CEREBELLUM

[SURIYA.KARTHIKEYHAN]

CEREBELLUM

CEREBELLUM -The Neuroscience behind Balance and Coordination by the little Brain.

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SYNOPSIS

• Introduction
• External features
• Subdivisions
• Internal features
• Microanatomy of cerebellum
• Connections of cerebellum
• Blood supply
• Applied- cerebellar dysfunction

 

Introduction 

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Cerebellum (L. cerebellum = little brain):

• Largest part of hindbrain.
• Weighs- 150 g. 
• Location- Posterior cranial fossa.

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The 3 primary functions of the cerebellum are:

1.    Maintenance of posture.
2.    Maintenance of muscle tone.
3.    Coordination of voluntary motor activity.

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• It also adjusts coordination of skillful movements.
• Sherrington regarded the cerebellum as the head ganglion of the proprioceptive system.

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External features

• It comprises
  • Three Parts.
  • Two Surfaces.
  • Two Notches.
  • Three well-marked Fissures.
• PARTS:
  • Two Cerebellar hemispheres.
  • United to each other by vermis.

Ventral and Dorsal view of cerebellum

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MEDIAN SAGITTAL SECTION OF CEREBELLUM AND BRAINSTEM

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Surfaces of cerebellum

• Superior surface.
• Inferior surface presents a deep median notch called vallecula.

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Notches

1. Anterior cerebellar notch - Accommodates pons and medulla.
2. Posterior cerebellar notch - Is deep and narrow, and lodges  the falx cerebelli.

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Fissures of cerebellum

1. Horizontal fissure - Runs along the lateral and posterior margins of the cerebellum. It marks the junction between the superior and inferior surfaces of the cerebellum.
2. Posterolateral/Dorsolateral fissure lies on the inferior surface of the cerebellum and separates the flocculonodular lobe from the rest of the cerebellum (corpus cerebelli).
3. 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.

Fissures of cerebellum

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Subdivisions - Anatomical

Anatomically the cerebellum is divided into three lobes:

• Anterior lobe
• Posterior/middle lobe
• Flocculonodular lobe

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Morphological classification

Based on phylogenetic and functional criteria the cerebellum is divided into three parts:

1. Archicerebellum (Vestibular cerebellum)
2. Paleocerebellum (Spinal cerebellum)
3. Neocerebellum (Cerebral cerebellum)

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Internal Structure

• Outer Grey matter and a central core of white matter.
• 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.

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• The central core of white matter being arranged in the form of the branching pattern of a tree, is called arborvitae cerebelli.

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Histology..

Distinct layers:

• Outer molecular layer(or) Plexiform layer.
• Intermediate Purkinje cell layer.
• Inner Granular layer.

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Outer Molecular layer:

• It mainly consists of numerous dendritic arborizations of Purkinje cells and relatively few nerve cells which are widely spaced.
• The nerve cells are of 2 types:
  • the Basket cells
  • the stellate cells

Basket cells :

• Small with extensive processes.                                                                       
• Axons of basket cells synapse with dendrites of many ( about 500 ) Purkinje cells.

Molecular layer-Contd…

• Stellate cells : with short processes are scattered near the surface.

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Purkinje layer

• Consists of single row of large flask shaped cells (Purkinje cells)A dendrite arises from the neck of the flask, passes upwards into the molecular layer and undergo further branches to form an elobrate dendritic tree.
• The dendrites of  Purkinje cells synapse with:
  •  the collaterals from the basket cells.
  •  the axons of the granule cells (parallel fibres ).
  •  the climbing fibres.

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Purkinje layer-Contd…

• The axons of the Purkinje cells pass through the granular layer into the white matter, where they relay into the intracerebellar nuclei.
• The outgoing Purkinje axons constitute the sole output from the cerebellar cortex and exert an inhibitory influence on the intracerebellar nuclei.

Granular layer Consists of:

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•  Granule cell : small, numerous cells.
•  Golgi cells : arge, scanty cells.

Granule cell : gives claw-like endings 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.
• They run at right angle to the dendritic processes of the Purkinje cells and make synaptic contacts with them. 

Golgi cells : the dendrites of these cells ramify in the molecular layer.

Intrinsic Neurons of Cerebellum

• All the intrinsic neurons of cerebellar cortex are inhibitory except granule cells.
• Such a collection of inhibitory neurons is not found anywhere in the CNS except in the cerebellum.

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Microanatomy of Cerebellum Identification points: Cerebellar folia with outer cortex and inner medulla.

• Cortex-3 layered.
  • Outer molecular
  • Middle purkinje
  • Inner Granular layer.
•  Main cell types - purkinje and granule cells.
• Medulla is made up of white matter.

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CAJAL STAIN- Nerve cell arrangement

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H and E Stain-High power Magnification

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Cerebellum – Special stains

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Intracerebellar nuclei

• It is also called as central nuclei.
• These are masses of grey matter embedded in the white matter of the cerebellum.

Intracerebellar nuclei-Contd….

