Corpus: Cerebellum

The cerebellum is a part of the metencephalon and serves as a control center for the coordination and fine-tuning of movement sequences.

2.1. Topography

The cerebellum is located in the posterior fossa and is separated from the cerebrum by the tentorium cerebelli. The falx cerebelli is situated between the two largely symmetrical cerebellar hemispheres. It borders the medulla oblongata and the pons, and forms the roof of the fourth ventricle. The cerebellomedullary cistern, located between the cerebellum and the medulla oblongata, is clinically significant as cerebrospinal fluid can be collected from it during a suboccipital puncture.

2.2. External outline

2.2.1. General structure

The cerebellum is connected to the brain stem via the three cerebellar peduncles. The afferent fibers run predominantly in the middle and lower peduncles, while the efferent fibers run predominantly in the upper cerebellar peduncle. The two cerebellar hemispheres are divided into three main lobes by two fissures:

• Anterior cerebellar lobe (Lobus anterior cerebelli)
• Posterior cerebellar lobe (Lobus posterior cerebelli)
• Flocculonodular lobe (Lobus flocculonodularis)

The primary fissure separates the anterior lobe from the posterior lobe, and the posterolateral fissure separates the posterior lobe from the flocculonodular lobe. The two hemispheres are divided by the cerebellar vermis (vermis cerebelli). Lateral to the vermis cerebelli are the flocculi, connected to the lower part of the cerebellar vermis via the nodulus, forming the flocculonodular lobe. The most caudal structure of the cerebellum is the tonsillae cerebelli, which lie directly above the foramen magnum. The surface of the cerebellum is divided by numerous leaf-shaped bulges (folia cerebelli) and fissures (fissurae cerebelli), increasing the surface area to an estimated 500 cm² when unfolded.

2.2.2. Detailed structure

The main lobes can be further subdivided into lobules. In addition to the classic naming of the lobuli cerebellares, the lobules are also labeled with Roman numerals. The numbering is based on the idea that each hemispheric lobule is assigned a vermis lobule. The hemispheric lobules are characterized by the prefix "H," while those of the vermis have no prefix.

2.3. Internal segmentation

In sagittal sections, the cerebellum shows strong convolutions and a clear division into cortex and medulla. The white matter spreads in a tree-like pattern called arbor vitae (tree of life).

The cerebellar cortex, consisting of gray matter, is divided into three layers (from inside to outside):

• Granular layer
• Purkinje cell layer
• Molecular layer

The cerebellar medulla consists of nerve fibers:

• Association fibers
• Commissural fibers
• Projection fibers

and cerebellar nuclei:

• Dentate nucleus
• Emboliform nucleus
• Globose nucleus
• Fastigial nucleus

Although the cerebellum accounts for only about 10 % of total brain volume, it contains more neurons than the rest of the brain due to the high density of small granule cells. In mammals, the neuron count in the cerebellum is in a relatively constant ratio to the number of nerve cells in the neocortex, approximately 3.6 to 1.

2.4. Functional organization

The cerebellum is divided into three parts based on function:

• Vestibulocerebellum: Includes the nodulus and flocculus. It receives afferent signals from the vestibular apparatus and sends efferent signals to the vestibular nuclei, influencing postural motor function and fine-tuning eye movements.
• Spinocerebellum: Comprises the vermis and paravermal zone. It receives afferent signals from the spinal cord about the position of the extremities, trunk, and muscle tone. Efferent signals are sent to the red nucleus and reticular formation.
• Pontocerebellum: Consists of the two hemispheres and interacts closely with the cerebrum via the pons. Afferent signals are transmitted from the pontine nuclei and the inferior olive, which receive indirect feedback about their own efferent neurons. Efferent signals are sent to the thalamus and red nucleus.

2.5. Vascular supply

The cerebellum receives arterial blood from the cerebellar arteries:

• Superior cerebellar artery (SCA)
• Anterior inferior cerebellar artery (AICA)
• Posterior inferior cerebellar artery (PICA)

The cerebellum builds in the 5th to 6th week of development from the rhombic lips of the metencephalon, which arise from the dorsolateral part of the wing plates. Initially, the rhombic lips protrude into the fourth ventricle and are widely separated. During further development, they converge cranially and form the cerebellar plate that covers the fourth ventricle. By the 12th week of development, the vermis can be distinguished as a central plate from two lateral thickenings, the future hemispheres.

From the 12th week, the posterolateral fissure forms on the back of the cerebellar plate, separating the future flocculonodular lobe, including the flocculus and nodulus, from the two hemispheres and the vermis. The primary fissure forms around the 14th week, separating the anterior lobe from the posterior lobe. The typical transverse fissures on the surface of the cerebellum appear around the 16th week.

Failure of the cerebellum results in symptoms such as:

• Ataxia
• Asynergy
• Dysdiadochokinesia
• Dysmetria
• Rebound phenomenon
• Gaze stabilization disorders with nystagmus and saccadic eye movements
• Reduced muscle tone
• Intention tremor