Sensory cell

“ The supporting cells like those referred to before, are cone-shaped, having a proximal fibrous process that runs into the underlying stratum of nerve fibres, and on the surface of the retina a broad distal pigmented termination. The sensory cells are spindle-shaped, the proximal processes becoming continuous with fibres of the underlying nervous mass, while the distal process runs up to the surface of the retina (the part toward the lens) in between the ends of the supporting cell. The two kinds of cells are accordingly designated as pigment and visual. ”

“ This last fact is the true analogue of what must happen between the nerve-cell and the sensory fibre. It is certain that cells are more inert than fibres, and that rapid vibrations in the latter can only arouse relatively simple processes or states in the former. The higher cells may have even a slower rate of explosion than the lower, and so the twenty thousand supposed blows of the outer air may be 'integrated' in the cortex into a very small number of cell-discharges in a second. This other diagram will serve to contrast this supposition with Spencer's. ”

“ In order that the brain cells may receive and store up memories, it is evidently necessary that nerve paths from the sense organs should lead into the brain; so, if we go back to the sensory nerve cells, we shall find that in nearly every one of them, if not in every one, one of the branches makes a nerve junction with a connecting cell, which either extends directly into the brain, or links with other connecting cells which do. ”

“ 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. ”

“ A cell which at an early stage was protective, locomotor and sensory in function may give rise to daughter cells in which these functions are distributed to different cells, cells which at an early stage were sensitive to many kinds of stimuli give rise to daughter cells which are especially sensitive to one particular kind of stimulus, such as vibration, light, or chemicals. ”

“ The neuron is but a highly differentiated cell or reservoir for the intake and outgo of energy. In this respect the nerve cell is entirely like other cells of humbler function. Every cell is a storage cell, accumulating energy and then liberating it at an appropriate occasion of a given stimulus, all cells working for the preservation of the organism as a whole. The rise and fall of the level of energy in the reservoir regulate the various manifestations, sensory, motor, emotional, mental which the individual displays to the various stimulations coming from his environment. ”

“ In the second type, usually called "nerve-eminences," the sensory cells are shorter than the supporting cells, and they are always innervated by the lateralis system. When first developed in the embryo they are quite superficial, like end-buds, but later the epidermis in which they lie sinks inwards so as to line a series of pits, closed sacs, tubes, open grooves, or closed canals. ”

“ The activity of an organ never depends on itself alone; the contraction of a muscle is induced by a nerve stimulus or by an electric current; the activity [Pg 382] of the nerve-cells of the brain requires the continual stimulus of the blood-stream, and cannot continue to exist without it; the specific sensory-nerves and sense-cells of the eye, ear, olfactory organ, and so on, are all prompted to activity by adequate stimuli. The same is true in regard to the determinants, they must be 'liberated' if they are to distribute themselves and migrate into the cell-body ”

“ According to certain histological researches of recent years we know that between the sense-organs and the central nervous system there exist closely connected chains of conductors or neurons, along which an impression received by a single sensory cell on the periphery is propagated avalanchelike through an increasing number of neurons until the brain is reached. ”

“ II, Figs. 19-21) will show that, except for the rather unimportant matter of the mucous cells, which he finds regularly and thickly disposed on each side of the nerve (’78, Fig. 21) , our only essential difference lies in the matter of sensory cells in the epithelium. His figures show a multitude of spindle-shaped sensory cells whose central ends are continued in processes that bend around into the mass of fibres of the nerve. In his Fig. 20 a relatively small number of nuclei, just one-third as many, are seen attached nearer to the surface, which represent the supporting cells. ”

“ C, Body of a ganglion-cell showing “Nissl’s granules.”
D, Sensory cells from olfactory epithelium. (After Schultze.)
E, Diagrammatic representation of the sensory epithelium of retina (rod and cone layer) . (After Schwalbe.)
(c) Specialization for the Reception and Conduction of Stimuli.—One of the most striking of the fundamental attributes of living protoplasm is its “irritability,” that is to say, its power of responding to external impressions, “stimuli,” by movement, which, both in kind and intensity, is wholly independent of the amount of energy expended by the stimulus. ”

“ The largest cells occur in the inferior horn of the spinal cord, in the spinal ganglia, in the large pyramidal cell layer of the cerebral cortex, in the Purkinjean cell layer of the cerebellum, and in Clark’s column of the spinal cord. Very small cells occur in the olfactory bulbs, in the granular layer of the cerebral and cerebellar cortex, and in the gliosum cornuale of the cord.
Nerve cells are classified according to the number of processes arising from the cell body, and neurones are referred to as unipolar, bipolar, and multipolar. ”

“ In this case the cells introduced, or more properly, some substances within the cells, act as stimuli to the body cells of the animal injected to cause them to produce more of the specific cell receptors which respond to the stimulus. ”

