Motor Reaction Units

Before we proceed to study the specific reflex response mechanisms, we may consider certain purely motor, or efferent systems, which are apparently in-built by ontogenetic forces. The preexistence of such "reaction units" greatly facilitates the development of learned response because they constitute ready-made coördination systems which can be appropriated as occasion demands by higher nervous centers. Thus, motor learning does not have to begin with a chaos of individual nerve and muscle fibre units, but finds such units already assembled into automatisms, which are approximately correct, in their general character, for voluntary action. These motor units can be conceived as being essentially independent of any particular afferent nerve paths, although it may happen that certain of them are actually connected anatomically with special afferent mechanisms, for the execution of particular reflex movements.

We have already given numerous specific examples of such motor units. In order to find many others, we have only to analyze our own movements (or postures) into gross physiological parts. Consider, for example, the case of the hand. There are several characteristic movements and postures which are possible for each of the fingers and the hand as a whole. Thus, a finger may be flexed, extended, lifted, depressed, or moved either right or left in the plane of the palm. The hand may be flexed at the wrist, either backwards or forwards; the fist may be clenched, the thumb and fore-finger may be brought together (as in grasping an object), etc. Of course, we cannot be certain that all of these possible elementary movements are based upon nervous automatisms; in fact, we may feel quite sure that some of the hand movements result only from prolonged practice which--more frequently than not--is directed towards overcoming such automatisms. Consider, for instance, the difficulty with which we learn to flex the fourth and fifth fingers independently of each other. This difficulty in itself demonstrates the existence of an innate coördinating device which operates to flex all of the fingers at once, although the index finger has a special independent control unit of its own.

It may seem to some readers that such simple movements as are above considered should not require any very special nervous arrangements to permit their execution. However, the necessity of considerable apparatus will be realized when one appreciates that even the simplest of these movements requires the simultaneous and balanced innervation of hundreds of individual muscle fibres, which may have a rather complicated anatomical distribution. In order that a smooth and coördinated movement shall occur, the impulses must not only be distributed to the appropriate outwardly conducting neurones, but they must be suitably regulated both with regard to their temporal course and their intensities.

Now, as a rule, we find that such regulation is not exclusively central in its origin, since ordinarily it involves so-called "proprioceptive" afferent nerve currents. These proprioceptive currents are in a class by themselves among afferent impulses, since they convey information to motor adjustment mechanisms in regard to the outcome of adjustments which have just been made. Therefore, they provide the essentials of an ideal mechanism for muscular control, since the control need not depend arbitrarily upon the activity of the central agency, but may involve the actual state of the motor organ, which reacts upon and modulates the innervations of the centers, through the medium of the proprioceptive impulses. Thus, we can imagine the innerration of a muscle to be throttled by the proprioceptive impulses, which result from its own contraction, in a quantitative manner so that the state of tension in the muscle is due to the balance between the proprioceptive and the essential adjustor energies, which react upon each other at the adjustor point. Now, it seems almost certain that motor regulation devices of this sort, involving proprioceptive impulses as cardinal features, are present in the organism as a direct hereditary endowment. As we have already indicated, the cerebellum--an important portion of the brain--appears to be the natural locus of many regulatory systems of this kind.

As a complement, or correlative of these purely motor control mechanisms, it would not be surprising to find hereditarily founded devices of an exclusively afferent nature, which are an addition to the sense-organs and their obvious nerve channels. The general mechanisms of the sensory side of the cerebral cortex may perhaps be included in this class, and, at a later point in our argument, we shall introduce the notion of further specific arrangements of an afferent character, which provide what we shall regard as "the hereditary basis of learning by experience."