STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XV.: IMMEDIATE REFLEX PHENOMENA RESULTANT UPON COMPOUND STIMULATION IN DECEREBRATE PREPARATIONS
Authors: Brown T. Graham 
Published: December 2014

The present paper itself is the summary of a large number of experiments, and it is not possible again to refer to all the points described in it. The following may, however, be noticed:—

1. Where the two antagonistic stimuli are commenced synchronously, “algebraic summation” (Sherrington) of the two “pure” reflexes may occur in the compound reaction. In some cases, however, complete suppression of one—usually the extension‐reflex—may occur. In such cases there may still be reduction of the extent of the contraction of the active muscle—for instance, the flexor.

2. Where the contralateral stimulus is applied first and allowed to run for a short period before the application of the interrupting ipsilateral stimulus, extensor relaxation occurs in the period of double stimulation, and this may or may not be accompanied by flexor contraction. The two may shew “algebraic summation,” and the values of flexor contraction and extensor relaxation increase with increase in the strength of the ipsilateral flexion‐producing stimulus, as well as with decrease in the value of the “background” contralateral extension‐producing stimulus. Sometimes, however, the flexor contraction of double stimulation is actually greater than in the “pure” flexion‐reflex.

3. When, in these circumstances, the temporal relations of the two stimuli are changed—so that the duration of the contralateral extension “background” before the commencement of double stimulation is varied,—it is found that often the extent of flexor contraction during double stimulation is greater the later the ipsilateral stimulus is applied. But at the same time the extent of extensor relaxation does not reciprocally increase—it usually shews progressive diminution of extent. In these experiments the duration of precurrent contralateral “background” has been varied within the limits of 0 seconds and 6 seconds.

4. When the ipsilateral stimulus is first applied the period of double stimulation which is conditioned by the addition of a contralateral stimulus is characterised by a reduction of the flexor contraction, and by the appearance of an extensor contraction which is smaller in extent than that of the “pure” contralateral extension‐reflex.

5. If, in these circumstances, the strengths of the compounded stimuli are varied, it is found that the flexor relaxation is smaller in extent, and the extensor contraction also smaller the stronger the ipsilateral “background” is.

6. If the temporal relations of the two stimuli are altered—but the stimuli kept of constant value—it is found that the component of extension in double stimulation is greater the longer the ipsilateral stimulus is allowed to run before the commencement of double stimulation.

7. Flexor relaxation during double stimulation is not the invariable rule. Sometimes augmentation occurs. When this is the case it may increase in value with increase in the strength of the interrupting contralateral stimulus, and at the same time there may occur a reduction of extensor contraction during double stimulation. When the temporal relations of the two stimuli are altered the flexor augmentation may be found to increase with increase of duration of the first applied ipsilateral stimulus. But a maximum may be attained, after which diminution of flexor augmentation occurs with further increase in the duration of time which separates the commencement of the ipsilateral stimulus from the commencement of double stimnulation.

8. If the effects of double stimulation are compared in cases in which the preceding “background” is flexion with the effects in cases in which that “background” is extension, it is found that the phenomena differ in the relative values of flexion and extension. Thus if a two‐second flexion “background” precede double stimulation, there may be a greater degree of flexion and a lesser degree of extension in the summation phenomena of double stimulation as compared with the same components when a two‐second extension “background” precedes the double stimulation. These differences may be referred to two conditioning factors. In the first place, the stimulus to be first applied (“background”) tends to “hold the field” to the comparative exclusion of the other. In the second place, this “capture of the field” tends rapidly to deteriorate in value.

9. When two series of reactions, in which the preceding “background” is flexion and in which it is extension, are compared, it is found that when the duration of the “background” before the commencement of double stimulation is small, the first applied stimulus (“background”) holds the field. Thus here there is a greater factor of flexion in double stimulation when the flexion‐producing stimnulus is the “background” and a greater factor of extension when the contralateral stimulus is first applied. With a greater duration of preceding “background” the factors of flexion and extension in double stimulation are similar whichever of the two stimuli is first applied. This may be termed the “neutral duration of preceding background.” With still greater durations of preceding “background” the “background” stimulus no longer “holds the field.” When these durations are used it is found that after a flexion “background” there is a greater extension factor in double stimulation than after an extension “background” of the same previous duration before the commencement of double stimulation.

10. Experiments in which the temporal relations of the two stimuli are kept constant, but in which their strengths are varied, seem to shew that the “capture of the field” is more complete, and lasts for a longer time in the period of a reflex, the stronger the evocative “background” stimulus.

11. Where the abnormal ipsilateral extension‐reflex is compounded with the normal contralateral extension‐reflex there may be augmentation of extensor contraction during double stimulation‐so that the level attained is greater than that in either of the compounded reflexes. This result is seen usually when the ipsilateral stimulus is very weak. The extensor contraction during double stimulation may even be greater in extent than the sum of the extents in the “pure” reflexes. Where the ipsilateral stimulus is relatively stronger the extensor contraction during double stimulation may be the mean of the extensor contractions in the two “pure” reactions. Where still stronger—even where the “pure” ipsilateral reaction is still one of extension alone—the extensor contraction during double stimulation may be smaller in extent than this mean.

