Cerebellum: Structure, Circuitry & Function

Question 1

What is the cerebellum subdivided into?

Answer 1

1. Flocculonodular lobe
2. Vermis
3. Anterior lobe
4. Posterior lobe

Question 2

Mid-sagittal section: arbor vitae (tree of life)

Answer 2

• Located posterior to the brain stem and the 4th ventricle
• Vermis:
  
  • lingual (next to the superior medullary velum)
  • nodule (next to the inferior medullary velum)
• Primary fissure
• Horizontal fissure
• Tonsil

Question 3

What is the blood supply to the cerebellum?

Answer 3

3 Pairs of Arteries:

1. Superior cerebellar (SCA):
   
   • ​​from basilar artery
2. Anterior inferior cerebellar artery (AICA):
   
   • from basilar artery
3. Posterior inferior cerebellar artery (PICA):
   
   • from vertebral artery

Question 4

External features of the cerebellum:

Answer 4

• folia: numerous convolutions
  
  • Latin, “leaves”

Question 5

Anatomical Lobes (3):

Answer 5

1. Anterior Lobe
   
   • Anterior to the primary fissure
2. Posterior Lobe
   
   • Posterior to the primary fissure
   • Includes cerebellar tonsils (medial inferior part):
     tonsillar herniation is of clinical significance
3. Flocculonodular Lobe

Question 6

How is the flocculonodular lobe subdivided?

Answer 6

1. 1 nodule
   
   • part of the vermis
2. 2 flocculi
3. Separated from the posterior lobe by the posterolateral fissure
4. Forms the vestibulocerebellum:
   
   • control of equilibrium, balance, and eye movements
   • mediates vestibulo-ocular reflex (VOR)

Question 7

Longitudinal subdivisions:

Answer 7

1. Vermis: along the midline
   
   • Part of the spinocerebellum:
     
     • control of axial and proximal limb movements
2. Intermediate zone of cerebellar hemisphere
   
   • Part of the spinocerebellum:
     
     • control of distal limb movements
3. Lateral zone of cerebellar hemisphere
   
   • part of the cerebrocerebellum:
     
     • planning and initiation of movements

Question 8

What are the excitatory afferents in the cerebellum?

Answer 8

Two excitatory afferents:

1. Climbing fibers:
   
   1. From contralateral inferior olivary nucleus
   2. Synapse directly with Purkinje cells
2. Mossy fibers:
   
   1. From all other sources
   2. Synapse with granule cells, which activate Purkinje cells

Question 9

What are the 5 major neuronal types?

Answer 9

1. Purkinje cells: inhibitory
   
   • output neurons of the cerebellar cortex
2. Granule cells: excitatory
3. Golgi cells: inhibitory interneurons
4. Basket cells: inhibitory interneurons
5. Stellate cells: inhibitory interneurons

Question 10

Cerebellar Cortical Layers (3):

Answer 10

1. Molecular layer
2. Purkinje cell layer
3. Granular layer

Question 11

What makes up the molecular layer?

Answer 11

1. Dendrites of Purkinje cells
2. Parallel fibers from axons of granule cells (excitatory)
3. Climbing fibers from the contralateral inferior olivary nucleus (excitatory)
4. Stellate cells (inhibitory)
5. Basket cells (inhibitory)

Question 12

How are the dendrites of Purkinje cells oriented in the molecular layer?

Answer 12

perpendicular to the axis of the folium

Question 13

Parallel fibers from axons of granule cells (excitatory):

Answer 13

• Each granule cell axon reaches the molecular layer
• bifurcate to form parallel fibers that run parallel to the axis of the folium
• Each parallel fiber can excite up to 500 Purkinje cells

Question 14

Climbing fibers from the contralateral inferior olivary nucleus (excitatory):

Answer 14

• Synapse with dendrites of Purkinje cells
• Each Purkinje cell receives input from only 1 climbing fiber
• Each climbing fiber innervates 1 – 10 Purkinje cells

Question 15

Purkinje cell layer:

Answer 15

• Purkinje cells (inhibitory; GABA):
  
  • output goes only to the deep cerebellar nuclei

Question 16

Granular layer:

Answer 16

• Granule cells (excitatory)
• Golgi cells (inhibitory)
• Mossy fibers (excitatory):
  
  • Synapse with many granule cells

Question 17

Deep Cerebellar Nuclei (DCN):

Answer 17

• Major output neurons of the cerebellum as a whole
  
  • not the Purkinje cells of the cerebellar cortex
• Receive excitatory input:
  
