Cerebellum Flashcards
Cerebellar
Movement Functions
- Starting movement
- Timing and fine tuning of movements
- Stopping at the right time
- Postural adjustments
- Motor planning (via outputs to premotor circuits)
- Motor adaptation/learning (via outputs to motor circuits)
Cerebellum
Anatomy
- Develops from alar plates of metencephalon
- Covers posterior surface of brainstem
- Forms roof of 4th ventricle
- Folia → transverse surface folds
- Medial to lateral: vermis → paravermis → lateral cerebellar hemisphere
- Primary fissure → seperates anterior from posterior lobes
- Posterolateral fissure → seperates posterior from flocculonodular lobes
Transverse Divisions
Based on phylogenetic trends.
Longitudinal Divisions
Based on function.
Flocculonodular Lobe
Includes nodulus and 2 flocculi.
Involved in control of balance and eye movements.
Brainstem Connections
Cerebellum connected to brainstem via 3 cerebellar peduncles:
ICP ↔ Medulla
MCP ↔ Pons
SCP ↔ Midbrain
Blood Supply
PICA
inferior half of cerebellum
inferior vermis
inferior cerebellar peduncle
AICA
middle cerebellar peduncle
strip of ventral cerebellum
flocculi
SCA
superior half of cerebellar cortex
deep cerebellar nuclei
superior vermis
superior cerebellar peduncle
Internal Anatomy
-
Cerebellar cortex
3 layers:- granule cell layer
- Purkinje cell layer
- molecular layer
-
Internal white matter
- afferent axons: mossy and climbing fibers
- efferent axons: from Purkinje cells
-
Three pairs of deep nuclei:
- fastigial nucleus
-
interposed nucleus
- globose n.
- emboliform n.
- dentate nucleus
Cerebeller Connections
Overview
-
Influence over motor neurons indirect
- cerebellum → thalamus → motor cortex → spinal cord
- cerebellum → UMNs of extra-pyramidal system in brainstem → spinal cord
-
Cerebral cortex informs cerebellum of command for movement
- mainly motor systems → pontine nuclei → cerebellum ⟾ cortico-ponto-cerebellar pathway
-
Influenced by all sensory systems (unconsciously)
-
direct projections
- DSCT ⟾ peripheral movements
- VSCT ⟾ motor commands to spinal LMNs
- polysynaptic pathways
- afferents from inferior olive = climbing fibers
- all other afferents = mossy fibers
- afferent bundles fan out in cerebellar white matter → folia
- afferent fibers send collaterals to deep cerebellar nuclei before terminating in cerebellar cortex
-
direct projections
Climbing Fibers
Inferior olive → ICP → climbing fibers → contralateral cerebellum.
-
Excitatory synapses with:
- deep cerebellar neurons
- dendrites of Purkinje neurons
- Each CF gives off 1-10 collaterals
- each collateral innervates only 1 Purkinje cell
- Each action potential in CF produces a complex spike in Purkinje cell
- Crucial for motor adaptation and motor learning
Mossy Fibers
- Mossy fibers arise from nuclei in spinal cord and brainstem
- sensory info from periphery
- commands from cerebral cortex
-
Excitatory to:
- deep cerebellar neurons
-
granule cells → parallel fibers → Purkinje neurons
- produces simple spikes
- Direct (MF) and indirect (granule cell) activation of Purkinje cell → steady stream of simple spikes
- encodes moment to moment input info
- Path supplies info about planned and ongoing movements.
Cerebellar Circuits
Summary
- Info about intended and ongoing movement → mossy and climbing fibers → cerebellum
- Projects to both cerebellar cortex and deep cerebellar neurons
- In cerebellar cortex circuits, differences between planned movements, stored info, and actual ongoing movements analyzed
-
Correction signal from Purkinje neurons adjusts output firing of deep cerebellar neurons
- Purkinje cells in flocculonodular lobe and vermis can directly control vestibular nuclei
- Alters movement as needed to complete task
Cerebellar
Internal Projections
Deep cerebellar n. also somatotopically organized.
Medial to lateral organization.
Trunk info:
vermis → fastigial n.
Limb info:
paravermis → interposed n.
lateral hemisphere → dentate n.
