The Cerebellum lies on the posterior aspect of the BrainStem, attached to it by three pairs of Cerebellar Peduncles that contain both Afferent and Efferent Nerve fibers. A centrally placed Vermis supports the two laterally placed Cerebellar Hemispheres. Small folds abound in all parts of the Cerebellum and are known as Folia.
These Folia let the Convolutions of the Cerebellum provide a vastly increased surface area for placement of Neurons. An outer covering of Gray Matter, the Cerebellar Cortex, overlies the Medullary Body of White Matter. Deep Nuclei lie more or less centrally within the organ.
The Cerebellum is Composed of Three Portions:
Cerebellar Cortex Cells
The Cerebellar Cortex contains the cells and circuitry that enable the organ to carry out its functions, and the structure is the same in all parts of the organ. The Inner or Deepest layer of the Cortex, called the Granule Layer, consists of many closely packed Granule Cells and cells called Golgi Cells.
The middle layer of the Cortex consists of a single row of large Purkinje Cells associated with Basket Cells. The Purkinje Dendrites ramify in the Outer Molecular Layer and are associated with Stellate Cells.
Input to these cells is provided by Mossy and Climbing Fibers. Mossy Fibers synapse with Golgi or Granule Cells (and also send branches to the Deep Nuclei), while Climbing Fibers reach all Five basic cell types (Granule, Golgi, Purkinje, Basket, and Stellate) and the Deep Nuclei.
This input is all excitatory. The Deep Nuclei receive impulses from the Mossy and Climbing Fibers and from the Purkinje cells. The Deep Nuclei generate the Output of the Cerebellum to all aspects of Motor Activity and the Impulses are apparently All Excitatory to the cells affected.
The Purkinje Cells, on the other hand, are invariably Inhibitory to the Nuclei they affect.
Functions Of The Cerebellum
In general terms, the function of the Cerebellum is; to compare intent and performance with regard to muscular activity and movement; to ensure that the movement is accurate and coordinated; and moves with appropriate force and direction. It operates entirely at a subconscious level.
Referring back to our circuit diagram, we can perhaps appreciate that the Purkinje Cell exerts a waxing and waning inhibitory effect on the Deep Nuclei that have been excited by Cerebral Cortical and Peripheral information and in this manner refine and control the movement.
Disorders Of Cerebellar Function
If one understands the four types of functions just described, it should be easy to predict that damage in the organ must and will have primary effects on the appropriateness and coordination of movement.
The severity of symptoms seems to depend on the amount of tissue destroyed and not on where the damage is. Among the most characteristic signs of Cerebellar damage are the following:
Review Of Major Motor & Sensory Systems
The basis of movement is the Skeletal Muscle and the Neurons that control it. Anything that affects the muscle must do so by impinging upon the Spinal Motor Neurons themselves.
The Motor Neurons act as final Common Pathways for signals of Voluntary, Involuntary, and Reflex nature. More critical is the balance between various inputs to the Motor Neurons so that no one influence can override the effects of another.
Where a given motor activity originates is not really known. Is it in the Cortex, the Basal Ganglia, the Cerebellum, or the BrainStem? Wherever the command to perform a Motor Task originates, all parts of the Motor System and certain parts of the Sensory System cooperate to ensure accurate and appropriate motion.
Action Potentials in the Voluntary Motor Pathways influence the Cord Motor Neurons, and by branches to Cerebellum, Basal Ganglia, and BrainStem they inform these areas of what the new directives are. These in turn are fed back to the Cortex to adjust the movement.
The Thalamus receives Sensory Input from the Muscles, Joints, and Tendons and feeds it to the Motor Areas for further Coordination and Control over the Movement. The closed loops formed by the Sensory Input and Motor Output of Spinal Segmental Arcs may act autonomously but are always under the influence of higher levels of the Motor System.
Most motor behavior is neither purely Voluntary nor Involuntary, containing components of each. Actions that are initially strongly voluntary can be reduced to being nearly automatic by Repetition and Learning.
Local control of motor activity is served largely by the Reflex Arcs involving the Muscle and Tendon Spindles, their Afferent Neurons, and the output back to the muscles via Efferent Alpha and Gamma Motor Neurons. Such activity tends to control the length of the muscle, particularly in Posture maintenance, and is basically involuntary in activity.
Higher Levels Of Control
The CorticoSpinal Pathways act as the direct pathways for muscle activation. The Neurons of this pathway reach from Cortex to Cord without synapses and serve as the Upper Motor Neurons for movement. The tracts provide the input necessary for initiation of a movement and are required for skilled movements.
Other Motor Pathways Consist Of
Several Neurons In The Pathway:
It should be clear that muscular performance must be relayed to the Motor Areas, and this is where Proprioceptive input and the Cerebellum enter the picture. Without precise information about what is actually going on, a movement cannot be controlled or adjusted.
Input passes to the Nucleus Dorsalis and then to the Ipsilateral Dorsal SpinoCerebellar Tract or to the ContraLateral Ventral SpinoCerebellar Tract. Final termination is in the Cerebellum to relay the performance of the movement.
Touch and Pressure pass by one of two routes to the Thalamus. Sense of Texture, Form, and Vibration are relayed directly from the Periphery via Ipsilateral Gracile and Cuneate Tracts to Nuclei in the Medulla, and from there to the Thalamus. (View Image)
Gross appreciation of Touch and Pressure sees Peripheral Neurons synapsing in the Substantia Gelatinosa and crossing to the ContraLateral SpinoThalamic Tracts. The tracts then proceed to the Thalamus. (View Image)
Pain and Thermal sensations follow a similar route. From the Thalamus, impulses for Touch, Pressure, Pain, and Thermal sensations are relayed to the Cerebrum. (View Image)
Note the interrelationships among local control that results in withdrawl from the painful stimulus, the crossed-extension reflex that maintains balance, and the appreciation of the nature of the stimulus.