Sensory receptor

“ We have now presented some evidence that nervous energy is discharged by the adequate stimulation of one or more of the various receptors that have been developed in the course of evolution. In response to an adequate stimulus, the nervous system is integrated for a specific purpose by the stimulated receptor, and but one stimulus at a time has possession of the final common path— the nerve mechanisms for action. The most numerous receptors are those for harmful contact; these are the nociceptors. ”

“ 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 ”

“ And in the process of specialization the sensorium itself is subdivided into higher and lower nerve centers—centers of conscious thought and automatic transfer of impulse into motion. This transfer indicates the real nature of all forms of nerve action. All are processes of transfer of sensation into movement. The sensorium or brain has no knowedge except such as comes to it from the sense organs through the ingoing or sensory nerves. It has no power to act save by its control of the muscles through the outgoing or motor nerves. ”

“ In saying that no action at all can be propagated by the agency of the nervous system, we do not make proper allowance for the vital properties of nerve-fibre. The vital nature of common nerve-fibre is such that two actions can be conducted along it;—an impulse producing sensation, or an impulse producing muscular action, at a distant point. It is known that an impression on the terminal extremity of a sensory nerve is capable of producing either sensation or motion at a distance, by what is called reflex nervous action. ”

“ The neurones concerned with the reception of these stimuli exhibit interesting relations. The olfactory neurones, as is well known, have their cell bodies in the olfactory epithelium, whence their neurites extend into the central olfactory apparatus. They reproduce in a most striking way the type of primary sensory neurone common to the invertebrates, and in this respect they represent the most primitive type of sensory neurone in the body of vertebrates. ”

“ Physiologically as well as anatomically the sea-anemone seems to possess a diffuse rather than a centralized nervous system, and its neuromuscular mechanism consists of receptors and effectors connected by a nervous net which is composed partly of the nervous processes of the receptor cells and partly of similar processes from ganglion cells. ”

“ It precludes one receptor from occupying for long periods an effector organ to the exclusion of all other receptors. It prevents long continuous possession of a common path by any one reflex of considerable intensity. It favours the receptors taking turn about. It helps to ensure serial variety of reaction. The organism, to be successful in a million-sided environment, must in its reaction be many sided. ”

“ 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. ”

“ The central organs or adjusters include the proximal ends of these elements, all the association neurones, and the proximal ends of the efferent neurones. The effectors are not neurones at all, but muscle fibers, gland cells or other types of cells under the control of nerves. Thus the ordinary reflex may be said to involve in sequence the activity of a receptor, adjustor and an effector, to use modern terminology, and these three elements are recognizable in every complete reflex arc.
Our own reflexes are sometimes associated with consciousness and sometimes not. ”

“ Just as men and animals in the world are only able to maintain their existence and constantly adapt themselves to changing circumstances by aid of the peripheral endings of their sensory nerves, so every organ, indeed every cell of every organ, can only maintain its place in the animal microcosm, and adapt itself to the activity of innumerable associates, as well as to the general life of the whole, by virtue of the fact that the peripheral end apparatus of its centripetal nerves possesses a specific excitability. ”

“ The message is transmitted to the brain, where it is received by a cluster of small sensory cells or nerve-centres, which are connected with another cluster of fewer and larger motor-centres, often at some distance from them, by a network of interlacing fibres. But it is always a case of a single circuit of wires, batteries, and accumulators, adapted for receiving, recording, and transmitting one sort of vibrations caused by and producing one sort of energy, and one only. The brain does not act as a whole, receiving indiscriminately impressions of light, sound, and heat ”

“ Being at once a conductor between the brain and nerves, and a reflex nerve centre, we must consider both rôles in seeking to locate lesions from symptoms. In passing from the nerves to and from the sensorium both sensory and motor currents cross so that one side of the brain presides over the other side of trunk and limbs. This crossing of the motor fibres takes place in the medulla oblongata, while that of the sensory fibres occurs in the spinal cord close in front of the nerve from which they have entered. ”

“ In the foregoing considera­tion of the formation of receptors of the skin it was assumed that certain common stimuli of the environment hammer out for themselves paths in the nerve-fibrils of the skin and by ceaseless repeti­tion lay down not only the receptor, which may be called the terminus a quo, but also the afferent fibres which ultimately find their way into the grey matter of the cord and brain. ”

