SELECTIVITY CONTROLLING THE CENTRAL‐PERIPHERAL RELATIONS IN THE NERVOUS SYSTEM

Summary 1. In 1922 it was discovered that a supernumerary transplanted limb in amphibia invariably duplicates the movements of the normal limb in whose nerve plexus it shares. This phenomenon, called “homologous response”, has been demonstrated to be an expression of the more fundamental fact that, in every reflex and spontaneous action, multiple muscles of the same name (synonymous muscles), innervated from the same functional district and the same side of the spinal cord, contract simultaneously and with approximately equal intensities, regardless of their anatomical position, the details of origin and distribution of their nerve supply, and the functional effects of their contractions with regard to the body as a whole. Homologous response of synonymous muscles, observed under such a variety of different conditions as enumerated in Section II (1), has led to the realization that the linkage between the nervous centers and the non‐nervous periphery is based upon mutual relations discriminative for each peripheral organ and much more specific than had previously been suspected. Further experimental analysis of this relationship has led to the following conclusions. 2. The specific relations between muscles and centers are not secondarily established by adjustments of the “conditioning” or “learning” type; for, the principal features of the latter, modifiability and adaptive functional value, are absent in the phenomenon of homologous response. In fact, the phenomenon is not even dependent upon the presence of sensory control at all, since it is likewise obtained from limbs with purely motor innervation (III (I)). 3. Nor is the specific relation between muscles and centers the result of morphological selectivity in the formation of peripheral nerve connections. Abundant evidence is on hand showing not only that in general no affinity whatever exists between a given nerve fiber and a particular muscle, but more specifically, that such an assumption can be decisively ruled out for the experimental cases exhibiting homologous response. Many new facts have been added in substantiation of this point and are discussed in Section III (2). 4. Other explanations being excluded, the specific relation between centers and muscles must be regarded as due to a primary physiological relationship, resembling in principle the specific linkage found in resonance‐like mechanisms. Every muscle is constitutionally (presumably biochemically) different from every other non‐homologous muscle, and to deal with this diversity the centers are endowed with a capacity to produce a corresponding variety of forms or modes of motor impulses, each one exclusively appropriate to a single muscle. Thus, the activation of a muscle becomes a matter of dual activities: release of discriminative emissions from the centers, and the selective reception of these by the periphery. 5. Experiments involving the transplantation of single supernumerary muscles in adult toads (IV (1) a ) have indicated that the peripheral muscle‐specific selection does not actually occur in the muscle fiber itself but in some part of the peripheral nervous system acquiring its muscle‐specific selectivity through a specifying (“modulating”) effect extending from the muscle. It is assumed that motor neurones subjected to these peripheral influences are gradually rendered specific to such an extent that they will no longer admit from the centers motor impulses other than those appropriate for the particular muscle at their peripheral ends. A study of the reappearance of motility in denervated and re‐innervated limbs in the toad has revealed that the specification of a nerve by its muscle is a slow process. Specification is, at least up to a certain age, reversible, the connection of a nerve with a new muscle resulting in corresponding respecification. Thus, the specific diversity of the muscles is projected into the motor nerves, and E. Hering's postulate of the diversity (“Ungleichartigkeit”) of nerves becomes, after all, a fact. 6. Concerning the spinal centers, the conclusion has been reached that their functioning cannot be satisfactorily envisaged in terms of switch‐board or other geometrical schemes (IV (2)). We have been forced to concede to the limb center, for instance, the property to produce, release, and circularize within certain intracentral limits, a variety of specific effects matching the existing variety of individual limb muscles. To judge from experimental evidence, the release of impulses specific for limb muscles is confined to the limb level of the spinal cord. 7. It has not yet been possible to offer any suggestion as to the presumable nature of the described selective affinity between central impulse and peripheral nerve fiber. But reasons have been advanced which, it would seem, allow us to reject the idea that specific frequencies or specific chronaxie relations (isochronism) are involved.