In order to understand the functions of the cerebrum, it is necessary, however, to have a clear knowledge of the general nature of the lower nerve centers. The spinal cord, as the lowest of these centers, contains the essential junction points for a large number of bodily reflexes which can occur quite perfectly without any participation by higher regulative centers, although they may be subject to interference or reinforcement through the action of the latter. The cord also acts as a conduit through which impulses are conducted from the body surface and internal organs to the brain. The functions of the medulla oblongata, which connects the spinal cord to the brain, are similar to those of the cord, but involve reflexes of the head-end of the body and the regulation of the more vital processes, such as those of circulation and respiration. The cerebellum, which is a portion of the brain adjacent to the medulla, receives afferent nerve impulses primarily from sense-organs located in the motor apparatus (muscles, tendons and joint-surfaces) and from the equilibrium sense mechanism of the inner ears. The efferent impulses which leave the cerebellum pass to all portions of the voluntary musculature, and are concerned in the automatic maintenance of tension and coördination between the various muscular units. The cerebellum appears to be a device for adjusting the details of motor innervation, usually under the guidance of the cerebral cortex. It is possible that it is endowed by heredity with a stock of "records," which enable it to produce and reproduce specific types of motor reaction when circumstances demand them, although these reaction forms are not linked with any definite stimuli. As examples of such reaction forms, we may consider some of the items on James' list of simple instincts: "sucking, biting, chew ing, licking, grimacing, etc." However, the prime duty of the cerebellum seems to consist in the maintenance of tonus and balance throughout the voluntary musculature.
In the mid-brain, which is enveloped by the mass of the cerebrum, we find further regions of reflex transfer, which, however, are usually more complex and variable in their action than is the case with the centers of the spinal cord or medulla. The thalamus, which is an important portion of the fore-brain, forms a kind of vestibule to the cerebral cortex, since practically all of the sensory nerve currents which are destined for the cortex, pass through the thalamus. Here, also, is found a synaptic center for all of the pain nerves of the body, and many of the mimetic expressions of instinct or emotion are probably controlled directly from centers in this general region of the brain. There is a very definite interaction between the thalamus and the cerebral cortex in regard to pain impulses, and possibly also with reference to impulses which give rise to pleasure. Concerning this relationship we shall have a great deal more to say in later chapters.
The cerebrum is by far the largest portion of the brain in the human being although it is practically absent in many lower vertebrates. It consists of a very intricate network of conducting fibres, which have myriads of junction points, located for the most part in the surface of the organ, a large part of which is adjacent to the bony case of the skull. The convolutions and fissures in this surface appear to have the function of increasing its area to a maximum. The cerebrum is divided, right and left, into two halves known as the cerebral hemispheres. The right hemisphere is connected almost exclusively with the left side of the body, while the left hemisphere deals with the affairs of the right side.
The cerebral cortex receives a very large number of nerve fibres from all of the sensory surfaces of the body and also gives rise to fibres which pass to all of the skeletal, or so-called voluntary muscles. These fibres are segregated and distributed to special zones, known as projection areas. Thus, we have surfaces in the cortex which are exclusively for visual, for auditory, for olfactory, for tactual, for motor impulses, and so on. The motor area is devoted to the transmission of impulses along the pyramidal neurones. These sensory and motor projection areas by no means exhaust the entire surface of the cortex, and it is natural to suppose that the remaining and intervening parts will be employed for purposes of association between the sensory and motor zones. This supposition has already been corroborated to a convincing extent by empirical observation. It is evidently in the association areas of the cortex that we should look for the principal basis of specificity in voluntary behavior.
Showing posts with label nerve fibres. Show all posts
Showing posts with label nerve fibres. Show all posts
Saturday, November 10, 2007
Response Conduction Patterns
The processes of convergence and of divergence are of particular interest in relation to the notion of conduction pattern. The idea of such patterns is of the utmost importance to an understanding of the more complex forms of response. It may be well to make this conception more vivid by means of a concrete example. Let us consider a case of visual response, which is naturally the easiest kind to "visualize." Now the notion of pattern can be applied to every stage of the response, beginning with the object and ending with the effect. In each stage it is a matter of the arrangement of parts in space; for the object, it is simply the spatial form of the latter with particular reference to the manner in which various portions of its surface are reflecting, or emitting, light in the direction of the observer's eyes. In the second, or stimulus stage, the pattern is comprised by the configuration of light rays which are passing from the object to the pupils of the eyes. This pattern is primarily an arrangement of directions of movement rather than of points on a surface. In passing through the eye the pattern is again changed as a consequence of the shifting of the directions of the rays, which is brought about by the refractive action of the ocular media. As a result of this an image or optical picture of the object is formed on the sensitive retina of the eye. This retinal image pattern is followed by the formation of a corresponding or registering pattern of visual receptor processes in the retinal rods and cones.
The response now passes into the fibres of the optic nerve, of which there are about a million. All of these are simultaneously excited in any act of vision, and the kind of behavior which results must obviously depend to a large degree upon the pattern of distribution of the excitations over the individual conducting components in these complex bundles of nerve fibres. At the moment when the nervous disturbance passes from the receptor cells to the conducting nerve fibres, the pattern has a close geometrical resemblance to the retinal image; but within the conducting layers of the retina itself there is almost immediately a radical condensation and change in geometrical form. This is referable to the fact that, except in the center of the retina, a considerable number of receptors connect in party line fashion with a single nerve conductor; and also to the convergence of the nerve fibres upon the point in the eye-ball where they emerge as a compact cable. Along the further course of the visual impulses from the eyes to the brain, there are other regions of transfer and of redistribution of the fibres, which cause still more radical changes in the exact geometrical arrangement of the individual nerve fibre excitations. When the impulses reach the cerebral cortex the pattern bears hardly any geometrical resemblance to the structure of the original object. Nevertheless, the exact configuration of the excitation in the cortex is determined in a fairly reliable manner by that of the object.
