Extrapyramidal movement disorders

Pyramidal system and peripheral motor neurons provide voluntary muscle contractions. Each complete motor act, no matter how simple it may be, requires a coordinated action of many muscles. The quality of the movement depends not only on the type and amount of the muscles, making it. The same muscles provide various movements. The same movement can be produced more slowly or faster with a greater or lesser force. To produce the movement it’s necessary to involve the mechanisms, regulating the sequence, the force and duration of the muscle contraction and regulating the selection of the necessary muscles. In other words, the motor act is formed as a result of a consistent, coordinated in power and time of inclusion of the separate neurons in cortico-muscular tract, which gives orders to the muscles and the large complex of neural structures outside the pyramidal system, which are united in the extrapyramidal system, operating in reflex-automated way.

Extrapyramidal system includes the cell groups of the cerebral cortex (primarily of the frontal lobes), subcortical ganglia (caudate nucleus - nucl, caudatus, putamen, lateral and medial pallidum - globus pallidus, subthalamic Luys’ body) in the brainstem - substantia nigra, red nucleus, quadrigeminal plate of the midbrain, the nucleus of the medial longitudinal fasciculus (Darkshevich nucleus), the coeruleus locus in the pons of the brain, reticular formation of the descending and ascending tracts cerebellum, y-motor neurons of the spinal cord, etc. Between these formations of the extrapyramidal system there are numerous bilateral contacts (closed neural circles).

Making some voluntary movement, a person doesn’t think about what kind of muscle should work at some particular moment, he doesn’t keep in his mind the scheme of the motor act sequence. Habitual movements are produced without paying any attention to it, the change of muscle contractions to another is automated. These motor automatisms facilitate the most economical use of muscle energy in the process of movement. The new, unknown motor act is always more wasteful energetically, than the automated act.

Improvement of the quality aspect of the movement with their shift to automated, more economical mode is provided by the extrapyramidal system mostly by its basal ganglia.

Morphologically and functionally the striatal - pallid system is divided into striatal and pallid. The pallid system is phylogenetically older, includes lateral and medial pallidum, substantia nigra, red nucleus, subthalamic nucleus. In both pallidas there is a large number of nerve fibers, the number of large neurons is relatively small. The striatal system is phylogenetically "young" and includes the caudate nucleus and putamen with many small and large neurons and a relatively small number of nerve fibers. There is somatotopical distribution in the striatal system: the head – in the anterior parts, upper limb and torso – in the middle parts, the lower limb – in back parts.

Pallid system of fish and striatal pallid system of birds are the supreme motor centers, determining behavior of these organisms. Striatal pallid apparatus provides diffuse body movement, coordinated work of all skeletal muscles in the process of movement, swimming, flying, etc. More delicate differentiation of the motor centers was required in case with higher animals and humans. The pyramidal system, which subdued the striatal - pallid system, emerged in the process of evolution. In human ontogenesis myelination of the striatal conductors ends by the 5th month of life (before the pyramidal system), so in the first months of the child’s life lateral and medial pallidas are the highest motor center. The baby’s motility has distinct pallid features: excess of the movements, rich mimicry with a smile, etc. In course of ageing many movements are getting more common, automated, energetically calculated. The adults’ solidity and steadiness are kind of triumph of striatal pallid system over pallid one.

When learning purposeful movements (including professional movements, such as playing a musical instrument, carpentry, plumbing work, driving, etc.) one can single out two phases. In the first phase (which is conventionally referred to as pallid) the movements are excessive, excessive in strength and duration of the muscle contraction. The second phase (pyramidal-striatal) includes the gradual optimization of the control of the movements. They become energetically rational, the most effective and automated.

Extrapyramidal system has many neuron connections between its formations, thalamus and segmental motor apparatus of the spinal cord (Fig.47).

All the afferent systems of striatal pallid system end in the striate body. These systems include the tracts from the most of the cortex parts, from the mid located nuclei of the thalamus (from the medial central and para-fascicular thalamic nuclei), from the compact zone of the substantia nigra and nuclei of the midbrain median suture. From the striatum the tracks go to the lateral and medial pallidas only and reticular zone of the substantia nigra, from which, the main efferent extrapyramidal systems start. From the medial globus pallidus the axons go to the nuclei of the thalamus (to the central medial, ventrolateral nuclei). The cell processes of these thalamic nuclei are projected on the motor cortex and other areas of the frontal lobe cortex.

