Motor neuron

“ The axons of the sensory and central neurones do not connect with any and every motor neurone indiscriminately, but link up with selected groups of motor neurones, and thus harness together teams that will work in definite ways, producing {39} flexion of a limb in the case of one such team, and extension in the case of another. Every reflex has its own team of motor neurones, harnessed together by its outfit of sensory and central neurones. ”

“ Extending as they do in the largest animals for some meters from their cell bodies, they afford an example of a cell process such as is seen in no other histological unit. Not only are the nerve cells or neurones thus highly specialized in their structure, but they also exhibit profound physiological differentiation. Thus among the primary sensory neurones each one is connected, as a rule, with a particular portion of the animal for which no other neurone is responsible, and among the motor neurones each one controls a group of muscle fibers not called into action by any other neurone. ”

“ This is an item of evidence that the “centres” of the cortex are connected with the motor nerves of antagonistic muscles in such a way that when the “centre” excites one set of the muscles to contract, it simultaneously under normal circumstances causes inhibition of the motor neurones of the opposed set of muscles (reciprocal innervation) . In the great majority of movements excited from the motor cortex of a single hemisphere of the cerebrum, the movement evoked is confined to one side of the body, namely to that opposite to the hemisphere stimulated. ”

“ The same motor neurone may however be harnessed into two or more such teams, as is seen from the fact that the same muscle may participate in different reflex movements; and for a similar reason we believe that the same sensory neurone may be utilized in more than one reflex arc. ”

“ 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. ”

“ 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. ”

“ There exist, it is true, older experiments by Uspensky, Belmondo and Oddi, &c., according to which the threshold of direct excitability of the motor root is lowered by stimulation of the afferent root. This points to an extension of the facilitation effect through the whole motor neurone, conversely to Verworn's demonstration for central inhibition. Verworn's experiment and its result is very clear. It leads us to search for some other mechanism common to A and B to which might be attributable their mutual influence on each other's reactions. ”

“ Thus the nerve current from a single sensory neurone is distributed to quite a number of motor neurones. Where there are central neurones in the arc, their branching axons aid in distributing the excitation; and so we get a big movement in response to a minute, though intense stimulus. ”

“ The area in the brain, where these neurones emerge, is near the top of the brain in the area marked Motor on the diagram. From here the fibers travel down through the spinal cord and out to the muscles. The nerve-cells in this group are called motor neurones and their function is to carry messages from the brain out to the muscles, for a muscle ordinarily does not act without a nervous current to set it off.
So far we have seen that the brain has the two functions of receiving impressions from the sense-organs and of sending out orders to the muscles. ”

“ The motor neurons of the anterior horns of the spinal cord are the junction of a222 great number of tracks, for example, the sensory neurons of the spinal cord at different levels, the neurons of the cerebellum, the pyramidal tracks from the motor areas of the cerebral cortex, etc. On the contrary, for example, the sensory neurons of the spinal cord are strictly “private paths” in the sense of Sherrington, for excitation can enter by this means only from the special paths of the spinal ganglia and, therefore, from the periphery. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The polarity of an individual input determines whether it shall be excitatory or inhibitory. This much of the circuitry is very similar to Harmon’s model.
Learning is postulated to take place in the following way: when the neuron fires, an outside influence (the environment, or a “trainer”) determines whether or not the result of firing was desirable or not. If it was desirable, the threshold of the neuron is lowered, making it easier to fire the next time. If the result was not desirable, the threshold is raised, making it more difficult for the neuron to fire the next time. ”

“ From the motor nerve-cells an impulse or command travels by motor nerve-fibres, one from each cell, to the muscles, which contract. If this took as long to happen as it takes to describe, even in outline, it would not be of much use to the earthworm. But the motor answer follows the sensory stimulus almost instantaneously. The great advantage of establishing or enregistering these reflex chains is that the answers are practically ready-made or inborn, not requiring to be learned. It is not necessary that the brain should be stimulated if there is a brain ”

