Neural tube

“ Again, the process of regeneration after section of a nerve is not in favour of the neuroblast theory. 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 nerve should be taken from the animal as soon as possible after death, and laid upon a glass slide, with a drop or two of serum if possible. The needles may be used to clean it, but extreme delicacy is necessary. It will be found that the nerve is tubular and filled with a substance which is readily ejected by very slight pressure. When the nerve is submitted to the action of acetic acid, the outer covering, which is very thin, is considerably contracted, whilst the inner tube is left projecting ”

“ The brain and spinal cord, since the days of the celebrated investigations of Karl Ernst von Baer, have been identified as modifications of a single long tube, the so-called medullary tube of embryology. This tube, as the embryo advances, gradually increases in complexity, especially in the region of the head, until it is converted into the brain and spinal cord. ”

“ The brain, like the rest of the nervous system, is developed from the ectoderm or outer layer of the embryo by the formation of a groove in the mid-dorsal line. The lips of this medullary groove unite to form a canal beginning at the place where the neck of the embryo is to be. The part of the neural canal in front of the earliest union forms the brain and very early becomes constricted into three vesicles, to which the names of prosencephalon, mesencephalon and rhombencephalon are now usually given. ”

“ There appear very early certain cells, which soon become recognizable as young nerve cells (neuroblasts) because of their size and pointed shape; the pointed end now elongates into a very delicate thread, the nerve fiber, which is at first very short but rapidly lengthens almost like a growing root; the growing fiber takes its course for a certain distance, varying according to circumstances, within the wall of the medullary tube, but ultimately passes outside the tube into the neighboring tissues together with other nerve fibers of similar origin. ”

“ Thus the nerve-tube differentiates into sense-organs, brain and spinal cord, the alimentary tube into mouth cavity, œsophagus, stomach, intestine, respiratory apparatus, liver, bladder, etc. This specialisation in development is bound up with increased or diminished growth" (p. 155) . Rapid growth concentrated at one point brings about an evagination; in this manner are formed the sense-organs from the nerve-tube, the liver and lungs from the alimentary tube. Or increased growth over a section of a tube causes it to swell out ”

“ The separateness of the tube from the nervous system at its first origin is shown especially well in the frog, where, as Assheton has pointed out, owing to the pigment in the cells of the external layer of epithelium, a pigmented tube is formed, on the outside of which the nervous tissue is lying, and step by step the gradual intermingling of the nerve-cells and the pigmented lining cells can be followed out. ”

“ The central nervous system is, however, the system which controls the movements of the tube-feet. As we have seen, extensions of the radial nerves run to the tip of each podium. Tube-feet are chiefly used in ordinary progression; when this is quickened the spines come into play exclusively. The extent to which these two organs of locomotion are used varies from genus to genus. Thus Centrostephanus uses its spines a good deal, Echinus and Strongylocentrotus very little. The last-named genus sometimes walks on its tube-feet entirely without touching the ground with its spines. ”

“ The cell element of the nerve system, called a neurone, is the developmental, structural, and functional unit of the nervous system. It is a single cell presenting unusual structural modifications. It comprises not only the nerve cell body with its numerous protoplasmic processes, or dendrites, but also the axone, which may vary in length from a fraction of a millimeter to fully half the bird’s length. The bulk of the axone is many times the bulk of the cell body. ”

“ The nervous parts have not suffered in their development; the mechanism for walking in the bird is as well developed as in any other animal; their position only is different, for they still retain the original ventro-lateral position, but the non-nervous tube, the remains of the old intestine, has undergone a peculiar gelatinous degeneration just where it has remained free from invasion by the nervous tissue. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ This theory is that which supposes incipient conductile tissues, or rudimentary nerve-fibres, to be differentiated from the surrounding contractile tissues, or homogeneous protoplasm, by a process of integration which is due simply to use; so that just as water continually widens and deepens the channel through which it flows, so molecular or nervous waves of stimulation, by always flowing through the same tissue-tracts, tend ever more and more to excavate for themselves functionally differentiated lines of passage. ”

“ 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. ”

