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BRAIN ANATOMY: CEREBELLUM


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A 4--year-old boy was taken to a neurologist because hismother was concerned about his attacks of vomiting on wakingin the morning and his tendency to be unsteady on standingup. The mother also noticed that the child walked with anunsteady gait and often fell backward. On physicalexamination, the child tended to stand with the legs wellapartthat is, broad based. The head was larger than normal

for his age, and the suture lines of the skull could be easilyfelt. A retinal examination with an ophthalmoscope showedsevere papilledema in both eyes. The muscles of the upperand lower limbs showed some degree of hypotonia. the childshowed no tendency to fall to one side or the other when

asked to walk.


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The cerebellum is situated in the posteriorcranial fossa. It is the largest part of the

hindbrain and lies posterior to the fourthventricle, the pons, and the medullaoblongata.

The cerebellum is somewhat ovoid in shape andconstricted in its median part. It consists of twocerebellar hemispheres joined by a narrow medianvermis. The cerebellum is connected to theposterior aspect of the brainstem by threesymmetrical bundles of nerve fibers called thesuperior, middle, and inferior cerebellar peduncles


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CEREBELLUM

The cerebellum is located behind the dorsal aspect of thepons and the medulla.

A midline portion, the vermis, separates two lateral lobes,

or cerebellar hemispheres. The cerebellum consists of the cerebellar cortex and the

underlying cerebellar white matter

Four paired deep cerebellar nuclei are located within thewhite matter of the cerebellum

Because of the location of the fourth ventricle, ventral tothe cerebellum, mass lesions or swelling of thecerebellum can cause obstructive hydrocephalus.


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The cerebellum is divided into three main lobes:the anterior lobe,the middle(posterior) lobe,

and the flocculonodular lobe.The anterior lobe may be seen on the superior surface ofthe cerebellum and is separated from the middle lobe by awide V-shaped fissure called the primary fissure.The middle lobe (sometimes called the posterior lobe),

which is the largest part of the cerebellum, is situatedbetween the primary and uvulonodular or posteriorfissures.The flocculonodular lobe is situated posterior to the

uvulonodular fissure.A deep horizontal fissure that is found along the margin ofthe cerebellum separates the superior from the inferiorsurfaces; it is of no morphologic or functional significance


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Structure of the CerebellumThe cerebellum is composed of an outer coveringof gray matter called the cortex and inner white

matter.

Structure of the Cerebellar Cortex

The cerebellar cortex can be regarded as a large sheet withfolds lying in the coronal or transverse plane. Each fold or

folium contains a core of white matter covered superficiallyby gray matter.A section made through the cerebellum parallel with themedian plane divides the folia at right angles, and the cutsurface has a branched appearance, called the arbor vitae.The gray matter of the cortex throughout its extent has auniform structure. It may be divided into three layers: (1)an external layer, the molecular layer; (2) a middle layer,the Purkinje cell layer; and (3) an internal layer, the

granular layer .


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Molecular LayerThe molecular layer contains two types of neurons: the

outer stellate cell and the inner basket cell

Purkinje Cell LayerThe Purkinje cells are large Golgi type I neurons. Theyare flask shaped and are arranged in a single layer. Thedendrites of these cells are seen to pass into themolecular layer, where they undergo profuse branching

Granular LayerThe granular layer is packed with small cells with denselystaining nuclei and scanty cytoplasm. Each cell gives rise tofour or five dendrites, which makeclawlike endings and have synaptic contact with mossyfiber input


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Functional Areas of the Cerebellar CortexClinicaly it is possible to divide up the cerebellar cortex

into three functional areas.The cortex of the vermis influences the movements ofthe long axis of the body, namely, the neck, theshoulders, the thorax, the abdomen, and the hips.

Immediately lateral to the vermis is a so-calledintermediate zone of the cerebellar hemisphere. Thisarea has been shown to control the muscles of thedistal parts of the limbs, especially the hands and feet.The lateral zone of each cerebellar hemisphere appears

to be concerned with the planning of sequentialmovements of the entire body and is involved with theconscious assessment of movement errors.


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Intracerebellar NucleiFour masses of gray matter are embedded in the whitematter of the cerebellum on each side of the midline.

From lateral to medial, these nuclei are the dentate, theemboliform, the globose, and the fastigial.

The dentate nucleus is the largest of the cerebellar nuclei. Ithas the shape of a crumpled bag with the opening facingmedially.The emboliform nucleus is ovoid and is situated medial to thedentate nucleus, partially covering its hilus.The globose nucleus consists of one or more rounded cell

groups that lie medial to the emboliform nucleus .The fastigial nucleus lies near the midline in thevermis and close to the roof of the fourth ventricle; itis larger than the globose nucleus


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White MatterThere is a small amount of white matter in the vermis; itclosely resembles the trunk and branches of a tree and

thus is termed the arbor vitae .There is a large amount of white matter in eachcerebellar hemisphere.The white matter is made up of three groups of fibers:(1) intrinsic, (2) afferent, and (3) efferent.

The intrinsic fibers do not leave the cerebellum but connectdifferent regions of the organ. Some interconnect folia ofthe cerebellar cortex and vermis on the same side; othersconnect the two cerebellar hemispheres together.

The afferent fibers form the greater part of the white matterand proceed to the cerebellar cortex. They enter thecerebellum mainly through the inferior and middle cerebellarpeduncles..


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The efferent fibers constitute the output ofthe cerebellum and commence as the axons

of the Purkinje cells of the cerebellar cortex.The great majority of the Purkinje cell axonspass to and synapse with the neurons of thecerebellar nuclei (fastigial, globose,

emboliform, and dentate).The axons of the neurons then leave thecerebellum. A few Purkinje cell axons in theflocculonodular lobe and in parts of the

vermis bypass the cerebellar nuclei and leavethe cerebellum without synapsing.


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Cerebellar PedunclesThe cerebellum is linked to other parts of

the central nervous system by numerousefferent and afferent fibers that aregrouped together on each side into three

large bundles, or peduncles.The superior cerebellar peduncles connectthe cerebellum to the midbrain,the middle cerebellar peduncles connect

the cerebellum to the pons,and the inferior cerebellar pedunclesconnect the cerebellum to the medulla

oblongata.


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CEREBELLAR FUNCTIONS

The cerebellum has several main functions:

coordinating skilled voluntary movements byinfluencing muscle activity,

controlling equilibrium and muscle tone.

There is a somatotopic organization of body partswithin the cerebellar cortex.

In addition, the cerebellum receives collateral input from

the sensory and special sensory systems.


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Each cerebellar hemisphere is connected bynervous pathways principally with the same

side of the body; thus, a lesion in onecerebellar hemisphere gives rise to signsand symptoms that are limited to the sameside of the body.


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Signs and Symptoms of Cerebellar Disease

Hypotonia

Postural Changes and Alteration of Gait

Disturbances of VoluntaryMovement (Ataxia)

Dysdiadochokinesia:inability to perform alternating movements

Disturbances of Reflexes

Disturbances of OcularMovement

Disorders of SpeechDysarthria


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FUNCTIONS

Co-ordination of movement

The cortex initiates it and the cerebellum co-

ordinates it.


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DAMAGE TO THE CEREBELLUM

Causes a lack of co-ordination:

Speaking

Walking

Tremor


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CEREBELLAR STROKE

Dizziness, vomiting

Unsteady so that walking is impossible

Power, tone and reflexes normal

Area of blood in the cerebellum would show on

aC

T scan

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