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White matter

• The white matter of cerebellum is made up of 3 types of fibres :
  • intrinsic
  • afferent
  • efferent
• The intrinsic fibres remain confined within the cerebellum. They 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.

Afferent connections of Cerebellum

• Structure of cerebellar cortex, showing intrinsic neurons and their processes.
• A single folium is shown cut longitudinally (right) and transversely (left).
• Note that cerebellar glomerulus is a synaptic complex involving four types of neurites: the mossy fibre terminal, the dendrites of granule and Golgi cells, and the Golgi cell axon terminal.

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Contd.(Cerebellar Glomerulus)

• Each Mossy fibre ends by forming 30 to 40 terminal swellings called rosette.
  • Each rosette forms a synaptic contacts with
  • dendrites of Granule cells and
  • axons of few Golgi cells.
• The rosette and its synaptic contacts together called as cerebellar glomerulus.
• Each glomerulus is surrounded by a Bergmann capsule of neuroglial cells and appears spherical or  ovoid in shape.

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Efferent connections

• The efferent fibres pass to the:
  •    Red nucleus
  •    Thalamus
  •    Vestibular complex
  •    Reticular formation.
• Sole output of Cerebellum - Through axons of the Purkinje cells.
• Fibres from dentate, emboliform and globose nuclei leave the cerebellum through the Superior cerebellar peduncle.
• Fibres from fastigial nucleus leaves through Inferior cerebellar peduncle.

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Intrinsic Neuronal Circuitry in The Cerebellum 

All the afferent fibres to the cerebellum (i.e. mossy and climbing fibres) are excitatory to the cells of cerebellar cortex and their collaterals are excitatory to the intracerebellar nuclei.

• Purkinje cells - Climbing fibres excite directly  Mossy fibres excite indirectly through  the granule cells.
• The granule cells in addition to Purkinje cells also excite the flanking basket and stellate cells which  inhibit the Purkinje cells a neural sharpening mechanism.
• The mossy fibres also excite the Golgi cells which provide the feedback inhibition to the granule cells.

The Purkinje cells exert an inhibitory influence on the neurons  of the intracerebellar nuclei which in turn modify the muscular  activity through the motor control areas of the brainstem   and cerebral cortex.

Note:

•  All Cells in the cerebellar cortex are inhibitory except the granule cells(sole Excitatory).
• The Purkinje cells are the only neurons in the cerebellar cortex which are capable of transmitting efferent impulses.(Sole output cell).

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Cerebellar Peduncles

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SAGITTAL SECTION OF CEREBELLUM SHOWING CEREBELLAR PEDUNCLES

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Arterial Supply

1. Superior cerebellar artery-supplies the superior surface.
2. Anterior inferior cerebellar artery-supplies the anterior part of the inferior surface.
3. Posterior inferior cerebellar artery(PICA)- supplies the posterior part of inferior surface of the cerebellum.

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Functions of Cerebellum

• Cerebellum - key role in accomplishing a smooth and coordinated movements.
• Comparator function:
• Cerebral cortex decides on a movement to do, and  impulses stimulate the anterior (LMNs) horn of spinal cord to initiate voluntary movement, and at the same time, the motor cortex tell to the cerebellum about the intended movement, the cerebellum also receives proprioceptive information from muscles and joints, telling it what movement is actually occurring.
• The cerebellum compares the intended movement with the actual movement occurring, and if a difference is detected, the cerebellum sends the information to the motor cortex and lower motor neurons to correct the discrepancy.
• The result is smooth and coordinated movements (i.e. movements produced are accurate in time, rate, range, force, and direction). With training, a person can develop highly skilled and rapid movements.
• The cerebellum thus plays an important role in learning of motor skills.

Applied Anatomy

• 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: 

The 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.

Note: The patient is unable to maintain balance while standing upright, and falls on closing the eyes (Romberg's sign).

• The involvement of paleocerebellum will lead to diminished muscle tone (hypotonia, asthenia) of limb muscles.
• This manifests in the form of:
• lack of stability of joints (flail joints).
• 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.
•  inability to maintain balance while walking  (ataxia).

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The involvement of neocerebellum leads to muscular incoordination called asynergia.

This manifests in the form of:    

(a) Dysmetria: i.e. inability to measure the distance correctly for reaching an intended target, described as past-pointing.  
  
(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 supina-tion 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.

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TAKE HOME MESSAGE

• Functions - Posture, coordination of motor activity and  Maintenance of muscle tone.
• Deep Cerebellar nucleus - Dentate, Emboliform, Globose and Fastigeal.
• Sole Output Neurons - Purkinje cells.
• Sole Excitatory Neurons - Granule cells
• Neuronal sharpening
• Cerebellar Glomerulus- Mossy fibres.Granule cells and Golgi cells
• Nystagmus, Romberg sign -  Archicerebellum
• Dysmetria, Dysdiadokinesis, Dysarthria - Neocerebellum
• Gait ataxia - Paleocerebellum