“ Yet this latter tube is one, if not the most striking, of the peculiarities which distinguish the vertebrate; a tubular central nervous system such as that of the vertebrate is totally unlike any other nervous system, and the very fact that the two nervous systems of the vertebrate and arthropod are so similar in their nervous arrangements, makes it still more extraordinary that the nervous system should be grouped round a tube in the one case and not in the other. ”

“ The numerous filaments came to be known as protoplasmic processes; the other fibre was named, after its discoverer, the axis cylinder of Deiters. It was a natural inference, though not clearly demonstrable in the sections, that these filamentous processes are the connecting links between the different nerve cells and also the channels of communication between nerve cells and the periphery of the body. The white substance of brain and cord, apparently, is made up of such connecting fibres, thus bringing the different ganglion cells everywhere into communication one with another. ”

“ The motor cells of the extensor muscles, when preoccupied by cerebral influence, appear refractory. T. Ziehen has noted exaltation of the jerk to follow extirpation of a cortical centre.
Although the cell body or perikaryon of the neuron, with its contained nucleus, is essential for the maintenance of the life of the cell branches, it has become recognized that the actual process and function of “conduction” in many neurons can, and does, go on without the cell body being directly concerned in the conduction. Conduction in Neurons. ”

“ The neural arch is strong, compressed from back to front, and placed as usual on the anterior part of the centrum. In outline it is sub-rhomboid with the sides concave. There is a strong process on each side above the neural canal for a rib, and apparently a neural spine, but all are broken. The transverse processes for the ribs are directed outward, and a little forward, flattened in front and behind, the surfaces being sub-parallel, so that in front the neural arch is concave from side to side. ”

“ It may be thought that the greater area of general tissue-mass in the larger segments than in the smaller, and not the lesser proportional area of tube-section, is the cause of the larger segments living longer than the smaller ones. I am led, however, to reject this hypothesis, because in Sarsia, where segmentation entails a comparatively small amount of tube-section, there is no constant rule as to the larger segments showing more endurance than the smaller ones—the converse case, in fact, being of nearly as frequent occurrence. ”

“ When we pass to the brain-region, to the anterior dilated portion of the tube, embryology tells a different story. Here, as in the spinal cord, the nervous masses are grouped at first laterally and ventrally to the epithelial tube, as is seen in Fig. ”

“ Bipolar cells are found almost exclusively in the peripheral sensory system, as in the olfactory membrane, in the retina, in the cochlear and vestibular ganglia, and in the cerebro-spinal ganglia of the embryo.
Multipolar cells are the most numerous and form the principal elements of nerve centers throughout the system. They are termed multipolar because of the greater or lesser number of dendrites given off in addition to the single axone.
The body of the nerve cell consists of a mass of protoplasm surrounding a nucleus. ”

“ While our knowledge of the cell is thus far indeed from having the precision which the text-books display, and in no sense warrants the current physiological interpretations, our knowledge of fibres and neuroglia is also too incomplete for theoretic purposes. We know that the axis cylinder is the essential element; but we are still at a loss what part is to be assigned to the medullary sheath. There is indeed a popular hypothesis which pronounces it to be the means of insulating the fibre, and thus preserving the isolated conduction of nerve-force. ”

“ Because of the greater plasticity of the neural mechanism in youth it is easier to open up and fix pathways of conduction than in later years. Moreover the earlier established lines of conduction become the more permanent.
Characteristic Arrangements of Nerve Cells Are as Subject to Inheritance as Other Structures of the Body.—That the main features of the nervous system [Pg 211] are inherited becomes obvious when we see that each kind of animal has its own distinctive numbers, arrangements and proportions of the various neural units. ”

“ The human brain contains approximately 1010 neurons to which the neuron theory assigns the primary role in central nervous activity. These cells occupy, however, a relatively small fraction of the total volume. There are, for example, approximately 10 times that number of neuroglia, cells of relatively indeterminate function. Each neuron (consisting of cell body, dendrites and, sometimes, an axon) comes into close contact with the dendrites of other neurones at some thousands of places, these synapses and “ephapses” being spaced approximately 5μ apart (1) . ”