Cerebellum

Original Editor - Lucinda hampton Top Contributors - Jess Bell, Lucinda hampton, Rahma Ahmed Ahmed Bahbah, Rusfid FM, Kim Jackson, Rucha Gadgil, Stacy Schiurring, Nikhil Benhur Abburi, Khloud Shreif, Mason Trauger and Jonathan Wong

Overview of the Cerebellum

The three areas of the brain: cerebrum, cerebellum, and the brainstem

The cerebellum is located at the base of the brain, under the cerebrum and posterior to the spinal cord. The cerebellum is relatively small, but it is neuron-rich, containing over 50% of the brain’s neurons in a dense cellular layer, called the cerebellar cortex.

Functions of the Cerebellum

The cerebellum plays a vital role in function and mobility. Traditionally known functions of the cerebellum include:

• motor movement regulation, including gait coordination and maintenance of posture
• balance control
• control of muscle tone and voluntary muscle activity
• motor learning

There is also growing research on the cerebellum's role in emotion and cognition, specifically with visual-spatial memory, the creation of generative grammar into the structure of the brain, and the conscious ability to manipulate cause-and-effect relationships.

The cerebellum makes fine adjustments to motor actions. Four principles important to cerebellar processing have been identified.

1. Feedforward processing
2. Divergence and convergence
3. Modularity
4. Plasticity

If you would like to learn more about these principles of cerebellar processing, please see this article.

Anatomical Position

The cerebellum is located at the back of the brain, immediately inferior to the occipital and temporal lobes, and within the posterior cranial fossa. It is separated from the occipital and temporal lobes by the tentorium cerebelli, a tough layer of dura mater. It is posterior to the pons. The fourth ventricle separates the pons from the cerebellum.

Cerebellar Structure

The cerebellum has two hemispheres. These hemispheres are connected by the vermis, a narrow midline area. The cerebellum receives input and transmits output via a limited number of cells. It is divided into thousands of independent modules, all with a similar structure.

The cerebellum is made up of grey matter and white matter:

• grey matter: located on the surface of the cerebellum. The grey matter forms the cerebellar cortex. It is tightly folded (i.e. convoluted to increase its surface area) and is divided into three layers: the molecular layer (external); the Purkinje cell layer (middle); and the granular layer (internal). There are two types of neurons in the molecular layer: the outer stellate cell and the inner basket cell.

• white matter: located below the cerebellar cortex. Four cerebellar nuclei (dentate, emboliform, globose, and fastigial nuclei) are embedded in the white matter.

The cerebellum can be subdivided in the following ways: (1) anatomical lobes, (2) zones and (3) functional divisions.

Anatomical Lobes

The cerebellar has three anatomical lobes, which are divided by two fissures: the primary fissure and the posterolateral fissure.

1. The anterior lobe
   • compares inputs from the periphery with motor plans from the cerebral cortex, modifying the command
   • regulates muscle tone
   • coordinates postural activities and gait
2. The posterior lobe
   • assists in premotor planning
   • coordinates fine, distal volitional movements
   • ability to judge time intervals and produce accurate rhythm
3. The flocculonodular lobe (archicerebellum)
   • the oldest part of the brain in evolutionary terms
   • primarily involved in balance and spatial orientation
   • its main connections are with the vestibular nuclei, although it also receives visual and other sensory input
   • coordinates and influences eye movement and the vestibulo-ocular reflex via the medial longitudinal fasciculus
   • influences postural muscles via the vestibulospinal tract to maintain an upright posture and horizontal head position

Zones

There are three cerebellar zones, from medial to lateral: the vermis, the intermediate zone, and the lateral zone. When examining the gross anatomy of the cerebellum, the intermediate zone and the lateral hemisphere are not distinguishable.

1. The vermis / vermal zone (medial) contains the fastigial nucleus and is involved in maintaining balance and the regulation of tone, posture, locomotion, and equilibrium of the axial muscles
2. The intermediate zone (paravermal zone) contains the emboliform and globose nuclei; it governs spatially organised, skilled movements, and posture and tone of the ipsilateral extremities
3. The lateral zone contains the dentate nucleus and governs spatially organised and skilled limb movement

Functional Divisions

The cerebellum can also be divided into three functional areas: (1) the cerebrocerebellum, (2) the spinocerebellum, and (3) the vestibulocerebellum.

