Extrapyramidal System

The extrapyramidal system is the oldest motor-tonic mechanism in phylogenetic terms, encountered as early as in fish. Its main part is the corpus striatum, which is why, while somewhat narrowing the anatomical-physiological substrate, it is sometimes also called the system of the corpus striatum, the striatal or striopallidal system. But the corpus striatum is only the effector part of this complex reflex mechanism, which extends far beyond the limits of the corpus striatum, and therefore the extrapyramidal system as a whole cannot be imagined without connection to its other parts—the optic thalami and hypothalamus, as well as the cerebellum (striocerebellar system) and mesencephalic formations. The anatomical basis of the extrapyramidal system thus consists predominantly of the basal ganglia, and its center is the corpus striatum (see Basal ganglia), which includes the nucleus caudatus (caudate nucleus) and the nucleus lenticularis (lenticular nucleus). The latter is divided into the putamen and globus pallidus. This nomenclature was replaced by the Vogts (C. and O. Vogt) with a shorter one, namely: striatum instead of nucleus caudatus and putamen, and pallidum instead of globus pallidus. Phylogenesis. In fish, there is still only the palaeostriatum (pallidum), which is a cluster of cells connected with the optic thalamus and caudally located nuclei. In amphibians, the neostriatum (putamen) already appears. In reptiles, the neostriatal part of the corpus striatum becomes even more developed. In birds, the changes in the corpus striatum consist mainly in an increase in its total mass and in a greater development of connections with various parts of the nervous system. In mammals, sharp changes occur in the structure of the corpus striatum—besides an increase in its volume, the internal capsule appearing here divides the neostriatum into two parts—the putamen and the nucleus caudatus. Ontogenesis. The pallidum is the first to be laid down. According to Kodama, in a human embryo 3 cm long, this latter can already be clearly seen, somewhat outpacing the formation of the putamen. In a 4.7 cm embryo, the pallidum divides into two parts. In a 10 cm embryo, the cells of the pallidum are already fully formed, and in a 5-month fetus, the first myelin fibers appear in it. By the 8th month of embryonic life, the myelination of the pallidum is almost finished. The putamen lags behind the pallidum in its development. The first differentiation of the cellular structure in it appears clearly only in an embryo 10 cm long. Myelination begins in the 9th month and ends in postnatal life. The formation of the nucleus caudatus begins even later; its primordium appears for the first time only in an embryo 5 cm long. Its cells develop slowly and reach their full development only by the end of the first year of life. Myelination reaches its maximum in the 11th month of life. Morphology. It is more correct to begin the description of the extrapyramidal system with its main, phylogenetically oldest link—the pallidum. By this is meant the former globus pallidus or those two internal segments of the nucleus lenticularis which, by means of a lamina of myelin fibers, are separated from the external segment of this nucleus or the putamen (see figure). The morphological structure of the pallidum is of a rather primitive character compared to other parts of the extrapyramidal system and reduces to a comparatively small cluster of gray matter, permeated by a dense network of myelin fibers. Its cytoarchitectonics is distinguished by the uniformity of the internal structure of the cells. In the sense of myeloarchitectonics, the pallidum also differs sharply from the striatum by its extreme richness in nerve fibers, characterizing it as a predominantly conductive organ. The striatum, by its morphological structure, appears to be a much more complex formation than the pallidum, especially in cytoarchitectonic terms. In any of its parts, it...

___, bus pallidus or those two internal segments of the nucleus lenticularis, which by means of a lamina of myelin fibers are separated from the external segment of this nucleus or the putamen (see figure). The morphological structure of the pallidum is of a rather primitive character compared to other parts of the extrapyramidal system and reduces to a comparatively small cluster of gray matter, permeated by a dense network of myelin fibers. Its cytoarchitectonics is distinguished by the uniformity of the internal structure of the cells. In the sense of myeloarchitectonics, the pallidum also differs sharply from the striatum by its extreme richness in nerve fibers, characterizing it as a predominantly conductive organ. The striatum, by its morphological structure, appears to be a much more complex formation than the pallidum, especially in cytoarchitectonic terms. In any of its parts, it...

Extrapyramidal system: 1-pedunculus thalami sup.; 2-tractus cortico-spinalis; 3-insula; 4-claustrum; 5-putamen; 6-globus pallidus; 7-ansa lenticularis; 8-substantia nigra; 9-nucleus subthalam.; 10-nucleus ruber; 11-pedunc. thalami inf.; 12-thalamus opticus; 13-nucleus caudatus.

