellum". The Latin name is "incisura cerebellaris posterior”.

The third of a total of three
cerebral surfaces is the front surface or the so-called "Petrosal
surface". It is in close relation to the petrosal parts of the temporal
bone. Each of these three surfaces has numerous shallow furrows and one deeper
furrow named after the surface on which it is located (hence, tentorial furrow,
suboccipital furrow, and petrosal furrow). The furrows separate the individual
lobes of the cerebral tissue.

In the classic neuroanatomy, in which the names and understandings about the structure of the cerebellum are based primarily on its development, these lobes in the vermis area also have their number tags, which were established by Larsell, neuroanatomist (1).

Such a division of the cerebellum results from its mediosagittal cross-section, which clearly shows a very regular pattern of distribution of gray and white matter which resembles a tree and is therefore called the "tree of life" (arbor vitae cerebelli).

Superior cerebellar surface

The superior or upper surface of the cerebellum is divided by the tentorial furrow (fissura prima) to the upper and lower part (2). Smaller furrows divide each of these two into lobes located on the upper surface. Because this surface is built of vermis in the middle and laterally located hemispheres, there are separate parts of the vermis that make up the central junction between the two lateral lobes of the hemispheres.

The most distal lobe of the hemisphere is the "rectangular lobe" (lobulusquadrangularis), adequate to the most rostral part of the vermis, the so-called culmen, including the IV and V lobes according to the Larsell division.

The so-called "simple lobe”
(lobulus simplex) is more caudal to this part, to which the part of the vermis
called “declive” corresponds (VI Larsell according to Larsell). The primary or
tentorial furrow (fissura prima) is located between these two-lobe groups. 

Inferior cerebellar surface

The lower or inferior surface of the cerebellum is the most complex surface of this part of our brain. The hemispheres are protruding above the vermis level located deeper in the posterior incision of the cerebellum.

Its most rostral hemispherical part is the continuation of the lower semilunar lobe, to which the folium vermis corresponds and represents a continuation of this hemisphere. Below it, the lobulus semilunaris inferior is located.

The main furrow on the petrosal
surface, the so-called petrosal furrow or the horizontal furrow
(fissurahorizontalis), lays between these two groups of lobes pass to the
suboccipital surface and laterally separates the two crescent lobes.

The next hemispherical lobe is a series of the so-called "two-trunk lobule" (lobulusbiventer), which forms a whole with the verbal part called "pyramid" (pyramis, VIII). The lowest lobes of the hemisphere on this surface are the "Cerebellar tonsils" (tonsilla cerebelli), a very elongated lobule that is connected with the rest of the cerebellum only by a small lateral segment of white matter, called the peduncle of the tonsil (pedunculus tonsillae).

The position of the cerebral tonsils
is clinically important because they can get trapped in the foramen magnum,
opening through which the cranial cavity communicates with the vertebrate
channel. This can happen if the pressure in the cranial cavity (Intracranial
Pressure - ICP) increases. 

Anterior cerebellar surface

The anterior surface of the cerebellum consists of two very important furrows that separate the cerebellum and the bridge (pons) of the brain stem and two parts of the anterior surface itself. The first furrow is called the “furrow between the bridge and cerebellum” (pontocerebellar furrow), which limits the so-called pontocerebellar angle and the bases of all infratentorial nerves (V to XII brain nerve).

Cerebellar nuclei

In addition to the surface
structures described in the previous sections, there are four small pairs of
gray matter deep in the cerebellum (even though its interior is made of white
matter). These are called the "cerebellum nucleus fastigii (it means
"deep cores of the little brain"). These cores or nuclei are very
important functional centers and their damage causes significant motoric
dysfunctions.

References:

1. Albert L. Rhoton, The Posterior
   Cranial Fossa: Microsurgical Anatomy and Surgical Approaches, Neurosurgery,
   Volume 47, Issue suppl_3, September 2000, Pages S5–S6, https://doi.org/10.1097/00006123-200009001-00005 Found online on: https://academic.oup.com/neurosurgery/article-abstract/47/suppl_3/S5/3774086/
2. Rai R, Iwanaga J,
   Shokouhi G, Oskouian RJ, Tubbs RS. The Tentorium Cerebelli: A Comprehensive
   Review Including Its Anatomy, Embryology, and Surgical Techniques. Cureus.
   2018 Jul 31;10(7):e3079. doi: 10.7759/cureus.3079. PMID: 30305987; PMCID:
   PMC6168052.
   Found online on: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6168052/