12. Where the extensor contraction during double stimulation in these abnormal cases is compared when the preceding “background” is the ipsilateral extension with that when the preceding “background” is contralateral extension, it is found that with weak ipsilateral stimuli there is a greater extensor contraction against the ipsilateral “background” than against the contralateral. With stronger ipsilateral stimuli there may be equal extents in the two cases. With yet stronger ipsilateral stimuli there may be less extensor contraction during double stimulation against the ipsilateral “background” than against the contralateral. As in these experiments the duration of the preceding “background” stimulus was kept constant, this seems to mean that with weak ipsilateral stimuli which give the abnormal direct extension‐reflex, the preliminary phase of flexion (not directly evidenced), if it occurs at all, is of short duration. In other words, the “neutral duration of preceding background” is very small—perhaps infinitely small—and thereafter the extension factors tend increasingly to “hold the field.” As the strength of ipsilateral stimulation is increased the primary flexion “capture of the field” either appears or, if previously present, increases in duration. The “neutral duration of preceding background” therefore increases, and the fixed duration of preceding “background” used in the experiments may coïncide with this— when the extension value of double stimulation will be the same for the two “backgrounds.” With still stronger ipsilateral stimuli the period of double stimulation may fall in the preliminary phase in which flexion (there directly evidenced in the stronger ipsilateral reflex) “holds the field.” In such cases the extensor contraction during double stimulation will be weaker against the ipsilateral “background” than against the contralateral.

13. Sometimes when the preceding “background” is contralateral extension, and even where the “pure” ipsilateral stimulus evokes extensor contraction alone, there may appear flexor contraction during double stimulation. In such cases the presence of the contralateral “background” seems actually to reverse the ipsilateral reaction from abnormal extension to normal flexion. Related to this are the cases in which extensor relaxation unaccompanied by flexor contraction occurs during double stimulation. A possible explanation of this phenomenon is that the “neutral duration of preceding background” having been passed in the contralateral reflex, the latent flexor element of the ipsilateral reaction was augmented by the contralateral extension “loss of the field.”

14. Where an abnormal contralateral flexion‐reflex occurs and is compounded with a normal ipsilateral flexion‐reflex, there appears usually flexor augmentation during double stimulation. Two subliminal stimuli when compounded in these circumstances may give a flexion reaction. If the preceding “background” is contralateral flexion, the flexor contraction during double stimulation is most often of smaller extent than that in the “pure” ipsilateral flexion‐reflex. But sometimes it is larger. When the ipsilateral stimulus is first applied there occurs most often flexor agumentation during double stimulation. It is possible that in such experiments the “neutral duration of preceding background” is comparatively great for the flexion‐reflex; that is to say, that flexion “holds the field” for a longer time than is usual.

15. In de‐afferented conditions the immediate phenomena of compound stimulation correspond to those in “normal” conditions. When the contralateral stimulus is first applied there occurs extensor relaxation and flexor contraction during double stimulation. These decrease in value with decrease in the relative strength of ipsilateral stimulation. It has been observed that there seems to be a greater tendency in de‐afferented conditions than in “normal” for a complete suppression of flexor contraction during double stimulation.

16. Where, in the de‐afferented condition, the ipsilateral stimulus is first applied, there is often the usual phenomenon of algebraic summation during double stimulation. Here again there seems to be a greater tendency than in the “normal” condition for complete flexor suppression to occur during double stimulation. In such cases, when the strength of contralateral stimulation is relatively increased, there may yet appear grading of the extent of extensor contraction during double stimulation although there is in every case complete flexor suppression. Sometimes, as in “normal” conditions, there may occur flexor augmentation during double stimulation. This usually is transient, and does not persist throughout the whole of the period of double stimulation. Where approximations to the abnormal direct extension‐reflex have occurred there has been observed to be extensor augmentation during double stimulation.

17. This phenomenon of flexor suppression is one of interest because it might have been supposed that the reverse would be the case. Thus in the period of double stimulation against an extension “background” the extension‐reflex is relatively depressed, and in the “normal” condition it may be supposed that in this period there is in being some factor which corresponds to the extensor after‐discharge of the “pure” extension‐reflex. In this phase there will thus be a compounding of one flexion factor with two extension factors. But in the de‐afferented condition this extensor after‐discharge is almost completely suppressed. It would therefore be supposed that de‐afferented conditions would demonstrate a greater extensor relaxation during double stimulation than “normal” conditions when the “background” is one of contralateral extension. In a similar manner there might be supposed to occur a less flexor relaxation during double stimulation against a flexion “background” in de‐afferented than in “normal” conditions—because in the “normal” condition there most often follows upon flexion suppression in the “pure” reflex an extensor rebound of maintained form, and this does not occur, or is more transient, in the de‐afferented condition.