one is struck by the huge accumulation of nerve cells. For the structure of the corpus striati and pallidi, see Basal ganglia. According to Vogt, the striopetal fibers, upon entering the striatum, end near small cells with short axons. The latter act on large cells with long axons, which are the source of the striofugal fibers that transmit excitation to the cellular apparatus of the pallidum. In addition to the striatum, the E. s. also includes other basal and mesencephalic formations—Luys' body, the red nucleus, the substantia nigra (corpus Luysi, nucleus ruber, substantia nigra). Of the nuclear formations of the subthalamic area, the largest is the corpus Luysi (see). It is a well-defined, almond-shaped nucleus, located dorsally to the pes pedunculi and substantia nigra. Histologically, when stained for myelin, it appears surrounded by a capsule, the fibers of which connect it to almost all the important formations of the E. s. A very important component of the E. s. is the red nucleus—nucleus ruber (see), located in the tegmentum of the cerebral peduncle at the level of the superior colliculi of the quadrigeminal body. Histologically, it presents the picture of a nuclear formation, not very densely populated with cells, but richly supplied with fibers going in various directions, especially from the superior cerebellar peduncle. According to Foix and Nicolesco, three types of cells are distinguished in the nucleus: 1) large cells, located in the caudal part of the nucleus, 2) medium-sized cells, which constitute the main mass of the nucleus and have connections with other parts of the brain, and 3) small cells, which belong to the proper apparatus of the nucleus. The capsule of the red nucleus is formed by the radiatio of the red nucleus, the radiatio of the tegmentum, and finally by fibers coming from the pallidum. From the caudal part of the nucleus, large cells give rise to one of the main extrapyramidal paths—the bundle of Monakow (tractus rubro-spinalis), which goes to the spinal cord. In the red nucleus, we have a very important motor-coordinative center of the E. s., which receives its excitations from the dentate nucleus of the cerebellum, from the cortex of the frontal lobe, and from the pallidum, with which it stands in close connection, serving as a place of switching of the strio-pallidal apparatus to the underlying mechanisms. A unique mesencephalic node of the E. s. is the substantia nigra. In its cytoarchitectonic aspect, it is divided into two zones: the dorsal, composed of groups of large cells containing melanin, and the ventral, formed by spindle-shaped cells resembling the cells of the pallidum. Its myeloarchitecture is still little studied. Among other formations of the E. s., the substantia nigra sharply differs by the presence of direct connections with the cerebral cortex. Extrapyramidal paths. At the center of the E. s. stand the thalamus and the subthalamic area as indirect and direct sources of impulses. As for the direct connections of the striatum with the cerebral cortex, this question cannot yet be considered finally resolved. A number of authors (Bechterev, Cajal, Economo, Marinesco, Minkowski, etc.) recognize the existence of such connections, while the majority (Vogt, Monakow, Wilson, etc.) deny them, allowing only indirect ones. The main connections of the striatum with the thalamus, subthalamic area, and mesencephalic formations are carried out by means of the following systems. The lenticular loop (ansa lenticularis) is a bundle of striopetal and striofugal fibers, which, crossing the lentiform nucleus, bend along the ventral surface of the pallidum and are directed from the outside inward and from behind forward, ending in the thalamus (or beginning in it for the striopetal fibers), as well as in a number of hypothalamic and mesencephalic formations, especially in the red nucleus. Crossing the internal capsule, the lenticular loop fans out at the border of the hypothalamus into a series of fibers—the anterior ones go to the area of the infundibulum, the middle ones to the thalamus, and the posterior ones, the most numerous, partly also to the thalamus, but mainly to the subthalamic area and to the red nucleus. Forel's thalamic bundle (fasciculus thalamicus Foreli) (H1) originates in the angle formed by the bundle of Vic d'Azyr and the lenticular bundle. It includes fibers of the superior cerebellar peduncle to the thalamus and from the thalamus to the red nucleus. Forel's lenticular bundle (fasciculus lenticularis Foreli) (H2) is a collection of fibers coming mainly from the radiatio of the pallidum, and also partly from the lenticular loop. It contains both striopetal and striofugal conductors. By giving off its posterior fibers to Luys' body, it contributes to the formation of its capsule, while its anterior fibers are closely connected