Cerebellar
Somatotopy
-
Head, neck, and trunk → vermis
- auditory input
- cortico-ponto-cerebellar relays
- visual and auditory projections
- frontal eye field projections
- somatosensory info from face
- trigeminal projection doubled → two head areas located midline
-
Limbs → paravermis & cerebellar hemispheres
- Anterior lobe: legs rostral, arms caudal
- Posterior lobe: legs caudal, arms rostral
- More concerned with where than what
- projections from cerebral cortex projects disynaptically to cerebellar cortex
- body representation more coarse
- shows divergence at cerebellar cortex
- Vestibular system (medial and inferior vestibular n.) → flocculonodular lobe > ant. and post. vermis
-
Olivo-cerebellar system → entire cerebellar cortex
- timing of movement
- inferior olive w/ input from cortical and spinal projections
- sends info to cerebellum that is already integrated
Direct vs. Indirect
Inputs
-
Direct sensory input
- DSCT/VSCT
- Cuneocerebellar tract
-
Disynaptic sensory input
- spino-olivo-cerebellar
- spino-reticulo-cerebellar
Functional Organization
Cerebellum divided into 2 distinct zones:
-
Medial zone ⟾ flocculonodular lobe and vermis
- spinal cord, vestibular, trigeminal, visual, and auditory inputs
- regulate output of fastigial n. and lateral vestibular n.
- involved in moment to moment control of
- balance
- eye movement
- posture
- locomotion
- trunk muscles
-
Lateral zone ⟾ paravermis and cerebellar hemispheres
- input from spinal cord & trigeminal and cerebral cortex inputs via pontine n.
- regulate output of interposed n. (globose + emboliform) and dentate n.
- involved in moment to moment control of distal muscles & motor planning
Medial Zone
Inputs
Input to vermis and flocculonodular lobe.
5 types:
-
Corticopontine input
- pontocerebellar fibers
-
Spinal and trigeminal input
- DSCT
- VSCT
- CuCT
- trigeminocerebellar tract (TCT)
-
Vestibular input
- vestibulocerebellar tract (VCT)
-
Reticular input
- reticulocerebellar tract (ReCT)
-
Inferior olive input
- olivocerebellar tract (OCT)
Medial Zone
Corticopontine Input
- Carries:
- motor commands from trunk area of motor cortex
- visual info from visual cortex and superior colliculus
- Projects to ipsilaterally pontine n.
- From pontine n., decussates as transverse pontine fibers
- Enter cerebellum via MCP
Medial Zone
Spinal and Trigeminal Input
All inputs enter cerebellum ipsilaterally via ICP.
-
Dorsal spinocerebellar tract (DSCT)
- carries ipsilateral lower body proprioception
- originate in Clarke’s n.
-
Cuneocerebellar tract (CuCT)
- carries ipsilateral upper body proprioception
- originates in lateral (acc.) cuneate n.
-
Trigeminocerebellar tract (TCT)
- carries info from face
- orginate in spinal trigeminal n.
DSCT
Input Pathway
CuCT
Input Pathway
Medial Zone
Spinal Cord Input
Ventral spinocerebellar tract (VSCT)
- Carries efferent copy of spinal cord LMN commands → “spy tract”
- Originates in spinal cord (border cells)
-
Decussates twice:
- ventral white commissure → forms VSCT
- SCP → enter cerebellum
- Carries ipsi info to ipsi cerebellum
Medial Zone
Vestibular and Reticular Inputs
Both tracts enter ipsilateral cerebellum via ICP.
-
Vestibulocerebellar tract (VCT)
- carries balance information
- originates from vestibular apparatus and vestibular n.
-
Reticulocerebellar tract (RetCT)
- carries cutaneous information
- originate from lateral reticular n. of medullary reticular formation
Medial Zone
Inferior Olive Input
Olivocerebellar tract (OCT)
- carries info used for timing and motor learning
- originate from inferior olivary n. in medulla
- fibers decussate and enter contralateral cerebellum via ICP
- terminates as climbing fibers in vermis and flocculonodular lobe
- sends collaterals to fastigial n.
Lateral Zone
Inputs
Input to lateral hemisphere and paravermis.