1. Cerebrocerebellum: this is the largest functional area, made up of the lateral portions of the cerebellar hemispheres. It is associated with the dentate nuclei. It is involved in movement planning and motor learning. In the cerebrocerebellar loop, the cerebellum receives information from the cerebral cortex via the pontine nuclei. The cerebellum processes this information and sends it to the thalamus and red nucleus.
2. Spinocerebellum: made up of the anterior lobe, vermis and the fastigial and interposed nuclei. It is involved in integrating sensory inputs and motor commands to coordinate truncal and limb movements. In the spinocerebellar loop, the cerebellum receives proprioceptive information from the dorsal column of the spinal cord. The cerebellum processes this information and sends it to the spinal cord and cortex to adjust and fine-tune movements.
3. Vestibulocerebellum: made up of the flocculonodular lobe and paravermis, and has connections to the lateral vestibular nuclei. It helps to control balance and the ocular reflexes, including fixation on targets. In the vestibulocerebellar loop, the cerebellum receives information about balance from the vestibular system. It then sends information to the vestibular nuclei and spinal cord to ensure trunk stability and balance are maintained.

To learn more about the divisions of the cerebellum, you might like to watch the following optional video:

Cerebellar Cells

Schematic representation of the cerebellar cytoarchitecture. The cerebellar cortex is organised into three layers: the molecular layer, the Purkinje cell layer, and the granular layer.

The image to the right depicts the cerebellar cells. The cerebellum is a multilayer structure with folds. Each of these folds has a white matter core surrounded by a grey matter layer.

The cortex of the cerebellum (i.e. grey matter) has three layers.

1. External or molecular layer: has outer stellate cells and inner basket cells, as well as granule cell axons and Purkinje cell dendrites
2. Middle or Purkinje cell layer: has Purkinje cell bodies
3. Internal or granular layer: has granule cell bodies, Golgi cells, Purkinje cell axons

Purkinje, Golgi, stellate, and basket cells are inhibitory while granule and unipolar brush cells are excitatory.

Granule cells are the most common cell type in the cerebellum and brain. The axons of granule cells extend into the molecular layer, forming parallel fibres. These parallel fibres synapse with the dendrites of inhibitory neurons found in the molecular layer, as well as with the dendrites of Golgi cells.

Purkinje cells are structurally distinct: they have large dendritic trees and are almost two-dimensional, so they can be arranged in parallel. Purkinje cells are the only output source from the cerebellar cortex.

Climbing fibres and mossy fibres provide excitatory input to the cerebellar cortex:

• climbing fibres are the endings of olivocerebellar tracts
• mossy fibres are the endings of the other cerebellar afferent tracts, arising from various parts of the nervous system, including the motor cortex and spinal cord

If you would like to learn more about the cells of the cerebellum, please watch the following optional video:

Cerebellar Deep Nuclei

The grey matter surrounds a highly-branched area of white matter, referred to as the arbor vitae (or tree of life). This white matter surrounds the paired cerebellar deep nuclei. From lateral to medial, they are:

Cross-section of the human cerebellum, showing the cerebellar deep nuclei.

1. Dentate nucleus: the largest nuclei. Its efferent fibres modulate motor neurons and neurons involved in conscious thought and visuospatial function.
2. Emboliform and globose nucleus (interposed nucleus): receive input from the intermediate zone and cerebellar afferents that carry spinal, somatosensory, auditory, and visual information. It sends outputs to the red nucleus.
3. Fastigial nucleus: has an important role in motor control and maintaining important functions, including feeding, immune, cardiovascular, and respiratory functions.

The fibres from the dentate, emboliform, and globose nuclei exit the cerebellum through the superior cerebellar peduncle while the fastigial nucleus fibres leave through the inferior cerebellar peduncle.

