Cerebellum: Movement Modulation

• Modulation of movement is controlled by the cerebellum

The Cerebellum

• Constitutes 10% of total brain volume
• Contains more than half of the brain's neurons
• Does not directly project to motor neurons in the spinal cord
• Modifies movement by regulating upper motor neurons in the motor cortex
• Consists of a cerebellar cortex and deep cerebellar nuclei
• Role in motor learning, error correction, and sensory-motor integration

Cerebellar Subdivisions

• The cerebellum includes the cerebrocerebellum, spinocerebellum, and vestibulocerebellum

Cerebrocerebellum

• Receives input from the cerebral cortex through the pons
• Involved in highly skilled movements like speech

Spinocerebellum

• Medial regions receive direct input from the spinal cord
• The vermis (median strip) is responsible for posture, locomotion, and eye movements
• Paramedian parts deal with movements of distal muscles

Vestibulocerebellum

• Receives input from the vestibular nuclei
• Important for balance, and vestibular reflexes

Cerebellar Peduncles

• Thick tracts connect the cerebellum to the rest of the CNS.

Deep Cerebellar Nuclei

• The nuclei project to upper motor neurons in motor and premotor cortices via the thalamus

Input to Cerebellum

• Cerebral cortex provides the largest input
• Input travels contralaterally through the middle cerebellar peduncle
• Sensory input travels ipsilaterally through the inferior cerebellar peduncle

Middle Cerebellar Peduncle ("MCP")

• Provides input only and contains transverse pontine fibers

Somatotopic Organization of Cerebellum

• Cerebellum is somatotopically organized, and maps are “fractured” with multiple representations
• Cerebellar hemispheres coordinate ipsilateral body movement

Functional Organization of Cerebellar Outputs

• Cerebellar cortex projects to deep cerebellar nuclei
• Pathways ultimately reach upper motor neurons, premotor cortex, and lower motor neurons

Cerebellar Outputs: Dentate and Interposed Nuclei

• Dentate and interposed axons exit the cerebellum via the superior cerebellar peduncle
• They cross the midline before synapsing with the thalamus/superior colliculus
• Send collaterals to the red nucleus, which then projects to the inferior olive

Cerebellar Outputs: Fastigial Nuclei

• Project via the inferior cerebellar peduncle to upper motor neurons
• These neurons control axial and proximal limb muscles

Inferior Cerebellar Peduncle

• Primarily contains afferent fibers from the vestibular nuclei and spinal cord
• Also contains efferents that project to the vestibular nuclei and reticular formation

Middle Cerebellar Peduncle

• Contains afferents from the contralateral pontine nuclei

Superior Cerebellar Peduncle

• Contains efferent fibers from the cerebellar nuclei to the thalamus and superior colliculus

Cerebellar Cortex

• Neurons are organized into three layers

Granular Layer

• Deepest layer
• Contains 100 billion granule cells
• Granule cells give rise to parallel fibers that ascend to the outermost molecular layer

Purkinje Cell Layer

• Middle layer and output layer of the cortex
• Dendrites of Purkinje cells extend upward into the molecular layer
• Receives input from many parallel fibers
• Output goes to the deep cerebellar nuclei

Molecular Layer

• Outermost and important processing layer of the cerebellar cortex

Cerebellar Cell Types

• There are five: granule (+), Golgi (-), Purkinje (-), stellate (-), and basket (-)

Purkinje Neurons

• Ultimate destination for afferent input in cerebellum cortex
• Integrate afferent input and modulate the output of deep cerebellar nuclei

Mossy Fibers

• A major source of input to the cerebellum from the brainstem and spinal cord

Granule Cells

• Give rise to parallel fibers

Climbing Fibers

• Inferior olive axons synapse onto Purkinje cells

Basket, Stellate, and Golgi Cells

• All inhibitory and provide input to Purkinje cells

Cerebellar Inputs

• Two: mossy fibers and climbing fibers
• Mossy fibers arise from pontine nuclei, brainstem, and spinal cord and synapse (excitatory) onto granule cells
• Granule cells then give rise to parallel fibers that have excitatory synapses onto Purkinje cell dendrites
• Purkinje neurons are contacted by 200,000 to 1 million granule cells

Climbing Fibers

• Arise from the inferior olive
• Synapse directly with, and excite, Purkinje cells and also excite the deep nuclei
• Each Purkinje cell receives numerous synaptic contacts from a single climbing fiber

Purkinje Cells

• Project to the deep cerebellar nuclei
• Only output cells of the cerebellar cortex
• Utilize GABAergic inhibitory output

Deep Cerebellar Nuclei

• Inhibitory inputs from Purkinje cells converge with excitatory inputs here
• The cerebellum coordinates ongoing movement by reducing motor error

Error Recognition

• Deep cerebellar cells and Purkinje cells recognize potential errors by comparing convergent activity concurrently available to both cell types

Induced Errors

• Potential for errors can be experimentally induced by cutting the lateral rectus muscle in the monkey eye, covering the normal eye
• The weak eye initially becomes hypometric, then hypermetric

Learned Changes

• The cerebellum contributes to experience-dependent modification
• No VOR adaptation occurs if the cerebellum is damaged

Cerebellar Lesions

• Do not cause paralysis
• Result in large movement errors and functional deficits associated with the damaged area

Cerebrocerebellum Damage

• Results in deficits in coordination and visuomotor integration

Vestibulocerebellum Damage

• Impairs the ability to stand upright and to maintain a direction of gaze

Spinocerebellum Damage

• Results in difficulty walking

Other Signs of Cerebellar Damage

• Dysmetria (over or under reaching a target)
• Action or intention tremors
• Speech deficits

Chronic Alcohol Use

• Can eventually degenerate the vermis
• Leads to difficulty walking (wide and staggering gait)

Movement Error Side

• Movement errors are on the same side of the body as the damage to the cerebellum

Dysmetria

• Is movement inaccuracy

Dysdiadochokinesia

• Difficulty performing rapid alternating movements

Ataxia

• Difficulty producing smooth, well-coordinated movement