  • from collaterals of afferents to the cerebellar cortex
  • climbing and mossy fibers
• Receive inhibitory input:
  
  • from Purkinje cells of the cerebellar cortex
• Output is excitatory

Question 18

DCN sets of nuclei (4):

Answer 18

1. Dentate nuclei:
   
   • Largest
   • Most lateral
   • Receive projections from the lateral zone of the cerebellar hemispheres
2. Interposed nuclei (made up of Emboliform nuclei + Globose nuclei):
   
   • Intermediate in position
   • Receive projections from the intermediate zone of the cerebellar hemispheres
3. Fastigial nuclei
   
   • Most medial
   • Receive projections from the vermis and some input from the flocculonodular lobe

Question 19

Describe the synaptic circuit involving climing fibers:

Answer 19

Climbing fibers ⇒ excitatory to Purkinje cells ⇒ inhibitory to DCN ⇒ excitatory output to other brain regions

Question 20

Describe the synaptic circuit involving mossy fibers:

Answer 20

Mossy fibers ⇒ excitatory to granule cells, which form parallel fibers ⇒ excitatory to Purkinje cells ⇒ inhibitory to DCN ⇒ excitatory output to other brain regions

Question 21

Describe the synaptic circuit involving inhibitory interneurons:

Answer 21

1. Molecular layer:
   
   • Basket cells and stellate cells ⇒ inhibitory to Purkinje cells
2. Granule cell layer:
   
   • Golgi cells ⇒ inhibitory to granule cells

Question 22

**What makes up the vestibulocerebellum? **

Answer 22

Flocculonodular Lobe and
inferior vermis

Question 23

Vestibulocerebellum:

Functions

Answer 23

• Controls balance and equilibrium while standing or moving
• Controls eye movements
• Coordinates movements of the head and eyes

Question 24

Vestibulocerebellum:

Afferent Inputs

Answer 24

Afferent inputs:

• Vestibular labyrinth (ipsilateral semicircular canals and maculae of the utricle and saccule) projects directly to the Vestibulocerebellum
• Vestibular labyrinth also projects to the vestibular nuclei in the medulla, which then projects to the vestibulocerebellum via the Juxtarestiform body (next to the Restiform body)
• Visual centers (LGN, superior colliculus, and the visual cortex) project to pontine nuclei, which then project to the vestibulocerebellum

Question 25

Vestibulocerebellum:

Efferent outputs

Answer 25

• Vestibulocerebellum projects back to the vestibular nuclei (reciprocal connections)

Note: Vestibular nuclei give rise to:

• Vestibulospinal tracts innervating LMNs of the axial muscles
  
  • Vestibulocerebellum is important for the control of axial muscles used in maintaining equilibrium and balance
• Medial longitudinal fasciculus, which projects to the oculomotor, trochlear, and abducens nuclei.
  
  • vestibulocerebellum is involved in coordinating movements of the head and eyes

Question 26

What makes up the spinocerebellum?

Answer 26

Vermis and Intermediate Zone

Question 27

Spinocerebellum:

Major afferent inputs

Answer 27

• Ipsilateral spinal cord and lower medulla project via:
  
  • Dorsal spinocerebellar tract
  • Cuneocerebellar tract
• **Contralateral inferior olivary nucleus (ION) project via: **
  
  • Olivocerebellar tract
• All 3 tracts: travel w/in the Inferior Cerebellar Peduncle (Restiform Body) to reach the Spinocerebellum
• Ipsilateral spinal cord:
  
  1. Also sends a ventral spinocerebellar tract that crosses the midline to the contralateral side
  2. At the pontine level, this tract joins the Superior Cerebellar Peduncle (Brachium Conjunctivum)
  3. Re-crosses the midline to end in the Anterior lobe of the Spinocerebellum on the original side (ipsilateral)

Question 28

Major efferent outputs from the Vermis part of the Spinocerebellum:

Answer 28

• Cerebellar cortex (Purkinje cells) ⇒ Deep cerebellar nuclei (major output neurons of the cerebellum) ⇒ other centers (via mainly the Superior Cerebellar Peduncle)
• Vermis relays information about the neck and trunk to the Fastigial nucleus (of the DCN):

1. Fastigial nucleus (via the Juxtarestiform body) ⇒ brain stem (via the Medial descending system) ⇒ spinal cord
   
   • innervate axial and proximal limb muscles
2. Fastigial nucleus (via the Superior Cerebellar Peduncle) ⇒ Thalamus ⇒ motor and premotor cortical areas
   
