Anatomy of Cerebellum

The cerebellum receives inputs from spinal cord, cerebrum, brainstem, and sensory systems of the body and controls the motor system of the body. The Cerebellum harmonizes the voluntary motor activities such as maintenance of posture and equilibrium, and coordination of voluntary muscular activity including learning of the motor behaviours. Cerebellum occupies posterior cranial fossa, and it is relatively a small part of the brain. It weighs about one tenth of the total brain. Cerebellar lesions do not cause motor or cognitive impairment. However, they cause slowing of movements, tremors, lack of equilibrium/balance. Complex motor action becomes shaky and faltering.


Introduction
The Cerebellum is the largest part of the hindbrain and develops from the alar plates (rhombic lips) of the metencephalon.
It lies between the temporal and occipital lobes of cerebrum and the brainstem in the posterior cranial fossa. It is attached to the posterior surface of the brainstem by three large white fibre bundles. It is attached to the midbrain by superior cerebellar peduncle, pons by middle cerebellar peduncle, and medulla by inferior cerebellar peduncle.
Cerebellum is concerned with three primary functions: a) coordination of voluntary motor functions of the body initiated by the cerebral cortex at an unconscious level, b) maintenance of balance, and posture, c) Maintenance of muscle tone. It receives and integrates the sensory inputs from the cerebrum and the spinal cord necessary for a planning and smooth coordination of the movements [1].
Cerebellar lesions result in irregular and uncoordinated, awkward intentional muscle movements. Cerebellar lesions often present as occipital headache that worsen at night, nausea, vomiting and unsteadiness. On examination the patients often have bilateral papilledema, nystagmus, slight slurred speech, irregular and ataxic movements [2].

External features of cerebellum
Cerebellum consists three parts; two laterally located hemispheres joined in the midline by the vermis (worm). Somatotopy: Vermis controls the central parts of the body (trunk) and its lesions produce truncal ataxia. And each hemisphere control ipsilateral limb, and their lesion causes ataxia of ipsilateral limbs. Superior surface of the cerebellum is separated from the occipital lobe cerebrum by tentorium cerebelli of dura mater. Superior vermis protrudes above the cerebellar hemispheres whereas the inferior vermis is buried in a deep groove present between the two bulging lateral lobes. The surface of the cerebellum features highly convoluted folds(folia) that are oriented transversely. These folds are separated by fissures of variable depths. Some of the deep fissures can be used as landmarks to anatomically divide the Cerebellum into three lobes: anterior, posterior and flocculonodular lobes. On the superior surface, a deep primary fissure separates the small anterior lobe from the large posterior lobe. On the inferior surface, a prominent posterolateral fissure isolates the flocculus of cerebellar hemisphere together with the nodule of the vermis from the rest of the cerebellum as flocculonodular lobe [2].

Cerebellar lobes (Phylogenetic/Evolutionary and Functional divisions)
a. Antezrior Lobe (Spinocerebellum-Spinocerebellar tract) • It lies anterior to the primary fissure. It regulates the muscle tone. It receives input from muscle spindles (stretch receptor) and Golgi tendon organs (GTOs) through spinocerebellar tract.
b. Posterior lobe (Neocerebellum-Corticopontocerebellar tract) • It lies between the primary fissure and posterolateral fissure. It regulates the voluntary motor activity. It receives enormous inputs from neocortex through cortico-pontocerebellar tract.
c. Flocculonodular lobe (Vestibulocerebellum-Vestibulocerebellar tract) • It consists of flocculus and the nodule (vermis). It regulates the maintenance of balance and posture.

Longitudinal organisation of Cerebellum
Cerebellum consists of three functional zones that are longitudinally oriented, and these zones are connected to specific cerebellar nuclei. a. Median (Vermal) zone of hemisphere consists of the cortex of the vermis and it is connected to fastigial nucleus.
b. Paramedian (Paravermal) zone consists of the cortex of the hemisphere that is immediately adjacent to the vermis and it is connected to nucleus interpositus (Globose and Emboliform nuclei) c. Lateral zone consists of cortex of the hemisphere that is exclusive of vermal and paravermal regions [3].

