Book - The Nervous System of Vertebrates (1907) 14
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Chapter XIV. Centers of Correlation
The whole nervous system has been treated thus far as consisting of four main divisions, each of which is connected with a special set of functions. For a review and summary of these functional divisions the student should turn to the outline given in Chapter V. The central portions of the functional divisions constitute four longitudinal zones of the brain and spinal cord. The four zones lie in the same relative position throughout the whole length of the central nervous system, except in certain segments of the brain where one or other zone is largely or wholly wanting. It may be supposed that the specialization of these four zones has taken place within a relatively undifferentiated neural tube which was possessed by the remote ancestors of vertebrates and that in all existing vertebrates the functional divisions are quite distinct from one another. The process of specialization of the functional divisions has left over, so to speak, a certain amount of material which has come to serve the purpose of connecting the functional divisions with one another and the centers of one segment with those of another. The elements which properly belong to the primary functional divisions as described in previous chapters may be summarized as follows, (i) In the afferent or sensory columns: (a) receptive neurones, which receive the end-branches of afferent fibers; (b) intrinsic neurones, whose neurites whether long or short are confined to the given column; and (c) extrinsic neurones, whose neurites go beyond the given column to carry impulses to other centers. These other centers are primarily located in the same column from which the neurites spring but they fall under the category of the centers of correlation which are here under consideration. The neurones (b) and (c) may both be at the same time receptive neurones. (2) In the efferent columns: motor or excito-glandular neurones whose neurites go to the periphery as fibers of efferent nerves.
Neurones which do not properly belong to any one division are at first scattered through all four divisions and in the spinal cord this condition is maintained, so that the material in question appears as cells scattered throughout the whole gray area. These cells have come to be known as homolateral and heterolateral tract cells (Fig. in). In the brain a large part of the corresponding material is retained in its embryonic position adjacent to the ventricle. The term central gray matter has been applied to this circum-ventricular zone of cells. Since many of the cells of the primary sensory and motor centers lie in this central gray, especially in lower vertebrates, it is necessary to use some other term to designate the material which serves functions of correlation. Whether these cells are situated adjacent to the ventricle or are scattered through the wall of the brain, their dendrites intermingle with the fiber tracts which form a large part of the brain wall. These areas consisting of mingled fiber tracts, cells and dendrites may be called the substantia reticularis. The portion in which the fiber tracts predominate is the substantia reticularis alba; that which is composed chiefly of cells is the substantia reticularis grisea.
Fig. in. Tract cells in the spinal cord of the trout. Combined from two figures by Van Gehuchten.
The tract cells in the spinal cord illustrate best the functions which this unspecialized material first served. Since these cells are widely scattered through the cord, some of them may receive impulses from one source, some from another. Their neurites enter the lateral tracts of the cord and run forward or backward for a longer or shorter distance, ending in relation with motor cells. There has been observed anatomically no order or system about these cells and their fibers. They seem to offer opportunities for the wide spread of all kinds of impulses from segment to segment of the cord. The heterolateral cells add the possibility of impulses reaching the opposite side of the cord. Whether any regularity in the relations of these neurones is constant in the species and is inherited from generation to generation is unknown. It seems more probable that these neurones offer a relatively indifferent material in the embryo, providing for the diffusion of impulses from segment to segment and from one side to the other, and that definite paths for impulses are set up chiefly as the result of the experience of the individual. If impulses traveling through certain cells and fibers serve for the performance of an act efficiently, ihe success attending the act will lead the young child to repeat the attempt. The repetition will render the impulse-pathway more easy for succeeding impulses to follow. Thus a habitual pathway is set up, while other possible pathways offered by the indifferent tissue of the embryonic nervous system become after a time unavailable through lack of use. So in the early life of the child it is probable that certain orderly sets of connections are established by way of these indifferent tract cells by means of which complex reflexes are carried out, and the actions of two divisions of the nervous system correlated. It is indeed just this development of orderly connections in the central nervous system which is going on during and as a result of the aimless movements of the infant in the first few months of its life.