   • control ongoing execution of voluntary movements

Question 29

Major efferent outputs from the Intermediate zone of the Spinocerebellum:

Answer 29

• Cerebellar cortex ⇒ deep cerebellar nuclei
  
  • DCN sends output fibers via the Superior cerebellar peduncle
• Intermediate zone relays information about the limbs to the Interposed nuclei of the DCN (Emboliform and Globose):

1. Interposed nuclei ⇒ brain stem (mainly the Red nucleus) ⇒ spinal cord (via the Lateral descending system)
   
   • innervate distal limb muscles
2. Interposed nuclei ⇒ Thalamus ⇒ Motor and Premoter cortical areas
   
   • innervate the distal limb muscles (via the corticospinal tract)

Question 30

What makes up the cerebrocerebellum?

Answer 30

Lateral Zone

Question 31

Cerebrocerebellum:

Major afferent inputs

Answer 31

• **Contralateral cortex **(via corticopontine tract) ⇒ synapse with pontine nuclei ⇒ pontine nuclei project via the Middle Cerebellar Peduncle (Brachium pontis) that crosses the midline ⇒ Cerebrocerebellum
  
  • This is the Cortico-ponto-cerebellar tract (a crossed pathway)

Question 32

Cerebrocerebellum:

Function

Answer 32

• Involved in the initiation, planning, and mental rehearsal of complex motor actions

Question 33

Cerebrocerebellum:

Major efferent outputs

Answer 33

• Lateral zone ⇒ Dentate nucleus (of the DCN) projects via the Superior cerebellar peduncle (Brachium conjunctivum) ⇒ contralateral Red nucleus (midbrain) ⇒ thalamus
  
  • Dentato-rubro-thalamic tract
• Dentate nucleus also projects directly to the thalamus
• Thalamus ⇒ Motor and Premotor cortical areas

Question 34

Motor and Premotor cortical areas send two
different types of projections:

Answer 34

1. Corticospinal tract:
   
   • crosses the midline at caudal medulla to innervate the spinal cord
   • i.e. back to the side ipsilateral to the original
     dentate nucleus
2. Corticopontine tract:
   
   • innervates the pontine nuclei
   • pontine nuclei project via the Middle Cerebellare Peduncle ⇒ contralateral Cerebrocerebellum
   • i.e. back to the side ipsilateral to the original Lateral zone of the cerebellar cortex

Question 35

Monoaminergic afferent fibers from the brain stem to the cerebellum:

Answer 35

• Serotoninergic fibers: from the raphé nuclei
• Noradrenergic fibers: from the locus ceruleus
• Both sets of fibers project to all parts of the cerebellum and play a modulatory role

Question 36

GENERAL FUNCTIONAL SIGNIFICANCE OF THE CEREBELLUM:

Answer 36

1. Control of balance and eye movements (vestibulo-ocular reflex)
2. Regulates movement and posture indirectly by modulating the output of the major descending motor systems
3. Compares intention with actual movement and compensates for errors in movement
4. The function of the cerebellum is changed by experience:
   
   • involved in motor learning

Question 37

How does the cerebellum compare intention with actual movement and compensate for errors in movement?

Answer 37

1. Receives information about plans for movement from motor and premotor cortex
   
   • via the corticopontocerebellar tract
2. Monitors the integration of descending and peripheral information regarding movement in the spinal cord
   
   • via the ventral spinocerebellar tract
3. **Receives feedback information from the sensory periphery **during the course of the movement
   
   • via the dorsal spinocerebellar tract
4. Projects to motor centers that send fibers to the spinal cord:
   
   • adjusts the output of the motor system
   • Ex: dentatorubrothalamic tract

Question 38

DYSFUNCTIONS OF THE CEREBELLUM:

Answer 38

• Lesions cause disruption of coordinated limb and eye movements, impairs balance, and reduced muscle tone
• Major signs: ataxia, hypotonia, intentional tremor, dysdiadochokinesia (irregular pattern of alternating movements), dysmetria, nystagmus, and titubation
• Damage to the cerebellum can impair motor learning

Question 39

Cerebellar lesions typically cause ___________ motor signs

Answer 39

ipsilateral

Question 40

Spinocerebellum:

Functions

Answer 40

• Vermis: control of axial and proximal limb muscles
• Intermediate zone: control of distal limb muscles
• Both control ongoing execution of movements