Internal structure of the cerebellum
Like the cerebral cortex, the Cerebellum also consists of outer shell of grey matter (cerebellar cortex) and the inner core white matter. The white matter consists of afferents and efferent fibres that go to and from the cortex. The white DOI: http://dx.doi.org/10.5772/intechopen.97579 mater present underneath the grey mater resembles branches of a tree, hence named arbor vitae cerebelli (tree of life). The fibres reach the cortex in a characteristic branch like projections.
The four cerebellar nuclei are distributed deeply within the white mater in each cerebellar hemisphere. The cerebellar nuclei, while connected to the cerebellar cortex, give off the outflow form the cerebellum to the other parts of the brain. The connections are primarily to brainstem nuclei and the thalamus.

Cerebellar cortex
On the surface of the cerebellum, a highly convoluted cortex forms numerous transversely oriented folium. The cerebellar cortex is filled with cerebellar neuronal cell bodies, dendrites, and various synapses. The cortex is histologically divided into three layers:

Inhibitory inputs: Golgi cells, basket cells and stellate cells inhibit (GABA) the Purkinje cells.
Excitatory input: Mossy fibres excite the granule cells.

White Fibres of cerebellum
Afferents travel through cerebellar peduncles and reach the cerebellar cortical neurons to stimulate them. Based on the origin, the afferents reaching cerebellar cortex are classified as: 1) Climbing fibres, 2) Mossy fibres.

Mossy fibres
The afferent fibres (excitatory) of spinocerebellar tract, pontocerebellar tract, and vestibulocerebellar tract are called as mossy fibres. Mossy fibres branch and terminate in an excitatory synapse with the granule cells as mossy fibre rosette, of several folia. The axons of granule cells enter the molecular layer, through Purkinje layer and split to form two parallel fibres which run along the long axis of the folium. Mossy fibres excite granule cells which discharge via their parallel fibres.

Climbing fibres
The afferent fibres (excitatory, aspartate) of olivocerebellar tract from contralateral inferior olivary nucleus of medulla are called as climbing fibres. They terminate on the dendrites of Purkinje cells and the deep cerebellar nuclei [4].

Cerebellar nuclei
Four cerebellar nuclei lie deep within the cerebellar white matter of each hemisphere. They are arranged from lateral to medial as follows: • Dentate Nucleus (Tooth like serrated edge) • Emboliform nucleus (Plug or Wedge-shaped) • Globose nucleus (Spherical shaped) • Fastigial nucleus (Peak of the Fourth ventricle: Fastigium)

Extracerebellar afferents of cerebellar nuclei
The collateral branches of Mossy fibers coming from: a) vestibular nuclei, b) reticular nuclei, c) pontine nuclei, d) spinocerebellar tract.
Among the deep cerebellar nuclei, the dentate nucleus with its crinkled bag-like appearance is the largest and the only nucleus visible to the naked eye. The dentate nucleus receives afferent fibers from the inferior olivary nucleus of the medulla, which also looks like a crinkled bag.

Intracerebellar afferents of cerebellar nuclei
Purkinje cells of the cerebellar cortex.

Efferent from Cerebellum
The majority of the efferent fibers leaving the cerebellum originate from the deep cerebellar nuclei. The efferent fibers reach: a) reticular nuclei, b) vestibular nuclei, c) red nucleus, ventral lateral nucleus of the thalamus. c. Neocerebellum consists of the remaining cerebellar hemisphere (except pyramid and uvula) and dentate nucleus. Function: Controls the highly skilled muscle coordination and trajectory, speed, and force of movements. Connections: Cortico-pontocerebellar: Pontine nuclei, cerebral cortex.

Cerebellar Peduncles
These are the white fibre bundles that join the different parts of the brain stem with the cerebellum.

Coordination of movement by Neocortex
• Lateral and paramedian reticular nuclei (Reticulocerebellar tract)

Blood supply of Cerebellum
The cerebellum is supplied by posterior circulation originated from vertebral arteries. The vertebral artery gives rise to the posterior inferior cerebellar artery (PICA), which supplies the posterior part of the inferior surface of the cerebellum. The basilar artery gives rise to the anterior inferior cerebellar artery (AICA), which supplies the anterior part of the inferior surface. The superior cerebellar artery (SCA) supplies the superior surface of the cerebellum [1].

