Lecture 13: Cerebellum

Cerebellum

Detects the difference (the motor error) between intended movement and actual movement

→ Influences the upper motor neurons to reduce the error

Cerebral cortical areas (rest of the brain) project to the two main gray matter cerebellar structures (cerebral cortex and deep cerebellar nuclei)

Deep cerebellar Nuclei

The main source of output from the cerebellum

→ output is integrated with input from the cerebellar cortex before being sent to upper motor neurons in the cerebral cortex

Cerebrocerebellum

Receives input indirectly from cerebral cortex

Regulation of highly skilled movements (planning execution of sequences of movements ex. speech)

Spinocerebellum

Receives direct input from the spinal cord

Medial-lateral regulation of muscles

Lateral → Distal muscles

Medial → Proximal muscles

Vestibulocerebellum

Receives input from vestibular nuclei

Regulation of posture and equilibrium

Vestibulo-ocular reflex

cerebellar peduncles

Cerebral pathways → 3 major pathways → Axons traveling in a tract/bundle

Middle, inferior, and superior pathway

Middle cerebellar peduncle

Afferent pathway

Receives cerebral cortical input via contralateral pontine nuclei

Inferior cerebellar peduncle

Afferent pathways from vestibular nuclei, spinal cord, and brainstem

Efferent pathways to vestibular nuclei and reticular formation (sleep-awake, cardiovascular)

Superior cerebellar peduncle

Efferent pathway (output)

Deep cerebellar nuclei projections to the thalamus, (which projects to upper motor neurons in motor cortex) and UMNs in the superior colliculus

Pontine Nuclei

Cerebral cortex and superior colliculus neurons synapse ipsilaterally on here

Relay inputs into the contralateral cerebellum via middle cerebellar peduncles

Inferior Olive

Receives inputs from cerebral cortex, reticular formation, and spinal cord

Project to contralateral cerebellum via inferior cerebellar peduncle

→ Participates in learning and memory

Ipsilateral representations

Vestibular Nuclei → vestibulocerebellum

Cuneate nucleus and Clarke’s nucleus → spinocerebellum

→ Somatosensory input remains topographically organized in spinocerebellum

Vestibular and spinal inputs remain ipsilateral

Dentate Nucleus

A deep cerebellar nuclei that receives input from the cerebrocerebellum

To the premotor cortex (motor planning)

Interposed and fastigial nuclei

Deep cerebellar nuclei that receives input from the spinocerebellum

Motor cortex and brainstem (motor execution)

Projections to Cortex

Responsible for voluntary movment

Cerebellar Cortex → deep cerebellar nuclei → Superior cerebellar peduncle → contralateral thalamus (+ superior colliculus) → upper motor neurons in the primary motor and premotor cortex

Descending cerebellar outputs

Dentate and interposed nuclei project to contralateral upper motor neurons in the superior colliculus (via superior cerebellar pundcle) → eye movements

Fastigial nuclei poject via the inferior cerebellar peduncle to nuclei of the reticular formation and vestibular complex

Motor Control Centers

Several structures in the brainstem contain circuits of upper motor neurons

UMNs are in the nuclei of the vestibular complex, the reticular formation, and the superior colliculus

Involved in balance, posture, locomotion, and gaze

Work with divisions of motor cortex that organize both skilled (voluntary) and supporting (reflexive) motor activities

Cerebellum Layers

3 distinct layers → molecular, purkinje, and granule

Mossy fibers → granule cells → parallel fibers (molecular layer) → purkinje cell dendrites

Purkinje Cells

Located in the ___ Layer → Projects GABAergic (inhibitiory) output to deep cerebellar nuclei

Receives input from parallel fibers and climbing fibers → excitatory output on deep cerebellar nuclei

GABAergic inhibition shapes discharge patterns generated by deep nuclei neurons that are being excited → error correction signal

Climbing Fiber (“Training Signal”)

Carry information via inferior olive

Neurons drive activation and promote adaptive plasticity in the inhibitory output

Relevant for long-term motor learning

Modulation of Movement

Cerebellum continually monitors and adjusts motor behavior → firing pattern closely follows movement

Both Purkinje cells and Deep nuclear cells are tonically (constant) active at rest & change their frequency of firing as movements occur (ex. flicking of the wrist)

Cerebellar ataxia

• Difficulty producing smooth, coordinated movements → instead: jerky, imprecise actions

• Problems are on the same side as the lesion

• Specific movements disrupted vary with the damage location

Wernicke's encephalopathy

Characterized by:

• involuntary, jerky eye movements or paralysis of eye muscles

• poor balance, staggering gait or inability to walk (ataxia)

• drowsiness and confusion

Korsakoff's psychosis

Characterized by:

• anterograde amnesia (inability to form new memories)

• retrograde amnesia (loss of existing memories)

• confabulation (false perceptions or memories)

• hallucinations

Korsakoff’s syndrome

(Wernicke-Korsakoff Encephalopathy → Wernicke’s Encephalopathy + Korsakoff’s Psychosis)

Vitamin B1 Deficiency associated with chronic alcoholism → poor eating habits and alcohol-induced inflammation of the stomach

Gain Adjustments

Muscles in the weakened eye adjust to make up for weakened muscle after patch is put on the healthy eye

After 5 days of practice, the muscle is sufficient again (motor learning)

→ Healthy eye now overcompensates

Motor learning is impaired after cerebellar lesions

vestibulo-ocular reflex

maintains gaze during head movements

Cerebellar lesions impair this ability to compensate