From the substantia nigra and globus pallidus the descending systems, which go to the nuclei of the midbrain operculum and brain stem, start. Then, they go to o the motor neurons of the spinal cord within the nigro-reticular-spinal and pallidal-reticular-spinal tracts. From the substantia nigra there are ascending tracks to the nuclei of the dorsal thalamus and to the subthalamic nucleus of the central thalamus. Descending and ascending efferent tracts of substantia nigra have different effects on motor activity of a man.

From the cell nuclei of the brain stem, axons, that pass in the spinal cord of the brain, start, and they end by synapses with cells of the anterior horn at different levels. They include, apart from mentioned above the main nigro - and pallidal - reticular - spinal tract, vestibular – spinal track (tr. vestibuospinalis), olive-cerebrospinal (tr. olivospinalis), rubrospinal- cerebrospinal (tr. rubrospinalis, or Monakow track), tegmental - cerebrospinal (tr. tectospicalis), the medial longitudinal fasciculus (fasciculus longitudinalis mediqalis). Reticular - cerebrospinal tract is the most powerful. It consists of axons of the reticular formation and in the spinal cord passes in the front funicle (the ventral tract) and in the lateral cord (its medial and lateral parts).

The fibers of the tegmental – spinal tract at the level of the medulla oblongata form synapses with cells of reticular formation and this fascicle is part of the reticular – spinal tract, with which it runs down in the anterior funiculus. The same must be said about rubrospinal fascicle, passing in the lateral funicle. A great number of its fibers pass into reticular substance within medulla oblongata and goes down in the reticular - spinal tract. Fibers of the extrapyramidal system, as well as of the pyramidal one, pass along the cerebrospinal axis, from the cortex to the lower spinal cord. However, anatomical - histological structure of the pyramidal and extrapyramidal systems is significantly different. The bodies of all neurons of the pyramidal system are grouped together in the cerebral cortex. Schematically, the pyramidal system is cortical neurons with long axons, approaching the different segments of the spinal cord. Extrapyramidal system is a long column of cells with a large number of nerve fibers throughout the whole brain and spinal cord.

Sometimes this column is increasing in volume (basal ganglia), dense weave of fibers with the cell bodies (globus pallidus, reticular substance of the brain stem, etc.) is formed on some levels. Clinical and especially clinical - anatomical observations facilitated the discovery of the functional significance of the extrapyramidal system; they revealed such forms of movement disorders, that cannot be explained neither by the pyramidal system lesion, nor by ataxia. Many syndromes of this kind has been described. In case with one of them, the most noticeable is slowness and poverty of movement, mask-like face, languid body language, rare winking, overall stiffness, lack of cooperative arm movements during the walking. This picture is called hypokinesis (from the Greek. Hypo - fall, failure, and kinesis - motion). Both in hypo-and hyperkinesis muscle tone disorder, quite different from that observed in lesions of the pyramidal system or peripheral motor neuron, takes place.

In the 20s of this century there was a concept that hypokinesis occur due to the lesions of the phylogenetically old formation - globus pallidus, and hyperkinesis occur due to the lesions of the caudate nucleus and putamen. It was thought that the caudate nucleus and putamen (new striatum) inhibit the old striatum (globus pallidus). However recently the mechanism of hypo-and hyperkinetic emergence was rejected. It was found that extrapyramidal disorder can occur in the lesions of the cerebral cortex and its trunk [E.K Sepp, 1938]. In the 40-60s, the new data on the function of that part of the extrapyramidal system, which is called the reticular formation, was found. (Reticular formation (formalio reticularis) is not only a part of the extrapyramidal system, but also a part of the autonomic-visceral system (limbic-hypothalamic-reticular complex)).

Animal experiments have shown that stimulation of this formation with electric shock leads to the activation of the cerebral cortex. Electroencephalogram shows that slow electrical activity of the cortex switches to high-frequency, low-amplitude (the reaction of desynchronization) electrical activity. Apart from that, there are areas in the reticular formation, stimulation of which activates the activity of the brain and spinal cord, leads to increase of the motor spinal reflexes. This facilitating effect on the spinal cord transmitted over reticular-spinal cord tract. [Magoun 1950]. There are areas within the reticular formation, stimulation of which causes inhibition of the cerebral cortex and spinal cord. It has been found that the impulses, passing reticular-spinal cord tract, reach not only a, but the y-motoneurons. [Granit R., 1973].

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