4 types:
-
Corticopontine input
- pontocerebellar fibers
-
Spinal and trigeminal input
- DSCT
- VSCT
- CuCT
- TCT
-
Reticular formation input
- RetCT
-
Inferior olive input
- olivocerebellar tract
Lateral Zone
Corticopontine Input
Cortico-Ponto-Cerebellar Tract
-
Carries:
- motor commands from limb area of motor cortex
- for coordination of ongoing movements of extremities
- info from all cortices including premotor cortex
- for motor planning
- motor commands from limb area of motor cortex
- Cortical input terminates in ipsilateral pontine n.
- From pontine n., fibers decussate as transverse pontine fibers
- Enter contralateral cerebellum via MCP
Lateral Zone
Spinal and Trigeminal Input
All tracts enter ipsilateral cerebellum via ICP.
Terminates in lateral hemisphere and paravermis.
-
Dorsal spinocerebellar tract (DSCT)
- carries ipsi lower body proprioception
- orginates in Clarke’s n.
-
Cuneocerebellar tract (CuCT)
- carries ipsi uuper body proprioception
- orginates in lateral (acc.) cuneate n.
-
Trigeminocerebellar tract (TCT)
- carries info from face
- originates in spinal trigeminal n.
Lateral Zone
Spinal Cord Input
Ventral spinocerebellar tract (VSCT)
- Carries efferent copy of spinal cord LMN commands → “spy tract”
- Originates in spinal cord (border cells)
-
Decussates twice:
- ventral white commissure → forms VSCT
- SCP → enter cerebellum
- Carries ipsi info to ipsi cerebellum
Lateral Zone
Reticular Formation Input
-
Reticulocerebellar tract (RetCT)
- carries cutaneous information
- originate from lateral reticular n. of medullary reticular formation
- enter ipsilateral cerebellum via ICP
Lateral Zone
Inferior Olive Input
Olivocerebellar tract (OCT)
- carries info used for timing and motor learning
- originate from inferior olivary n. in medulla
- fibers decussate and enter contralateral cerebellum via ICP
- terminates as climbing fibers in lateral hemisphere and paravermis
Cerebellar Afferents
via
Inferior Cerebellar Peduncle
-
Dorsal spinocerebellar tract
- from Clarke’s n.
- ipsilateral lower limb proprioception
-
Cuneocerebellar tract
- from lateral (acc.) cuneate n.
- ipsilateral upper limb proprioception
-
Reticulocerebellar tract
- from reticular formation
- complex sensory inputs including visual and auditory
-
Vestibulocerebellar tract
- from vestibular n. and ganglion
- gravity and momentum info
- balance info
-
Trigeminocerebellar tract
- from spinal trigeminal and chief sensory n.
- ipsilateral proprioception from face
-
Olivocerebellar tract
- from contralateral inferior olive
- via climbing fibers
- integrated sensory info
Cerebellar Afferents
via
Middle Cerebellar Peduncle
Only caries afferents from corticopontocerebellar tract.
No efferents.
Cerebellar Afferents
via
Superior Cerebellar Peduncle
Only carries afferents from VSCT.
“Spy tract” compares what MN is “told” to do and what it is doing.
Many efferents.
Deep Nuclei
- Receives collaterals from afferents to cerebellum
- Receives processed info from cerebellar cortex via Purkinje cell axons
- Glutamatergic (excitatory) projections to brainstem and thalamus
- Many projections form part of polysynaptic loops feeding back to cerebellum
Fastigial Nucleus
Receives input from Purkinje neurons in vermis of all cerebellar lobes.
Bilateral projections via ICP to:
medial reticular formation (inhibitory)
vestibular nuclei esp. lateral vestibular (Deiter’s) n.
Interpositus nucleus
&
Dentate nucleus
Receive afferents from cortex of cerebellar hemispheres.
Represents limbs.
Cerebellar Cortex
Structure
Three-layer cortex with several interneuron types and Purkinje cells which projects out.
-
Granule layer (deepest)
-
Granule cells → glutamateric → excitatory
- axons ascend to molecular layer
- bifurcate to form parallel fibers
- only excitatory interneuron
- Golgi cells → GABAergic → inhibitory
-
Granule cells → glutamateric → excitatory
-
Purkinje Layer
-
Purkinje cells → GABAergic → inhibitory
- extensive dendritic arborization to molecular layer
- synapses with parallel fibers, basket cells, and stellate cells
- up to 100k connections
- major integrator of cerebellum
- project to deep cerebellar n.
- some cells from flocculonodular lobe and anterior vermis project directly to vestibular n.
