STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XIII: THE COMPOUNDING OF STIMULI IN TEMPORAL SUCCESSION

1. In the main, the experiments described in this paper agree with those of Sherrington—shewing that after a flexion‐reflex there is a period in which a following extension‐reflex is augmented; and that after an extension‐reflex there is a period in which a following flexion‐reflex is augmented. 2. The experiments demonstrate this in spinal as well as in decerebrate preparations; and, in addition, they shew for the first time that the phenomena of “successive spinal induction” of this type occur in the deafferented condition as well as in the normal. 3. But the rule of successive spinal induction cannot be expressed in such simple terms, for instance, as “the termination of a reflex stimulus which evokes a reaction at a limb joint in the mammal is succeeded by a period in which the effect of a stimulus which evokes an opposite reaction is augmented.” In some cases there seems to be a primary period of depression instead of augmentation. 4. For instance, where a contralateral extension‐reflex is followed, on termination of the stimulus which evokes it, by a rebound contraction of the extensor muscle itself (and sometimes when not obviously so followed), there may be a depression of a flexion‐reflex if it is applied immediately on the termination of the preceding extension‐reflex. This period of depression is succeeded shortly by a period of augmentation; it seems to be proportional in its depth to the strength of the preceding reflex stimulus; and it corresponds in time to the period of the extensor rebound contraction, although it occasionally lasts longer and may even occur when there is no such extensor rebound contraction. 5. But where the extension‐reflex is followed not by an extensor rebound contraction but by an extensor residual tonus, there is usually no preliminary phase of flexor depression at the termination of the extension‐reflex. This seems to indicate that the extensor rebound contraction is a phenomenon different from the extensor residual tonus. It may well be that the former is a central phenomenon, while the latter may be peripheral (“shortening reaction” of Sherrington), or may be due to the blending of a peripheral proprio‐ceptive reflex upon a central extensor after‐discharge. 6. Again, there appears sometimes to be a short primary phase of extension depression upon the termination of a flexion‐reflex. This is followed by the phase of extension augmentation; it is usually of short duration; it seems to be proportional to the strength of the preceding flexion‐producing stimulus; and it seems to correspond in time to the flexor after‐discharge. Sometimes, however, the effect may be more prolonged. It may perhaps be evidenced by an increase in the flexion factor of the extension‐reflex of “decerebrate” type. 7. Just as it seems probable that in every flexion‐reflex there is a factor of extension, and in every extension‐reflex one of flexion—that is a compounding of opposite activities in the “half‐centres” which subserve these acts—so, too, it is possible that the after‐effects of a reflex stimulus may be compounded of opposite factors. Thus the after‐effect of an extension‐reflex may be compounded of the factors of flexion depression and augmentation (that is, extension augmentation and depression). At first the factor of flexion depression preponderates over that of flexion augmentation, but it rapidly decreases and allows the flexion augmentation eventually to preponderate. And the same may be the case after a flexion‐reflex with the factors of extension depression and augmentation (that is, flexion augmentation and depression). The primary depression may in some manner be connected with a central after‐discharge of the precurrent reflex; the secondary augmentation perhaps with the rebound contraction which may follow in the inhibited muscle of that reflex. The factors at any rate seem to be of central locus, for both the depression and the augmentation may occur in de‐afferented preparations. Some of the rebound phenomena seem to suggest that even when the augmentation has become predominant the latent depression factor may be seen at the end of the second reflex in increasing the rebound effect from it. But here it must be remembered that the extent of that rebound contraction of the inhibited muscle may be increased by the augmentation of the reflex of which it is a part; and also it must be noticed that the rebound contraction after excitation may also be increased, and that this looks rather as if the augmentation factor applied not only to the reflex itself but also to the rebound, which may sometimes occur in the same sense as the reflex. 8. The phenomena of successive spinal induction as they occur in abnormal reactions are of interest in the evidence which they lend to the idea of the compounding of opposite activities in the simple reflex. Thus augmentation of extension may be obtained shortly after the termination of an ipsilateral reaction of nearly pure extension. Again, a contralateral flexion‐reflex may augment a succeeding ipsilateral flexion‐reflex, but an ipsilateral flexion‐reflex may also augment a succeeding contralateral flexion‐reflex. If the contralateral flexion‐reflex be of “decerebrate” type and exhibit therefore a factor of extensor contraction, that has been observed to be augmented by a precurrent ipsilateral flexion‐reflex. 9. The view has been put forward in previous papers that rhythmic phenomena may be due to a balancing in the centres of more or less equal and opposite activities. If this be correct, it might be expected that occasionally a second reflex, when applied in the depression phase which follows a preceding reflex of opposite type, would exhibit rhythmic phenomena. This in fact occurs. It occasionally happens, for instance, that an extension‐reflex applied immediately on cessation of a flexion‐reflex may exhibit rhythmic movements in the first part of the period of application of its evoking stimulus; or it may even be rhythmic throughout the whole period of that application.