Cerebellum Dr Taha Sadig Ahmed MBBS Ph D

Cerebellum Dr Taha Sadig Ahmed , MBBS , Ph. D ( England ). Consultant , Clinical Neurophysiology. Associate Professor , Physiology Department , College of Medicine 2/25/2021 1

4 th Ventricle The cerebellum is located at the bottom of the brain , in the posterior cranial fossa , over the occipital bone. It is separated from the overlying cerebrum by a layer of leathery dura mater called Tentorium Cerebelli Anterior to it is Pons and 4 th ventricle 2/25/2021 2

Ø It consists of (1) two Cerebellar Hemispheres , connected by (2) a middle part called Vermis , and a (3) a posterior Flocculonodular Lobe. Posterolateral Fissure 2/25/2021 Flocculonodular Lobe 3

Functionally , the Cerebellum is divided into 3 parts : Neocerebellum (Lateral parts of hemispheres ) Spinocerebellum (Vermis + medial parts of hemispheres ) Hemisphere Posterolateral Fissure (1) Neocerebellum ( Cerebrocerebellum ) (2) Spinocerebellum ( Paleocerebellum) , & Flocculonodular Lobe (3) 2/25/2021 Vestibulocerebellum ( Floculonodular Lobe , Archicerebellum ) 4

It is connected to the brainstem on each side by 3 peduncles : Superior , Middle & Inferior ar l l e reb e C cle r o eri edun p Su P Middle Cerebellar Peduncle Inputs to the cerebellum from the Cerebrum Inputs to the Cerebellum from the Pons Inf eri or Pe Cere du be nc le llar 2/25/2021 Inputs to the Cerebellum 5 from the Medulla Oblongata

Although it weighs only 10 % as much as the cerebral cortex , its surface area is about 75 % of that of the cerebral cortex. Cerebellar motor pathways are uncrossed, so that injuries on one side will result in symptoms & signs on the same side of the body. 2/25/2021 6

Neocerebellum ( (Cerebrocerebellum ) üComprises the lateral parts of cerebellar hemispheres. üIt closely cooperates with the cerebrum ( including motor cortex ) cerebral cortex in planning/ programming + execution of voluntary movements. üIt is involved in motor learning of voluntary , skilful movements. üIt coordinates activity of muscles involved in execution of skilled movements. üThese skilled movements are mainly performed by distal limb muscles ( namely hand muscles ). 2/25/2021 üThus the Neocerebellum helps us to monitor , coordinate , and execute fine , skilled movement ( which is primarily voluntarily initiated by the motor cortex ). üAnd makes us able to perform complex skilful movements such as writing , drawing , playing musical instruments , singing etc

Spinocerebellum ( Paleocerebellum) üConsist of Vermis & Medial Parts of the Cerebellar Hemispheres . üThe vemis projects to the brainstem areas concerned with control of axial and proximal limb muscles (which are concerned with gross postural adjustments ). üThe Spinocerebellum receives (1) Proprioceptive inputs afferents from all parts of the body + (2) a copy of the “ Motor Plan ”from the motor cortex ü It functions to (1) Regulates & maintains muscle tone (2) Acts as a comparator 2/25/2021 8

Vestibulocerebellum ( Floculonodular Lobe) üIt has connections to the vestibular nuclei , consequently , it is concerned with maintenance of body balance & equilibrium ü It also controls the Vestibulocular Reflex ( VOR) 2/25/2021 9

Efferents from Cerebellar Cortex 2/25/2021 10

Neocerebellar Cortical Efferents ü Neocerebellar cortex projects to to the Dentate Nucleus ü& from there to the Thalamus ( its Ventrolateral Nucleus ), üwhich , in turn , will project to the Motor & Premotor Areas of the Cerebral Cortex. Ø in order to Ø(1) aid in motor planning Ø(2) coordinate activity of different muscle groups during execution of voluntary , skilled movement

Spinocerebellar Cortical Efferents : üThe Vermis projects to the Fastigial Nucleus & from there to Brainstem Nuclei üThe Hemispheric Portions of the Spinocerebellum project to Nucleus Interpositus ( i. e. , Emboliform and Globose nuclei ) ü& from there to Brainstem Nuclei üThese Brainstem Nuclei receiving inputs from the spinocerebellum are concerned with control of axial and proximal limb muscles ( in order to adjust muscle tone & body posture ).

