Axon

“ B is a touch corpuscle, consisting of a coiled axon-end surrounded by a little cone of other tissue.
C is an end-bulb, presumably belonging to the temperature sense. It has, again, a coiled axon-end surrounded by other tissue. The "coils" are really much more finely branched than the diagram shows.
D is a free-branched nerve end, consisting simply of a branched axon, with no accessory apparatus. It is the pain-sense organ. ”

“ Many of these small cells possess numerous processes, but no axon. It is difficult to see, therefore, that their function can be conductive in the ordinary sense. Most of our ideas about nervous functioning are based on the assumption that each neuron acts essentially as a link in some chain of conduction, but there is really no warrant for this in the case of cells with many short branches. ”

“ It has been known for half a century that the axone degenerates when cut off from the cell of which it is a process. The fact that the cell suffers also is a striking illustration of the essential unity of the nerve-cell and all its branches, whether they are long or short—three feet in length, as the axones that run from the lower part of the spinal cord to the foot may be, or a thousandth of an inch, like some which arise and terminate within the gray matter of the cord and brain. ”

“ The living protoplasm of the nerve cell has a gray color, so that wherever this shows we have what is commonly called gray matter. We saw a moment ago that every axon is inclosed in a sheath. Some of these sheaths are transparent, so that the gray color can be seen underneath, but most of them have a layer of white material, which makes them look white instead of gray. The bundles of axons corresponding to the telephone cables make up what we call the nerves. Nearly all nerves are white in color because of the white material in the sheaths. ”

“ The "white matter" of the brain and cord is composed of axons. Axons afford the means of communication between the nerve centers and the muscles and sense organs, and between one nerve center and another. ”

“ With parameters adjusted to closely match the data of Hodgkin and Huxley, the electronic model exhibits all of the important properties of the axon. It produces spikes of 1 to 2 msec duration with a threshold of approximately 5% to 10% of the spike amplitude. The applied stimulus is [Pg 13] generally followed by a prepotential, then an active rise of less than 1 msec, followed by an active recovery. ”

“ 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. ”

“ The vocal movement of saying the name is made in response to the auditory stimulus of hearing the name, but when the neurone in the "speech center" is thus made active, it takes up current also from the axon that reaches it from the visual center, even though the synapse between this axon and the speech neurone is far from close. This particular synapse between the visual and the speech centers, being thus exercised, is left in an improved condition. Each neurone in the diagram represents hundreds in the brain, for brain activities are carried on by companies and regiments of neurones. ”

“ The neuron is described as having a cell body or perikaryon from which the cell branches—dendrites and axon—extend, and it is this perikaryon which, as its name implies, contains the nucleus. It forms the trophic centre of the cell, just as the nucleus-containing part of every cell is the trophic centre of the whole cell. ”

“ A—Diagram to illustrate neurons and their method of connection; a, axon; d, dendrite; s, synapse. To simplify the diagram the medullary sheathes of such fibers as would have them have been omitted. The arrows indicate the direction in which the impulse travels. The lower series shows diagrammatically how from the same neuron in the cortex two subordinate neurons may be affected, the one excited to cause contraction of a certain group of muscle fibers, the other inhibited so that the antagonistic fibers may relax and thus not hinder the movement of a given part. ”

“ There is no evidence that the cut end of a nerve can grow down and attach itself to a muscular or epithelial element without the assistance of a nerve tube down which to grow. When the cut nerves connected with the periphery degenerate, that applies only to the axis-cylinder and the medullary sheath, not to the neurilemma; the connective tissue elements remain alive and form a tube into which the growing axon finds its way, and so is conducted to the end-plate or end-organ of the peripheral structure. ”

“ Each nerve cell is trophic to its processes and to the tissue cells to which these processes are distributed. The growth, nutrition and repair of each cell of the body is dependent upon the integrity of the axon which supplies it. The effect of nerve impingement upon the single cell is a weakening of cell structure and a disturbance, slight or great, of the special function possessed by that cell. ”

“ An impulse is supposed to travel normally only in one direction through a neuron, the dendrites being the receiving and the axon the discharging terminals. There are various types of neurons. Some, particularly within the brain, have their main processes so provided with branches and brushes that they may come into physiological connection with a number of other neurons. ”

“ The axons which make up the motor nerves are branches of nerve cells situated in the cord and brain stem; they extend from the reflex center for any muscle out to and into that muscle and make very close connection with the muscle substance. A nerve current, starting from the nerve cells in the reflex center, runs rapidly along the axons to the muscle and arouses it to activity. ”

“ In addition to the axon properties, however, the electronic model is able to reproduce all of the somatic and dendritic activity outlined in the section on subthreshold activity. Simulation of the pacemaker and graded-response potentials is accomplished without additional circuitry. ”

“ At the present time, owing to the researches of Golgi, Ramón y Cajal, and others, the nervous system is considered to be composed of a number of separate nerve-units, called neurones, each neurone consisting of a nerve-cell with its various processes; one of these—the neuraxon—constitutes the nerve-fibre belonging to that nerve-cell, the other processes—the dendrites—establish communication with other neurones. The place where these processes come together is called a synapse, and the tangle of fine fibres formed at a number of synapses forms the 'neuropil.' ”

