Cerebellar microarchitecture

Cerebellar microarchitecture

1. Cerebellar function

   • Early clues
     • Cerebellar damage does not cause paralysis, but makes many movements inaccurate, slow and uncoordinated (similar to effects of alcohol)
       • No abnormal tremor at rest but dramatic intention tremor (inverse effect to Parkinson‘s disease)
   • Influential suggestion
     • Appears that other parts of the brain issue movement commands; the role of cerebellum is to ensure they are carried out properly
     • Brindley (1964) suggested in an abstract that the purpose of the cerebellum is to learn motor skills, so that when they have been learned a simple or incomplete message from the cerebrum will suffice to provoke their execution.
     • Related to ideas of user-friendliness; automaticity; freeing up cerebral cortex

2. Cerebellar orginisation

   • Two parts
     • Extensive cerebellar cortex
     • Compact deep nuclei = cerebellar output stage
       • Exception: Vestibular nuclei = output of Vestibulocerebellum
   • Vestibulocerebellum
     • (oldest part, appearing in fish)
     • balance, eye movement
     • Vestibular input, projects to lateral vestibular nuclei
   • Spinocerebellum
     • Motor execution
     • Spinal cord somatosensory inputs
       • Vermis: visual, auditory, vestibular input
   • Cerebrocerebellum
     • Well developed in primates
     • Motor planning
     • Exclusive input from cerebral cortex

3. Structure of cortex

   • Granule cell layer
     • 50 billion granule cells (> 50% neurons in entire brain) + Golgi cells
   • Purkinje cell layer
     • Only 1 cell thick
   • Molecular layer
     • Axons of granule cells
     • Dendrites of purkinje cells and interneurons

4. Purkinje cells

   • Largest cells in cerebellar cortex
   • Distinctive dendritic field - flattened out like a fan
   • Sole output cells of cerebellar cortex
   • Spiking activity
     • Simple spikes
       • spontaneous firing rates ~50 Hz
     • Complex spikes
       • fire at low frequencies ~1 Hz
       • lead to strong Ca2+ influx!

5. Cerebellar circuitry

   • Output
     • Purkinje cells inhibit cells in cerebellar nuclei (or vestibular)
   • Input
     • Basic information flow through cortex is simple
     • 2 Inputs
       • Mossy fibres excite granule and Golgi cells
       • Climbing fibres excite Purkinje cells
   • Granule cells
     • Granule cells form axons (parallel fibres) that excite all cell types
   • Golgi cells
     • Project back to the synapses between mossy fibres and granule cells
   • Stellate and basket cells
     • Both inhibitory
     • Both get input from parallel fibres
     • Basket cells synapse with Purkinje cell BODY
     • Stellate cells synapse with Purkinje cell DENDRITES

6. Synaptic Plasticity in the cerebellum

   • LTD in cerebellum was found first! LTP only recent discovery
   • PF + CF activation > large Ca2+ influx > LTD
   • PF activation > less Ca2 influx > LTP

7. Control system

   • Feed-forward
     • Requires experience in order to learn the appropriate actions
     • Better for fast (movement) tasks
   • Feed-back
     • Accurate control, but slow (due to delay)!
     • Feed-back control is not effective for fast (movement) tasks

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