Nerve cell

“ Fig. 110.—Nerve Cells from the Spinal Cord.
Nerve cells, which may be regarded as the central organs of the nerve fibers, consist of masses of cell protoplasm, with a large nucleus and nucleolus. They bear a general resemblance to other cells, but vary much in size and shape. Nerve cells grow, become active, and die, as do other cells. A number of processes branch off from them, some cells giving one or two, others many. The various kinds of nerve cells differ much in the shape and number of processes. ”

“ The last cell divisions among nerve cells are believed to occur within a few months after birth. As most of us have observed in our own cases, bodily injuries, if at all severe, are followed by the formation of scars. This means that connective tissue has grown in to fill the place of the cells destroyed by the injury, which cannot be replaced by cells of their own sort, since they have lost the power of cell division. ”

“ If it be too metaphysical to talk of the conversion of bodily heat into force, and of force into muscular motion, it is, nevertheless, a fact of observation that a nerve-centre becomes exhausted by over-excitation and over-action, and, being exhausted, becomes incapable of energizing till its power or force is renewed by rest or food. In the following observations I shall try to illustrate the theory that a nerve-cell is called into action by stimulation applied to it from without, and that, according to its special quality, it will then energize and act upon other cells or structures. ”

“ A most interesting body it proved, this nerve cell, or ganglion cell, as it came to be called. It was seen to be exceedingly minute in size, requiring high powers of the microscope to make it visible. It exists in almost infinite numbers, not, however, scattered at random through the brain and spinal cord. On the contrary, it is confined to those portions of the central nervous masses which to the naked eye appear gray in color, being altogether wanting in the white substance which makes up the chief mass of the brain. ”

“ All the experience of living merely modifies, the state of the cells already present. The modification is memory.
But though a nerve-cell cannot divide, it can send forth new processes, or nerve-fibres from itself—what we call a nerve being simply a collection of processes from a nerve-cell. Throughout the brain and spinal cord we find great numbers of nerve processes which simply run from one set of nerve-cells to another, instead of running to a sense-organ, or a muscle, or a gland. Such fibres are called association fibres, their business being to associate different sets of nerve-cells. ”

“ We must remember that the nerve junctions are exceedingly delicate things, consisting, as they do, of the interweaving of the almost inconceivably tiny featherings at the tips of sensory or connecting nerve cells with the equally tiny featherlike branches from the cell bodies of connecting or motor nerve cells. It is likely, also, that on account of their delicacy they are easily affected by the waste products that may be circulating about in the blood stream from the active muscles. ”

“ A nerve cell is not a secreting cell—that is, it does not derive from the blood or surrounding fluid a pabulum which it elaborates into a visible, palpable secretion characteristic of the organ of which the cell is a constituent element, to be in due course discharged into a duct which conveys the secretion out of the gland. Nerve cells, through the metabolic changes which take place in them, in connection with their nutrition, are associated with the production of the form of energy specially exhibited by animals which possess a nervous system, termed nerve energy. ”

“ The whole of the nerve structure is composed of the nerve tissue and supporting connective tissue. The neurones constitute the nerve tissue, while the supportive tissue is composed of neuroglia and of white fibrous tissue derived either from the investing membrane or from the sheaths of its numerous vascular channels.
The neurones, or nerve cells, exhibit marked variations as to external characters, dimensions, and form. The neurone presents a swollen cell mass and a nucleus; it is known as the ganglion cell. ”

“ Feeling may be supposed to exist, wherever a nerve-cell is in an excited condition. The disturbance of feeling, or sense of reaction, accompanies the transmission of disturbance between nerve-cells or from a nerve-cell to a muscle-cell or the external stimulation of a nerve-cell. General conceptions arise upon the formation of habits in the nerve-matter, which are molecular changes consequent upon its activity and probably connected with its nutrition. ”

“ There is evidence to show that in the nerve cell the nucleus, as well as the cell plasm, is an effective agent in nutrition. When the cell is functionally active, both the cell body and the nucleus increase in size (Vas, G. Mann, Lugaro) ; on the other hand, when nerve cells are fatigued through excessive use, the nucleus decreases in size and shrivels ”

“ Not only does the evidence which points to the essential continuity in structure of nerve cell and fibre discredit the notion of their physiological diversity, but it is further supported by the fact that although the whole nervous system is structurally continuous, an immense mass of nerve-fibres have no immediate connection with ganglionic cells:—neither springing from nor terminating in such cells, their activity cannot be assigned to them. ”

“ 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. ”

“ 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. ”

“ It has been observed that the ganglionic nerve cells elongate and become spindle-shaped; each pointed end of the cell grows out into a nerve fiber; as the nerve cell connects the two fibers, we may describe the actual condition accurately as resulting in a single nerve fiber, which has a nerve cell interpolated in its course. Each group of nerve cells forms a bundle of nerve fibers, which constitute the posterior (or so-called dorsal or sensory) root of the anatomists. ”

“ Only by the most careful measurements has it been shown that nerve cells that are functioning have a greater metabolism than those that are at rest. For a long time it was thought that a nerve cell acted very much like a telephone or a telegraph wire, transmitting some kind of a disturbance which was set up in it, but not having any active part itself in the process. ”

