8. Cerebellum

The **cerebellum** sits at the base of the brain in the posterior cranial fossa. It is the primitive brain with important functions related to the co-ordination of movement and balance. It has the following features:

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

Surfaces

* Superior (tentorial) surface
	+ Faces & conforms to inferior surface of tentorium
	+ Transition between vermis & hemispheres is smooth
	+ Primary fissure divides superior surface into anterior & posterior parts
* Inferior (suboccipital) surface
	+ Located below, between lateral & sigmoid sinuses
	+ Vermis is contained within a deep vertical depression that separates the cerebellar hemispheres
	+ Tonsil is part of hemisphere, located on inferomedial part of inferior surface
* Anterior (petrosal) surface
	+ Faces the posterior surface of petrous bone, brainstem & fourth ventricle
	+ Vermis lies dorsal to fourth ventricle
	+ Horizontal fissure divides anterior (petrosal) surface into superior & inferior parts

Connections to brainstem

* to midbrain: superior cerebellar peduncles - efferent from the dentate to the red nucleus/thalamus/contralateral cortex; afferent from anteriorspinocerebellar tract and tectocerebellar fires.
* to pons: middle cerebellar peduncles - contains fiber mass originating from pontine nuclei & represent continuation of corticopontine tracts afferent from contralateral pontine nuclei
* to medulla: inferior cerebellar peduncles - contains spinocerebellar tracts and connections to vestibular nuclei.

Vermis

Consists of superior and inferior parts.

* The superior vermis form a ridge between the anterior lobes. Consists of lingula, central lobule, culmen, declive, folium (anteroposteriorly). The inferior vermis, lies in the deep groove between the inferior surface of the posterior lobe and consists of the tuber vermis, pyramid, uvula and nodule (posterioanteriorly).

* Floculonodular lobe: consists of the nodule, and the flocculus ( a slender band of white and grey matter that project laterally from each side of the nodule between the cerebellum and the pons)

Cerebellar hemisphere

* Extend laterally and posteriorly
* Surfaced with cortex of grey matter and white matter internally. The surface of the cerebellum is indented by fine slit sulci and between lies more parallel folds called termed **folia**.
* Horizontal fissure: extends around the posterolateral border of each hemisphere and divide it into superior and inferior halves.
* Primary fissure: shallow groove on the superior half, divide it into anterior and posterior lobes.
* Each of the nine vermis lobules is associated in both sides with two cerebellar hemisphere lobules with 18 resultant cerebellar hemisphere lobules.

**Lobules related to the superior vermis:**

* wing of lingula (lingula)
* wing of central lobule (central lobule)
* quadrangular lobule (culmen): primary (tentorial) fissure
* simple lobule (declive)
* semilunar superior lobule (folium): horizontal (petrosal) fissure

**Lobules related to the inferior vermis:**

* Semilunar superior lobule (tuber): prebiventral/prepyramidal (suboccipital) fissure
* biventral lobule (pyramis)
* tonsil (uvula): a spherical lobule on the inferior aspect of the posterior lobe. This part can be displaced down through the foramen magnum in severe raised ICP or hindbrain malformations.
* flocculus (nodulus)

Relations:

* Posteriorly: the base of the occipital bone, falx cerebelli with occipital sinus.
* Superiorly: tentorium cerebelli, straight sinus, occipital lobe
* Inferiorly: cisterna magna, foramen magnum
* Laterally: walls of the posterior cranial fossa and transverse sinuses
* Anteriorly: the brainstem (fourth ventricle)

Functions:

* The anterior lobe is the main recipient of spinal and trigeminal afferents.
* The posterior lobe, including inferior vermis governs tone, and lesions can result in hypotonia, diminished muscle jerks, intention tremor and clumsy movements and nystagmus.
* Flocculonodular lobe forms connection with vestibular nuclei and responsible for equilibrium.

Cerebellar Nuclei

* Located deep in cerebellar white matter
* Nuclei project fibers to coordinate goal directed movement
* Fastigial nucleus: Medial group (vermis)
	+ Fibers from vermis cortex, vestibular nuclei and other medulla nuclei
* Globose (posterior) nucleus: Intermediate group
	+ Fibers from vermis cortex, sends fibers to medulla nuclei
* Emboliform (anterior) nucleus: Intermediate group
	+ Fibers from cerebellar cortex between vermis and hemispheres, sends fibers to thalamus
* Dentate nucleus: Lateral group
	+ Fibers from hemispheric cortex, sends fibers to red nucleus and thalamus
	+ Largest nucleus, shaped as a heavily folded band with medial opening (hilum)

CSF cisterns

* cerebellopontine angle cistern
* cisterna magna
* quadrigeminal plate cistern
* superior cerebellar cistern

Blood supply

The cerebellum is essentially supplied by three vessels:

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

**Superior cerebellar (SCA):** arise near the termination of the basilar and passes laterally to wind around the cerebral peduncle below the fourth nerve. It 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

**Anterior inferior cerebellar (AICA):** arise at the lower part of the pons from the basilar and pass back on the anteroinferior surface of the cerebellar hemisphere. Area supplied usually includes:

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

**Posterior inferior cerebellar (PICA):** The PICA is very tortuous and is the largest branch of the vertebral. It spirals back around the medulla below the huypoglossal rootlets and then between the rootlets of the glossopharygneal and vagus nerves. It 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
* choroid plexuses of the fourth ventricle

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.

**Venous drainage**: from the surface of the cerebellum, into the nearest venous sinus of the dura mater. The superior and posterior surfaces drain into the straight and transverse sinuses, the inferior surfaces into the inferior petrosal, sigmoid and occipital sinuses.

* Three major posterior fossa/midbrain drainage systems
	+ Superior (galenic) group drains up into vein of Galen, has three major named veins
		- Precentral cerebellar vein: Single, midline; lies between lingula/central lobule of vermis; terminates behind inferior colliculi
		- Superior vermian vein: Originates near declive of vermis, courses up/over top of vermis (culmen)
		- Anterior pontomesencephalic vein: Superficial venous plexus covers cerebral peduncles, anterior surface of pons
	+ inferior group:
		- inferior petrosal, sigmoid and occipital sinuses via petrosal veins
	+ Posterior group
		- straight and transverse sinus via Inferior vermian veins

Variant anatomy

* cerebellar agenesis
* vermian pseudotumour
* low lying cerebellar tonsil (cerebellar ectopia)
* dentate nucleus calcification