Starting at the cerebral cortex and proceeding in an efferent or outgoing direction, there will ordinarily be a complex outflow of nerve impulses along the so-called pyramidal system of neurones; and the exact pattern of this outflow may be controlled by that of the visual processes. However, the motor pattern will never have any resemblance whatsoever to the sensory one, the connection between the two being entirely arbitrary, so far as similarity of process is concerned. The pattern of the impulses in the pyramidal neurones will determine that in a larger number of fibres which innervate the skeletal muscles and in turn will govern the behavior pattern, or the posture and movements of the organism. All along the course of the conduction from the object to the "effect," there is a continuous and at many points radical modification of the pattern, but, nevertheless, the pattern in each stage determines that in the next following stage, working of course in conjunction with the inherent structures which characterize the stages in question.
The pattern of the response on the entrant or sensory side of the cerebral cortex is correlated very closely with the psychical pattern, or configuration which is found in the accompanying consciousness. This psychical pattern is identical, as a rule, with the object, as the latter is presented in consciousness. It bears, however, only a remote resemblance to the physical object which initiates the response.
Now the transformations of response patterns which occur between the object and the brain are of great technical interest in general physiology, but the transformation which is of primary interest in the theory of motivation is that occurring between the entrant and the emergent brain processes. This is the relationship which determines the specificity of the response. From the standpoint of the brain, it is quite immaterial how the entrant activities are determined, and from the standpoint of the specification of behavior the only thing which counts is the form of the impulses which leave the centers. In order to understand behavior, we must know how the input and output phases of the central activity are associated, and what the specific causes of such association are in particular cases. For example, suppose that we react to an apple by taking a bite out of it. This involves a linkage of the cortical representation of the apple with a complex series of motor innervations which are prerequisite to the grasping of the apple, the lifting of the same to the mouth and the application of the teeth thereto. General physical principles do not enable us to infer the behavior from the object, so that we shall be forced to appeal to the peculiar structure and properties of the cerebral machine.
The response now passes into the fibres of the optic nerve, of which there are about a million. All of these are simultaneously excited in any act of vision, and the kind of behavior which results must obviously depend to a large degree upon the pattern of distribution of the excitations over the individual conducting components in these complex bundles of nerve fibres. At the moment when the nervous disturbance passes from the receptor cells to the conducting nerve fibres, the pattern has a close geometrical resemblance to the retinal image; but within the conducting layers of the retina itself there is almost immediately a radical condensation and change in geometrical form. This is referable to the fact that, except in the center of the retina, a considerable number of receptors connect in party line fashion with a single nerve conductor; and also to the convergence of the nerve fibres upon the point in the eye-ball where they emerge as a compact cable. Along the further course of the visual impulses from the eyes to the brain, there are other regions of transfer and of redistribution of the fibres, which cause still more radical changes in the exact geometrical arrangement of the individual nerve fibre excitations. When the impulses reach the cerebral cortex the pattern bears hardly any geometrical resemblance to the structure of the original object. Nevertheless, the exact configuration of the excitation in the cortex is determined in a fairly reliable manner by that of the object.
Starting at the cerebral cortex and proceeding in an efferent or outgoing direction, there will ordinarily be a complex outflow of nerve impulses along the so-called pyramidal system of neurones; and the exact pattern of this outflow may be controlled by that of the visual processes. However, the motor pattern will never have any resemblance whatsoever to the sensory one, the connection between the two being entirely arbitrary, so far as similarity of process is concerned. The pattern of the impulses in the pyramidal neurones will determine that in a larger number of fibres which innervate the skeletal muscles and in turn will govern the behavior pattern, or the posture and movements of the organism. All along the course of the conduction from the object to the "effect," there is a continuous and at many points radical modification of the pattern, but, nevertheless, the pattern in each stage determines that in the next following stage, working of course in conjunction with the inherent structures which characterize the stages in question.
The pattern of the response on the entrant or sensory side of the cerebral cortex is correlated very closely with the psychical pattern, or configuration which is found in the accompanying consciousness. This psychical pattern is identical, as a rule, with the object, as the latter is presented in consciousness. It bears, however, only a remote resemblance to the physical object which initiates the response.
Now the transformations of response patterns which occur between the object and the brain are of great technical interest in general physiology, but the transformation which is of primary interest in the theory of motivation is that occurring between the entrant and the emergent brain processes. This is the relationship which determines the specificity of the response. From the standpoint of the brain, it is quite immaterial how the entrant activities are determined, and from the standpoint of the specification of behavior the only thing which counts is the form of the impulses which leave the centers. In order to understand behavior, we must know how the input and output phases of the central activity are associated, and what the specific causes of such association are in particular cases. For example, suppose that we react to an apple by taking a bite out of it. This involves a linkage of the cortical representation of the apple with a complex series of motor innervations which are prerequisite to the grasping of the apple, the lifting of the same to the mouth and the application of the teeth thereto. General physical principles do not enable us to infer the behavior from the object, so that we shall be forced to appeal to the peculiar structure and properties of the cerebral machine.
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