Neuron

“ 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. ”

“ A neuron may range from 1/300 to 1/3000 of an inch in diameter, and there are several thousand millions of these cells in the nervous system. A developed neuron consists of a cell body with numerous prolongations in the form of white, thread-like fibres. The neuron with its outgoing fibres is the unit of the nervous system. Neurons are supposed to be of three classes, sensory to receive stimulations, motor to send out impulses to the muscles, and association to connect sensory and motor centres. ”

“ We should turn our attention to the properties of real neurons, and see which of them are the most important ones for us to imitate. Obviously, we cannot hope to imitate all the properties of a living neuron, since that would require a complete simulation of a living, metabolizing cell, and a highly specialized one at that ”

“ The most dramatic aspect of neuron function is, of course, the axon discharge. It is this which gives the neuron its “all-or-nothing” character, and it is this which provides it with a means for propagating its output pulses over a distance. Hodgkin and Huxley (1) have developed a very complete description of this action. Their model is certainly without peer in describing the nature of the real neuron. ”

“ 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) . ”

“ It is just because of this community in their concepts that there actually is a certain “point to point” correspondence or parallelism between the psychical and the neural: as an organ a neuron is a unit; physically regarded, it ceases to be one. ”

“ 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. ”

“ There are two very compelling reasons why mathematical and physical models of the neuron should be built. Model building, while widely used in the physical sciences, has been largely neglected in biology. However, there can be little doubt that building neuron models will increase our understanding of the function of real neurons, if experience in the physical sciences is any guide. Secondly, neuron models are extremely interesting in their own right as new technological devices. ”

“ Each can make the selections it prefers, because the parameters of real neurons which are necessary and sufficient for learning have not been exhaustively identified.
From the point of view of one whose interests include real neurons, this lack of identification is unfortunate. I once wrote a book which included some guesses about the essential attributes of neurons. Since that time, many neuron simulation programs have been written. But these programs, although interesting and worthwhile in their own right, have done little to answer the question of the necessary parameters. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ If the intensity of the stimulus is below this threshold, the neuron does not fire.
c. The neuron is capable of temporal and spatial integration. Many subthreshold stimuli arriving at the neuron from different sources, or at slightly different times, can add up to a sufficient level to fire the neuron.
d. Some inputs are excitatory, some are inhibitory.
e. There is a refractory period. Once fired, there is a subsequent period during which the neuron cannot be fired again, no matter how large the stimulus. This places an upper limit on the pulse rate of any particular neuron. ”

“ It is incredible that the neurone-action corresponding to the perception of a response in another, or to the idea of a response in oneself, or to the first term in an association by similarity, should have, in and of itself, a special power to determine that the next neurone-action should be that paralleling the response in question. ”

“ But the association neurones are not only the most numerous in the vertebrates; they also constitute the basis of the most significant evolution. The central nervous organs that show the most conspicuous progressive changes in the vertebrates are the cerebellum and the cerebrum, particularly their cortical portions, and when it is remembered that few or no primary sensory or motor neurones contribute to these two organs, but that they are made up of association neurones almost exclusively, it will be seen how enormously important these neurones become. ”

“ It will help in understanding both the structure and the working of the nervous system to keep in mind that it contains but one fundamental unit of structure. This is the neurone. Just as the house is built up by adding brick upon brick, so brain, cord, nerves and organs of sense are formed by the union of numberless neurones. ”

“ 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. ”

“ The chains of neurones passing through the cortex are more complex and connected with greater numbers of associate complex chains than are those of the spinal centres. But just as the reflex centres of the cord are each attached to afferent channels arriving from this or that receptive-organ, for instance, tactile-organs of the skin, or spindles of muscle-sense, &c., so the regions of cortex whose function is to-day with some certainty localized seem to be severally related each to some particular sense-organ. ”

