Cerebellum

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Introduction

The cerebellum represents only 10% of the CNS by volume, but contains roughly 50% of its neurons.

The cerebellum is located behind the pons and medulla, and is separated from the occipital lobe by the dural tentorium.

The cerebellar cortex is gray matter and covers the white matter. Four nuclei lie within the white matter.

The cerebellum lies behind the 4th ventricle, and swelling within the cerebellum (from masses or edema following infarct) can lead to obstructive hydrocephalus.

The cerebellum is divided into two symmetric hemispheres.

 

White matter is derived from peduncles (tracts) superior, middle, inferior

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Functions

The cerebellum has several main functions. It receives an exceedingly large number of inputs from essentially all types of receptors and processes them before sending signals back to the body. Input connections exceed output connections roughly 40:1.

 

Skilled movement coordination occurs through the cerebellum's influence on muscle activity, tone, and equilibrium, through connections with the vestibular system and gamma motor neurons. The cerebellum is ipsilaterally somatotopically organized.

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Peduncles

The cerebellum is attached to the brainstem through three pairs of peduncles, through which fibres enter and leave.

Afferent fibres enter mostly through the inferior and middle peduncles, and end mostly in climbing or mossy fibres in the cortex. Both fibre types are excitatory.

 

Inferior cerebellar peduncle

• largely afferent
• connects the medulla with the cerebellum
• contains many fibre systems from the spinal cord
• sensory nuclei, especially trigeminal
• restiform body receives olivocerebellar fibres and posterior spinocerebellar tract
• juxtarestiform body: incoming fibres from vestibular fibres, efferents involved with head and eye movements

Middle cerebellar peduncle

• largest, almost exclusively afferent
• consists of fibres from the contralateral pontine nuclei, which receive information from corticopontine fibres originating across the cortex.

 

Superior cerebellar peduncle

• composed mostly of efferent fibres of information processed in the cerebellum
• wide outputs: to thalamus and spinal cord, with some relays in the red nuclei; effect on both UMN and LMNs
• afferent fibres enter from the anterior spinocerebellar tract, mesencephalic nucleus of V (face proprioreception), locus ceruleus

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Cerebellar Processing

 

vestibulocerebellum

• coordinate equilibrium and eye movements
• input: vestibular nuclei and inferior olivary nucleus
• consists of cortical floculonodular lobe and uvula of vermis; fastigial nucleus
• outputs through ICP go to vestibular nuclei, which mediate lower motor neurons via the MLF and lateral vestibulospinal tract
• other outputs to thalamus project to upper motor neurons in the motor cortex
• problems here (rare) affect
  • balance, resulting in a wide-spaced gait
  • eye movements (nystagmus)
  • limb movements spared

 

spinocerebellum

• most of the rest of the vermis and adjacent cortex, and globose and emboliform nuclei
• input: anterior and posterior spinocerebellar tracts (muscle stretch receptors), CN V nuclei, inferior olivary nucleus
• involved in posture, gait, and muscle tone
• output via SCP: red nucleus, reticular formation, which project to LMNs via brain stem and spinal cord
• output to thalamus: upper motor neuron signaling in motor cortex
• problems cause:
  • rigidity
  • increased reflexes
  • difficulty with coordinated locomotion (cerebellar ataxia)

 

pontocerebellum

• lateral cerebellar cortex and dentate nucleus
• input: sensoriotor cortex, via pontine nuclei and inferior olivary nuclei
• outputs to: thalamus
• coordinating and synchronyzing complex, learned movements
• problems cause:
  • hypotonia
  • decreased reflexes
  • intention tremor
  • dysmetria (no knowledge of where limbs are in space)
  • lack of rapid alternating movements (dysdiadochenesis) (sp)
  • people know what they want to do, but just don't know how to

 

 

 

Gray Matter Organization

Purkinje cells are the only output; rest of layers are involved in its modulation.

Climbing fibres from olive form a strong excitatory synapse with Purkinje cells.

Mossy fibres also synapse with Purkinje cells and can be excitatory or inhibitory.

 

Molecular Layer

• two cell types: stellate and basket cells
• also contains dendrites of Purkinje cells

Purkinje Cell Layer

 

 

Granular Layer

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Outputs

The cerebellar nuclei are the predominant output of the cerebellum. Output from cortex is determined solely by Purkinje cells, overwhelmingly GABA-erbic (inhibitory) inhibition of nuclei, which are the predominant

spinal, brainstem inputs to nuclei are excitatory.

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Resources and References

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