Pyramidal System

PYRAMIDAL SYSTEM (syn.: pyramidal tract, tractus cortico-spinalis) from the perspective of phylogenesis and ontogenesis, in contrast to the extrapyramidal or paleokinetic system, as associated with the neencephalon, is called the neokinetic system. Being a neencephalic formation, it is completely absent in lower vertebrates and appears for the first time only in mammals. However, even in lower representatives of these, it is still poorly developed and occupies a modest place in the total mass of brain tissue, noticeably increasing in size as it ascends the ladder of phylogenetic development. Initially, it does not have a long caudal extension and terminates at high levels of the central nervous system. Therefore, the degree of its development in the spinal cord and its quantitative ratio with other systems can serve as a phylogenetic indicator. Thus, in an elephant this ratio is 4.8%, in a dog 6.7%, in higher primates 20.1%, and in humans 30%. As for the topography of the P. s. in the spinal cord, it is by no means the same in different species of animals. According to Kappers, the P. s. in lower stages of phylogenesis lies mainly in the posterior columns and only in higher animals moves to the lateral columns. In the hedgehog, only individual fibers go to the spinal cord, as most of the pyramidal bundle already ends in the medulla oblongata at the level of motor nerve nuclei. In mice and rats, the entire pyramidal tract still goes entirely in the posterior columns of the spinal cord. In sheep and goats, pyramidal tracts are also still rather weakly expressed and reach only the cervical region, being partly laid in the anterior columns and partly within the anterior commissure, although some of their fibers already pass through the lateral columns, disappearing near the processus reticularis. In the rabbit, the P. s. is already located mainly in the lateral columns. In cats, dogs, and monkeys, the relationships resemble those found in humans. In the latter, due to the high development of the motor zone of the brain, the P. s. occupies a much larger volume, located in the lateral and partly in the anterior columns. Thus reaching its structural maximum and functional predominance over the extrapyramidal system, the P. s. ontogenetically in terms of the time of its development noticeably lags behind the latter. While many pathways of the extrapyramidal system complete their formation and even function before birth, the pyramidal tract begins its development in humans during extrauterine life, barely appearing in the first weeks in the form of individual descending fibers from the brain cortex. As already proven by the research of Flechsig using the myelogenetic method, myelination of the pyramidal tract occurs quite late - its fibers in humans receive their myelin only at the end of the 5th month of extrauterine life. As both old and recent research in the field of cyto- and myeloarchitectonics of the brain (Brodmann, Vogt, Economo, Gurevich) show, the P. s. originates in the cells of the 5th layer of the cortex of the anterior central and paracentral gyrus, as well as the posterior parts of the 1st and 2nd frontal gyri. Economo and Koskinas on the basis of their research (1925) consider the indicated territory a typical motor area due to the presence here of large pyramidal and spindle-shaped cells and the absence of round-celled and granular elements. The area of the anterior central gyrus as the source of the P. s. consists of two fields differing from each other in cytoarchitectonic relations: area gigantocellularis, or Brodmann's 4th field, located in its upper half, and the posterior part of areae agranularis frontalis, or Brodmann's 6th field, occupying the lower half of this gyrus. Both fields functionally represent a series of centers served by the P. s. The giant pyramidal cells of Betz (1874) belonging to the 5th layer of the first field are distinguished by their large size, sharply defined nucleus containing a nucleolus; their main dendrite from the apex of the pyramid goes upward to the surface of the cortex, and the long axon arising from the base has the opposite direction. The axis cylinders of these as well as other deep pyramidal cells in their aggregate constitute the system of the projection pathway of voluntary movements going to the cells of the anterior horns of the spinal cord (tractus cortico-spinalis, the pyramidal tract in the proper sense). To this pathway in the brain area are added tractus cortico-nucleares - pathways going to the nuclei of motor cranial nerves from cells located in the lower third of the anterior central gyrus. Initially they go in the general mass to the lower edge of the superior olive, where part of the fibers separates from them, going to the nucleus of the facial nerve of its and even more the opposite side. Another part of the fibers goes to the nucleus of the hypoglossi of the opposite side. Fibers starting even lower in the cortex to the motor nucleus of the trigeminal nerve separate below the posterior colliculus and go dorsally to the nucleus of the trigemini of both sides. From there also probably go fibers to the motor nucleus of the vagus. From the cerebral cortex, all this mass of cortico-nuclear and cortico-spinal pathways goes together and, converging, forms part of the corona radiata, even more closely connected in the internal capsule. In the latter they occupy the anterior two-thirds of the posterior crus and knee, and, descending into the brainstem, lie in the ventral, i.e., neencephalic part of the cerebral peduncle and in the basis (basis) of the pons Varolii. In the medulla oblongata, the cortico-spinal system forms the pyramids, from which it received its name. In their lower part, where cortico-nuclear fibers are no longer present, as they have gone to the nuclei of cranial nerves, the P. s. makes its crossing (decussatio pyramidum). This crossing