Cerebellar microarchitecture

Nota:

• See lecture for examples and summary

1. Cerebellar function
   1. Early clues
      1. Cerebellar damage does not cause paralysis, but makes many movements inaccurate, slow and uncoordinated (similar to effects of alcohol)
         1. No abnormal tremor at rest but dramatic intention tremor (inverse effect to Parkinson‘s disease)
   2. Influential suggestion
      1. Appears that other parts of the brain issue movement commands; the role of cerebellum is to ensure they are carried out properly
      2. 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.
      3. Related to ideas of user-friendliness; automaticity; freeing up cerebral cortex
2. Cerebellar orginisation
   1. Two parts
      1. Extensive cerebellar cortex
      2. Compact deep nuclei = cerebellar output stage
         1. Exception: Vestibular nuclei = output of Vestibulocerebellum
   2. Vestibulocerebellum
      1. (oldest part, appearing in fish)
      2. balance, eye movement
      3. Vestibular input, projects to lateral vestibular nuclei
   3. Spinocerebellum
      1. Motor execution
      2. Spinal cord somatosensory inputs
         1. Vermis: visual, auditory, vestibular input
   4. Cerebrocerebellum
      1. Well developed in primates
      2. Motor planning
      3. Exclusive input from cerebral cortex
3. Structure of cortex
   1. Granule cell layer
      1. 50 billion granule cells (> 50% neurons in entire brain) + Golgi cells
   2. Purkinje cell layer
      1. Only 1 cell thick
   3. Molecular layer
      1. Axons of granule cells
      2. Dendrites of purkinje cells and interneurons
4. Purkinje cells
   1. Largest cells in cerebellar cortex
   2. Distinctive dendritic field - flattened out like a fan
   3. Sole output cells of cerebellar cortex
   4. Spiking activity
      1. Simple spikes
         1. spontaneous firing rates ~50 Hz
      2. Complex spikes
         1. fire at low frequencies ~1 Hz
         2. lead to strong Ca2+ influx!
5. Cerebellar circuitry
   1. Output
      1. Purkinje cells inhibit cells in cerebellar nuclei (or vestibular)
   2. Input
      1. Basic information flow through cortex is simple
      2. 2 Inputs
         1. Mossy fibres excite granule and Golgi cells
         2. Climbing fibres excite Purkinje cells
   3. Granule cells
      1. Granule cells form axons (parallel fibres) that excite all cell types
   4. Golgi cells
      1. Project back to the synapses between mossy fibres and granule cells
   5. Stellate and basket cells
      1. Both inhibitory
      2. Both get input from parallel fibres
      3. Basket cells synapse with Purkinje cell BODY
      4. Stellate cells synapse with Purkinje cell DENDRITES
6. Synaptic Plasticity in the cerebellum
   1. LTD in cerebellum was found first! LTP only recent discovery
   2. PF + CF activation > large Ca2+ influx > LTD
   3. PF activation > less Ca2 influx > LTP
7. Control system
   1. Feed-forward
      1. Requires experience in order to learn the appropriate actions
      2. Better for fast (movement) tasks
   2. Feed-back
      1. Accurate control, but slow (due to delay)!
      2. Feed-back control is not effective for fast (movement) tasks