CEREBELLUM

A. Introduction

1. Part of the motor system
2. Activity is unconscious
3. Located superior to the pons
4. Communicates with the remainder of the CNS by way of the three pairs of cerebellar peduncles (superior, middle, inferior)

B. Structure

1. Can be divided into lobes based upon convolutions

a. anterior lobe
b. posterior lobe
c. floccular-nodular lobe (flocculus + modulus); evolved from fish

2. Anterior and posterior lobes can be subdivided into zones based on mediolateral location

a. cerebellar hemispheres (lateral zone) -- comprises 90% of cerebellar cortex in man; phylogenetically newest part of the cerebellum

b. vermis – narrow medial region of the cerebellum; unpaired (fr. L. worm)

c. intermediate zone – area between the hemispheres and the vermis; paravermal area

3. Can be subdivided by function

a. cerebrocerebellum (= cerebellar hemispheres)
b. spinocerebellum (= vermis + intermediate zone)
c. vestibulocerebellum (= flocularnodular lobe)

C. Cerebellar cortex

1. 3 cell layers: Molecular layer, Purkinje cell layer, Granule cell layer

D. Input and Output (via the cerebellar peduncles)

Note: cerebellar effects on muscle tone and movement are mainly ipsilateral

1. Afferent (input): mossy fibers & climbing fibers

a. motor cortex
b. proprioceptors, especially from skeletal muscles, tendons, and joints
c. vestibular (equilibrium) organs
d. other brainstem nuclei

2. Efferent (output)

a. from deep cerebellar nuclei

1) fastigial
2) interpositus
3) dentate

Note: each deep nucleus is somatatopically organized

b. connections to

1) motor cortex by way of the thalamus (via thalamic synapses)
2) brainstem motor centers -- red nucleus & descending reticular formation (extrapyramidal system)
3) vestibular nuclei (associated with vestibular organs, respond to body position and movement)

E. Functional Divisions (named according to their efferent connections)

1. Vestibulo-cerebellum

a. structures: flocculo-nodular lobe (flocculus and nodulus)
b. input: vestibular organs
c. output: vestibular nuclei, which connect to alpha- and gamma-motoneurons of somatic muscles and eye muscles
d. role: posture and muscle tone, balance, coordination of gait; eye movements

2. Spino-cerebellum

a. structures: vermis and intermediate zone of the cerebellar hemispheres
b. input: most sensory modalities, especially proprioceptive endings (e.g. muscle spindles, tendon organs, joint receptors)
c. output: deep cerebellar nuclei which project to brainstem motor nuclei (red nucleus and reticular motor nuclei)
d. role: adjust force, direction, and rate of movement to match intended action (feedback system)

3. Cerebro-cerebellum

a. structures: lateral cerebellar hemispheres
b. input: cerebral cortex, particularly the cerebral cortex premotor area, supplementary motor area, and parietal lobe sensory areas
c. output: thalamus (ventrolateral nucleus), which in turn projects to the primary motor cerebral cortex and the brainstem red nucleus
d. role: modify the motor output to muscles as the action is being planned
e. motor learning (plasticity): long term modification of motor coordination based upon motor experience; possibly associated climbing fiber synapses mediated by increase Purkinje cell calcium concentration

D. Pathophysiology

1. Deficits apparent only upon movement

2. Difficulty maintaining equilibrium upon standing

Test:  stand with heels together with eyes closed (sensory deficit, positive Romberg test) and with eyes open (cerebellar deficit, negative Romberg)

Note: Effects of ethanol in imparing cerebellar function

3. Ataxic (staggering) gait

4. Atonia; hyporeflexia; pendular response to tendon tap

5. Intention tremor; e.g. unsteady hand-arm motion when reaching for an object

6. Bradykinesia (inability to perform rapid movements); difficulty initiating and terminating movements

Example: alternating pronation and supination of hand

7. Difficulty with adjusting dynamic movement; e.g. past pointing

Example: nose-finger tracking

8. Eye position errors (nystagmus, vertigo)