Cerebellum

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General features of the cerebellum

'the small brain'

accounts for approx. 10% brain volume, but contains over 50% of brain's neurons

Functions of the cerebellum

Maintain equilibrium - through vestibular input

Adjust posture using muscles - through spinal input

Programming & fine-tuning movements - through cortical input - 'motor learning', typically done through trial & error (i.e. learning how to use a tennis racket)

Co-ordinates muscles - co-ordinates the timing & force of different muscle groups to produce smooth limb/body movements

**See internal anatomy, iv) evolutionary development

Cerebellum always receiving info from different sensory pathways & motor pathways

Incongruence between intended & actual movement - cerebellum will create error-correcting signal

This signal will be sent to pre-motor & primary motor cortices via brainstem & thalmus

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Surface anatomy

i) General features

Located in posterior cranial fossa

underneath the occipital & temporal lobes

covered superiorly by tentorium cerebelli

Falx cerebelli - posterior cerebellar notch - separates hemispheres

attaches to mid-brain by 3 peduncles

superior cerebellar peduncle

Middle cerebellar peduncle

Inferior cerebellar peduncle

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ii) Divisions

2 cerebellar hemispheres joined by a narrow median vermis

Uvulva = inferior portion of vermis on posterior lobe - located between tonsils

nodule = located superior to uvulva - touches against the roof of the 4th ventricle

* uvulva close to foramen magnum - herniation - could press on MO

3 lobes - Anterior, Posterior & Flocculonodular

Anterior lobe - superior surface

Posterior/Middle lobe - largest part - 'curls' around cerebellum

Flocculonodular lobe - Located on inferior surface, between tonsils & peduncles

3 fissures - primary, posterolateral & horizontal

Primary - separates the posterior & anterior lobes

Posterolateral - seperates the flocculonodular lobe from the posterior lobe

Horizontal - located along margin - seperates superior & inferior surfaces - not hugely sig.

See above images for locations

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Internal anatomy

i) Structure

Surface folds = folia (foliage)

Folia consist of outer covering gray matter (cerebellar cortex) & inner core of white matter - folia separated from one another by fissures

Arbor vitae = 'tree of life' - internal branches of cerebellum - white matter

Superior medullary velum = thin lamina stretched between superior medullary peduncles - forms roof of 4th ventricle

Inferior medullary velum = same as sup. except stretched between inferior medullary peduncles

ii) Gray matter

4 nuclei located on each side of mid-line (listed medial to lateral)

Fastigial nucleus

Globose nuclei

Embiloform nucleus

Dentate nucleus - largest nucleus - 'crumpled bag appearance' - visible to naked eye - WM fibres leave dentate nucleus through sup. cerebellar peduncle

(Interposed = embiloform + globose nuclei)

Afferent motor fibres relay in cerebellar nuclei

Forward Going Every Direction

Cerebral cortex composed of 3 layers:

Outer layer (Molecular layer)

2 types of neurons - outer stellate cell & inner basket cell - both inhibitory

contains glial cells

Middle layer (Purkinje layer)

Arranged in single cell layer

Contains large Golgi type I neurons

Dendrites of Purkinje cell (inhibitory efferent) branch up into molecular layer

Branches Purkinje axons synapse dendrites basket cells & stellate cells in Mol. layer

Purkinje dendrites also synapse with parallel fibres from granule cell axons, which pass down into granular layer

Passes through WM & synapses with cells in cerebellar nuclei - Efferent

Inner layer (Granular layer)

Interneurons of cerebellar cortex

Densely packed with unmyelinated axons (don't need to extend v far - low conduction)

Golgi (inner stellate cells) - inhibitory - activated by Mossy fibres - excited by parallel fibres - Golgi cells inhibit granule cells (Feedback inhibition)

Long axons of granule cells (excitatory afferent) called parallel fibres that reach up into Mol. layer - run horizontally along folia (@ right angle to dendritic processes of Purkinje cells)

Parallel fibres synapse with dendrites of purkinje cells

AFFERENT = ARRIVING

EFFERENT = EXITING

2 main lines afferent info to cerebellar cortex

Climbing fibres

Mossy fibres

Efferent info = Purkinje cells

Most end in deep cerebellar nuclei

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iii) White Matter

3 groups of WM fibres:

Intrinsic fibres - do not leave cerebellum - connect diff. cerebellar regions

Afferent fibres - enter through mid & inf cerebellar peduncles - go to cortex - make up most of WM

Efferent fibres - Purkinje cell axons - cerebellar nuclei - leave through sup cerebellar peduncle (some fibres don't synapse in nuclei)

iv) Evolutionary development

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Relationship to other parts of brain

i) Cerebellar peduncles

Superior cerebellar peduncle - links cerebellum with nuclei in midbrain - mostly efferent fibres from cerebellar nuclei

Middle cerebellar peduncle - links cerebellum with pons - largest peduncle - transverse pons fibres pass through

Inferior cerebellar peduncle - links cerebellum with medulla oblongata - mostly afferent fibres from muscles & spinal cord

Afferent & efferent fibres pass through them

See page 2 for picture of peduncles & pg.5 for peduncular tracts!

ii) Cerebellar pathways

Voluntary movements - primary motor cortex + basal nuclei send impulses through pons nuclei to cerebellum

Assessment of voluntary movements - Proprioceptors in skeletal muslces + joints report degree of movement to cerebellum

Integration & analysis - cerebellum compares planned movements (motor sig.) against results of actual movements (sensory sig.)

Corrective feedback - cerebellum sends impulses through thalamus to primary motor cortex & motor nuclei in brainstem

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Cerebellar syndromes

Flocculonodular lobe lesion (vestibulocerebellum)

Responsible for co-ordination of para-axial muscles (equilibrium)

Dysfunction = postural instability (truncal ataxia) - person has no control over axial muscles

person seperates feet upon standing (wider base to avoid falling forwards/backwards)

Reeling side to side / swaying

Often seen in children with medulloblastomas arising in roof of 4th ventricle

Abnormal Gait Exam : Ataxic Gait Demonstration (Einbetten)

Anterior lobe syndrome (spinocerebellum)

Mostly maintain co-ordination of limb movements

Commonly caused by malnutrition accompanying chronic alcoholism - damages Purkinje fibres

Loss of co-ordination of lower limbs - 'drunken' walk, stiff-legged stagger - gait ataxia

Sliding heel of one foot down shin of opposite leg = extremely difficult

Abnormal Coordination Exam ; Heel-to-shin (Einbetten)

Posterior lobe syndrome (Cerebrocerebellum)

Learning & storage of all sequential components of skilled movements (how to pick up a cup, how to ride a bike etc)

Caused by cerebrovascular accidents, tumors, trauma or degeneration

Loss of co-ordination of voluntary movements +decreased muscle tone

Person unable to direct limb to target without swaying to & fro - swaying perpendicular to direction of movement (intention tremor) - not present at rest, only when voluntary movement being performed

Dysmetria - patient over/undershoots when attempting to touch a target

Dysdiadochokinesia - patient unable to perform rapid alternating movements i.e. repetitive hand pronation & supination

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3 is the magic number!!

3 areas!

3 lobes!

3 fissures!

3 peduncles!

3 layers in cortex!

3 deep nuclei!

3 cerebellar syndromes!