The Cerebellum Contributes To Voluntary Movements:
- It correlates incoming muscle and other sensory information
- It computes the most effective deployment of muscular effort necessary to accomplish a required task
- It composes the necessary outgoing commands to the Spinal motor Neurons and the Motor Cortex
- Some zones and associated internal nuclei show electrical activity only after the onset of movement (e.g. the Interpositus Nucleus)
- Their purpose is thought to be compensation on the basis of Sensory Feedback
Other zones show electrical activity before the onset of movement (e.g. the Dentate Nucleus). They probably participate in the generation of motor sequences.
Their particular function is control of the relative timing of Agonist and Antagonist Alpha Motor Neuron activity to effect a smooth pattern of limb Acceleration, Deceleration, Stop, and Acceleration in the Opposite Direction.
Mild Cerebellar Dysfunction results in inability to judge the range of limb movements without watching them.
Severe Cerebellar Dysfunction results in inability to perform limb movements smoothly and efficiently even while watching them.
Internal Cerebellar Organization
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A - Incoming tracts
- Mossy fibers
- Climbing fibers
B - Purkinje cells
C - Granule cells
D - Internal Nuclei along with their outflow tracts
There are also three populations of local Inhibitory InterNeurons that modulate the function of Purkinje & Granule Cells:
- Basket cells
- Stellate cells
- Golgi cells
Input & Output Neurons
Simple Spike Discharges:
Incoming Mossy Fibers form excitatory Synapses with Granule Cells:
- Granule Cells are the major input cells
- Axons project toward the surface, bifurcate, and form a layer of parallel fibers in the direction of the Folia
- Each parallel fiber forms excitatory synapses with a sequence of several dozen Purkinje Cells at their flattened Dendritic bushes
- Purkinje Cells are the major Cortical Output Cells
Excitation of a sufficient number of parallel fibers contacting a given Purkinje Cell will cause that Cell to discharge trains of simple spikes that inhibit the muscles to which the Cell projects.
Complex
Spike Discharges
Purkinje Cells also receive excitatory input from Climbing Fibers that originate mostly in the opposite Inferior Olive. (Each region of the Inferior Olive projects to a seperate longitudinal strip of Cerebellar Cortex).
Climbing Fibers synapse extensively with dendrites of Purkinje Cells, but not with the Cell Body. Climbing Fiber input to a Purkinje Cell produces in that Cell a large, prolonged complex spike discharge.
Internal Nuclei
In addition to the obvious anatomic division of the Cerebellum into horizontal folds, there is a functional subdivision into three vertical strips:
1 - Vermis
2 - Intermediate Hemisphere
3 - Lateral Hemisphere
Different body parts are topographically mapped in these zones:
* Trunk and Head in the Vermis
* Limbs stretched out toward the Lateral Hemisphere.
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Nearly all Purkinje cells in any one of the three strips project to the same internal Cerebellar Nucleus:
- Vermis to the Fastigial Nucleus
- Intermediate Hemisphere to the Interpositus Nucleus (Globose & Emboliform Nuclei)
- Lateral Hemisphere to the Dentate Nucleus
- ( Purkinje Cells in the Flocculonodular Lobe are an exception because they project directly to the Lateral Vestibular Nucleus in the MidBrain, where they participate in the control of Eye movements.)
since July 16, 1998.
