Cerebellum and Motor Control

• Chapter 19 discusses the modulation of movement by the cerebellum.

Cerebellum Overview

• The cerebellum makes up 10% of the brain volume.
• It contains more than half the brains neurons.
• The cerebellum does not directly project to motor neurons in the spinal cord.
• Movement is modified by regulating upper motor neurons, such as those in the motor cortex.
• The cerebellum is made up of a cerebellar cortex and deep cerebellar nuclei and plays a roll in motor learning, error correction, and sensory-motor integration.

Cerebellum Subdivisions

• The cerebellum consists of three major subdivisions: cerebrocerebellum, spinocerebellum, and vestibulocerebellum.
• Cerebrocerebellum receives input from the cerebral cortex via the pons and is involved in skilled movements (e.g. speech).
• Spinocerebellum's medial portion receives input directly from the spinal cord.
• Spinocerebellum, Vermis portion, is responsible for posture, locomotion, and eye movements.
• The spinocerebellum's paramedian part is responsible for distal muscle movement.
• Vestibulocerebellum receives input from the vestibular nuclei.
• Vestibulocerebellum is responsible for balance, and vestibular reflexes, eye movements.
• Cerebellar peduncles are thick tracts that connect the cerebellum with the rest of the central nervous system.
• Deep cerebellar nuclei project to the upper motor neurons in the motor and premotor cortices via the thalamus.
• Output from the cerebellum goes via the deep cerebellar nuclei.
• The three pathways between the cerebellum and the CNS include the superior, middle, and inferior cerebellar peduncles.
• Middle Cerebellar Peduncle provides input to the cerebellum.
• The Cerebral cortex has the largest input to the cerebellum.
• The Cerebral cortex travels contralaterally through the middle cerebellar peduncle.
• The sensory input travels ipsilaterally through the inferior cerebellar peduncle.
• The cerebellum is somatotopically organized, however, the maps are fractured with multiple representations.
• Cerebellar hemispheres coordinate movement of the ipsilateral body.

Cerebellar Outputs

• The cerebro-cerebellum projects to the dentate nucleus, then to the premotor cortex, for motor planning and learning.
• The spino-cerebellum projects to the interposed and fastigial nuclei, then to the motor cortex and brainstem, for motor execution.
• The vestibulo-cerebellum projects to the vestibular nuclei and then to the lower motor neurons in the spinal cord and brainstem, for balance and vestibulo-ocular regulation.
• Dentate and interposed axons exit the cerebellum via the superior cerebellar peduncle and cross the midline before synapsing with the thalamus or superior colliculus.
• Eye movement-related neurons in the superior colliculus receive input from the contralateral cerebellar cortex via the superior cerebellar peduncle.
• The fastigial nuclei project via the inferior cerebellar peduncle to upper motor neurons that control axial and proximal limb muscles.

Cerebellar Peduncles

• The inferior cerebellar peduncle contains afferent fibers from vestibular nuclei and spinal cord, and efferents to the vestibular nuclei and reticular formation.
• The middle cerebellar peduncle contains afferents from the contralateral pontine nuclei.
• The superior cerebellar peduncle contains efferent fibers from the cerebellar nuclei to thalamus and superior colliculus.

Cerebellar Microcircuit

• The granular layer is the deepest layer with 100 billion cells give rise to parallel fibers which send signals to the molecular layer.
• The middle or Purkinje cell layer is the output layer of the cerebellar cortex.
• Purkinje cell dendrites extend upwards and receive input from parallel molecular-layer fibers.
• The output of the Purkinje cells goes to the deep cerebellar nuclei.
• The molecular layer is the outermost processing layer of the cerebellar cortex.
• There are 5 cell types in the cortex: granule (+), Golgi (-), Purkinje (-), stellate (-), basket (-).
• The ultimate afferent destination for the cerebellar cortex is the Purkinje neuron.
• Mossy fibers are a major input from the brainstem and spinal cord.
• Granule cells give rise to parallel fibers.
• Climbing fibers from the inferior olive synapse onto Purkinje cells.
• Basket, stellate, and Golgi cells are all inhibitory with regards to Purkinje.
• The mossy fibers arise from the cell bodies from the pontine nuclei, brainstem, and spinal cord, and synapse on to granular cells, which give rise to parallel fibers, and have excitatory synapses onto the dendritic spines of Purkinje cells.
• Each Purkinje neuron is contacted by 200,000 to 1 million granule cells.
• The climbing fibers arise from the inferior olive, contact Purkinje cells directly and excite the deep nuclei, there is an excitatory synapse onto Purkinje cells.
• Each Purkinje cell receives several synaptic contacts from a single climbing fiber.
• Purkinje cells project to the deep cerebellar nuclei and are the only output cells of the cerebellar cortex.
• The output is GABAergic and inhibitory.
• The deep cerebellar nuclei receive inhibitory inputs from Purkinje cells that converge with excitatory input.
• This inverts the sign of the input signal and has the ability to generate the correct signal that can modify movements.

Reduction of Motor Error

• The cerebellum is able to coordinate ongoing movement by reducing motor error by way of cerebellar circuitry and coordination.
• Deep cerebellar cells and Purkinje cells recognize potential errors by comparing convergent activity concurrently available to both cell types.
• Reduced motor error has been observed in cases where errors are induced.

Cerebellar Damage

• No VOR adaptation occurs if the cerebellum is damaged.
• Lesions of the cerebellum do not cause paralysis but large movement errors instead.
• Cerebrocerebellum damage: deficits in coordination and visuomotor integration.
• Vestibulocerebellum damage: impairs ability to stand upright and maintain a direction of gaze.
• Spinocerebellum damage: difficulty walking.
• Additional signs of cerebellar damage: dysmetria, action or intention tremors, and speech deficits.
• Chronic alcohol use can degenerate the vermis, leading to difficulty walking (wide and staggering gait).
• Cerebellar damage leads to movement errors that that occur on the same side of the body.
• Dysmetria is a movement inaccuracy.
• Dysdiadochokinesia is difficulty performing rapid alternating movements.
• Ataxia is difficulty producing smooth, well-coordinated movement.