Cerebellum

The cerebellum, meaning "the little brain", sits at the base of the brain in the posterior cranial fossa below the tentorium and behind the brainstem. 

Gross anatomy

The cerebellum has the following features:

• three surfaces: anterior (petrosal), superior (tentorial), inferior (suboccipital)
• three fissures: primary (tentorial), horizontal (petrosal), prebiventral/prepyramidal (suboccipital)
• two hemispheres
• single median vermis

Vermis

The vermis is divided into nine lobules: (in a clockwise rotation, looking at the patient sagittally from his left), and separated into groups by fissures:

• lingula
• central lobule
• culmen: primary (tentorial) fissure
• declive
• folium: horizontal (petrosal) fissure
• tuber: prebiventral/prepyramidal (suboccipital) fissure
• pyramid
• uvula
• nodulus

The subdivisions of the cerebellar vermis can be remembered by this mnemonic.

Cerebellar hemisphere

The cerebellar folia run parallel to the calvaria in an onion-like configuration.

Each of the nine vermis lobules is associated in both sides with two cerebellar hemisphere lobules and therefore the cerebellum has 18 cerebellar hemisphere lobules:

• wing of lingula (lingula)
• wing of central lobule (central lobule)
• quadrangular lobule (culmen): primary (tentorial) fissure
• simple lobule (declive)
• superior semilunar lobule (folium): horizontal (petrosal) fissure
• inferior semilunar lobule (tuber): prebiventral/prepyramidal (suboccipital) fissure
• biventral lobule (pyramid)
• tonsil (uvula)
• flocculus (nodulus)

Connections to brainstem

• to the midbrain via the superior cerebellar peduncles (brachia conjunctiva)
• to the pons via the middle cerebellar peduncles (brachia pontis)
• to the medulla via the inferior cerebellar peduncles (restiform bodies)

CSF cisterns

• cerebellopontine angle cistern
• cisterna magna
• quadrigeminal cistern

Blood supply

The cerebellum is essentially supplied by three bilateral vessels from the vertebrobasilar system:

1. superior cerebellar artery (SCA): branch of the distal basilar artery
2. anterior inferior cerebellar (AICA): branch of the proximal basilar artery
3. posterior inferior cerebellar (PICA): branch of the distal vertebral arteries

Superior cerebellar (SCA)

The SCA supplies:

• whole superior surface of the cerebellar hemispheres down to the great horizontal fissure
• the superior vermis
• dentate nucleus
• most of the cerebellar white matter
• superior cerebellar peduncle
• middle cerebellar peduncle

Anterior inferior cerebellar (AICA)

The amount of tissue supplied by the AICA is variable (PICA-AICA dominance) but usually includes:

• middle cerebellar peduncle
• inferolateral portion of the pons
• flocculus
• anteroinferior surface of the cerebellum

Posterior inferior cerebellar (PICA)

The PICA has a variable territory depending on the size of the AICA, but usually supplies:

• posteroinferior cerebellar hemispheres (up to the great horizontal fissure)
• inferior portion of the vermis
• inferior cerebellar peduncle

There are some variations in the PICA:

• 18% arise extracranially, inferior to the foramen magnum
• 10% arise from the basilar rather than vertebral artery
• 2% bilaterally absent
• occasionally loops around the cerebellar tonsil
• occasionally a small vertebral artery will terminate into a common PICA/AICA trunk

It divides into lateral and medial branches that supply the inferior portion of the vermis and cerebellar hemispheres respectively.

Variant anatomy

• cerebellar agenesis
• tonsillar ectopia: asymptomatic protrusion of the tonsils through the foramen magnum by no more than 3-5 mm