20 - Cerebellum

Question 1

Cerebellum General Functions, Size, Anatomy

Answer 1

• Functions:
  
  • Alters timing of firing of M1 neurons
    
    • ​Fine tunes activity in all motor pathways
  • Coordinates, corrects errors in movements
  • Maintains uright posture
  • Maintains tension/tone of muscle
• Size
  
  • 150g (10% brain weight)
  • 1000 sq cm (40% of cerebral cortex)
• Anatomy: 3 lobes
  
  • Anterior: receives spinocerebellar info about nonconscious proprioception
  • Posterior: receives corticopontine signals
  • Flocculonodular: receives vestibular inputs

Question 2

Cerebellum Functional DIvisions

Answer 2

• Vermis (midline)
  
  • Muscle tone, posture
  • Head, trunk, limb movements (not digits)
• Hemispheres:
  
  • Planning, initiation, timing of movements
  • Fine motor control of digits
  • Memory, language
• Flocculonodular Lobe:
  
  • Balance, eye movements

Question 3

Cerebellum Input Systems

Answer 3

1. Mossy FIber System
   
   • Have dendrites called rosettes to synapse with granule cell and golgi cells in granule cell layer
2. Climbing Fiber system
   
   • Inferior olive input only
   • Drives cerebellar plasticity
   • Helps cerebellum generate error signal and adjust connections between Purkinje cells and parallel fibers via calcium spikes

Question 4

Mossy FIber Pathways Entering ICP/MCP/SCP

Answer 4

SCP: ventral spinocerebellar fibers (ipsilateral)

MCP: Pontocerebellar fibers (contralateral)

ICP (all ipsilateral):

• Dorsal spinocerebellar fibers
• Cuneocerebellar fibers
• Vestibulocerebellar fibers
• Reticulocerebellar fibers

Question 5

Cerebellar Cortex Anatomy

Answer 5

1. Molecular Layer
   
   • Dedrites of Purkinje cells
   • Parallel fibers of granule cells
   • Stellate, basket inhibitory neurons (for insulation of each arboritic P dendrite)
2. Purkinje Layer
   
   • Large Purkinje cell bodies
     
     • Send out inhibitory projections to deep cerebellar nuclei
3. Granule Cell Layer
   
   • Granule cells: MOST ABUNDANT IN BRAIN/SC (60-80 billion)
     
     • 70% of all brain, SC neurons
     • Synapse with 200k Purkinje cells
   • Golgi Cells: inhibitory feedback to granule cells
   • Rosettes of mossy fibers

Question 6

Granule Cell Feedforward vs Feedback Inhibition

Answer 6

• Feedforward Inhibition
  
  • Moss fiber excites a Golgi cell that inhibits the granule cell
    
    • Use sensory info in advance of movement (fast)
• Feedback Inhibition
  
  • Granule cell excites the Golgi cell that inhibits the granule cell
    
    • Use sensory info during movement (slow)

Question 7

IOC Inputs (3)

Answer 7

1. Motor cortex > ipsilateral red nucleus via central tegmental tract > IOC via rubro-olivary fibers
   
   • Coarse motor plan
2. Motor cortex > IOC
   
   • Active UMN signaling to cord
3. Spinal cord contralateral proprioceptors via spino-olivary tract > IOC
   
   • Coarse signals about proprioception

Question 8

IOC Projection

Answer 8

TO CONTRALATERAL ICP

Question 9

Deep Cerebellar Nuclei & Functions

Answer 9

• ALL OUTPUTS LEAVE VIA SCP
• Dentate
  
  • Project to contralateral VL thalamus
  • Alter timing of firing of M1 neurons
• Emboliform
  
  • Project to contralateral red nucleus
  • Completes myoclonic triangle
• Globose
  
  • Project to contralateral red nucleus
  • Completes myoclonic triangle
• Fastigial
  
  • Projects to ipsilateral bestibular, reticular nuclei
  • Feedback circuits for maintenance of posture and muscle tone via MVST and LVST, especially axial

*Interposed nuclei = emboliform and globose nuclei

Question 10

Dentato-rubro-thalamic tract

Answer 10

Cerebellar outputs from dendate, decussate and run through red nucleus to the VL thalamus

Question 11

Myclonic Triangle

Answer 11

• Feedback loop
• Red nucleus -> ipsilateral inferior olive -> contralateral cerebellum -> output via interposed nuclei -> original red nucleus
• Lesions to triangle components result in:
  
  • Palatal myoclonus (palate contractions)
  • Ocular myoclonus
  • Holmes’ tremor (wing-beating)

Question 12

Motor Results of Cerebellum

Answer 12

• Error detection and correction of cortically-originating movement
• Motor learning: increased Purkinje cell firing during learning of a new motor task
• Initiation of movement: deep cerebellar nuclei fire simultaneously with pyramidal cortical neurons prior to movement
• Sensory-motor integration: generates prediction of the sensory consequence of movement

Question 13

Non-motor Function of Cerebellum

Answer 13

• Midline
  
  • Autonomic function (due to hypothalamic interconnections)
  • Behavior, mood, sexuality, affectively important memory
• Hemispheres
  
  • Cognition, planning, language
  • Memory and learning
  • Sequencing of non-motor tasks

Question 14

Vasculature of Cerebellum

Answer 14

• SCA: most extensive cerebellar territory
  
  • Superior surface of cerebellum down to the horizontal fissure
  • Most of the cerebellar white matter (a watershed area between all 3 areas)
  • Superior cerebellar peduncle
• AICA
  
  • Anteroinferior surface of cerebellum
  • Flocculus
  • Middle cerebellar peduncle
• PICA
  
  • Posterioinferior surface of cerebellum
  • Tonsils
  • Inferior cerebellar peduncle

Question 15

Cerebellar Lesions

Answer 15

• Produce ipsilateral deficits
• Midline lesions
  
  • Truncal ataxia
  • Dysarthria
• Hemisphere lesions
  
  • Limb ataxia
  • Tremor WHEN MOVING
    
    • Different from Parkinson’s, tremor at rest
  • Dysmetria = past pointing
  • Dyssynergia = jerky movements
  • Dysdiadochokinesia = difficulty making rapidly alternating hand movements
  • Hypotonia (because of reduced input to tone-managing reticulospinal tracts)
• Flocculonodular lesions
  
  • Visual tracking deficit
  • Oculomotor control deficit
    
    • Nystagmus
    • Vertigo
  • Vestibular signs