with the periventricular nuclear formations, and its middle fibers with the centers of the hypothalamic area. Fibrae strio-luysianae originate from the radiatio of the pallidum and go along the same path as the previous bundle to Luys' body, also participating in the formation of its capsule. Fibrae strio-thalamicae connect the striatum directly with the thalamus. Commissura Meynerti, according to Déjerine, is an interstriatal system, i.e., connecting both striata with each other. Of the pallidofugal paths, it is necessary to note the tractus pallido-peduncularis—a system of fibers going mainly to the substantia nigra, and the tractus pallido-rubralis, as a further development and continuation of the powerful bundle H3 to the red nucleus. From a group of large cells of the red nucleus begins the fasciculus rubro-spinalis (Monakow's bundle)—one of the main efferent paths of the E. s., going after the decussation in the direction of the spinal cord. Among other descending paths, it is necessary to note the connections of the pallidum through the posterior commissure with the quadrigeminal body, from which the tractus tecto-spinalis begins, going to the spinal cord, as well as the tractus vestibulo-spinalis from the nuclei of Deiters and Bechterev. The main of the indicated paths should be considered the pallidopetal paths from the thalamus to the pallidum of the same side, the paths from the thalamus to the neostriatum, from the neostriatum to the pallidum, and from the latter to the various hypothalamic and mesencephalic formations included in the E. s. At the same time, one cannot ignore other connections of the E. s., in particular with the cerebral cortex, even if indirect ones. Physiology. To form a general idea of the basic functions of the E. s. and of the evolution of the latter, it is necessary to take at least a brief look at the phylogenetic development of these functions in the animal world and on their ontogenesis in man. In fish, the entire motor apparatus is still represented only in the form of the pallium. The latter, receiving impulses from the thalamus, transforms them into motor and tonic ones. In accordance with this simple mechanism, the movements of fish are characterized by the following features: 1) elementariness, 2) constancy, 3) diffuseness of distribution, 4) automatism, 5) rhythm. The movements are smooth, slow, stereotyped, and consist in the sequential contraction of the entire musculature. In reptiles, in connection with the presence, in addition to the pallidum, of an elementary striatum, which plays the role of a brake and regulator of the pallidal apparatus, the movements no longer have a constant character, although they still retain elementariness. In birds, where there is a high development of the striatum, the movements are much more complex and differentiated. In mammals, along with the further evolution of the striatum, the motor cortex and the pyramidal path (see Pyramidal system) appear as a neokinetic system. However, in most higher representatives of mammals (dog, monkey), removal of the cortex does not yet deprive them of motoricity. As Rothmann showed, a dog without a cortex but with the striatum preserved is still capable of independent movements, whereas a dog after removal of the cortex together with the striatum, according to Dresel's data, is already completely incapable of them, although elementary locomotor functions and posture are preserved. In man, removal of the motor cortex causes complete paralysis. The data of ontogenesis in man depict almost the same picture of the development of motoricity as the data of phylogenesis. As Minkowski showed, a 2-5 month embryo already has certain movements of the choreoathetotic type. By the time the child is born, the thalamo-pallidal paths and the pallido-fugal ones are already myelinated, whereas the thalamo-striatal and strio-pallidal ones are not yet fully developed. In accordance with this, the movements of the newborn have clear pallidal features. Only after 5 months, thanks to the participation of the striatum, more complex and differentiated acts appear, for example, the transition from a lying to a sitting position. Even later, the ability to assume and maintain an orthostatic position appears, and finally the ability to walk, which in its differentiated forms is already closely connected with the neokinetic-pyramidal system. The latter, however, by no means takes upon itself the fullness of motor functions. It acquires in man only a dominant significance in motoricity, insofar as it carries out volitional, purposeful, and strictly differentiated movements. Therefore, every complex motor act of man should be considered as a complex act, containing in itself a whole series of components of different origins, of different historical age, and finally of different functional value.

