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#20-02

Physiology Of The Human Body



The Cerebellum
Functional Anatomy

ch 9
p.110

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:

  • The VestibuloCerebellum is the phylogenetically oldest part of the organ and anatomically corresponds to the Flocculus and Nodule. This portion is present in all Vertebrates.

    As the name Vestibulo suggests, this portion of the Cerebellum has Afferent and Efferent connections mainly with the Vestibular Apparatus of the Inner Ear (Semicircular Canals and Maculae).

    The information conveyed is related to changes in head position, acceleration, decelerration and angular movements. Recently, fibers from the Retina and from the parts of the Brain concerned with Eye movements have been shown to terminate in the VestibuloCerebellum.

  • The SpinoCerebellum corresponds anatomically to the Vermis and receives general Sensory (Touch, Pressure, Thermal) and Proprioceptive impulses chiefly from the Ascending Pathways of the Spinal Cord.

    These types of impulses give information concerning the Rate, Force, and Direction of movements as detected by Skin, Muscle, and Tendon Receptors. This aspect of movement is sometimes called Performance.

  • The PontoCerebellum corresponds anatomically to most of the Anterior and Posterior Lobes of the organ and is the largest part of the Cerebellum in animals capable of skilled or complex types of movement.

    The major input to this part of the Cerebellum is via PontoCerebellar fibers originating from Nuclei in the Pons that, in turn, have received impulses from the Motor regions of the Cerebral Cortex. This part of the system conveys Cortical Intent.


Cerebellar Cortex Cells

p.111

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.

p.112

    The net effect of this circuitry is summarized as:
      • All Nuclear Output is Excitatory
      • Cortical input to the Nuclei is Inhibitory
      • All input to the Cortex is Excitatory


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.

Error Control
By comparing intent and performance, the Cerebellum ensures that a movement goes where it is supposed to go, at a proper rate and with a force appropriate to the resistance being overcome. Such an action usually involves initial strong contraction of one set of muscles and subsequent contraction of Antagonist Muscles to control the movement.

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.

Damping
Most body movements are pendular in nature, with a movement in one direction opposed by a force applied in the opposite direction. There is thus a tendency for a back and forth motion to occur, called Tremor. The Cerebellum cancels or damps this tendency, for smooth movements.

Prediction
By comparing information received from Eyes, body parts and Cerebrum, the Cerebellum calculates when a motion should be slowed and ultimately stopped.

Progression

p.112
If a movement is to be accurate and purposeful, particular groups of muscles must be made to contract in a particular order. This function, called Progression, is also handled by the Cerebellum.


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:

Asthenia
This refers to a lack of muscular strength, either during voluntary muscle contraction or in holding posture.

Fatigability
Muscles on the same side as where Cerebellar damage has occurred tire more easily and have slower than normal contraction and relaxtion times, leading to slowed movements.

Hypotonia
The muscles feel flabby and offer less resistance to passive displacement. This may be from lack of response to Spinal Tract Input.

Dysmetria
Literally "difficulty measuring" this term refers to failure to stop a motion at the intended point with overshoot occurring. Prediction would seem to be faulty here.

Ataxia
This term indicates incoordination of muscular activity involving Tremor, failure of progression, and failure accurately to perform rapid alternating movements such as tapping a finger. A swaying, unsteady and wide based Gait is often the most obvious sign.


Review Of Major Motor & Sensory Systems

Motor Activity
p.167
ch.14

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.

p.167

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

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

p.170

    The Cerebrum, Basal Ganglia, BrainStem, & Cerebellum utilize three mechanisms to exert their control via Descending Pathways on the Alpha Motor Neurons:

    • By direct synapse with the Motor Neurons. This gives speed to the activation process.

    • By direct synapse with the Gamma Motor Neurons. This provides a degree of control on the length/tension relationships of the muscle.

    • By synapses with InterNuncial Neurons that can confer Reciprocal Inhibition and other aspects of movement.

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:

  • Basal Ganglia
    The basic function of the Basal Ganglia appears to be the control of Tremor, Balistic Movements, and Muscle Tone.

  • BrainStem
    Superimposed primarily on Spinal Cord activity, the BrainStem, and particularly the Reticular Formation, adds to, subtracts from, and combines the influences of the areas from which it receives signals.

  • Cerebellum
    It acts as the overseer for coordination and the final adjustor of movement. It compares what should be happening via information from the periphery and ensures accurate movement.


Sensory Contribution

p.170

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.

p.171

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.

_________________
Continued_In_(20-03)






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