“ In the bony fishes the optic tract reaches the grey matter of the optic lobe, a part of the mid-brain, to which the so-called anterior colliculus is equivalent in the mammalian brain. In the optic lobe the axones of the neurones of the optic tract meet neurones whose axones pass in turn to the motor neurones of the muscles moving the eyeballs, and also to other motor neurones. But in these fish the optic tract has no obvious connexion with the fore-brain or with any cerebral pallium. ”

“ Tracts of association are probably formed in definite lines through the nervous system, as during the life of a child repeated waves of sense impulses beat against and overcome resistances, and make smooth pathways here and there through the brain structure. Thus may be produced growth of axons in certain directions, and synapses of this cell with that. If the same stimulus be often repeated, the synapses between groups of cells may become permanent. A memory, a definite line of action which is manifested by a certain muscular response, may thus become structurally fixed. ”

“ 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. ”

“ The connection between a sense organ and the brain consists of a series of separate links, which always remain distinct units. Each cell, or neurone, as one of these units is called, consists of a body, which contains the nucleus, and two sets of branches. In one set the branches are very short and numerous, called dendrites; in the other there is usually only one branch, called the axone, which is relatively long. A group of cell bodies is called a ganglion. In practically all sensory nerves these ganglia are situated outside of the central nervous system. ”

“ The apices of the pyramidal perikarya are turned towards the free surface of the cortex. The figure as interpreted in terms of functional conduction means that the cortex is beset with conductors, each of which collects nerve-impulses, from a minute but relatively wide field by its branched dendrites, and that these nerve-impulses converge through its perikaryon, issue by its axone, and are carried whithersoever the axone runs. In some few cells the axone breaks up into branches in the immediate neighbourhood of its own perikaryon in the cortex. ”

“ There is only one thing that a sense organ always and necessarily contains, and that is the termination of a sensory nerve. Without that, the sense organ, being isolated, would have no effect on the brain or muscles or any other {189} part of the body, and would be entirely useless. The axons of the sensory nerve divide into fine branches in the sense organ, and thus are more easily aroused by the stimulus. ”

“ The neurons of a nerve pathway are arranged end to end with the axons all pointing in one general direction so that the nerve energy travels always in the same direction over the entire nerve path. Impulses are transferred from the first neuron in the chain to the second, and from second to third, etc., by contact of the telodendria of the one neuron with the dendrites or receptive processes of the next. Part of the nerve pathway may be within the central axis and part within the trunk of a peripheral nerve. ”

“ If we dissect the body of any animal or bird, we can find nerves passing to various parts. For example, a large nerve goes down each leg; this nerve subdivides here and there. Since we know that the nerve consists of a great many axons bundled together, we will realize that this subdivision is not a real branching, but simply a passing of some of the axons away from the main trunk along the smaller stem. Some of these smaller stems can be traced to endings in the skin; these contain the axons connecting with sense organs. Others lead directly into muscles. ”

“ Some of these axons may also connect with sense organs, since, as we have already seen, every muscle has embedded in it the organs of muscle sense, but in addition any nerve that leads to a muscle contains a great many axons which pass directly to the muscle fibers. ”

“ Its growth is faster than the rest of the neural tube and from its walls, by proliferation, develop the structures of the cerebrum, mid-brain, and hind-brain. It also gives off ganglionic masses from which grow sensor processes to form the afferent elements of the cranial nerves and contains, like the cord, motor nuclei, or nidi, from which motor axons grow toward the periphery to come into relation with definitely predetermined organs. ”

“ The heavy border represents the medullary, or enveloping sheath, which becomes thicker in the larger fibers.—After Donaldson.
Neurone Fibers.—The neurone fibers are of two kinds, dendrites and axons. The dendrites are comparatively large in diameter, branch freely, like the branches of a tree, and extend but a relatively short distance from the parent cell. Axons are slender, and branch but little, and then approximately at right angles. They reach a much greater distance from the cell body than the dendrites. Neurones vary greatly in length. ”

“ The court could of course vacate the FTC’s order. But Axon’s separation-of-powers claim is not about that order; indeed, Axon would have the same claim had it won before the agency. The claim, again, is about subjection to an illegitimate proceeding, led by an illegitimate decisionmaker. And as to that grievance, the court of appeals can do nothing: A proceeding that has already happened cannot be undone. Judicial review of Axon’s (and Cochran’s) structural constitutional claims would come too late to be meaningful. ”

“ This is what a win looks like under Thunder Basin. When you replace clear jurisdictional rules with a jumble of factors, the room for disagreement grows. The incentive to litigate increases. Years and fortunes are lost just figuring out where a case belongs. Ms. Cochran and Axon have already endured multi-year odysseys through the entire federal judicial system—and no judge yet has breathed a word about the merits of their claims. Nor can I fault the district court in Ms. Cochran’s case, or all of the lower courts in Axon’s case, for thinking the Thunder Basin factors required dismissal. ”

“ The Article II challenges in Axon’s and Cochran’s cases would likewise prevent ALJs—through whom the Commissions do much of their work—from exercising any power, unless they lose their double-for-cause tenure protection. And Axon’s combination-of-functions claim similarly goes to the core of the FTC’s existence, given that the agency indeed houses (and by design) both prosecutorial and adjudicative activities. The challenges here, as in Free Enterprise Fund, are not to any specific substantive decision—say, to fining a company (Thunder Basin) or firing an employee (Elgin) . ”