Cerebellum

The cerebellum is the part of the hindbrain that lies within the posterior cranial fossa, inferior to the occipital lobes and dorsal to the brainstem. It consists of three lobes (the anterior, posterior, and flocculonodular lobe). Functionally, it is divided into three zones. These are the cerebrocerebellum (the lateral zones), which receives sensory information from the cerebral cortex and is responsible for the planning of movements that are just about to occur (motor planning); the spinocerebellum (the vermis and intermediate zones), which receives proprioceptive information from the dorsal columns of the spinal cord and is responsible for the coordination of movements (motor execution); and the vestibulocerebellum (the flocculonodular lobe), which receives information from the vestibular nuclei and visual cortex and is responsible for balance and ocular movements. The cerebellar cortex transmits the information received from the three afferents (i.e., the cerebrum, spinal cord, and vestibular nuclei) to the four cerebellar nuclei (dentate, emboliform, globose, and fastigial) within the inner white matter. The cerebellar nuclei send efferent impulses to the thalamus, red nucleus, and reticular formation to execute cerebellar functions. The cerebellar tracts travel through the cerebellar peduncles.

Overview ^[1]

• Location
  • Posterior cranial fossa above the foramen magnum
  • Inferior to the occipital and temporal lobes
  • Covered by the tentorium cerebelli (dura mater)
  • Connected to brainstem via three paired peduncles
  • Separated from the medulla and pons by the fourth ventricle
• Function
  • Control of balance and ocular movements
  • Planning of movements that are about to occur
  • Coordination of complex and sequential movements
  • Maintenance of muscle tone

Structure ^[2][3]

• The 2 hemispheres of cerebellum are separated in the midline by the cerebellar vermis.
• The cerebellar tonsil is an ovoid lobule located on the inferomedial aspect of each cerebellar hemisphere, above the foramen magnum
• Sulci and fissures further divide the cerebellum into lobes (see “Lobes and fissures” below)

Cerebellar tonsil herniation through the foramen magnum can occur due to a Chiari malformation or an increase in intracranial pressure, resulting in brainstem compression and cardiopulmonary arrest.

Lobes and fissures

• Three lobes
  • Anterior lobe
  • Posterior lobe
  • Flocculonodular lobe
• Main fissures
  • The primary fissure separates the anterior and posterior lobes.
  • The posterolateral fissure separates the posterior lobe from the flocculonodular lobe.

Cerebellar peduncles

• Three pairs of cerebellar peduncles connect the cerebellum to the brain stem.
• Afferent and efferent cerebellar tracts travel through the peduncles to and from the ipsilateral cerebellar cortex.

Cerebellar lesions cause neurological deficits on the ipsilateral side!

Vasculature of the cerebellum ^[2]

• Arteries
  • Basilar artery
    • Superior cerebellar artery (SCA) → superior surface of the cerebellum
      • Also supplies the superior and middle cerebellar peduncles and the midbrain
    • Anterior inferior cerebellar artery (AICA)→ anterior surface of the cerebellum
      • Also supplies the middle cerebellar peduncle and inferolateral pons
  • Vertebral artery
    • Posterior inferior cerebellar artery (PICA)→ posterior surface of the cerebellum
      • Also supplies the inferior cerebellar peduncles and the inferolateral medulla
• Veins: the cerebellar veins drain into the transverse sinuses and the superior petrosal sinuses.

For further information on the effects of cerebellar lesions, see “Clinical features” in cerebellar syndromes.

A tip to remember the symptoms of cerebellar lesions: lesions of the medial part of cerebellum (i.e., vermis, flocculonodular lobe, and corresponding deep nuclei) affect medial structures (i.e., axial and proximal limb musculature), resulting in symptoms including truncal ataxia and nystagmus. Lesions of the lateral parts of cerebellum (i.e., the hemispheres) affect lateral structures (distal limb musculature), resulting in symptoms such as ipsilateral limb ataxia.

References:^[4][5]

The cerebellum is composed of the outer gray matter (cerebellar cortex) and inner white matter (cerebellar medulla).

Cerebellar cortex ^[5]

• Receives afferent inputs from the cerebrum, spinal cord, and vestibular nuclei
• Sends neural impulses to the cerebellar nuclei
• Composed of 5 types of neuronal cells, densely packed and arranged in 3 layers
• The cortex is primarily an inhibitory structure; all cerebellar cells except granule cells are inhibitory.

Cerebellar medulla

• Composed of climbing fibers, mossy fibers, Purkinje cell axons, and the deep cerebellar nuclei
  • Mossy fibers : afferent axons from the cerebral cortex, pons, spinal cord, and vestibular nuclei to the cerebellum. Terminate on granule cells → send excitatory stimuli to the Purkinje cells.
  • Climbing fibers: afferent axons from the inferior olivary nuclei of the medulla → terminate on Purkinje cells
  • Four deep cerebellar nuclei (from lateral to medial): dentate, emboliform, globose, and fastigial nucleus
• For information on the function and neural pathways of the cerebellar medulla, see “Cerebellar tracts” below.

Glutamate is the neurotransmitter of granule cells, which are the only excitatory cerebellar cells. GABA is the inhibitory neurotransmitter of all other cerebellar cells.

Purkinje cells provide the only efferent effect from the cerebellar cortex to the cerebellar nuclei, and, thus, play an essential role in the regulation of cerebellar output!

Purkinje cells are inhibitory!

References:^{[4][5][6][7][8][9]}

Afferent cerebellar tracts (input) ^[4][10]

• Afferent tracts arise from three main sources: the cerebral cortex, the spinal cord, and the vestibular nerve.
• Afferent tracts are excitatory and travel to the cerebellum via mossy fibers and climbing fibers.
• Afferent tracts travel mainly through the inferior and middle cerebellar peduncles.

Excitatory input to the cerebellar cortex travels through mossy fibers and climbing fibers!

Mossy fibers terminate on granule cells. Climbing fibers terminate on Purkinje cells.

Efferent cerebellar tracts ^[10]

• Efferent tracts originate from the four deep cerebellar nuclei and travel mainly via the superior cerebellar peduncle.
• The tracts of the fastigial nucleus travel via the inferior cerebellar peduncle.

Deep cerebellar nuclei (from lateral to medial): "Don't Eat Greasy Food" (Dentate, Emboliform, Globose, Fastigial).

The deep nuclei are the only output from the cerebellum.

References:^{[1][11][12][13][14][15][16][17][18]}

The cerebellum derives from the hindbrain (rhombencephalon) and metencephalon.

For more information on the embryology of the nervous system, see “Embryology” in “The cerebral cortex, meninges, basal ganglia, and ventricular system.”

• Cerebellar defects
  • Ataxia
  • Dysmetria
  • Dysarthria
  • Scanning speech
  • Dysdiadochokinesis
  • Tremor
  • Nystagmus
  • Hypotonia
• Hereditary ataxias
  • Friedreich ataxia
  • Spinocerebellar ataxia
  • Ataxia-telangiectasia
• Acquired ataxias
  • Multiple sclerosis
  • Cerebellar stroke
  • Vitamin B12 deficiency
  • Wilson disease (accumulation of copper in the cerebellum)
  • Traumatic brain injury
• Congenital cerebellar malformations
  • Chiari I malformation
  • Chiari II malformation
  • Dandy-Walker malformation
• Cerebellar tumors
  • Cerebellar medulloblastoma
  • Pilocytic astrocytoma
  • Cerebellar metastases
  • Hemangioblastoma
• Other
  • Foramen magnum herniation
  • Neurological exam (e.g., finger-to-nose test, Romberg test, rapid alternating movement tests, heel-to-shin test)