“ Hence one of the goals to which each afferent fibre from a receptive organ leads is a number of motor nerves. Their conducting paths must, therefore, converge in passing to the starting-points of the motor nerves; because these latter are instruments common to the use of a number of different receptive organs in so far as they excite reflex actions. On the other hand those of their conducting paths which are concerned in the genesis of sensation, instead of converging, diverge, at least as far as the cortex cerebri, or if not divergent, remain separate. ”

“ 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. ”

“ By little and little the rat is educated, and at last recognizes the exact point that is excited; thus showing that our so-called innate knowledge of the place occupied in space by every point of our bodies is, like all our knowledge, merely the result of repeated experiences.
To conclude, the experiment I have just recounted shows that excitation of a sensor nerve at any point of its length is propagated in two directions, both centrifugally and centripetally. The same is doubtless true of a motor nerve. ”

“ On passing through the network of nerves in the brain area, the current passes back again through a complicated maze of connections to a motor nerve which insures a muscular response. The first time a stimulus passes through this network the resistance offered at the nerve junction or synapse is very high; at succeeding repetitions of the stimulus the resistance is reduced, the nerve current passes more rapidly and fluently over the paths it has already traveled, and the action resulting becomes as direct and automatic as if it were an original reflex action. ”

“ Having listed which seem to be the important properties of a neuron, it is possible to synthesize a simple model which has all of them. [Pg 73]
A number of input stimuli are fed to the neuron through a resistive summing network which establishes the threshold and accomplishes spatial integration. The voltage at the summing junction triggers a “one-shot” circuit, which, by its very nature, accomplishes pulse generation and exhibits temporal integration and a refractory period. ”

“ 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. ”

“ How The Brain Produces Muscular Movements
The motor area is itself aroused to action by nerve currents entering it through axons coming from other parts of the cortex; and it is by way of the motor area that any other part of the cortex produces bodily movement. There are a few exceptions, as, for example, the movements of the eyes are produced generally by the "visual area" acting directly on the lower motor centers for the eye in the brain stem; but, in the main, any motor effect of brain action is exerted through the motor area. ”

“ It is, in the cases noted, at once sensory and motor, or at any rate involves motor elements. And the effect of the activity of such motor elements is both to increase the distinctness of the image and to prolong the duration of the process by which it is apprehended. The sensory process thus stands in intimate dependence on the motor. Nor would failure to move the eyes or any other organ with the movement of attention, if established, be conclusive as against the presence of motor elements. A motor impulse or idea does not always result in apparent peripheral movement. ”

“ In terms of functional operation, we can think of the synapse as a transducer. The input to this transducer is a spike or series of spikes in the presynaptic axon. The output is an accumulative, long-lasting potential which in some way (perhaps not uniquely) represents the pattern of presynaptic spikes. The pacemaker appears to perform the function of a clock, producing periodic spikes or spike bursts or producing periodic changes in the over-all excitability of the neuron. The graded-response regions appear to act as nonlinear amplifiers and, occasionally, spike initiators. ”

“ Second, the designers of Artron see a similarity in its operation and Darwin’s theory of natural selection.
Another new component in the bionics approach is the “neuristor.” This semiconductor diode simulates the axon, the nerve fiber that connects with the neuron. Another device is the “memistor,” unique in that it uses electrochemical phenomena to function as a memory unit. A different kind of artificial neuron called MIND is made up of magnetic cores.
There is another plus factor in this duplication of what we think is the system used by the brain. ”

“ Now, in speaking of reflex action a few pages back, we said that the nerves which bring in stimuli from the periphery distribute them about the neural canal to those cells whose activity, by sending out fresh stimuli to the muscles, produces the requisite movements. These motor cells, however, are not scattered about the spinal cord anyhow. They are collected into clusters, or nuclei, as they are sometimes called, and each cluster has special duties—i.e., a special organ to control. Thus, we say that there are in the central nervous system centres—a nervous centre to control the leg ”

“ The "weaker and purer" sensory currents furnish "ideas of the environment," while the "motor feelings" only furnish "ideas of the organism." What he means by motor feelings as distinguished from motor impulses following normally upon the discharges of the sensori-motor centers it is difficult to see. He seems to think there can be motor without sensory phenomena, and to forget that all sensory currents, whether peripheral or internal may produce motor effects. ”