Neuroplasticity

“ Nerve fluid from living animals had been tested and found to behave properly even in the fine glass tubing. But it was highly unstable. If a synthetic brain capable of integrated thought processes was to be constructed, a non-deteriorating nerve fluid would have to be found. One that duplicated the performance of the actual nerve threads of the human brain.
All that held back Project Brain was the proper synthetic nerve fluid! Maybe it's one of those ten, Earl thought. But he entertained that thought with every ten he tested.
But right now there was a more pressing problem. Nadine Holmes. ”

“ Now of all the tissues the nervous tissue is that which takes longest to repair when injured; and it seems a not improbable inference that it is a tissue which is slower in its histological development than others. If this be so, we may see why, in the embryos of the higher vertebrates, the central nervous system quickly grows large in comparison to the other systems—why by pre-adaptation the brain of a chick develops in advance of other organs so much more than the brain of a fish. ”

“ From it in turn come a great many connecting nerve cells whose axons pass along the spinal cord and form nerve junctions with the motor nerve cells; and so nervous disturbances coming from the cerebellum can arouse the muscles to activity. We all know that walking or any other locomotor act, after it has once been learned, is carried on successfully without our performing any intellectual work in connection with it. What we do have to do is to start the act, and this is done through a discharge from the cerebrum ”

“ Then, abruptly, I was aware of myself—not the gross mechanism of clumsy bone and muscle, but the neuro-electric field generated within the massive structure of a brain alive with flashing currents and a lightning interplay of molecular forces. A sense of orientation grew. I occupied a block of cells ... here in the left hemisphere. The mass of neural tissue loomed over me, gigantic. And "I" ... "I" was reduced to the elemental ego, who possessed as a material appurtenance "my" arms and legs, "my" body, "my" brain.... ”

“ Such is the case in the nerve cells of the brain and spinal cord, a loss in which tissue is never repaired by the formation of new cells; and in the muscles the same is true. The least differentiation is seen in those cells which serve the purpose of mechanical protection only, as the cells of the skin, and in these the formative activity is very great. Not only must the usual loss be supplied, but we are all conscious of slight injuries of the surface which are quickly repaired.
Repair, other things being equal, takes place more easily in the young than in the old. ”

“ This gray matter is indeed a series of such nerve-centres, each capable of discharging energy precisely as the central nerve-mass of the polyp does. The white fibres are the conductors of impulses: during life they are continually occupied in carrying messages to and fro between different portions of the spinal gray matter, or up and down—that is, to and from the brain.
In order to study the functions of the spinal cord the physiologist takes a frog from the ditch and with one sweep of a sharp knife beheads it, or, what is the same thing, severs the spinal cord just as it enters the skull. ”

“ Sometimes, indeed, the two sets of filaments are combined into one nerve cord, but if traced to their origin these are found to arise from different brain centres. Thus it was clear that a hitherto unrecognized duality of function pertains to the entire extra-cranial nervous system. Any impulse sent from the periphery to the brain must be conveyed along a perfectly definite channel; the response from the brain, sent out to the peripheral muscles, must traverse an equally definite and altogether different course. ”

“ The largest nerve cells of all are found in the motor area, and are called, from their shape, the "giant pyramids". They have large dendrites and very long axons, which latter, {53} running in a thick bundle down from the cortex through the brain stem and cord, constitute the "pyramidal tract", the principal path of communication from the cerebrum to the lower centers. The motor area of the brain has no direct connection with any muscle, but acts through the pyramidal tract on the lower centers, which in turn act on the muscles. ”

“ Through his efforts, and with the invaluable aid of such other workers as Remak, Purkinje, Henle, Muller, and the rest, all the mystery as to the general characteristics of nerve tracts was cleared away. It came to be known that in its essentials a nerve tract is a tenuous fibre or thread of protoplasm stretching between two terminal points in the organism, one of such termini being usually a cell of the brain or spinal cord, the other a distribution-point at or near the periphery—for example, in a muscle or in the skin. ”

“ In other words, there is every reason to believe that numerous "centres," in this restricted sense, exist in the brain that have as yet eluded the investigator. Indeed, the current conception regards the entire cerebral cortex as chiefly composed of centres of ultimate co-ordination of impressions, which in their cruder form are received by more primitive nervous tissues—the basal ganglia, the cerebellum and medulla, and the spinal cord.
This, of course, is equivalent to postulating the cerebral cortex as the exclusive seat of higher intellection. ”

“ In the most complex case, that of man, there are a number of nerve-endings, or small lumps of protoplasm, embedded in the tissues all over the body, or highly specialised and grouped together in separate organs such as the eye and ear, from which a nerve-fibre leads direct to the brain, or to the spinal cord and so up to the brain. These nerve-endings receive the different vibrations by which outward energy presents itself, which propagate a current or succession of vibrations of nerve-energy along the nerve-fibre. ”

“ All bone appears first as membrane, then cartilage, and finally ossifies. This is the order both in past evolution and in present embryonic development. The brain is at first a bulbous expansion of the spinal nerve-cord. It is at first simple, but gradually, both in the scale of nature and in the embryo, divides into five parts. One of these parts, the cerebrum, is mainly connected with mental life. We find it [Pg 134] increasing in size, in proportion to the animal's intelligence, until in man it comes to cover the whole of the brain. ”

“ Work and Waste of Brain Tissue Necessary––This, speaking broadly, is the rationale of the matter: just as actually as we sew or write through the instrumentality of the hand, so the child learns, thinks, feels, by means of a material organ––the very delicate nervous tissue of the cerebrum. Now this tissue is constantly and rapidly wearing away. The more it is used, whether in the way of mental effort or emotional excitement, the more it wears away. Happily, rapid new growth replaces the waste, wherefore work and consequent waste of tissue are necessary. ”