Vestibulocerebellar Cortical Efferents : ü Efferents from the Vestibulocerebellum pass directly to Brainstem Nuclei ( & not through the DCN) üThese vestibulocerebellar efferents function to regulate of body balance & equilibrium + the VOR ). Ø Hence , it should be remembered that , though the DCN provide the only outlet for the Neocerebellum and Spinocerebellum , they are not the outlet/gateway for Vestibulocerebellum 2/25/2021 13

Functions of Cellular Elements of Cerebellar Cortex 2/25/2021 14

Cerebellar Cellular Organization ü The cerebellum comprises Cortex and Deep Cerebellar Nuclei (DCN) üThe DCN are 4 in number , & are named : (1) Dentate , (2) Fastigial. (3) Globose , (4) Emboliform , NB : Globosus & Emboliform are collectively called Interpositus Nucleus. Cerebellar Afferents : üThese are Mossy Fibers & Climbing Fibers. v Climbing Fibers come from the Inferior Olivary Nucleues , & they provide indirect proprioceptive inputs to the cerebellar cortex 2/25/2021 v Mossy Fibers, come from ü(1) all parts of the body , + ü(2) cerebral cortex. üAs they ascend , they send collaterals to the DCN , and thereafter proceed ( pass on ) to synapse on cerebellar cortical cells. 15

Layers of the Cerebellar Cortex The cerebellar cortex is made of layers (1) External Molecular layer , ( 2) Middle Purkinje Cell layer that is only one cell thick , (3) Internal Granular ayer layer 2/25/2021 16

Cells of the Cerebellar Cortex The cerebellar cortex contains mainly 5 types of neurons : Purkinje , Granule , Basket , Stellate & Golgi cells. (1) Purkinje Cells : Are amongst the biggest neurons in the body. Have very extensive dendritic arbors that extend throughout the Molecular Layer. Their axons , which are the only output from the cerebellar cortex , pass to the deep nuclei. 2/25/2021 17

(2) Granule Cells Their cell-bodies are situated in the Granular layer. They receive inputs from the Mossy fibers and they innervate the Purkinje cells. Each sends an axon to the Molecular layer , where the axon bifurcates to form a T. Because the branches of this “ T ” are straight and run for long distances , they are called Parallel Fibers. 2/25/2021 18

The other 3 types of neurons in the cerebellar cortex are inhibitory neurons : (3) Basket cells : Are located in the Molecular layer They receive inputs from Parallel fibers , and each projects to many Purkinje cells. Their axons form a basket around the cell-body and axon hillock of each Purkinje cell they innervate. They are excited by Parallel fibers of Granule cells , & their output inhibits Purkinje cell discharge by a process of Feed-Forward Inhibition. (4) Stellate cells Similar to Basket cells , these cells are excited by Parallel fibers of Granule cells , & their output inhibits Purkinje cell discharge by a process of Feed-Forward Inhibition. They differ from Basket cells only in being more superficially located in the cortex than Basket cells. 2/25/2021 19

Golgi cells are located in the Granular layer. Their dendrites , which project into the Molecular layer , receive inputs from the Parallel fibers. Their cell bodies receive input via collateralsfrom the incoming Mossy fibers and the Purkinje cells Their axons project to the dendrites of the Granule cells. They are excited by (1) Mossy fibers (2) Purkinje cells , & (3) Parallel fibers. They inhibit the action of Mossy fibers on Granule cells. 2/25/2021 20

The Main Inputs (Afferents ) to the Cerebellar Cortex (1) There are 2 main inputs to the cerebellar cortex : the Climbing Fibers and Mossy Fibers , both of which are excitatory. Climbing Fibers : The climbing fibers come solely from the Inferior Olivary Nucleues They provide an indirect proprioceptive input to the cerebellar cortex bringing to it proprioceptive information from all parts of the body via relays in the Inferior Olive (which receives proprioceptive inputs from all over the body parts ) 2/25/2021 Each climbing fiber projects to the dendrites of Purkinje cells , around which it entwines like a climbing plant. 21

The Main Inputs (Afferents ) to the Cerebellar Cortex (2) Mossy Fibers : (1) These , unlike Climbing Fibers ( which provide an indirect proprioceptive input ) do provide a direct proprioceptive pathway ( input ) to the cerebellar cortex , from all parts of the body , and , in addition (2) Provide inputs from the Motor Area ( M 1) & related areas of the Cerebral Cortex ( indirectly , via relays in the pontine nuclei ). They end on the dendrites of Granule cells in complex synaptic groupings called Glomeruli. The Glomeruli also contain the inhibitory endings of the Golgi cells. Climbing Fibers 2/25/2021 Mossy Fibers 22