In the brain the same kind of processes have been at work but the indifferent material is proportionately greater in amount and in certain regions special brain centers have been formed. In order to understand these it is necessary to look at them from the genetic point of view, especially as regards their relations to the functional divisions. Strictly speaking, no fast line of division can be drawn between the elements of the functional divisions proper and these of the substantia reticularis. Not only are the neurones of the reticularis scattered throughout the four longitudinal zones, but in the brain region at least, the reticularis neurones are especially related to the functional division in which they lie. In all segments of the brain cephalad from the VII nerve one or other of the primary functional divisions is either greatly reduced or wholly wanting. Throughout this region of the brain special centers are formed which in most cases clearly lie within the bounds of one of the primary longitudinal zones. Whether neurones belonging primarily to the functional divisions have entered into the formation of special centers, and to what extent this may have happened are questions which cannot now be decided. For the present the following practical criterion may be applied ; where the structure of centers and the disposition of their tracts do not follow the typical arrangement of centers and tracts in one of the functional divisions, the centers in question are treated as centers of correlation. Since these centers are at least largely derived from the substantia reticularis it may be stated that the most fundamental difference between the brain and spinal cord is the presence in the former of a relatively larger volume of this indifferent material for the formation of correlating centers. It will be the purpose of this and the following chapters to give in a broad and general way an account of the phylogenetic history, morphology and functional relations of these special brain centers.
The reason for the use of the term centers of correlation is very simple. As has already been shown (p. 81), simple reflexes are mediated by direct connections between sensory and motor nuclei. Even large movements of a vague and ill-directed sort may be carried out in this way. But when complex movements involving the action of many muscles directed to a definite end are called forth by a stimulus, the intervention of secondary and tertiary centers with their fiber paths is necessary for the control of the muscles as to the time, extent and force of contraction. The experiments with the brainless frog (p. 82) show that for movements of short duration up to a certain grade of complexity the tract cells of the spinal cord serve the purpose of correlating centers, but for the direction of movements for a longer period of time involving adjustment and readjustment of muscles with reference to some whole act destined to reach a given end, the better organized brain centers are necessary. It is the function of relating several simple actions with reference to some common end, the co-relation of activities, which these brain centers serve.
The degree of complexity of the activities controlled by the centers of correlation in various animals is directly paralleled by the complexity of the brain itself. The efforts at escape made by a normal frog when seized are much more complex and long continued than those of a frog whose brain has been destroyed. To the human observer, however, the efforts of the normal frog are very simple. The frog has no cerebral hemispheres related to somatic sensation and somatic movement. The efforts of a mammal with its^cerebral cortex are enormously more complex and may involve keen observation, connected effort through relatively long periods, employment of indirect means, etc.
In the lowest vertebrates, cyclostomes, a large part of the substantia reticularis of the brain remains in its primitive indifferent condition; few special nuclei are developed and the activities of the animals are correspondingly simple. To any stimulus that may come the animal can respond only in a very limited number of ways. To two similar stimuli little or no difference in response is to be expected. In the whole hindbrain region no special nuclei in the substantia reticularis have been found. Even the secondary gustatory nuclei are so little developed that they have not yet been seen. The cerebellum is wholly unspecialized. The tectum opticum and the nucleus of the tractus habenulo-peduncularis in the mesencephalon, the nucleus habenulae, the nucleus of the posterior commissure and the inferior lobes in the diencephalon, and the striatum and epistriatum in the telencephalon probably comprise all the special nuclei at present known in the cyclostome brain. These will be considered in connection with the brain of true fishes below.
