- Cerebellum = little brain.
- second largest part of brain.
- largest part of hindbrain.
- about 150 grams.
- numerous fine slit-like sulci or fissures and parallel folds or folia.
STRUCTURE OF THE CEREBELLUM
- The cerebellum is composed of an outer covering of gray matter called the cortex and inner white matter.
- Embedded in the white matter of each hemisphere are three masses of gray matter forming the intracerebellar nuclei.
- The cerebellar cortex can be regarded as a large sheet with folds lying in the coronal or transverse plane. Each fold or folium contains a core of white matter covered superficially by gray matter.
- A section made through the cerebellum parallel with the median plane divides the folia at right angles, and the cut surface has a branched appearance, called the arbor vitae
- The gray matter of the cortex throughout its extent has a uniform 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.
- 3 parts = 2 lateral hemispheres and 1 worm-like vermis
- Surfaces = superior, inferior
- Notches = anterior and posterior cerebellar notch
- Fissures = horizontal, posterolateral, V-shaped fissura prima.
- The cerebellum consists of two large lateral hemispherical lobes, the cerebellar hemispheres which are united to each other by a narrow median worm-like portion, called vermis.
- The superior and inferior aspects of vermis are termed superior and inferior vermis respectively.
- The ridge-like superior vermis is continuous on either side with the superior surface of cerebellar hemisphere imperceptively.
- The inferior vermis is more clearly demarcated from the hemispheres in the floor of vallecula cerebelli.
- The superior surface of the cerebellum is convex. The two cerebellar hemispheres are continuous with each other on this surface.
- The inferior surface presents a deep median notch called vallecula which separates the two cerebellar hemispheres.
- The floor of the vallecula is formed by inferior vermis and is limited on each side by sulcus valleculae.
- The anterior aspect of cerebellum is marked by a wide shallow anterior cerebellar notch which accommodates pons and medulla.
- The posterior cerebellar notch is deep and narrow, and lodges the falx cerebelli.
- The horizontal fissure is most conspicuous and runs along the lateral and posterior margins of the cerebellum. It marks the junction between the superior and inferior surfaces of the cerebellum.
- The posterolateral fissure lies on the inferior surface of the cerebellum and separates the flocculonodular lobe from the rest of the cerebellum (corpus cerebelli).
- The V-shaped fissura prima on the superior surface cuts the superior vermis at the junction of its anterior two-third and posterior one-third. It divides the corpus cerebelli into anterior and posterior (middle) lobes.
Subdivisions of Cerebellum
- Anatomically the cerebellum is divided into three lobes – anterior, posterior and flocculonodular.
- Based on phylogenetic and functional criteria the cerebellum is divided into three parts, archicerebellum, paleocer-ebellum, and neocerebellum.
1) Archicerebellum (vestibular cerebellum)
- Oldest part of the cerebellum.
- The fishes and lower amphibians possess only this component of the cerebellum.
- It consists of flocculonodular lobe and lingula.
- Chiefly vestibular – concerned with the maintenance of equilibrium, tone and posture of trunk muscles.
2) Paleocerebellum (spinal cerebellum)
- In terrestrial vertebrates with the appearance of limbs, viz. reptiles and birds.
- Consists of anterior lobe (except lingula) and pyramid and uvula of inferior vermis.
- Chiefly spinocerebellar in connections and concerned with the tone, posture and crude movements of the limbs.
Neocerebellum (cerebral cerebellum)
- Most recent part of the cerebellum to develop- prominent in higher mammals.
- It is made up of middle lobe, the largest part of the cerebellum (except the pyramid and the uvula of inferior vermis).
- Chiefly corticoponto-cerebellar in connections and is concerned with smooth performance of skilled voluntary movements.
- Thin surface layer of grey matter, the cerebellar cortex and a central core of white matter. Embedded within the central core of white matter are masses of grey matter called intracerebellar nuclei.
- The cerebellar cortex is folded in such a way that the surface of cerebellum presents a series of parallel transverse fissures and intervening narrow leaf-like bands called folia. Each folium consists of a slender branched lamina of central core of white matter covered by a thin layer of grey matter.
- The central core of white matter being arranged in the form of the branching pattern of a tree, is called arbor vitae cerebelli (arbor vitae = tree of life).
