cerebellum

• centre of motor coordination

• cerebellar disorders cause –incoordination or ataxia

structure

• Cerebellum is divided into 3 lobes by 2 transverse fissures– anterior lobe– posterior lobe– flocculonodular lobe

structure

– anterior lobe (paleocerebellum)– large posterior lobe (neocerebellum)– flocculonodular lobe (archicerebellum is the oldest

lobe)

• Anterior cerebellum and part of posterior cerebellum

– receives information from the spinal cord

• Rest of the posterior cerebellum – receives information from the cortex

• Flocculonodular lobe – involved in controlling the balance through vestibular

apparatus

• Functionally cerebellum is divided into 3 areas medial to lateral– lateral zone– intermediate zone– vermis

• lateral zone– this is concerned with overall planning of sequence

and timing

• intermediate zone– control muscles of upper and lower limbs distally

• vermis– controls muscles of axial body, neck, hip

Inputs

• corticopontocerebellar• from motor and premotor cortex (also sensory cortex)

• these tracts supplies the contralateral cerebellar cortex

• olivocerebellar• from inferior olive

– excited by fibres from

» motor cx

» basal ganglia

» reticular formation

» spinal cord

Inputs (cont’d)

• vestibulocerebellar• to the flocculonodular lobe

• reticulocerebellar• to the vermis

• spinocerebellar tracts– dorsal spinocerebellar tracts

• from muscle spindle, prorpioceptive mechanoreceptor (feedback information)

– ventral spinocerebellar tarcts• from anterior horn cell

– excited by motor signals arriving through descending tracts (efference copy)

Outputs

• through deep cerebellar nuclei: dentate, fastigial, interpositus– 1. vermis -> fastigial nucleus -> medulla, pons– 2. intermediate zone

-> nucleus interpositus-> thalamus -> cortex

-> basal ganglia-> red nucleus

-> reticular formation– 3. lateral zone -> dentate nucleus

-> thalamus -> cortex

Outputs

• Functions

– 1. vermis -> fastigial nucleus -> medulla, pons– control balance and equilibrium with the

vestibular pathways

Outputs

• Functions

– 2. intermediate zone -> nucleus interpositus

-> thalamus -> cortex-> basal ganglia

-> red nucleus -> reticular formation– coordinate reciprocal contractions of agonist &

antagonist muscles in limbs

Outputs

• Functions

– 3. lateral zone -> dentate nucleus -> thalamus -> cortex

– coordinate sequential motor activities initiated by the cerebral cortex

Neuronal circuitry of the cerebellum

• Main cortical cells in cerebellum are known as Purkinje Cells (large cells).

• There are about 30 million such cells.

• These cells constitute a unit which repeats along the cerebellar cortex.

• Somatotopic representation of the body is present in cerebellar cortex although it is not as clear as cerebral cortex.

Topographical representationvermis

intermediatezone

Functional unit of the cerebellar cortex

• a Purkinje cell

• a deep nuclear cell

• inputs

• output from the deep nuclear cell

Purkinje cell

Inputfrom Inferiorolive

Inputfrom otherafferents

Climbingfibre

Mossy fibre

Granule cells

Deep nuclearcell

Output

excitationexcitation

inhibition

OutputInputsInputs

• Purkinje cells & deep nuclear cells fire continuously

• afferents excite the deep nuclear cells

• Purkinje cells inhibit the deep nuclear cells

Functions of cerebellum

• planning of movements• timing & sequencing of movements• particularly during rapid movments such as

during walking, running• from the peripheral feedback & motor cortical

impulses, cerebellum calculates when does a movement should begin and stop

Motor Cortex

ThalamusCerebellum

Muscles

brain stem nuclei

proprioceptivetactilefeedback

‘Error correction’• cerebellum receives two types of information

– intended plan of movement• direct information from the motor cortex

– what actual movements result• feedback from periphery

– these two are compared: an error is calculated

– corrective output signals goes to• motor cortex via thalamus• brain stem nuclei and then down to the anterior horn cell through extrapyramidal

tracts

• ‘Prevention of overshoot’– Soon after a movement has been initiated– cerebellum send signals to stop the movement at the

intended point (otherwise overshooting occurs)

• Ballistic movements– rapid movements of the body, eg. finger movements during

typing, rapid eye movements (saccadic eye movements)– movements are so rapid it is difficult to decide on feedback– therefore the movement is preplanned

planning of movements

• mainly performed by lateral zones

• sequencing & timing– lateral zones communicate with premotor areas,

sensory cortex & basal ganglia to receive the plan

– next sequential movement is planned– predicting the timings of each movement

features of cerebellar disorders

• ataxia – incoordination of movements– ataxic gait

• broad based gait• leaning towards side of the lesion

• dysmetria– cannot plan movements

• past pointing & overshoot• decomposition of movements• intentional tremor

features of cerebellar disorders

• dysdiadochokinesis– unable to perform rapidly alternating

movements

• dysarthria– slurring of speech

• nystagmus– oscillatory movements of the eye

features of cerebellar disorders

• hypotonia– reduction in tone

• due to excitatory influence on gamma motor neurons by cerebellum (through vestibulospinal tracts)

• decreased reflexes

• head tremor

• head tilt