“ The nervous system controls many more kinds of effectors than muscle and its sensory elements are vastly more complex than is implied in the preceding sketch. To gain a more comprehensive view of the evolution of these organs, it is necessary to consider a subsidiary but important process, the appropriation of effectors and receptors.
The nervous system of many of the higher animals not only acts upon the musculature; it may also control electric organs, luminous organs, chromatophores, glands, etc. ”

“ The sensory nerve cell, the recipient of the stimulation, like the muscle cell, the reagent to stimuli, has evolved from the primitive cell by greater and greater differentiation, both of structure and function. In the crustaceans, invertebrates, and lower vertebrates where motor reactions to stimuli are more or less complex and varied, the sensory aspect of the moment is probably correspondingly complicated,―organic connections are present giving rise to secondary sensory elements, constituting the material of perceptual life. ”

“ This also illustrates how one nerve center influences another. (Figure text: white matter, gray matter, lung, respiratory center in the brain stem, diaphragm, motor center in cord for the diaphragm)
Very often, and possibly always, the reflex arc really consists of three neurones, a "central" neurone intervening between the sensory and motor neurones and being connected through synapses with each. The central neurone plays an important rôle in coördination. ”

“ 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. ”

“ The afferent neurones are not in most cases so constituted as to be excitable themselves directly by the environment - for instance, they cannot be stimulated by light. Their amenability to the environment, their sensitization to environmental agencies, is effected by special cells adjunct to their peripheral ends. These cells from organs are called receptors. ”

“ 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. ”

“ The first link of each reflex chain is a neurone which starts in a receptor organ, e. g., a sense-organ. A receptive field, e. g., an area of skin, is always analyzable into receptive points, and the initial nerve-path in every reflex arc starts from a receptive point or points. A single receptive point may play reflexly upon quite a number of different effector organs. It may be connected through its reflex path with many muscles and glands in various parts. ”

“ In left-handed persons, on the contrary, it seems to attach to the right hemisphere.
Auditory Region of the Cortex.—Besides the two great organs of distance-receptors, namely, the nose and eye, whose cerebral apparatus for sensation has just been mentioned, those of a third great distance-receptor have to be considered. The agents of stimulation of the two former are respectively chemical (olfactory) and radiant (visual) ; the mode of stimulation of the third is mechanical, and the sensations obtained by it are termed auditory. Their cerebral localization is very imperfectly ascertained. ”

“ 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. ”

“ 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. ”

“ From what has been said, it is plain that a sensory nerve is one which conveys a sensory impulse from the peripheral or outer part of a nerve to the spinal cord or brain, and which is, therefore, termed afferent; and that a motor nerve is one which transmits an impulse from the nerve centre, or is efferent. So difference in structure, or in chemical or physical composition, can be discerned between the afferent and the efferent nerves. A certain period of time is required for the transmission of all impulses. ”

“ 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. ”

“ The primary elements are initiated directly by incoming peripheral stimulations, while the secondary sensory elements are brought about indirectly, through the mediacy of the primary elements, the secondary elements themselves being really derived from sense-organs others than the ones directly stimulated by the peripheral excitation. ”

“ He discovered that in the nervous trunks there are special sensory filaments, the office of which is to transmit impressions from the periphery of the body to the sensorium, and special motor filaments which convey motor impressions from the brain or other nerve centre to the muscles. He also showed that some nerves consist entirely of sensory filaments and are therefore sensory nerves, that others are composed of motor filaments and are therefore motor nerves, whilst a third variety contains both kinds of filaments and are therefore to be regarded as sensory-motor. ”

“ Thirdly, The vibrations thus excited in the æther, and particles of the sensory nerves, will be propagated along the course of these nerves up to the brain. For the æther residing in the medullary substance, being of an uniform density on account of the smallness of the pores of the medullary substance, and uniformity of its texture, before taken notice of, will suffer the excited vibrations to run freely through it. And the same uniformity, together with the continuity, softness, and active powers of the medullary substance, will farther contribute to the free propagation of the vibrations ”