“ Nerve cells and often large areas of gray matter are replaced by connective tissue (resembling scar tissue) , which grows and increases in what would otherwise be vacant spaces. All areas which contain this connective tissue, this filling which has no function, of course, cease to join with other parts of the brain in concerted action, and so the power of the brain is diminished, and certain of its activities are restricted or abolished. ”

“ The cerebrum, or fore brain, is divided into two hemispheres, each of which is further divided into numerous lobes consisting of grey and white nervous substance and millions of nerve cells and fine blood vessels. The hemispheric ganglia are the instruments through which the intellectual powers manifest themselves. If these instruments have become imperfect in structure through retarded development by wrong environment and education, or have been damaged in any manner by severe shock, violence or disease, the manifestations of intelligence are affected in a corresponding degree. ”

“ Such a process might be relied upon either to equip the cortical center of the accessory nerve for its new and more varied functions of control, or to prepare new paths of connection between this center and that which had formerly exercised exclusive control of the muscles of the face through the facial nerve. In a word, tin-building process in the brain, finding much of the old mechanism thrown out of its accustomed use, may have made ready another center and new tracts for the same use. ”

“ In the medulla the nerves that arise in the cerebrum cross over from one side of the body to the other on the crossed pyramidal tracts. The importance of this crossing of the nerve fibers is seen in apoplexy, when a blood-vessel is ruptured in the brain and hemorrhage causes pressure, generally on the motor tract. Paralysis of the nerves and of the muscles to which they go results. The paralysis is generally of one side of the body, the opposite side from that on which the injury occurred. ”

“ In the lowest vertebrates, as in the amphioxus, the nervous system consists of nothing more. It may therefore be said to have only two parts—the cord and the sensory ganglia, and to have two functions—the automatic, attributable to the former, and the instinctive, attributable to the latter.
But as we advance from the low vertebrates upward in the animal scale, we begin to detect new organs; on the medulla oblongata a cerebellum, and on the sensory ganglia a cerebrum. Their intellectual apparatus. ”

“ These neurons are controlled for the most part by fibers from the bulb and spinal cord, and cannot for this reason be looked upon as forming an independent system. Their chief purpose seems to be that of spreading the influence of neurons from the central system over a wider area than they would otherwise reach. For example, a single neuron passing out from the spinal cord may, by terminating in a sympathetic ganglion, stimulate a large number of neurons, each of which will in turn stimulate the cells of muscles or of glands. ”

“ It proceeds too and does obtain blood abundantly to and from the heart, but the results obtained are not satisfactory, and another leaf is opened of why no good results are obtained and where is the mystery, what quality and element of force and vitality has been withheld? A thought strikes him that the cerebro spinal fluid is the highest known element that is contained in the human body, and unless the brain furnishes this fluid in abundance a disabled condition of the body will remain. ”

“ The medulla is part of the brain stem.
This was only one of the difficulties which had to be overcome. The eyes are set on the pre-fulcral lever of the head. For our safety we must be able to look in all directions—over this shoulder or that. We must also be able to turn our heads so that our ears may discover in which direction a sound is reaching us. In fashioning a fulcral joint for the head, then, two different objects had to be secured: free mobility for the head, and a safe transit for the medullary part of the brain stem. ”

“ Muscular action, however, cannot take place without nervous action. These two tissues are dependent one on the other. Yet the muscular tissue may be considered as subordinate to the nervous tissue. While the muscular tissue may become totally inactive or incapacitated, or even removed, the brain tissue may retain its activity and continue to perform its functions. The very reverse takes place when the brain is either injured or removed. We know by experience, experiments, that injuries or other pathological changes will cause impairment to muscular tissue. ”

“ Nerve Stimuli.—While the properties of irritability and conductivity supply a necessary cause for the production and transmission of nervous impulses, these alone are not sufficient to account for their origin. An additional cause is necessary—a force not found in the nerve protoplasm, but one which, by its action on the protoplasm, makes it produce the impulse. In this respect, the neuron does not differ essentially from the cell of a muscle. Just as the muscle cell requires a stimulus to make it contract, so does the neuron require a stimulus to start the impulse. ”

“ They dwindle in size; they do not, however, die. Similarly, when the axons of the motor spinal cells are by severance of the nerve trunk of a muscle broken through, the muscle cells undergo “degeneration”—dwindle, become fatty, and alter almost beyond recognition. This trophic influence which one neuron exerts upon others, or upon the cells of an extrinsic tissue, such as muscle, is exerted in that direction which is the one normally taken by the natural nerve impulses. ”

“ They will be expended for the purpose of furnishing the necessary energy to the vital functional activity, muscular [Pg 231] contraction, secretion, heat, etc., just as coal is expended to set the steam engine going. The proof as far as the muscle is concerned does not stand alone. There are other examples. In particular, micrographic physiologists who have studied nervous phenomena say that the anatomical elements of the brain last indefinitely, and that they continue as they are, without renewal from birth to death. ”

“ When a young child first endeavours to stand upright, the sensation of falling is doubtless conveyed to the brain, and thought taken of how the erect position can be maintained. But it is not until after many experiments and failures that the brain-cells can send messages to the right cells in the cord, and these set the necessary muscles in motion. Experience [68] teaches what must be done, and constant practice eventually enables the spinal cord to act for itself without referring for orders to the brain. It is on the same principle that we learn to ride the bicycle. ”