“ The business of the cells individually and of the gray matter as a whole is to receive sensory messages from the external organs of the senses and to transform or to co-ordinate their impulses into the proper movements—as, for example, when we see a fruit or flower, and stretch out our hands to pick it. The white substance is shown to consist of numerous interlacing fibers, having for their function the conveyance of such information from without inward, or the carrying down of such motor impulses from within outward. ”

“ But, when the individual becomes multicellular and bears only one kind of germinal cells, the attractive energy which was originally limited to these cells is transmitted to the whole organism, and this necessitates the existence of sensory and motor nerve centers. ”

“ 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. ”

“ Living tissue can only be set into activity by stimuli of certain minimal intensity; should the stimulus fall below that minimal intensity, the living protoplasm does not react. This holds true of all cells, from the lowest to the highest,―from the bacterium and infusorium to the highly differentiated cell, such as muscle cell, or neuron. The reaction of the living protoplasm to the stimulus shows the irritability or sensitivity of the cell. This sensitivity has its physiological threshold, so that a stimulus falling below a certain intensity cannot call forth any reaction in the cell. ”

“ The most powerful microscopes disclose in this protoplasm a certain definite structure, a very fine, thread-like network spreading from the nucleus throughout the semi-fluid albuminous protoplasm. It is certainly in line with the broad analogies of life, to suppose that in each cell the nucleus with its network is the brain and nervous system of that individual cell.
All living organisms consist, then simply of cells. Those consisting of but one cell are termed unicellular; those comprising more than one cell are called pluricellular.
The unicellular organism is the unit of life on this earth. ”

“ When a gland enters into activity, a certain quantity of liquid flows into the channels of the gland, and this liquid is caused by a physical and chemical modification of the cellular protoplasm; it is a melting, or a liquefaction, which likewise is material. The function of the nerve cell is to produce movement, or to preserve it, or to direct it; ii is material like the cells. There is therefore nothing in all those functional phenomena which might lead us to understand how a material cause should be capable of engendering a conscious effect. ”

“ As a matter of fact, it is not necessary for single cells to have this great length, because it is possible for them to be arranged end to end, making a path of living protoplasm consisting not of one cell but of a chain of them. If we look at a nerve cell under the microscope we see that it is made up of a little central mass of protoplasm to which has been given the name of “cell body.” From this cell body extends a tiny thread of living protoplasm. ”

“ Undeveloped Cells.—Professor Donaldson tells us on this point that: "At birth, and for a long time after, many [nervous] systems contain cell elements which are more or less immature, not forming a functional part of the tissue, and yet under some conditions capable of further development.... For the cells which are continually appearing in the developing cortex no other source is known than the nuclei or granules found there in its earliest stages. ”

“ Diagram 67.—Showing the System of Nerve Cells in the Cortex.
A, Afferent fibre; B, distributing cell; C, small pyramidal cell; D, large pyramidal cell.
Returning to general considerations, an important point is the way in which the different centres are connected by fibres, which put them into relation. The brain may consist of many centres, just as the body consists of many organs, but both body and brain must live as a whole. ”

“ As I have explained, the tissues and organs of the higher animals break up under the microscope into a most ingeniously constructed network of tiny living gelatinous corpuscles with a nucleus in the centre—the cells. But our radiolarian has no more got tissues composed of cells than it has stomach or lungs or any other organ. It is merely a single cell with a nucleus and a jelly-like body. Yet in this case the single cell is a whole individual, a complete animal, that lives, moves, eats, breathes, and so on. ”

“ Certain sensory nerve-cells in the earthworm's skin are stimulated by vibrations in the earth; the message travels down a sensory nerve-fibre from each of the stimulated cells and enters the nerve-cord. The sensory fibres come into vital connection with branches of intermediary, associative, or communicating cells, which are likewise connected with motor nerve-cells. ”

“ This universal cell-function was largely [66] given over when the animal (a collection of cells) began to develop special organs of touch, sight, and hearing; but there is no indication that the original power of sensibility has ever been wholly destroyed in any cell. It is, therefore, still within the range of biological possibility that a man should hear with his fingers or smell with his toes, since every cell of both finger and toe once did a work corresponding to the present functions of the five animal senses. ”

“ For, let it be granted that nerve-cells are the origins of the fibres and the sources of their nutrition—a point which is eminently disputable—this would in no sense help the physiological hypothesis of the cell as the fountain of Neurility. If the fibre is simply the cell-contents drawn out longitudinally, if its essential element is identical with the essential element of the cell, then we can no more ascribe to the cell the exclusive property of Neurility than we can draw a lump of lead out into a wire, and then ascribe different properties to the thin end and the thick end. ”

“ 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. ”

“ The physiologist has many possibilities of conceiving the further stages of the process, in order to satisfy the demand of explanation. He may say the excitement of each [Pg 43] of these two brain cells, the one in the olfactory center, the other in the auditory center, irradiates in all directions through the fine branches of the brain fibers. Each cell has relations to every other cell in the brain ”