- extensive dendritic arborization to molecular layer
-
Purkinje cells → GABAergic → inhibitory
-
Molecular Layer (superficial)
- Stellate cells → GABAergic → inhibitory
- Basket cells → GABAergic → inhibitory
- parallel fibers & Purkinje cell dendritic arbors
White matter contains axons of Purkinje cells and afferent fibers going to cortex.
-
Climbing fibers
- contra. inferior olive → directly and repeatedly on Purkinje cell dendrites
-
Mossy fibers
- terminates on granule cells dendrites
- indirect contact w/ Purkinje cells
Climbing & mossy fibers → Glutamatergic → excitatory.
Cerebellar Output
Overview
Medial zone → correcting regulatory output → smooth coordination of trunk muscles → balance and equilibrium.
Lateral zone → correcting regulatory output → smooth coordination of distal muscles + motor planning.
Cerebellum → brainstem motor n. + thalamus → corticospinal, reticulospinal, vetibulospinal, rubrospinal tracts.
Medial Zone Output
Pathway
Vermal-Fastigial System
-
Purkinje cells in vermis and flocculonodular lobe
- ⊖ → fastigial n. and lateral vestibular neurons
- ∆ firing to correct for errors in trunk movement, balance, position
- ⊖ → medial vestibular neurons and oculomotor n. (not shown)
- ∆ MVST for coordination of head and eye movements
- ⊖ → fastigial n. and lateral vestibular neurons
- To fastigial n.
- Via ICP → ⊕ medullary reticular formation
- facilitate extensor inhibition by MRST
- Decussate in SCP → ⊕ contralateral VL thalamus
-
⊕ → motor cortex (trunk area)
- ⊕ → VSCT controlling trunk muscles during movement
-
⊕ → motor cortex (trunk area)
- Via ICP → ⊕ medullary reticular formation
- To lateral vestibular neurons via ICP
- ∆ LVST output to extensor antigravitic muscles
Vermal-Fastigial System
Characteristics
Bilateral control of axial and proximal movement via lower brainstem nuclei.
For postural adjustments including trunk fixation & head position to maintain gait, balance, and posture.
Efferents via ICP.
-
Cerebelloreticular tract
- fastigial n. → medullary reticular formation → MRST
-
Cerebellovestibulo tract
- fastigial n. → vestibular n. → LVST
- Inhibitory Purkinje fibers (flocculonodular/vermis) → lateral vestibular n.
Lesion produces hypertonicity.
Disinhibition of trunk extensors.
Lateral Zone Output
Pathway
Hemisphere-Lateral System
-
Purkinje cells in paravermis and lateral hemisphere
-
⊖ → interposed n. & dentate n.
- ∆ firing to provide regulatory signal
- correct for errors in ongoing limb movement
- participate in motor planning
- ∆ firing to provide regulatory signal
-
⊖ → interposed n. & dentate n.
-
Interposed & Dentate nuclei
- ⤮ in SCP → contra. VL thalamus → motor cortex (limb area) → ∆ LCST → distal muscles
- ⤮ in SCP → contra. red nucleus magnocellular neurons → RST → upper limb flexors
- ⤮ in SCP → contra. red nucleus parvocelluar neurons → inferior olive → ⤮ in ICP → climbing fibers → cerebellum
- ∆ cerebellar circuits for timing and motor learning
Lesion results in hypotonia.
Hemisphere-Lateral System
Characteristics
Functions in coordinated movements and motor planning.
- Dentate & Interpositus n. project via SCP
- ⤮ in lower midbrain
- maintains somatotopy
- Exerts control over all supra-spinal motor pathways.
- Dentatothalamo tract (aka cerebellothalamo tract)
-
Dentatorubro tract (aka cerebellorubro tract)
- interpositus n. → magnocellular red n.
-
Dentatorubroolivary tract (aka cerebellorubroolivary tract)
- dentate n. → parvocellular red n. → inferior olive
Cerebellar Output
Summary
Cerebellar Lesion
Symptoms
- Hypertonia
- Ataxia
- Hypotonia
- Cerebellar nystagmus
-
Intentional tremors
- tremors when trying to move
- error in movement end-point → attempt to correct → error in opposite direction
-
Dysmetria (past-pointing)
- mid-course correction made en route esp. w/ eyes closed
-
Dysarthria
- inability to coordinate speech muscles
-
Dysdiadochokinesia
- uncoordinated rapid antagonist exercise
- Dysrhythmia
Medial Zone or ICP
Lesion
Lesion of ICP, vermis, flocculonodular n., or fastigial n.