Corollary ( summary ) of effects of different cells & afferents on Purkinje cells (A) Excitatory The fundamental circuits of cerebellar cortex are thus relatively simple : (1) climbing fiber inputs exert a strong excitatory effect on single Purkinje cells , whereas (2) Mossy fiber inputs exert a weak excitatory effect on many Purkinje cells via the Granule cells. (B) Inhibitory Both are excited by Parallel fibers of Granule (1) Basket cells , & their output inhibits Purkinje cells (2) Stellate cells ( Feed-Forward Inhibition ). (3) Golgi cells are excited by (1) Mossy fibers (2) Purkinje cells , & (3) Parallel fibers. They inhibit the action of Mossy fibers on Granule cells 2/25/2021 23

Q : What are the Neurotransmitters Secreted by in the Cerebellar Cortex Neurons ? Purkinje cells Basket cells Stellate cells Golgi cells Secrete GABA Granule cells Glutamate 2/25/2021 24

Summary of Connections & Functions Deep Nuclei Cortex Inputs Outputs Function Corticopontine/ pontocerebellar SCP to VA/VL In cooperation with cerebral cortex Planning , orogramming and execution of voluntary & skilled movements Neocerebellum Dentate Lateral portions of Cerebellar Hemisphere Spinocerebellum Interposed ; Fastigial Vermis & Spinal and Medial portions brainstem paths of Cerebellar hemispheres SCP to Red Nucleus; Fastigial to RF (1) Muscle tone (2) Postural adjustmen& (3) Coordination of different muscle groups activity for smooth performance of movement Vestibulocerebellum Fastigial Flocculo Nodular Lobe Vestibular nuclei; RF Balance , equilibrium & VOR 2/25/2021 Vestibular nuclei 25

Summary of Cerebellar Cortex Efferenrts Projections of Neocerebellar Cortex : The Neocerebellar Cortex projects ( sends its efferents ) to the Dentate Nucleus & from there to the Ventrolateral Nucleus of the Thalamus. Projections of Spinocerebellar Cortex : The medial part of the Spinocerebellum ( Vermis ) projects to the Fastigial Nuclei & from there to the brainstem. The hemispheric portions of the Spinocerebellum ( i. e. , medial parts of the cerebellar hemispheres ) project to the Emboliform and Globose nuclei & from there to the brainstem nuclei. Projections of the Vestibulocerebellar Cortex : Most of the Vestibulocerebellum cortex outputs ( efferents ) pass directly to the brainstem ( to regulate balance , equilibrium & the VOR ). However , unlike it , the rest of the cerebellar cortex ( i. e. , that of the Neocerebellum and Spinocerebellum ) sends fibers first to the deep cerebellar nuclei , and then , these in turn will project ( send their efferents ) to the brainstem. Consequently , the Deep Cerebellar Nuclei provide the only 2/25/2021 26

Summary The cerebellum influences movement on the ipsilateral side of the body. Neocerebellum is involved , in conjunction & close cooperation with the cerebral cortex , in programming , planning & execution of voluntary movement ; especially skilled movements ( which is mostly performed by the distal limb muscles ( e. g. , hand ) Spinocerebellum is primarily involved with (1) regulation & maintenance of muscle tone , (2) gross postural adjustments , & (3) coordination between different muscle groups for smoothening of movement ( it acts as a comparator ) (The vermis helps coordinate movements of the axial and proximal limb muscles ). Floculonodular lobe is mainly concerned with balance equilibrium as well as VOR. 2/25/2021 27

Contd Summary ……. . Climbing fibers originate in the inferior olivary nuclei and synapse directly onto Purkinje cells. All other sources of input to the cerebellum enter as mossy fibers. Purkinje cells are the main output neurons of the cerebellar cortex & project to the deep nuclei of the cerebellum. They are inhibitory to the DCN. The deep cerebellar nuclei ( DCN ) project out to brainstem and thalamic targets via the superior cerebellar peduncles. They are excitatory , but in turn , are themselves inhibited ( switched off ) by Purkinje cells. 2/25/2021 28

