Cerebellum

Question 1

Function and anatomical location of vestibulocerebellum

Answer 1

• Orientation of head and body (balance/posture), eye movements
• Flocculonodular lobe

Question 2

Function and anatomical location of the spinocerebellum

Answer 2

• Control of axial and limb musculature
  
  • Motor execution
• Vermis and paravermis

Question 3

Function and anatomical location of cerebrocerebellum

Answer 3

• Planning and timing of precise movements
• Lateral parts of the hemispheres

Question 4

Orientation of head and body (balance/posture), eye movements

Answer 4

• Function of the vestibulocerebellum
• Flocculonodular lobe

Question 5

• Control of axial and limb musculature
  
  • Motor execution

Answer 5

• Function of the spinocerebellum
• Vermis and paravermis

Question 6

• Planning and timing of precise movements

Answer 6

• Cerebrocerebellum
• Lateral parts of the hemispheres

Question 7

What are three gross functional subdivisions of the cerebellum? What are their roles?

Answer 7

• Vestibulocerebellum
  
  • Flocculonodular lobe
  • Regulates balance and eye movements
• Spinocerebellum
  
  • ​Vermis and paravermis
  • Regulates posture and locomotion
• Cerebrocerebellum
  
  • Lateral parts of the hemispheres
  • Planning and timing of precise and complex movements

Question 8

Describe the inputs of the vestibulocerebellum

Answer 8

• The vestibulocerebellum to the flocculonodular lobe from direct and indirect routes
• Direct route
  
  • Transmitted via primary sensory afferents directly from the semi-circular canals
• ​Indirect route
  
  • ​Transmitted via secondary sensory afferents from the vestibular nuclei of the medulla

Question 9

Describe the outputs of the vestibulocerebellum

Answer 9

• Vestibular purkinje cell output is delivered either to back & neck muscles or to extraocular muscles
• Back & neck muscles
  
  • Vestibular nucleus
    
    • ​Direct pathway directly to the vestibular nucleus
    • Indirect pathway via fastigial nucleus
• ​​Extraocular muscles
  
  • Via oculomotor nuclei to extraocular muscles

Question 10

Describe the inputs to the spinocerebellum

Answer 10

• Three differnet inputs SIS
  
  • Somatosensory & motor cortex
  • Inferior olivary nucleus
  • Spinocerebellar tract
    
    • Conveys somatosensory information importantly proprioception from the neck, trunk and limb muscles

Question 11

Describe the outputs of the spinocerebellum

Answer 11

• Vermis sends information to axial muscles (neck and trunk)
  
  • Via thalamic relay
  • Along corticospinal, reticulospinal and vestibulospinal tracts
• Paravermis sends signals to control limbs
  
  • Via interposed nucleus to the descending corticospinal and rubrospinal tracts in the brainstem
    
    • Rubrospinal tract is responsible for some involuntary movement
      
      • large muscle movement regulation flexor
      • inhibiting extensor tone
      • fine motor control.

Question 12

Describe the inputs to the cerebrocerebellum

Answer 12

• Receives input from the cortex which is transmitted along corticopontine fibres to pontine nuclei that input to the cerbellum via the middle cerebellar peduncle

Question 13

Describe the outputs of the cerebrocerebellum

Answer 13

• Transmitted from the dentate nucleus and ascends to the ventrolateral thalamus and then to the prefrontal and motor cortex

Question 14

List the two types of cerebellar input

Answer 14

• Mossy fibre system
• Climbing fibre system ​

Question 15

Explain mossy fibres

Answer 15

• They stimulate Purkinje cells indirectly through the granule-cell parallel fibre pathway
• As mentioned above, these parallel fibres form excitatory glutamatergic synapses with purkinje cell dendrites and provide the major input that dictates purkinje cell output.
• Through the summation of around 200 parallel fibres onto a single purkinje cell, they can induce conventional action potentials known as simple spikes.
• These simple spikes occur at the rate of approximately 40 Hz.

Question 16

Explain climbing fibres

Answer 16

• Climbing fibres are the axons of neurons in the inferior olivary nucleus of the medulla
• Upon detection of a mismatch between cerebellar output and ascending sensory systems, the inferior olivary nucleus stimulates a large EPSP in the purkinje cell by generating a prolonged Ca²⁺-dependent action potential pattern known as the complex spike

Question 17

Outine an expeiremtn which shows that learned adaptation in Purkinje cell output is facilitated through error detection by climbing fibres

Answer 17

• In 1977, Gilbert et al conducted an experiment in which monkeys were trained to grasp a handle and move it in a horizontal arc to a central position by flexing or extending the wrist.
  
  • A torque motor applied forces to the handle that switched at random intervals to alternately load flexor and extensor muscles
  • At each load switch, the handle was displaced transiently from the central position and then moved back by the monkeys and held there steadily again
  • Recordings were made from cerebellar Purkinje cells whose simple spike activity was related to the task
  • The magnitude of one of the oppositely directed load was then altered and the monkeys took about 12 to 100 trials with the novel load before performing as regularly as before
  • As the new motor task was being learned, there was an increased frequency of complex spikes and gradual decrease in simple spikes as shown in the diagram .
  • Once the task was learned, the frequency of complex spikes (from CF) returns to normal and the frequency of simple spikes decreased (from MF)

Question 18

Explain how long-term depression is thought to underlie motor adaptations

Answer 18

• Long-term depression is defined as a long-lasting decrease in the amplitude of an excitatory postsynaptic potential (EPSP) elicited by a single presynaptic action potential.
  
  • This is thought to be induced by climbing fibres
  • Climbing fibres deliver the motor error signal to the cerebellum via the inferior olivary complex that receives information from both the sensory systems (proprioceptive, visual, vestibular) and cerebellar output
  • This then feeds into the climbing fibre input system and provides feedback information about the state of movement.
  • Climbing fibre activity stimulates long-term depression which decreases the sensitivity of the purkinje cell to the parallel fibres which stimulated them.
  • This makes them less responsive to the error that mediated the wrong motor output.
• Because of this motor adaptation, it could be argued that tumours developing the cerebellum often present motor symptoms very late.
• Evidence that climbing fibres are involved in long-term depression comes from an experiment by Ito et al.
  
  • Microelectrodes were used to electrically stimulate mossy fibre (MF) and climbing fibre (CF) afferents to the flocculus of decerebrate rabbits and the E_m of PCs was measured with time.
  • The results mirrored that achieved by Gilbert et al. Stimulation of CFs and MFs at the same time was associated with a decrease in the purkinje cell EPSP amplitude as a percentage of control
  • After stimulation was terminated, the MF-induced EPSP amplitude increased but remained depressed below control.
  • CF-only and MF-only stimulation had no long-term effect on EPSP amplitude suggesting that LTD requires simultaneous input.