- The grey matter of the cerebellum is represented by: (a) the cerebellar cortex, and (b) the intracerebellar nuclei.
- Cerebellar cortex
- 3 layers :
- Outer molecular / plexiform = numerous dendritic arborizations of purkinje cells, few widely spaced two types of nerve cells ; basket cells and stellate cells.
- Purkinje cell layer = single row of large flask-shaped purkinje cells.
- Granular layer = numerous closely packed small granule cells, few large golgi cells.
Molecular (plexiform) layer
- Consists of numerous dendritic arborizations of Purkinje cells and nerve cells.
- The nerve cells are of two types: (a) the basket cells, and (b) the stellate cells.
- The Basket cells – small in size with little cytoplasm and extensive processes.
- The longest of these processes (axon) of each cell assumes a transverse course parallel to the cortical surface and synapse with dendrites of Purkinje cells.
- The Stellate cells – short processes. Scattered near the surface. Their axonsarborise with dendritic spines of the Purkinje cells.
Purkinje cell layer
- Consists of a single row of large flask-shaped cells, the Purkinje cells.
- A dendrite arises from the neck of the flask, passes upwards into the molecular layer, where it undergo profuse branching to form an elaborate dendritic tree.
- The dendrites of Purkinje cells synapse with: (a)basket cells, (b) granule cells, and (c) the climbing fibres.
- The outgoing Purkinje axons constitute the sole output from the cerebellar cortex and exert an inhibitory influence on the intracerebellar nuclei.
- Consists of numerous closely packed small Granule cells.
- Alsocontains few large Golgi cells.
- Each granule cell gives rise to four or five short dendrites, which make claw-like endings which synapse with the terminals of the mossy fibres.
- The axon of each granule cell passes into the molecular layer where it bifurcates at a T junction, and its branches run parallel to the long axis of the cerebellar folium. These fibres are known as parallel fibres.
- The Golgi cells are large and prominent but scanty. Their den-drites ramify in the molecular layer.
Intrinsic neurons of the cerebellar cortex
- There are five types of intrinsic neurons in the cerebellar cortex:
(a) Purkinje cells,
(b) Granule cells,
(c) Stellate cells,
(d) Basket cells,
(e) Golgi cells.
- All the intrinsic neurons of cerebellar cortex are inhibitory except granule cells. Such a collection of inhibitory neurons is not found anywhere else in the CNS except in the cerebellum.
- Four masses of gray matter are embedded in the white matter of the cerebellum on each side of the midline.
- From lateral to medial, these nuclei are the dentate, the emboliform, the globose, and the fastigial.
- The dentate nucleus is the largest of the cerebellar nuclei. It has the shape of a crumpled bag with the opening facing medially .
- The interior of the bag is filled with white matter made up of efferent fibers that leave the nucleus through the opening to form a large part of the superior cerebellar peduncle.
- The emboliform nucleus is ovoid and is situated medial to the dentate nucleus, partially covering its hilus.
- The globose nucleus consists of one or more roundedcell groups that lie medial to the emboliform nucleus.
- The fastigial nucleus lies near the midline in the vermis and close to the roof of the fourth ventricle; it is larger than the globose nucleus.
- The dentate nucleus- most prominent and largest in primates, especially in humans.
- It is the nucleus of neocerebellum and therefore receives afferent fibres from it.
- Fibres include dentorubral and dentothalamic fibres relaying in the red nucleus and ventral lateral nucleus of the thalamus respectively.
- Fibres from red nucleus and thalamus project to the spinal cord and cerebral cortex respectively.
- Oval in shape and situated medial to the dentate nucleus, partially covering its hilum.
- It is the nucleus of paleocerebellum, hence receives afferent fibres from it and gives fibres to the red nucleus via superior cerebellar peduncle.
- The red nucleus projects to the spinal cord through rubrospinal tract, which facilitates the flexor muscle tone.
- Rounded in shape and lies between the emboliform and fastigial nuclei.
- It has similar connections to that of emboliform nucleus.
- The globose and emboliform nuclei together are sometimes referred to as nucleus interpositus.
- Lies near the midline in the vermis and close to the roof of the fourth ventricle.
- Smaller than the dentate but larger than the emboliform or globose nuclei.
