CEREBELLUM OF BRAIN – ANATOMY OF ITS LOBES, STRUCTURE AND FUNCTIONS

The cerebellum of brain is located in the posterior cranial fossa behind pons and medulla oblongata. It is the largest part of hindbrain. A tent shaped fold of duramater called tentorium cerebelli forms roof of cerebellum.

Cerebellum consists of a central part called vermis and the right and left cerebellar hemispheres. Anterior cerebellar notch lies anteriorly and separates cerebellum from fourth ventricle. Posterior cerebellar notch is present posteriorly which lodges a fold of duramater, the falx cerebelli.

The surface of cerebellum is composed of closely packed thin sheets called folia with intervening transverse fissures. Cerebellum is divided into three lobes by three fissures.

The cerebellum is connected to brainstem by three paired peduncles – the superior cerebellar peduncle connects cerebellum to midbrain, middle cerebellar peduncle connects it to pons and inferior cerebellar peduncle connects it to medulla oblongata. Afferent and efferent fibers pass through these peduncles.

FISSURES OF CEREBELLUM

• HORIZONTAL FISSURE – Is at the junction of superior and inferior surface of cerebellum.
• POSTEROLATERAL FISSURE – It is located on the inferior surface. It separates the flocculonodular lobe from the rest of cerebellum called corpus cerebelli.
• PRIMARY FISSURE – It divides the superior surface into anterior and posterior parts.

LOBES OF CEREBELLUM

The cerebellum has three lobes – anterior lobe, posterior lobe and flocculonodular lobe.

FLOCCULONODULAR LOBE – It is located at the anterior most part of inferior surface. It has a central part called nodule and flocculi on each side of nodule.

ANTERIOR LOBE – Includes the portion of upper surface which lies anterior to primary fissure. Lingula, central lobule and culmen are located medially on this lobe. Ala and quadrangular lobules are present laterally. Lingula is the anterior most part of superior vermis.

POSTERIOR LOBE – It is also called the middle lobe. It consists of the part present behind primary fissure on superior surface and entire inferior surface except except flocculonodular lobe. Its superior surface has declive and folium vermis medially and lobus simplex and superior semilunar lobules laterally. On its inferior surface, tuber vermis, pyramid and uvula are located in middle and inferior semilunar lobule, biventral lobule and tonsil form lateral part.

FUNCTIONAL DIVISION OF CEREBELLUM

VESTIBULO CEREBELLUM – It is made by the flocculonodular lobe. Functionally, it is related to vestibular apparatus. It is concerned with maintenance of equilibrium, body posture and visual fixation (vestibulo-ocular reflex).

SPINO CEREBELLUM – It receives inputs from the spinal cord. It is made by anterior lobe (except lingula), uvula, pyramid and lobus simplex. The spino cerebellar connections carry unconscious proprioceptive impulses from limb muscles to cerebellum. It controls axial muscles, limb muscles and postural reflexes.

NEO CEREBELLUM – It is formed by posterior lobe (except uvula, pyramid and lobus simplex). It receives information from cerebral cortex and pons (cortico-ponti-cerebellar connections). It is concerned with performance of skilled voluntary movements.

PHYLOGENETIC DIVISION OF CEREBELLUM

It is based on development of various parts of cerebellum at different times during evolution.

ARCHICEREBELLUM – It is represented by flocculonodular lobe. This was the first part that developed during evolution. Thus, it is the oldest part.

PALEOCEREBELLUM – It includes the anterior lobe (except lingula) and uvula and pyramid of posterior lobe. It evolved next, after archicerebellum.

NEOCEREBELLUM – It comprises of posterior lobe (except uvula and pyramid). It was last to evolve and is hence the most new. This part shows greatest development in humans.

STRUCTURE OF CEREBELLUM

The cerebellum consists of – an outer layer of grey matter, the cerebellar cortex, central core of white matter and four pairs of cerebellar nuclei embedded in white matter.

The cerebellar cortex comprises of following three layers –

OUTER MOLECULAR LAYER – Consists of :

• Stellate cells (superficial)
• Basket cells (deep). Dendrites of these cells receive input from the parallel fibers.
• Dendrites of purkinje cells.
• Axons of granule cells.
• Climbing fibers – afferent fibers from inferior olivary nucleus which form synapse with dendrites of purkinje cells.

MIDDLE PURKINJE CELL LAYER – Consists of large flask shaped Purkinje cells. They have extensive network of dendrites that extend vertically into the outer molecular layer. Their axons end in cerebellar nuclei.

INNER GRANULE CELL LAYER – Consists of :

• Granule cells -Their axons ascend upwards into the outer molecular layer where it bifurcates into ‘T’ shaped manner and are called parallel fibers. Their dendrites make synapse with mossy fibers, making the so called glomerulus.
• Golgi cells

AFFERENT FIBERS OF CEREBELLUM

• CLIMBING FIBERS – These fibers arise mainly from cells in the inferior olivary nucleus. They are excitatory in nature. They form one-to-one connection with dendrites of purkinje cells and excite them. Its collaterals also excite golgi cells.
• MOSSY FIBERS – It is constituted by axons of spinocerebellar, vestibulocerebellar, reticulocerebellar, cuneocerebellar and cortico-ponto-cerebellar tracts. Each mossy fiber makes synaptic contact with dendrites of many granule cells, called glomeruli. They are also excitatory in nature.

CEREBELLAR NUCLEI

The cerebellum has four pairs of nuclei embedded in its white matter. From medial to lateral they are fastigial, globose, emboliform and dentate nuclei. Dentate nucleus is the largest.

All these nuclei receive axons from Purkinje cells. The fastigial nucleus also receives afferents from vestibular nerve and nuclei. Efferent fibers from cerebellar nuclei innervate other parts of brain.

The fastigial nucleus receives afferents from archi-cerebellum and gives efferents to vestibular nuclei and reticular formation via inferior cerebellar peduncle.

The globose and emboliform nuclei receive impulses from paleocerebellum. Dentate nucleus gets afferents from neocerebellum. The efferents from these nuclei pass through superior cerebellar peduncle and cross to the opposite side in midbrain. Most fibers arising from these nuclei end in red nucleus. Most efferent fibers arising from dentate nucleus terminate in the ventral lateral nucleus of thalamus which projects to motor and premotor areas of cerebral cortex.

FUNCTIONS OF CEREBELLUM

• Cerebellum controls body posture and equilibrium.
• Plays important role in regulation of muscle tone and stretch reflexes.
• Coordination of body movements.
• Control of fine motor activities.