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Essentials Of Human Physiology

    22-02
    1992


    by: Uwe Ackermann, Ph.D.





Basal Ganglia

p194

The Basal Ganglia exert their influence over networks that link the Motor Cortex to other Cortical areas. They behave as a variable filter, with two primary functions:

  • They match the performance specifications of motor programs to the criteria that have been established by the Motivational and Sensory cues that define a particular circumstance

  • They facilitate the selection of only those motor programs that meet the specific criteria

Basal Ganglia participate in motor control only if incoming signals are facilitated by Dopaminergic input from the Substantia Nigra Compacta. The loss of this facilitation leads to Parkinson's Disease.

In their subsequent function, the Basal Ganglia facilitate or inhibit the incoming signals and modify them by signals from the SubThalamic Nucleus.

The overall effect of Basal Ganglia activity on motor activity is inhibition of inappropriate networks that link the Motor Cortex to the entire Non-Motor portion of the Cerebral Cortex.

They are of particular importance for the selection of bridging (preparatory) subprograms that move a Limb or Muscle from its initial position to one from which a standard Motor program can continue.

As a result, Basal Ganglia Diseases such as Parkinson's Disease are accompanied by impaired ability to perform preparatory movements, and patients appear to freeze before they execute major Motor tasks.


p.195

    Basal Ganglia Nuclei are extensively interconnected:
    1. Striatum (Caudate Nucleus, Putamen, & Nucleus Accumbens)
    2. Substantia Nigra Compacta
    3. Substantia Nigra Reticulata
    4. Pallidum (external & internal)
    5. SubThalamic Nucleus
               (View: Interior Image & Larger Image)

    They receive input from the CentroMedian Nucleus of the Thalamus and from several regions of the Cortex:
      1. Association Cortex projects to Caudate Nucleus
      2. SensoriMotor Cortex projects to Putamen
      3. Limbic Cortex projects to Accumbens

Modification of Striatal output by the SubThalamic Nucleus may be of particular importance. The SubThalamic Nucleus receives inhibitor input from the External Pallidum.

It, therefore carries an inverted image of the External Pallidum. This image is modulated by Cortical input to yield facilitatory output to the Internal Pallidum and to the Substantia Nigra Reticulata.

All Basal Ganglia output is inhibition of ongoing activity


Cerebellum

P.196

    The Cerebellum consists of:
    1. A Three Layered Cortex that is extensively folded from side to side into Folia
    2. Pairs of Internal Nuclei
    3. The White Matter, composed of Fibers entering, leaving, and transversing the Cerebellum
    4. It's attached to the BrainStem by Three distinct Fiber Bundles (Cerebellar Penducles)


Internal Nuclei

    The Major Internal Cerebellar Nuclei
    1. Fastigial Nucleus
    2. Dentate Nucleus
    3. Interpositus Nucleus, is divided into:
      • Globose
      • Emboliform

Most Purkinje Cells in any of the three functional Cerebellar zones (Vermis, Intermediate Hemisphere, & Lateral Hemisphere) project to the same Internal Cerebellar Nucleus.

    Projections from the Cerebellar Nuclei terminate in specific loci and, therefore, modulate specific aspects of motor function:

    1. Fastigial Nucleus to the Lateral Vestibular and Reticular Nuclei (for balance and posture)

    2. Interpositus Nucleus to the Red Nucleus and the VentroLateral Nucleus of the Thalamus (for posture, gait, and coarse movements)

    3. Dentate Nucleus to the VentroLateral Nucleus of the Thalamus (for skilled movements of hands and fingers)

Internal Nuclei are excitatory on muscle tone & motor activity


Cerebellum Functions In Movement Control

    Contributes To Voluntary Movements:
    1. It correlates incoming muscle and other sensory information

    2. It computes the most effective deployment of muscular effort necessary to accomplish a required task

    3. It composes the necessary outgoing commands to the Spinal Motor Neurons and the Motor Cortex

    4. Some zones and associated internal nuclei show electrical activity only after the onset of movement (e.g. the Interpositus Nucleus)

    5. 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

p.197


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.

p.197

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.)


Inhibitory InterNeurons

Stellate Cells
Are excited by parallel fibers and inhibit Purkinje Cells.

Basket Cells
Are excited by parallel fibers. The Axons of each of these form baskets of inhibitory synapses around 20 or more Purkinje Cells.

Golgi Cells
Have dendritic trees that spread in all directions among the parallel fibers (unlike the flattened Purkinje Dendrites, which are confined to a narrow longitudinal layer); are excited by input from parallel fibers and Mossy fibers; act to inhibit Granule Cells.


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