“ The cortical neurone therefore collects impulses in the region of cortex just about its perikaryon and discharges them to other regions, some not cortical or even cerebral, but spinal, &c. One question which naturally arises is, do these cells spontaneously generate their impulses or are they stirred to activity by impulses which reach them from without? The tendency of physiology is to regard the actions of the cortex as reactions to impulses communicated to the cortical cells by nerve-channels reaching them from the sense organs. ”

“ It is hard to teach an old dog new tricks, because the old tricks employ familiar, much-used neural paths, while new tricks require the connecting up of groups of neurones not in the habit of working together; and the flow of nerve energy is more easily accomplished in the neurones accustomed to working together. ”

“ Neurones are devoted to the maintenance of functions. Reproductive neurones are so arranged as to receive afferent nerve impulses from other tissues; emissive neurones give off efferent nerve impulses. The former are sensory neurones; the latter are motor neurones if connected with muscles, and excito-glandular if connected with gland cells. ”

“ That neurone forms the sole avenue which impulses generated at its receptive point can use whithersoever may be their distant destination. That neurone is therefore a path exclusive to the impulses generated at its own receptive points, and other receptive points than its own can not employ it.
But at the termination of every reflex arc we find a final neurone, the ultimate conductive link to an effector organ, gland or muscle. ”

“ The strongest adherents of computational neuroscience argue that not only do mental states depend on brain states, but that (as a methodological dictum) we ought to focus our scientific efforts on mapping neuronal firings only. That is, it’s not just necessary to understand the brain in order to get a grip on psychology—understanding how neurons work just is understanding psychology. ”

“ What happens in a train of thought is that first one group of neurones is aroused to activity, and then this activity, spreading along the axons that extend from this group of neurones to another, arouses the second group to activity; and so on. The brain process may often be exceedingly complex, but this simple scheme gives the gist of it.
The way nerve currents must go shooting around the brain from one center or group of neurones to another, keeping it up for a long time without requiring any fresh peripheral stimulus, is remarkable. ”

“ Recovering the paper, he read for the last time those words which he had copied. "All the functions of man are controlled by the brain, yet less than a fourth of the brain tissues are used. Size has no importance relative to brain capacity, which exists solely because of synapses among the neurons, more simply explained as connections among the different cells. Thus, the more of these connections, the more brain capacity and proportionately the more intelligence is produced." ”

“ The central neurons of the cerebral cortex are, as it were, the tablets, upon which the excitations transmitted thither by the sensory neurons, record the extended neurograms that constitute the physical basis of the concrete imagery of memory and imagination. ”

“ 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. ”

“ It is an accepted principle of physiology that when a number of neurons, involved, let us say, in a coördinated sensori-motor act, are stimulated into functional activity they become so associated and the paths between them become so opened or, as it were, sensitized, that a disposition becomes established for the whole group, or a number of different groups, to function together and reproduce the original reaction when either one or the other is afterward stimulated into activity. ”

“ It is believed that certain cells of the grey substance only, the great and the little pyramidal-shaped cells, are associated with the psychical functions, and that each of these, forming with its axis-cylinder, dendrons and other branching prolongations what is called a neuron, is not in constant connection with, and does not occupy a fixed position once for all in regard to, other similar neurons, but may by means of its prolongations place itself alternately in contact with a great number of these. ”

“ The brain centers through which the labyrinth elicits the various appropriate muscular reactions of the head, body, limbs, and eyes—the righting, the postural, and the ocular reflexes—represent an intricate mechanism. Before we can hope for a satisfactory understanding of their functional organization, we will have to know their anatomy in more detail. Thus, we are confronted with a fruitful field for the exploration of basic mechanisms of neuronal activity. ”

“ All the psychic activities of the brute, such as sensation, object-perception, imagination, associative memory, sensual emotion, etc., are organic functions of the sensitivo-nervous type. In all of them the agent and recipient is not the soul alone, but the psycho-organic composite of soul and organism, that is, the soul-informed sensory and central neurons of the cerebrospinal system. The sensory neurons are nerve cells that transmit centerward the excitations of physical stimuli received by the external sense organs or receptors, in which their axon-fibers terminate. ”