Cerebellar Syndromes 2/25/2021 29

Signs of Cerebellar Disorders Deficit Manifestation Ataxia Reeling, wide-based gait Decomposition of movement Inability to correctly sequence fine, coordinated acts Dysarthria Inability to articulate words correctly, with slurring and inappropriate phrasing Dysdiadochokinesia Inability to perform rapid alternating movements Dysmetria Inability to control range of movement Hypotonia Decreased muscle tone Nystagmus Involuntary, rapid oscillation of the eyeballs in a horizontal, vertical, or rotary direction, with the fast component maximal toward the side of the cerebellar lesion Scanning speech Slow enunciation with a tendency to hesitate at the beginning of a word or syllable Tremor Rhythmic, alternating, oscillatory movement of a limb as it approaches a target (intention tremor) or of proximal musculature when fixed posture or weight bearing is attempted (postural tremor)

The Difference Between Lesions of the Cerebellar Cortex & Lesions of DCN If only the cortex of the cerebellum is involved , the movement abnormalities gradually disappear as “ compensation ” occurs. However , lesions of the DCN produce more generalized defects , and abnormalities are permanent. For this reason , care should be taken to avoid damaging the DCN when surgery is undertaken to remove a tumor involving part of the cerebellar cortex. 2/25/2021 31

Cerebellar Hemispheric Lesions Cerebellar lesions cause no paralysis or sensory deficit. When not moving , there are no externally obvious signs. However , upon physical examination , signs such as hypotonia and pendular reflexes can be elicited. Once the patients attempts movement , ataxia appears. What is ataxia ? Ataxia is incoordination of due to errors in the rate , range , force and direction of movement. Wth circumscribed lesions , the ataxia may be confined/localized to only one part of 32 the body. 2/25/2021

A/ Hemispheric Lesions I/Ataxia (lack of coordination of muscle movement ) , which is manifested by (1) Wide-based , unsteady “ drunken , or staggering “ gait. (2) Scanning speech (3) Dysmetria ( also called Past-Pointing ) : attempting to touch an object with a finger results in overstretching to one side or the other this promptly initiates a gross correction action ( corrective action ) , but the correction overshoots to the other side Consequently , the finger oscillates back and forth. This oscillation is the (4) “ Intention Tremor ” , which is characteristic of cerebellar disease. This cerebellar tremor , unlike that of Parkinson’s 2/25/2021 33 disease , is absent at rest.

Hemispheric Lesions ( Contd ) II/ Inability to “ put on the brakes ” i. e. , inability to stop movement promptly. Normally , for example , flexion of the forearm against resistance is quickly checked when the resistance force is suddenly broken off. The patient with cerebellar disease can not break the movement of the limb , and the forearm flies back in a wide arc. This abnormal response is known as the “ Rebound Phenomenon ”. III/ Adiadochkinesia ( Dysdiadochkinesia ) : Inability to perform rapidly alternating opposite movements such as repeated pronation and supination of the hands. IV/ Difficulty in performing actions that involve simultaneous motions at more than one joint. The patient dissects such movements and carries them out one joint at a time , a phenomenon known as “ Decomposition of Movement ”. 2/25/2021 34

B/ Flocculonodular Lobe Lesions Midline cerebellar tumors in children , arising from the “ Nodule ” , early in their course (& before affecting the rest of the cerebellum) , damage first the Flocculonodular lobe. Such a child is afraid ( & reluctant ) to stand erect and move without support. This is because if he tries to walk , he does so in a staggering fashion on a broad base , & tends to fall. Moreover , selective Flocculonodular lobe lesions may cause vertigo 2/25/2021 35

Role of the Cerebellum in Learning (1) The cerebellum is concerned with learned adjustments that make coordination easier when a given task is performed over & over As a motor task is learned , activity in the brain shifts from the Prefrontal ( cerebral ) Cortex to the (1) Parietal Cortex , (2) M 1 , & (3) Cerebellum. The basis of learning in the cerebellum is the input via the Olivary Nucleus. It is worth noting , in this regard , that each Purkinje cell receives inputs from 250, 000 to 1, 000 Mossy fibers. By contrast , each Purkinje cell receives only a single ( only one ) Climbing fiber from the inferior olive , and this fiber makes 200 -3000 synapses on the Purkinje cell. Climbing fiber activation produces a large complex spike in the Purkinje cell This spike produces lomg-term modification of the pattern of Mossy fiber input to that particular Purkinje cell. Climbing fiber activity is increased when a new movement is being learned , and lesions of the Olivary Nucleus abolish the ability to produce long-term adjustments in motor responses. . 2/25/2021 36

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