Midline lesions
The midline lesions of the cerebellum (vermis) cause loss of control of trunk posture resulting in truncal ataxia. Patients present with the inability to sit or stand, as there would be involuntary swinging of the body back and forth to stabilize around the center of gravity.

Unilateral cerebellar lesions
Cerebellar tracts do not decussate like the cerebrum. The symptoms (limb ataxia) produced by the lesions of cerebellar hemispheres are ipsilateral. Unilateral lesions of cerebellar hemispheres cause ipsilateral loss of arm or leg coordination resulting in an unsteady gait (No motor or sensory loss). Limb ataxia can be tested by asking the patient to do a "heel to shin" test. When patients with limb ataxia try to walk, the body has difficulty coordinating muscle movements, leading to shifting the center of gravity. When there is a fall due to a significant shift in the center of gravity, the fall is usually towards the same side of the lesion. The patient often compensates for this by lowering their center of gravity by wide stepped gait [5].

Bilateral cerebellar dysfunction
Bilateral cerebellar dysfunction causes the following symptoms: Diseases in which cerebellum is affected bilaterally: a) hypothyroidism, b) alcoholic intoxication, c) multiple sclerosis, d) degenerative diseases, e) metabolic disorders).
Charcot's triad: A characteristic combination of nystagmus, dysarthria, and intentional tremor are observed in multiple sclerosis.

Conclusion
Cerebellum consists of median vermis and prominent lateral hemispheres. It forms the roof of the fourth ventricle behind the brain stem. It is attached to the parts of brainstem by cerebellar peduncles which are large white fibre bundles carrying afferent and efferent fibres of cerebellum. Afferent systems of cerebellum include climbing fibres and mossy fibres. It also receives fibres from brainstem reticular formation. Climbing fibres are connected to the contralateral inferior olivary nucleus of medulla oblongata, at one end, and the proximal dendrites of a single Purkinjee cell in the cerebellar cortex, at the other end. Mossy fibres are connected to spinal cord, brain stem, at one end and multiple Purkinjee cells of cerebellar cortex at another end.

Cerebellum and Spinocerebellar ataxia
Case study: 55 old male presents with a history of poor hand coordination, slurred speech, rapid eye movements, reduced intellectual function. Physical examination reveals cerebellar ataxia, spasticity, negative Babinski sign. Brain CT scan showed mild cerebral and marked cerebellar atrophy.

Spinocerebellar ataxia
Introduction: Spinocerebellar ataxias (SCA), are a group of hereditary ataxias transmitted by autosomal dominant inheritance, in which there is a progressive and slow degeneration of cerebellum and certain parts of spinal cord. Among the many types of SCAs, they are classified based on the gene mutation responsible for a specific type of SCA. The types are described as SCA1 through SCA40.
Symptoms: The signs and symptoms across the different types generally include abnormal speech (dysarthria), uncoordinated walk (gait), poor handeye coordination, vision problems and difficulty in processing, learning and remembering information. The main symptom include ataxia, where smooth coordination of voluntary motor functions is lost, and there is also nystagmus where the vestibulo-cerebellar fibres and vestibulo-cerebellum are involved. Owing to the degenerative nature of the disease, not only dorsal and ventral spinocerebellar fibres carrying proprioceptive fibres from skeletal muscles and joints, almost all the functions of the cerebellum are affected in Spinocerebellar ataxias.
Etiology: Certain types of SCA are caused due to mutation called trinucleotide repeat expansion, where a particular segment of DNA is repeated number of times beyond the tolerable limit. Such nucleotide repeats are unstable and alter their length while passing through generations and often lead to early age onset of the disease. The risk of transmission of the disease from the affected generation to the next is 50%.
Diagnosis: If the disease-causing mutation is known then the carrier testing for at-risk relatives and prenatal testing can be done to diagnose the disease.
Treatment: There is no specific treatment for SCA. For ataxia, physiotherapy to strengthen the muscles can be done. Physical aids such as crutches and walkers can be used to assist daily activity of the patient [2].