- It is nucleus of archicerebellum, hence receives afferent fibres from flocculonodular lobe (archicerebellum) and conveys efferent fibres
- The cerebellum is linked to other parts of the central nervous system by numerous efferent and afferent fibers that are grouped together on each side into three large bundles, or peduncles.
- Superior cerebellar peduncles connect the cerebellum to the midbrai.
- Middle cerebellar peduncles connect the cerebellum to the pons.
- Inferior cerebellar peduncles connect the cerebellum to the medulla oblongata.
- Made up of three types of fibres: (a) intrinsic, (b) afferent, and (c) efferent.
- The intrinsic fibres- remain confined within the cerebellum.
- Connect different regions of the cerebellum either in the same hemisphere or of the two cerebellar hemispheres.
- Through afferent and efferent fibres the cerebellum is connected with the other parts of the CNS.
- Superior cerebellar = branch of basilar, supplies superior surface.
- Anterior inferior cerebellar = branch of basilar, supplies anterior part of inferior surface.
- Posterior inferior cerebellar = branch of vertebral, supplies posterior part of inferior surface.
- The cerebellar lesions due to trauma, vascular occlusion, tumours, etc. produce a number of signs and symptoms, which together constitute the cerebellar syndrome.
- The signs and symptoms produced by cerebellar lesions are as follows:
Involvement of archicerebellum will lead to:
(a) Trunkal ataxia and staggering gate: The patient is unsteady while standing, and staggering while walking [he walks with legs well apart (i.e. on a wide base) and sways from side to side].
(b) Nystagmus: The involuntary, to and fro, oscillatory movements of the eyeballs while looking to either side.
Romberg’s sign- The patient is unable to maintain balance while standing upright, and falls on closing the eyes.
- Involvement of paleocerebellum will lead to diminished muscle tone (hypotonia asthenia) of limb muscles. This manifests in the form of:
(a) lack of stability of joints (flail joints).
(b) Disturbances of tendon reflexes, viz. tapping of patellar tendon produces oscillating movements of the leg (i.e. leg swings backwards and forward) called pendular knee jerk.
(c) inability to maintain balance while walking (ataxia).
- The involvement of neocerebellum leads to muscular incoordination called asynergia.
(a) Dysmetria, i.e. inability to measure the distance correctly for reaching an intended target. Clinically this is tested by the finger-nose test. If the patient is asked to touch the tip of his nose by his finger, he either undershoots or overshoots the tip of his nose. When patient tries to correct the mistake, it results in, to and fro movements.
- (b) Intention tremors probably due to dysmetria: The tremors occur during purposeful movements and disappear with rest.
- (c) Adiadochokinesis/dysdiadochokinesis, i.e. inability to execute alternate movements in rapid succession, e.g. pronation and supination of the forearm.
- (d) Dysarthria or scanning speech: The speech is slurred, prolonged, explosive and with pauses in wrong places.
- (e) Rebound phenomenon: The patient is unable to check the action of agonist muscles by the corresponding antagonist muscles.
If the patient is asked to push the palm of a doctor by his palm, the force that the patient can develop slowly rises and now if the doctor suddenly removes his hand the limb of patient flies back (i.e. rebounds) and hits against his chest because he is not able to stop his pushing act immediately.
- The most common cause of vermis syndrome is a medulloblastoma of the vermis in children.
- Involvement of the flocculonodular lobe results in signs and symptoms related to the vestibular system.
- Since the vermis is unpaired and influences midline structures, muscle incoordination involves the head and trunk and not the limbs.
- There is a tendency to fall forward or backward.There is difficulty in holding the head steady and in an upright position. There also may be difficulty in holding the trunk erect.
Cerebellar Hemisphere Syndrome
- Tumors of one cerebellar hemisphere may be the cause of cerebellar hemisphere syndrome.The symptoms and signs are usually unilateral and involve muscles on the side of the diseased cerebellar hemisphere.
- Movements of the limbs, especially the arms, are disturbed.Swaying and falling to the side of the lesion often occur.
- Dysarthria and nystagmus are also common findings.
- Disorders of the lateral part of the cerebellar hemispheres produce delays in initiating movements and inability to move all limb segments together in a coordinated manner but show a tendency to move one joint at a time.
Other Anatomy Notes
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