Download Objectives 36 - u.arizona.edu

CEREBELLUM –
1. ANATOMY OF CEREBELLUM
- consistent with function of providing coordinating signals for movement
- receives sensory inputs from many areas of body (as well as visual and vestibular sensory info)
- cortex associated with movement projects to cerebellum
- output of cerebellum projects to motor areas of cerebral cortex (via thalamus) and brainstem
- cerebellum has convoluted outer cortex of gray matter covering inner core of white matter; it is
folded over upon itself so superior and inferior ends meet in the roof the 4th ventricle
- Three pairs of deep cerebellar nuclei: dentate nuclei, interposed (globose and emboliform), and
fastigial nuclei (from lateral to medial as seen in caudal pons)
- Cerebellar cortex  projects to deep cerebellar nuclei  cerebellar outputs to CNS
- inputs and outputs of cerebellum travel via superior, middle, and inferior peduncles
- more fibers enters cerebellum than leave it; input: output ratio is 40:1; cerebellum integrates
- afferents travels in middle cerebellar peduncle and come from the cerebral cortex via
contralateral pontine nuclei (in basal pons, crossed pontocerebellar fibers)
- other afferents from inferior olivary nucleus (crossed) and direct spinocerebellar tracts found
in inferior cerebellar peduncle (with visual and vestibular inputs)
- efferent fibers travel through superior cerebellar peduncle, decussate, and terminate in the
thalamus, VA/VL, (to motor cortex), red nucleus, and other brainstem sites
Two subdivision schemes
1. Anatomically divided into 3 lobes: anterior and posterior divided by primary fissure;
flocculonodular lobe separated by posterior lobe by another deep fissure – vestibular nuclei project
here
2. Based on behavioral observations from ablation studies and on anatomical connections; each
half of cerebellum divided into longitudinal zones, each associated with deep cerebellar nucleus:
Lateral zone – lateral hemispheres of cerebellar cortex  dentate nucleus
Intermediate (paravermal) zone – lateral to vermal zone, medial to lateral zone  interposed
nucleus
Vermal zone – in midline  fastigial nucleus; fastigial nucleus projects to vestibular nuclei via
inferior cerebellar peduncle
- somatosensory map: head/trunk occupy midline of anterior lobe; limbs occupy intermediate
zones
- lesions of midline/vermal zone  abnormalities of gait and posture (limbs not involved)
- lesions of intermediate/lateral zones  abnormalities of ipsilateral limb movement
1, 4.
Deep cerebellar
nucleus projected to
Afferents/inputs –
MCP/ICP
Efferents/outputs SCP
Timing of changes
in neuronal firing
Putative functions
Clinical signs if
lesion
Medial zone
(vermis)
Fastigial
Spinal cord (trunk),
brainstem (vestibular
nuclei)
Brainstem
After motor cortex,
after movement onset
Modify posture and
balance during
movement
Gait and trunk
ataxia
(incoordination)
Intermediate zone
(paravermal zone)
Interposed (globose and
emboliform)
Spinal cord (limbs),
brainstem, and motor
cortex via pons
Motor cortex, red
nucleus, brainstem,
thalamus (VA/VL)
After motor cortex, after
movement onset
Modify movement as it
progresses, limb
feedback
Limb ataxia during
volitional movement,
dysmetria (defective
correction of
movement)
Lateral Zone
(hemispheres)
Dentate
Cerebral cortex,
association cortex via
pons, inferior olive
Thalamus, brainstem,
premotor cortex via
thalamus (VA/VL)
Before motor cortex,
before movement onset
Planning of skilled
volitional movement,
especially the limbs
Limb ataxia during
volitional movement
(defective planning of
movement)
2. Cerebellar dysfunction
- syndromes involve abnormalities of movement (little or no weakness); cerebellar syndromes
associated with disorders of speed and spatial accuracy of movement; uncoordinated movements
such as dysmetria (mistakes in extent of movement), ataxia, tremor, trouble with alternating
movement
- fundamental abnormality is timing of muscle activities  errors in speed and spatial accuracy
3. Microscopic structure of cerebellar cortex
- cortex made up of regular folds called folia
- molecular layer (sparse nerve cell nuclei, heterochromatin), purkinje layer (large multipolar
cells/euchromatin with axon to granular layer and white matter), granular layer (dense nerve cells
with dark nuclei, heterochromatin)
- Parallel to length of folia are unmyelinated axons called parallel fibers, which synapse on
dendrites of Purkinje cells, excitatory (arranged in planes at right angles to long axis of folium)
- Purkinje cells are sole output neurons of cerebellar cortex and synapse on deep cerebellar
nuclei
- input to cerebellum arrives as climbing fibers (originating in contralateral inferior olivary
nucleus) and mossy fibers (originating everywhere else)
- mossy fibers are excitatory  terminate on granule cells in cortex  form parallel fibers
- mossy and climbing fibers send collaterals to deep cerebellar nuclei (excitatory, glutamate)
- input from mossy and climbing fibers causes increased excitation in deep cerebellar nuclei,
keeping them continuously active
- mossy fibers (via parallel fibers from granule cells) and climbing fibers input excites Purkinje
cells  synapse on deep cerebellar nuclei (inhibitory, GABA)  pauses of activity out of the
continuosly active deep cerebellar nuclei
4. Look at table above
Lateral/Intermediate zones – regulate timing of movement by 1) planning timing in advance of
movement and 2) modifying the plan as it unfolds
Planning-in-advance : projections from cerebellum to cerebral cortex (via thalamus) conveying
info regarding timing of muscle; deficit in this function  difficulty initiating and stopping
muscular contractions (inappropriate timing)
hypotonia – decreased resistance to passive movement; pendular reflexes – undamped response to
tendon taps which causes limb to oscillate in a pendular notion
Modify plan – instructions from motor cortex amended before the movement is finished; errors:
Dysmetria - mistakes in extent of movement; ataxia; errors in control of timing of muscle
activities; movement is inaccurate in extent and repeated efforts to reprogram result in oscillations
or tremor; dysarthria – errors in speech (prosody and intonation)
Vermal cortex
- errors in postural adjustments; gait ataxia, staggering
- midline zone specialized to provide coordination of posture with movements; input for periphery
- neuronal activity in fastigial nucleus occurs after onset of the movement  vermis helps to adjust
movements as they occur  affects proximal muscles
Flocculonodular lobe
- extension of vestibular system; principal connections with vestibular nuclei
- maintenance of equilibrium; lesions  disequilibrium and vertigo; nystagmus
- helps coordinate slow eye movements during smooth tracking of an object
- receives input regarding head and eye movement as well as indirect visual information
Motor learning
- normal adaptation of vestibuloocular reflex after prism glasses; if flocculonodular lobe is
lesioned  no adaptation
- normal eye blink reflex in response to puff of air (conditioned by bell); interposed nucleus is
lesioned  conditioned response (blink to sound stimulus) was lost even though unconditioned
response (blink to air puff) remained
- Cerebellar patients fall, not because they make insufficient responses, but because they fail to
adapt (learn)  incorrect responses that throw them off balance
- cognitive or behavioral abnormalities; lateral cerebellum implicated in cognitive learning
because it is connected with association cortex; midline cerebellum implicated in higher order
autonomic and affective functions, because it is connected with limbic cortex and associated
structures
- dramatic mood swings, giddiness
5. Identify: cerebellar vermis and hemispheres, flocculus, nodulus, inferior, middle, superior
cerebellar peduncles, dentate, interposed, fastigial nuclei, inferior olivary nucleus