in humans is incomplete. The larger part of the fibers from the pyramids crosses to the opposite side and lies in the lateral column, in order to gradually emerge from it and end at the cells of the anterior horns. A small part of the P. s. (direct pyramidal bundle, bundle of Turck), subject to great individual variations in its size, remains uncrossed and descends downward in the anterior column of its side. As for the further course of this uncrossed bundle, part of its fibers goes through the anterior commissure to the cells of the anterior horns of the opposite side and thus still forms a preterminal crossing, while another part goes to the cells of its own side. Descending, both bundles, direct and crossed, gradually decrease in size; the anterior bundle disappears mostly already at the level of the thoracic region, while the lateral one reaches to the sacral segments inclusive. Being a later superstructure over the extrapyramidal apparatus, the P. s. even in higher animals plays an unequal role in motor function. In this regard, the experimental-physiological research on animals aimed at turning off their new motor mechanisms and artificially transferring them to a lower stage of phylogenetic development is of great interest. Golz's dog, in which both hemispheres were removed, lived for 18 months, retaining the ability to walk and jump. Similar experiments were carried out by Lewy on monkeys. Removal of the cortex in the motor area did not cause loss of the ability to move and generally had little effect on motor function. The hemiparesis that occurred with unilateral shutdown of the cortex quickly equalized, leaving only small traces in the form of awkwardness, clumsiness. Complete paralysis was obtained only on the condition of simultaneous removal or destruction of the extrapyramidal system. But at the height of phylogenetic development of the pyramidal system - in humans - its destruction always leads to a sharp and persistent loss of voluntary movements. Lesions of the P. s. clinically manifest in two types of syndromes - irritation and loss phenomena. Symptoms of irritation of the P. s. are observed mainly with its lesion in the area of the cerebral cortex or areas closest to the cortex and manifest as clonic convulsions in the muscles of the opposite half of the body, giving a picture of so-called partial or Jacksonian epilepsy. The syndrome of loss or pyramidal paralysis is the result of destructive changes in the P. s. In this case, for cortical lesions, monoparesis or monoplegia is most characteristic, for lower parts of the P. s. - hemiparesis or hemiplegia (see), for the stem part - the so-called alternating hemiplegias, and with lesions below the crossing, paralysis of the body on the side of the focus and quadriplegia or paraplegia with bilateral lesions of the spinal cord will be observed. The most basic symptom in lesions of the P. s. is thus a greater or lesser degree of loss of voluntary movement. The latter suffers both in terms of strength and in terms of volume. In the facial musculature, movement is disturbed mainly only in its lower part. Simultaneous closing of both eyes is performed quite satisfactorily, but isolated closing of the eye on the paralyzed side is impossible (Revilliod's symptom). In the extremities, motor disturbances predominate in the distal parts.

Unlike extrapyramidal paralysis, here the most finely differentiated movements are affected. As a rule, in such cases, the so-called Wernicke-Mann 'predicative type' of paralysis is observed, characterized by its predominance in the extensors of the arm and flexors of the leg. Due to the hypertonia that appears in the antagonists of the most paralyzed muscles as a result of damage to the P. s., a contracture develops, which further delineates the aforementioned type of pyramidal paralysis. The upper extremity in this case is in a state of rudimentary grasping action - the fingers and hand are flexed, the arm is pronated and flexed at the elbow joint; the foot of the paralyzed leg, extended at the knee joint, is in a state of plantar flexion, turned inward, which also reveals a similar rudimentary posture, particularly sharply manifested when the inner edge of the sole is irritated (Girshberg's symptom). The loss of the inhibitory influence of the cortex on the segmental apparatus in such cases is manifested by a sharp increase in tendon reflexes on the side of the paralysis. But an even more characteristic feature for damage to the P. s. is the loss of skin reflexes and in particular the plantar flexion reflex. The latter is replaced by the dorsiflexion of the big toe (see Babinski, Oppenheim, Rossolimo reflexes), which is normally observed only in early childhood, i.e., before the functional maturation of the P. s. Along with these primitive forms of reactions from the disinhibited segmental apparatus, in cases of damage to the P. s., we encounter irradiation of irritations, which clinically manifests as crossed reflexes and the so-called associated movements or synkineses. The latter are understood as automatically performed movements when attempting to perform a motor act with the help of some other muscle group. An example can be the Strümpell phenomenon, consisting in the contraction of the tibialis anterior muscle when attempting to flex the paralyzed limb at the hip joint. Thus, the exclusion of the P. s. in humans is not limited to the phenomena of paralysis alone, but also causes a number of other symptoms, which in their nature represent nothing more than manifestations of functions inherent to the segmental apparatus as such, which, due to the loss of cortical influences, has been placed in conditions of independent activity. Therefore, from an evolutionary-biological point of view, the semiotics of P. s. lesions can be considered as a transfer of motor function to a lower stage of phylo- and ontogenetic development, i.e., prepyramidal.