Cerebellum

Question 1

The subcortical motor controllers

Answer 1

• basal ganglia and cerebellum (regulate planning and execution of voluntary movement)
• both receive cotical inputs a both project via thalamus to motor cortex
• both needed for somooth movement and posture
• lesions - positive and negative motor symptoms

Question 2

The cerebellum

Answer 2

• 10% of brain volume but has more than half of the brain’s neurons
• CBM has expanded during our evolution - suggests it has done a good job
• 40x more input axons than outputs
• most outputs project to motor areas
• but NOT to the spinal cord (doesnt wire to the alphaMN in the spinal cord at all, goes to descending nuclei)

Question 3

Cerebellar lesions

Answer 3

• give great difficulty in performing tasks
• affect ipsilateral side
• dont lead to weakness - just clumsiness
• hypotonia (low muscle tone and uncoordinated contraction)
• postural ataxia (cant keep stable posture)
• intention (action) tremor - overshhot, oscillation of voluntary movement

Question 4

What is the function of the CBM?

Answer 4

• improves performance in the future - motor learning

- rapid on-line refinement of a rapid movement

Question 5

CBM as a feedback comparator

Answer 5

• Cortex sends out a set of commands - causes muscles to contract and sensory receptors are activated
• if you make a clumsy movement, the sensory receptors will send a signal back to the brain via CBM giving feedback
• CBM sends a signal to cerebral cortex next time to improve the movements
• way too slow for online control where you need to be able to change movements instantly - even if you have never dealt with it before

Question 6

Feed-forward control

Answer 6

• There is time for this to happen in ballistic movements
• as the cerebral cortex sends a signal for movement, collateral fibres get sent to the cerebellum
• CBM has a blueprint of the body so it can send feedback, telling the motor cortex how a movement will go before it even happens

Question 7

What are the three anatomical/functional domains?

Answer 7

• spino-cerebellum (central zone) = modulates descending motor systems in brainstem via reticular formation and vestibular nuclei
• Vestibulo-cerebellum = regulates balance and eye movements via vestibular nuclei
• cerebro-cerebellum = high level planning of movement, regulates cortical motor programs via thalamus

Question 8

Cerebro-cerebellar pathways

Answer 8

• commands come down from motor area
• get sent to spinal cord, but a branch sends a copy to cerebellum
• Signal goes down to the CBM through the pontine nucleus
• feedback then comes out from the deep cerebellar nuclei to the brain stem, via the thalamus and back up to the cortex

Question 9

Somatotopic map

Answer 9

• we have a very crude homunculus of the body that we can somatotopically map onto the cerebellum

Question 10

Cellular structure

Answer 10

• highly regular structure
• purkinje cells - huge neurons
• purkinje cells (inhibitory) project to the deep cerebellar nuclei (excitatory) - project out via thalamic relays to the cortex
• granule cells - very numerous

Question 11

Purkinje neurons

Answer 11

• cell body is very big
• tree-like growth of dendrites - very narrow tree (like a plate)
• purkinjes project down to deep nuclei
• lined up in layers
• there are two influences, parallel fibres and climbing fibres

Question 12

Climbing fibres

Answer 12

• climbing fibres come from ccx and go via inferior olive
• each purkinje cell has one climbing fibre that wraps around it
• each climbing fibre wraps around about 10 purkinjes
• if climbing fibre fires an AP, so does the purkinje cell

Question 13

Parallel fibres

Answer 13

• Mossy fibres from the pons/brainstem nuclei synapse onto the granule cells - these produce the parallel fibres
  = these go through the plates of dendrites of the purkinje cells at a 90 degree angle
• each fibre come from a different granule neurone
• each one forms a synapse as it goes through
• there are about 1 million parallel fibres per purkinje, and each parallel fibre will synapse onto many purkinje cells

Question 14

Cerebellar learning

Answer 14

• as the motor cortex sends out a set of commands to make a movement, the commands are copied via mossy fibres to the granule cells
• carry the movements out to the purkinje dendrites
• the climbing fibre receives input from the rewards system
• when you make a good movement, the reward system fires
• the climbing fibres carry APs and so the purkinje cells that were activated at the time of the good movement, will get a stimulus
• all the synapses active at the time will get reinforced > synaptic strengthening
• the ones fired when you do a clumsy movement will get weaker and eventually die away

Question 15

Summary

Answer 15

• motor learning - when the movement is repeated, CBM gives corrective feedback
• feed-forward comparator - CBM refines a rapid movement on-line