Cerebellum – anatomy of cerebellum



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CEREBELLUM
1. ANATOMY OF CEREBELLUM

- consistent with function of providing coordinating signals for movement

- receives sensory inputs from many areas of body (as well as visual and vestibular sensory info)

- cortex associated with movement projects to cerebellum

- output of cerebellum projects to motor areas of cerebral cortex (via thalamus) and brainstem
- cerebellum has convoluted outer cortex of gray matter covering inner core of white matter; it is folded over upon itself so superior and inferior ends meet in the roof the 4th ventricle

- Three pairs of deep cerebellar nuclei: dentate nuclei, interposed (globose and emboliform), and fastigial nuclei (from lateral to medial as seen in caudal pons)

- Cerebellar cortex  projects to deep cerebellar nuclei  cerebellar outputs to CNS

- inputs and outputs of cerebellum travel via superior, middle, and inferior peduncles

- more fibers enters cerebellum than leave it; input: output ratio is 40:1; cerebellum integrates
- afferents travels in middle cerebellar peduncle and come from the cerebral cortex via contralateral pontine nuclei (in basal pons, crossed pontocerebellar fibers)

- other afferents from inferior olivary nucleus (crossed) and direct spinocerebellar tracts found in inferior cerebellar peduncle (with visual and vestibular inputs)

- efferent fibers travel through superior cerebellar peduncle, decussate, and terminate in the thalamus, VA/VL, (to motor cortex), red nucleus, and other brainstem sites
Two subdivision schemes

1. Anatomically divided into 3 lobes: anterior and posterior divided by primary fissure; flocculonodular lobe separated by posterior lobe by another deep fissure – vestibular nuclei project here
2. Based on behavioral observations from ablation studies and on anatomical connections; each half of cerebellum divided into longitudinal zones, each associated with deep cerebellar nucleus:

Lateral zone – lateral hemispheres of cerebellar cortex  dentate nucleus

Intermediate (paravermal) zone – lateral to vermal zone, medial to lateral zone  interposed nucleus

Vermal zone – in midline  fastigial nucleus; fastigial nucleus projects to vestibular nuclei via inferior cerebellar peduncle
- somatosensory map: head/trunk occupy midline of anterior lobe; limbs occupy intermediate zones

- lesions of midline/vermal zone  abnormalities of gait and posture (limbs not involved)

- lesions of intermediate/lateral zones  abnormalities of ipsilateral limb movement

1, 4.





Medial zone (vermis)

Intermediate zone (paravermal zone)

Lateral Zone (hemispheres)

Deep cerebellar nucleus projected to

Fastigial

Interposed (globose and emboliform)

Dentate

Afferents/inputs – MCP/ICP

Spinal cord (trunk), brainstem (vestibular nuclei)

Spinal cord (limbs), brainstem, and motor cortex via pons

Cerebral cortex, association cortex via pons, inferior olive

Efferents/outputs - SCP

Brainstem

Motor cortex, red nucleus, brainstem, thalamus (VA/VL)

Thalamus, brainstem, premotor cortex via thalamus (VA/VL)

Timing of changes in neuronal firing

After motor cortex, after movement onset

After motor cortex, after movement onset

Before motor cortex, before movement onset

Putative functions

Modify posture and balance during movement

Modify movement as it progresses, limb feedback

Planning of skilled volitional movement, especially the limbs

Clinical signs if lesion

Gait and trunk ataxia (incoordination)

Limb ataxia during volitional movement, dysmetria (defective correction of movement)

Limb ataxia during volitional movement (defective planning of movement)

2. Cerebellar dysfunction

- syndromes involve abnormalities of movement (little or no weakness); cerebellar syndromes associated with disorders of speed and spatial accuracy of movement; uncoordinated movements such as dysmetria (mistakes in extent of movement), ataxia, tremor, trouble with alternating movement

- fundamental abnormality is timing of muscle activities  errors in speed and spatial accuracy


3. Microscopic structure of cerebellar cortex

- cortex made up of regular folds called folia



- molecular layer (sparse nerve cell nuclei, heterochromatin), purkinje layer (large multipolar cells/euchromatin with axon to granular layer and white matter), granular layer (dense nerve cells with dark nuclei, heterochromatin)

- Parallel to length of folia are unmyelinated axons called parallel fibers, which synapse on dendrites of Purkinje cells, excitatory (arranged in planes at right angles to long axis of folium)

- Purkinje cells are sole output neurons of cerebellar cortex and synapse on deep cerebellar nuclei

- input to cerebellum arrives as climbing fibers (originating in contralateral inferior olivary nucleus) and mossy fibers (originating everywhere else)

- mossy fibers are excitatory  terminate on granule cells in cortex  form parallel fibers

- mossy and climbing fibers send collaterals to deep cerebellar nuclei (excitatory, glutamate)

- input from mossy and climbing fibers causes increased excitation in deep cerebellar nuclei, keeping them continuously active

- mossy fibers (via parallel fibers from granule cells) and climbing fibers input excites Purkinje cells  synapse on deep cerebellar nuclei (inhibitory, GABA)  pauses of activity out of the continuosly active deep cerebellar nuclei


4. Look at table above
Lateral/Intermediate zones – regulate timing of movement by 1) planning timing in advance of movement and 2) modifying the plan as it unfolds
Planning-in-advance : projections from cerebellum to cerebral cortex (via thalamus) conveying info regarding timing of muscle; deficit in this function  difficulty initiating and stopping muscular contractions (inappropriate timing)

hypotonia – decreased resistance to passive movement; pendular reflexes – undamped response to tendon taps which causes limb to oscillate in a pendular notion
Modify plan – instructions from motor cortex amended before the movement is finished; errors:

Dysmetria - mistakes in extent of movement; ataxia; errors in control of timing of muscle activities; movement is inaccurate in extent and repeated efforts to reprogram result in oscillations or tremor; dysarthria – errors in speech (prosody and intonation)
Vermal cortex

- errors in postural adjustments; gait ataxia, staggering

- midline zone specialized to provide coordination of posture with movements; input for periphery

- neuronal activity in fastigial nucleus occurs after onset of the movement  vermis helps to adjust movements as they occur  affects proximal muscles


Flocculonodular lobe

- extension of vestibular system; principal connections with vestibular nuclei

- maintenance of equilibrium; lesions  disequilibrium and vertigo; nystagmus

- helps coordinate slow eye movements during smooth tracking of an object

- receives input regarding head and eye movement as well as indirect visual information
Motor learning

- normal adaptation of vestibuloocular reflex after prism glasses; if flocculonodular lobe is lesioned  no adaptation

- normal eye blink reflex in response to puff of air (conditioned by bell); interposed nucleus is lesioned  conditioned response (blink to sound stimulus) was lost even though unconditioned response (blink to air puff) remained

- Cerebellar patients fall, not because they make insufficient responses, but because they fail to adapt (learn)  incorrect responses that throw them off balance

- cognitive or behavioral abnormalities; lateral cerebellum implicated in cognitive learning because it is connected with association cortex; midline cerebellum implicated in higher order autonomic and affective functions, because it is connected with limbic cortex and associated structures

- dramatic mood swings, giddiness



5. Identify: cerebellar vermis and hemispheres, flocculus, nodulus, inferior, middle, superior cerebellar peduncles, dentate, interposed, fastigial nuclei, inferior olivary nucleus


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