In the different classes of vertebrates the process of differentiation of many of the special centers which are well marked in the mammals is seen in various stages of completeness. In some selachians and ganoids parts of the brain are little more highly specialized than in cyclostomes. Other parts of the brain, however, as the olfactory centers in selachians and the gustatory centers in some bony fishes, are very largely developed and appear more complex than in other fishes. The cerebellum also is extremely large in many selachians and bony fishes. The study of these structures in those forms in which they are highly developed has led to some results which could not be so well attained from the study of any other forms. In bony fishes the development of special centers from the indifferent gray substance has gone farther than in other lower vertebrates. There has been a collecting and sorting of elements which are more diffusely placed in ganoids and selachians. This fact makes the brain of the teleost an especially rich field for the study of the centers and fiber tracts constituting the apparatus for the performance of specific functions. Since the study of less specialized brains has given the fundamental plan of structure of the vertebrate brain, the brain of the teleost should now be subjected to careful and detailed study in order to determine the early form of the central apparatus which directs known activities. Herrick has just done this for the gustatory apparatus, and it is much to be desired that the centers and fiber paths involved in other functions should be worked out in the same way. Generally speaking, any vertebrate in which any system of organs is unusually highly developed presents special opportunities for the study of the central apparatus of that system and also of the process and method of brain differentiation in general.
Although much remains to be done in the way of rendering our knowledge of the special centers and their relation to the main functional divisions complete and exact, the description of these centers will be given as far as possible in the form of an account of the functional system of neurones of which they form parts.
First there are to be mentioned a number of neurones which seem to be a vestige of invertebrate structures which are quite lost in higher vertebrates. These are the Miillerian cells and fibers of cyclostomes and the cells of Mauthner of fishes and amphibia. There are in the brain of Petromyzon over twenty gigantic cells lying in the somatic and visceral motor columns in the region of the cranial nerve roots. Their neurites run back into the spinal cord. In other fishes a pair of such cells lies adjacent to the motor root of the VII nerve, whose neurites cross and run back into the spinal cord. These elements remind one of the large cells and thick fibers characteristically found in the nervous system of invertebrates and the fact that they are found only in lower vertebrates and most numerous in the cyclostomes, suggests that they are very archaic elements which are not to be counted among the typical elements of the vertebrate nervous system.
In the region of the myelencephalon the substantia reticularis has on the whole the same relations as in the spinal cord. In lower vertebrates the formation of special nuclei from this material has not proceeded far, but it is probable that further study will show a tendency even in fishes for the segregation of the neurones related to somatic centers from those related to visceral centers. Especially is this to be expected in any forms in which one system, such as the acustico-lateral in selachians or the gustatory in bony fishes, is greatly developed. Indeed it seems clear that the inferior and superior secondary gustatory nuclei, which have already been described, are specially developed parts of the substantia reticularis which were primitively closely related to the visceral sensory column.
At the caudal end of the myelencephalon in fishes a collection of substantia reticularis cells about the roots of the first ventral spinal nerves (hypoglossus) forms the inferior olive. This nucleus becomes of great size and complexity in mammals but its functional relations are not yet well understood in any vertebrate.
In the ventral part of the cerebellar segment occurs a collection- of several nuclei which in mammals causes the large protuberance known as the pons. These nuclei are highly developed only in the mammals, where they are related to the pyramidal tracts and the cerebellum. A part of them receive numerous collaterals and terminal branches from the pyramidal tracts and are believed to send fibers to the motor centers of the spinal cord and to the cerebellum. These pontial nuclei are thus interpolated in the direct pathway between the cerebral hemisphere and the spinal cord and also in the indirect path by way of the pons and the cerebellum. The mesial part of the pontial nuclei receives collaterals from the lemniscus and hence is related to the somatic sensory apparatus.
Demonstration or Laboratory Work
- Study the tract cells of the spinal cord in Golgi sections of young fish fry, frog tadpoles or chick embryos of the fifth to eighth day of incubation.
- Study the lower olive and pontial nuclei in Golgi sections of the. brain of the mouse or other small mammal.
- Study the tract cells of the spinal cord of a fish or amphibian and the cells and their relations to the four primary columns in the medulla oblongata.
Cajal, S. R.: Beitrage zum Studium der Medulla oblongata. Leipzig 1896.
Cajal, S. R.: Textura del sistema nervioso del Hombre y de los Vertebrados. Madrid 1904.
Van Gehuchten, A.: La moelle epiniere de la truite (Trutta fario). La Cellule, Tome ii. 1895.
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Cite this page: Hill, M.A. (2021, March 3) Embryology Book - The Nervous System of Vertebrates (1907) 14. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Nervous_System_of_Vertebrates_(1907)_14
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