The extrapyramidal system in these motor acts primarily provides a series of kinetic forms of a reactive order, which are instinctive reactions caused by sudden changes in body position or general situation, danger, etc. This includes a whole series of orienting and protective reactions. At the same time, the extrapyramidal system provides a number of auxiliary kinesthetic and automatic components for complex voluntary movements; this is especially true for learned, stereotyped forms of the latter. The close connection of the extrapyramidal system with vegetative centers determines its role in carrying out expressive, mimetic reactions and its lively participation in the emotional-affective life. In its elementary form, expressive movements already appear in the newborn child. Later they become significantly differentiated. Thus arise those automatic changes in tone and reflex movements that manifest themselves in the lively play of facial and eye muscles, in gesturing, which accompany any experience. But inseparably connected with these kinetic functions, the extrapyramidal system performs the statotonic function. Proper tonic innervation of the motor apparatus, normal myostasis are the necessary basis for any complex motor act. To develop maximum dynamics, the motor apparatus with the help of the extrapyramidal system is set in the most favorable kinetic conditions, in conditions of better statotonia and optimal posture. In this case, the body position must be flexible, adapted not only to a given form of movement, but also to a given moment of the developing action. This reflex body position, these postural reflexes are carried out not by the intermediate brain, but mainly by the mesencephalic parts of the extrapyramidal system (a special role here belongs, as Rademaker emphasized, to the red nucleus), which are however closely connected with its central parts. The coordination-tonic function is served by several apparatuses. The main role here is played by the fronto-bridge-cerebellar system, the cerebellum, and the motor-tonic mechanisms of the midbrain and trunk parts. Although the latter work on the basis of proprioceptive impulses, nevertheless they are influenced by the regulating influence of the cerebellum—via paths going to the tectum, i.e., to formations included in the extrapyramidal system, and by the extrapyramidal system itself—via its descending paths. With the help of this complex tonic-coordinating apparatus, the various postural changes in tone, the various body positions are carried out reflexively and automatically. Considering the extrapyramidal system as a whole, we see that, representing a phylogenetically very ancient mechanism, it simultaneously performs kinetic, tonic, and positioning functions. Receiving both nervous and humoral impulses, being in close relations with vegetative centers, it is, as it were, the center of the entire nervous system. Pathology. In accordance with the data presented above, it is quite natural that under pathological conditions we do not encounter a complete splitting of the functions of the extrapyramidal system into purely kinetic and purely tonic ones. In it and here, the ancient connection between motoricity and tone continues to exist. From this point of view, the term 'amiostatic symptom complex' introduced by Strümpell into clinical practice, as not covering the entire sphere of its action, at present can no longer serve as an exhaustive representation of its disorders. In any syndrome with damage to the extrapyramidal system, there are necessarily violations of both of its basic functions. Thus, with damage to the pallidum, a peculiar rigidity of the muscles (pallidal rigidity) occurs, which, in addition to stiffness in the joints, is characterized by cogwheel or step-like movements, persistent spasms (pallidum-spasmus), general stiffness, and amimia. But the matter is not limited to only the violation of tone and myostasis. Along with an increase in plastic tone, there is also a loss of extrapyramidal kinesthetic movements. Among the pallidal syndromes, one should first of all name the paralysis agitans, or Parkinson's disease (see Paralysis agitans and Parkinsonism). The basis of this disease, which has a chronic progressive course, consists of changes mainly in the pallidum (Lewy), which however also extend partly to the striatum (and in severe cases to more diffuse areas). With the same picture of 'parkinsonism', one also encounters in most of the most typical forms of epidemic encephalitis and in other diseases of the extrapyramidal system, e.g., in arteriosclerosis—the so-called 'arteriosclerotic muscular rigidity' of Förster. Parkinsonism was also noted in syphilitic lesions of the pallidary part of the extrapyramidal system. Striatal syndromes, in contrast to pallidal ones, are characterized by hyperkinetic-hypotonic syndromes. One of the typical hyperkinetic striatal syndromes is chorea, especially its chronic progressive form, the so-called chorea Huntingtoni, which is patho-anatomically limited mainly to the lesion of small cells of the striatum. Another characteristic clinical form of striatal syndromes is the so-called double athetosis (see Athetosis, athetose double). To the striatal forms also belongs the one-sided athetosis, which has the most varied etiology. Clinically, this form resembles the previous one, but differs from it in the limitation of hyperkinesia. Patho-anatomically, the basis of the athetotic forms is a process that destroys the large cells of the striatum. The group of striatal diseases also includes torsion spasm and tics, and among the latter also torticollis spastica, which can be considered as a reaction to a narrowly focal lesion of the striatum, in which only a very limited group of muscles suffers (Jakob). To the mixed strio-pallidal syndromes belongs the Hallervorden-Spatz disease (see), characterized by diffuse damage to the medullated fibers (status dysmyelinisatus) in the pallidum and partly in other parts of the extrapyramidal system. Among other extrapyramidal diseases, one should note the so-called hepatolenticular degenerations. These include very close to each other Wilson's disease (see) and Westphal-Strümpell pseudosclerosis (see Westphal-Strümpell disease).