The Cerebellum

Question 1

Which 2 subcortical motor controllers are required for smooth movement and posture?

Answer 1

Basal ganglia and cerebellum

Question 2

The cerebellum contributes 10% to total brain volume. What proportion of all neurons in the brain does it contain?

Answer 2

More than 50%

Question 3

Name the classic triad of symptoms for a cerebellar lesion.

Answer 3

1. Hypotonia - low muscle tone, uncoordinated muscle contraction
2. Postural ataxia - can’t keep stable posture, especially with eyes closed
3. Intention (action) tremor - overshoot, oscillation of voluntary movements

Question 4

What are the 2 main functions of the cerebellum?

Answer 4

1. Improves future performance - motor learning
2. Rapid on-line refinement of a “ballistic” (rapid, explosive) movement

Question 5

How does the cerebellum act as a “feed-forward comparator”?

Answer 5

• Cerebellum receives collateral projections from the motor cortex
• It has an internal model of the body for on-line control
• Cerebellar loop - receives from cortex and feeds “forward” to cortex before motor activity is executed

Question 6

Describe the basic characteristics of the 3 anatomical/functional domains of the cerebellum.

Answer 6

• Spino-cerebellum - modulates descending motor systems in brainstem (via reticular formation)
• Vestibulo-cerebellum - regulates balance and eye movements (via vestibular nuclei)
• Cerebro-cerebellum - high level planning of movement; regulates cortical motor programs (via thalamus to cortex)

Question 7

Describe the characteristics of the Purkinje and granule cells of the cerebellum.

Answer 7

Purkinje cells -

• Huge output neurons
• GABAergic, inhibitory
• Project to deep cerebellar nuclei, thalamus (and hence cortex), reticular formation, vestibular nucleus
• Deep cerebellar nuclei contain excitatory glutamatergic neurons

Granule cells:

• Very numerous, approx 10¹¹

Question 8

Name the 2 excitatory inputs to the cerebellar Purkinje fibres and explain the interaction between these cells.

Answer 8

Climbing fibres:

• Cerebral cortex > inferior olive > cerebellum
• Approx 10 Purkinje fibres to 1 climbing fibre
• 1 climbing fibre synapses many times with each Purkinje cell - huge spatial summation, meaning Purkinje cell invariably excited

Parallel fibres:

• Mossy fibres from pons/brainstem nuclei > parallel fibres from granule cells
• 1 million parallel fibres per Purkinje cell, many Purkinje cells per parallel fibre
• Each parallel fibre forms very few synapses with the Purkinje cell, but many parallel fibres synapse with the same cell

Question 9

What happens to the cerebellar parallel fibres and Purkinje cells when a motor command is successful?

Answer 9

• Synapses between parallel fibres and Purkinje cells are strengthened
• Repetition of successful movement causes long-term synaptic potentiation

Question 10

Outline how the cerebellum contributes to motor learning and acts as a feed-forward comparator.

Answer 10

• Motor learning - when movement is repeated, CBM gives corrective feedback
• Feed-forward comparator - CBM refines a rapid movement on-line