Impairs ability to use vestibular and proprioceptive info to coordinate movement.
Unilateral lesion may result in bilateral deficits.
- Truncal ataxia
- Wide-based unstable gait
- Vertigo, nystagmus
- Deficit in eye movements
- altered smooth persuit
- impaired suppresion of VOR
- Dysarthria
- Lesion of ICP ⟾ hypertonia
- Unilateral lesion of fatigial n. ⟾ hypertonia, nystagmus, and ipsilateral stumbling
Dentate, Interpositus, or SCP
Lesion
Unilateral lesion of SCP, lateral hemisphere, interposed or dentate n. ⟾ ipsilateral deficits.
Characterized by errors in direction, force, speed, and amplitude of movements.
-
Hypotonia
- Due to loss of tonic cerebellar facilitation of motor cortices
- Delays in initiating movements
-
Decomposition of multiple-joint movements
- breaks down smooth movement into several jerky components
-
Intentional tremors
- oscillations caused by “intentional” or goal-directed movement
-
Dysmetria
- past-pointing which occurs due to overshoot/undershoot of target
-
Dysdiadochokinesia
- inability to perform rapid alternating movements
Ice Skating
Application
Person is skating on ice:
-
Balance and movements continuously checked and adjusted smoothly by medial and lateral zones
- Uses spinal, vestibular, and visual cues
- Regluation of axial/proximal muscles
- To jump, premotor and supplementary motor send motor plan to lateral zone
- Lateral zone also receives info from all cortices about the state of skater
-
Lateral zone
- processes info
- compares with internal stored components of learned skilled movement
- provides premotor and motor cortices with motor plan adjusted for timing sequence and activation of each muscle
- interposed/dentate n. → VL thalamus → premotor/motor cortex
- As person jumps, medial and lateral zones compare:
- intended motor commands (from cortex)
- real-time feedback from muscles (DSCT, CuCt…)
- what spinal cord motor neurons are being told to do (VSCT)
- Checks if movement is occuring as intended
-
Corrections made via tune up of motor centers controlling:
- axial/proximal muscles
- fastigial/vestibular → MRST / LVST
- fastigial → VL thalamus → trunk area of motor cortex
- distal muscles
- interposed/dentate → VL → limb area of motor cortex
- interposed/dentate → red nucleus
- axial/proximal muscles
- Ensures motor sequence is done correctly and smoothly
- As skater repeats sequence, climbing fibers generate teaching signal → modifies cerebellar circuits → motor learning → stores new learned skilled motor sequences
Cerebellum
Higher Functions
May also be critical for thought, behavior, and emotion.
Disruption of cerebellar influences on higher functions may lead to dysmetria of thought and impairment of mental agility.
- classical conditioning
- navigational skills
- cognitive flexibility
- modulation of emotion, sham rage, predatory attack, and agression
- linguistic and sensory processing
- verbal working memory, shifting attention, mental imagery
Tidal Wave
Hypothesis
- Mossy fibers ⟾ focal set of granule cells ⟾ beam of parallel fibers ⟾ activation of longitudinal strip of Purkinje cells and associated basket/stellate cells
- Basket/stellate cells produces lateral wave of inhibition to surrounding Purkinje cells
- Deselects competing set of Purkinje cells which may affect needed set of muscles for action
- Each parallel fiber may represent a diffferent program of percisely timed motor sequences
Actual function of parallel fibers still unclear.
Climbing Fibers
Hypotheses
Climbing fiber entering cerebellar cortex branches sagitally to excite 1-10 Purkinje cells.
CF sagittal lines and beams of parallel fibers form a matrix.
CF activation ⟾ robust complex spike in Purkinje cell ⟾ LTD of parallel fibers.
Purposed hypotheses:
- CF reset purkinje cell to baseline
- CF conditions Purkinje/parallel fiber synapse through LTD ⟾ role in motor adapation and learning
- CF carry an error signal with repeated movements about the inappropriate mossy fiber/Purkinje cell synapses to silence
- CF with internal timing function d/t capacity for synchronous and rhythmic firing ⟾ role in motor coordination
