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Thence it pushes on, over association neurones in the brain to motor neurones, over which it passes down the spinal cord again to muscles, and ends in some movement. In the pathway which it traverses it leaves its impression, and, thereafter, when the first neurone is excited, the nervous current tends to take the same pathway and to end in the same movement.
Physiologically stated, free attention is given when a neurone series which is ready to act is called into activity. The situations which do this, other things being equal, will be those which appeal to some instinctive tendency or capacity, or to the self-activity or the personal experience of the individual and which therefore are in accord with his stage of development and his experience.
In this case the action is double, for it implies the disuse of certain connections which have been made and the forming of others; for the breaking of a bad habit means the beginning of a good one. The plasticity of neurone groups seems to vary in two respects as to modifiability and as to power to hold modifications.
The neurone groups controlling the reflex and physiological operations are least easily modified, while those controlling the higher mental processes are most easily modified. The neurone groups controlling the instincts hold a middle place.
So far as permanence goes, connections between sensorimotor neurone groups seem to hold modifications longer than do connections between either associative-motor or associative-association. It is probably because of this fact that habit in the minds of so many people refers to some physical activity. Of course this is a misconception. Wherever the nervous system is employed, habits are formed.
The second property of nerve-cells which is important in study is conductivity. As soon as a neurone is stimulated at one end, it communicates its excitement, by means of the nervous current, to the next neurone or to neighboring neurones. Just as an electric current might pass along one wire, thence to another, and along it to a third, so the nervous current passes from neurone to neurone.
There should be a difference, as our mind demands it; but our eye fails to note it. Let us suppose, however, that to-morrow, or several centuries hence, an improved technique should show us a material difference between the visual and the auditory neurone. There is no absurdity in this supposition; it is a possible discovery, since it is of the order of material facts.
The Organ of Mind. Gross Structure. Microscopic Structure. The Neurone. The Nervous Impulse. The Synapse. Properties of Nervous Tissue Impressibility, Conductivity, Modifiability. Pathways Used in Study Sensory, Motor, Association. Study is a Process of Making Pathways in Brain. Definition of Habit. Examples. Inevitableness of Habits in Brain and Nervous System.
It is important to note that neurones never act singly; they always act in groups, the nervous current passing from neurone to neurone. It is thought that the most important changes in the nervous system do not occur within the individual neurones, but at the points where they join with each other.
"A second change in fatigue has been found in the cell body of the neurone. Hodge showed that the size of the nucleus of the cell in the spinal cord of a bee diminished nearly 75 per cent, as a result of the day's activity, and that the nucleus became much less solid. A third change that has been demonstrated as a result of muscular work is the accumulation of waste products in the muscle tissue.
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