“ These ganglion cells extend each of them an axone process which attains the organ (muscular cell or gland cell) , which it is the office of the sympathetic path to reach and influence. The axone-process of the first nerve cell is a myelinated nerve-fibre extending from the spinal cord to the ganglion; it constitutes the pre-ganglionic fibre of the conduction chain. The axone-process of the second nervecell, that is the neurone whose cell-body lies in the ganglion, is usually non-myelinate and constitutes the post-ganglionic fibre of the chain. ”

“ In the case of nerve paths to skeletal muscles the cell body of the motor nerve cell lies either in the spinal cord or the brain and its axon extends all the way from there directly to the muscle fibers. In the case of smooth muscles and glands a connecting nerve cell extends out from the central nervous system some distance and makes its nerve junction with the motor nerve cell part way between the central nervous system and the muscle or gland. These connecting cells are much branched, as are all connecting cells ”

“ It varies greatly with different persons. During this reaction-time, the cell or cells attacked upon the surface of the hand have conveyed news of the assault through numberless intermediate sensory nerve cells to the brain. The brain in turn has sent out its mandate through the appropriate motor nerve cells to all the muscle and other cells surrounding the injured cell, commanding them to remove it from the point of danger. ”

“ Complicated as is the nervous system, modern advances in the sciences of anatomy and physiology have made the comprehension of the subject easier. It is now known that the nervous system, in spite of its wide ramifications, is also made up of cells which are structurally and functionally related to each other, and make connection with every part of the whole community, the body. A nerve-cell, or neurone, may be very complicated [Pg 39] in its structure because of its many branches or extensions from the main body of the cell. ”

“ The nervous organs of the higher animals, including man, consist of enormously intricate systems of interwoven nerve cells or neurones whose unique character was first fully grasped some twenty years ago by Waldeyer. These neurones, like other cells, possess a nucleated cell-body, the ganglion cell of the older neurologists, from which extremely attenuated processes, the nerve fibers, reach out to the most distant parts of the animal. ”

“ To sum up, the development of nerves is not, as has been recently urged, an outgrowth of cell processes from certain cells, but is a differentiation of a substance which was already in position, and from which all other organs of the body have been and are developed. It frequently happens that the young nerve tracts can be seen sooner near the central organ than elsewhere, but it is doubtful if any importance can be attached to this fact, since it is not constantly observed. ”

“ But it is admitted that the nerve fibres constitute an uninterrupted network which admits of no endings--that is, whose ultimate reticulations lie beyond the microscopic limit. But there is a cell in every hundredth part of an inch of these ultimate reticulations, in each of which one of these bioplastic weavers sits plying his threads into the warp and woof of nerve tissue, if not of nerve force. ”

“ In order to control the functions of the various organs and tissues and to regulate the rate at which they reproduce themselves, the nerves extend their terminal branches, not only into every tissue, but into every microscopical unit of such tissue, and the part of the cell which represents the nerve terminal is the inner structure called the nucleus. ”

“ A mere glance at nervous tissue in any part will show that cells are far from forming the principal constituents. In the epidermis or a gland the cell is obviously the chief element, forming the bulk of the tissue, and being the characteristic agent. In nerve-tissue, as in connective tissue, the reverse is the case. We must therefore cease to regard the cell as having the importance now attached to it, and must rather throw the emphasis on the fibres and neuroglia. ”

“ Our nerve-arc is not a nervous system. We need only one additional element, however, to form such a system, and this is an ascending or "centripetal" fiber which shall proceed to another collection of cell-matter. In the cerebro-spinal system we have the lowest nerve-arc brought into close anatomical relation with the large cell-mass of the cerebral hemispheres, as well as with the lower cell-masses, by ascending and descending fibers. ”

“ A nerve consists of a bundle of such tiny axons, bound together by connective tissue. As a nerve ganglia is a center of activity in the nervous system, so a cell body is a center of activity which may send an impulse over this thin strand of protoplasm (the axon) prolonged many hundreds of thousands of times the length of the cell. ”

“ In the first (1) we have the epithelium of the nerve (scn in Fig. 47) in surface view, the cells appearing polygonal in outline, with here and there a mucous cell. In (2) we find a very slight layer of ganglion cells and fibres having a transverse direction (gc and fp in Fig. 47) . These are continuous with the plexus of fibres and ganglion cells which lie above the muscle layer all over the subumbrella, and which represent the motor part of the nervous system. This connection with the nerve shows how co-ordination is effected. ”

“ Their extension from cell to cell would indicate nerve communication, and it may be that the undoubted nerve and muscle function of many low animals, in which no nerves and muscles have been discovered, may be due to these interlacing fibrils. And the widely extended nerve-system of the higher animals, by which the whole body is made one interrelated unit, may be but a final outgrowth of the fibrillar-cell system. The fibril reticulum of the isolated cell becomes the nerve reticulum of the complex body, which is virtually converted into a single cell, with its intricate network of fibers. ”

“ There is no adequate warrant for the assumption that all nerves have their origin in ganglia, all fibres in cells. Such evidence as at present exists is against that supposition, and in favor of the supposition that both cell and fibre are differentiations of a common neuroplasm, sometimes directly,303 sometimes indirectly continuous. Fibres, and plexuses of fibres, interspersed with cells irregularly distributed—now singly, now in small groups, now in larger and larger groups—constitute the figured elements of nerve-tissue ”