Cerebellar Peduncles

The cerebellum is connected to the brainstem by the superior, middle and inferior cerebellar peduncles. Afferent and efferent fibres pass through these peduncles:

• superior cerebellar peduncle: primarily an efferent pathway
• middle cerebellar peduncle: an afferent pathway; most of the cell bodies that form this pathway are located in the pons
• inferior cerebellar peduncle: the smallest cerebellar peduncle, with afferent and efferent pathways

Cerebellar Tracts

The afferent pathways are from the spinal cord, cerebral cortex, brainstem and vestibular system:

• Spinal cord pathways:
  • anterior (ventral) spinocerebellar pathway - enters the cerebellum via the superior cerebellar peduncle
  • posterior (dorsal) spinocerebellar pathway - enters the cerebellum via the inferior cerebellar peduncle
  • cuneocerebellar pathway - enters the cerebellum via the inferior cerebellar peduncles
  • rostral spinocerebellar tract - enters the cerebellum via the inferior cerebellar peduncles

• Cerebral cortex pathways:
  • corticopontocerebellar pathway - enters via the middle cerebellar peduncle
  • cerebro-olivocerebellar pathway - enters via the inferior cerebellar cerebellar peduncle
  • cerebroreticulocerebellar pathway - enters via the middle and inferior cerebellar peduncle
• Vestibulocerebellar pathway (this pathway also receives visual input)

The efferent pathways include:

• globose-emboliform-rubral pathway
• dentatothalamic pathway
• fastigial vestibular pathway
• fastigial reticular pathway

More information on these pathways is available in the following optional video, from around 7:02:

Cerebellar Blood Supply

The cerebellum receives its blood supply from three main arteries that arise from the vertebrobasilar arterial system (also known as the posterior circulation of the brain):

• superior cerebellar artery
  • supplies the anterior lobe, part of the posterior lobe and the superior vermis
• anterior inferior cerebellar artery
  • supplies the cerebellar peduncles, flocculonodular lobe and part of the posterior lobe
• posterior inferior cerebellar artery
  • supplies part of the posterior lobe and the inferior vermis

The superior cerebellar artery and the anterior inferior cerebellar artery are branches of the basilar artery. The posterior inferior cerebellar artery is a branch of the vertebral artery.

The main veins of the cerebellum are the superior and inferior cerebellar veins. These veins drain into the great cerebral vein or venous sinuses (i.e. the superior petrosal, transverse and straight dural venous sinuses).

Clinical Signs of Cerebellar Dysfunction

The cerebellum has a key role in the coordination, timing and fine-tuning of movements, balance, posture, motor learning and regulating muscle tone. Damage to the cerebellum can affect these functions, resulting in a lack of movement coordination. Because each cerebellar hemisphere regulates movement on the ipsilateral (same) side of the body, symptoms associated with cerebellar dysfunction occur ipsilaterally.

Cerebellar dysfunction can produce a wide range of signs and symptoms, which vary depending on which area of the cerebellum is affected.

Signs associated with cerebellar dysfunction include:

• ataxia: decreased control or coordination of movements
• issues with gait / ataxic gait: individuals often present with a "staggering gait" - i.e. wide base of support, unsteady, stumbling gait
• intention tremor: a low-frequency tremor that isn't present at rest, but is associated with voluntary limb movements. The tremor worsens as the limb reaches the endpoint of the movement (e.g. when trying to push a button, there is increased oscillation as the hand gets closer to the button)
• dysdiadochokinesia: individuals are unable to perform rapidly alternating movements in a coordinated, rhythmic manner (e.g. they cannot rapidly pronate and supinate the forearm and hands)
• dysmetria: affects the ability to gauge distance, speed and the force required for a movement. Individuals will overshoot or undershoot a target (e.g. missing a cup and knocking it over when attempting to pick it up)
• dysarthria: difficulty speaking, or articulating words
• movement decomposition: an individual is unable to complete a movement smoothly, so the movement is broken down into its parts

It can help to remember the mnemonic DANISH for the main signs of cerebellar dysfunction:

• D: Dysdiadochokinesia and Dysmetria
• A: Ataxia
• N: Nystagmus
• I: Intention tremor
• S: Slurred speech
• H: Hypotonia

Signs associated with vermal and flocculonodular lobe lesions:

• gait ataxia
• titubation: a tremor (or noodling) of the head or axial body, which, when severe, can affect a person's ability to sit or stand unassisted
• nystagmus: involuntary, rapid, repetitive eye movements

Signs associated with cerebrocerebellum lesions:

• limb ataxia, potentially presenting as dysmetria, dysdiadochokinesis, hypotonia or intention tremor
• dysarthria may be present

The following optional videos provide additional information on signs of cerebellar dysfunction. Signs of cerebellar syndrome are discussed from around 6:37 in the second video: