STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XVI.: DECEREBRATE PREPARATIONS, SUCCESSIVE PHENOMENA IN COMPOUND REACTIONS—STIMULI OF SYNCHRONOUS TERMINATION AND STIMULI OF ASYNCHRONOUS TERMINATION WHERE THE EXTENSION‐REFLEX IS LEFT IN ACTION

1. Where the two antagonistic stimuli are synchronously terminated, and where there is no rebound phenomenon in the “pure” flexion‐reflex and only extensor after‐discharge in the “pure” extension‐reflex, there often occurs extensor relaxation on withdrawal of the two stimuli. Complete suppression of extensor after‐discharge may occur in those cases in which there is complete extensor suppression during double stimulation. Where complete extensor suppression does not occur during double stimulation there may yet occur extensor relaxation on synchronous termination of the two stimuli. An incomplete extensor after‐discharge may occur. This has been observed to be diminished if the strength of the ipsilateral stimulus is increased, and to be increased if the contralateral stimulus is first applied late in the period of the ipsilateral stimulus—both stimuli being synchronously withdrawn. The later the contralateral stimulus is applied the greater is the extent of the extensor after‐discharge. It is possible that this phenomenon of suppression (either partial or complete) of extensor after‐discharge points to the flexor relaxation, which is the terminal phenomenon in these flexion‐reflexes, as an active phenomenon and not merely as a passive one. 2. Where flexor rebound contraction occurs as a terminal phenomenon in the contralateral extension‐reflex and the two antagonistic stimuli are compounded there seems usually to occur an augmentation of that rebound on synchronous withdrawal of the two stimuli. In the section in which these phenomena are described there is instanced a curious case of reversal of the contralateral reaction to abnormal flexion when the contralateral stimulus is applied in time shortly after an ipsilateral. 3. Where flexor rebound contraction occurs in the “pure” ipsilateral flexion‐reflex as a terminal phenomenon, and where the “pure” contralateral extension‐reflex is followed by an extensor after‐discharge, there may appear a depression of the flexor rebound as the terminal phenomenon in a compound reaction in which the stimnuli are synchronously withdrawn. But where in the “pure” extension‐reflex extensor relaxation has appeared as the terminal phenomenon, there has been observed to be augmentation of the flexor rebound contraction. Perhaps these two observations may be reconciled on the hypothesis that in the terminal phenomena of the compound reactions in the two cases there is a compounding of the factors present in the terminal phenomena of the “pure” reactions. Thus in the one case they are antagonistic—flexor rebound and extensor after‐discharge—and the compound terminal phenomena shew algebraic summation of the two; while in the other case the two phenomena are not antagonistic, and augmentation of the common terminal phenomenon (flexor contraction: extensor relaxation) occurs. 4. Where in the “pure” reflexes there is flexor rebound in both, synchronous withdrawal of the two stimuli seems to be followed by an augmentation of the flexor rebound—so that it is greater either in extent or duration than that in either “pure” reflex. 5. Where the “pure” ipsilateral flexion‐reflex is not followed by any terminal phenomenon save flexor relaxation, and where there is extensor after‐discharge in the contralateral extension‐reflex, on synchronous withdrawal of the two stimuli in a compound reaction there may ensue an extensor rebound when during double stimulation there is complete suppression of extensor contraction. Where there is incomplete suppression there may ensue an augmented extensor after‐discharge. In some cases this rebound or after‐discharge has been observed to be greater than the after‐discharge of the “pure” extension‐reflex in extent; but more commonly it is of less extent. 6. Where extensor rebound contraction occurs in the “pure” flexion‐reflex, and extensor after‐discharge in the “pure” extension‐reflex, in compound reactions there may occur augmentation of the extensor terminal phenomenon on synchronous cessation of the two stimuli. The extent of extensor contraction in the terminal phenomena of the compound reaction may be greater than the sum of the extents of the extensor terminal phenomena in the “pure” reflexes. In other cases depression of the extensor terminal phenomenon may occur. In series of “pure” flexion‐reflexes evoked with increasing strengths of ipsilateral stimuli the extensor rebounds may be found to decrease in value. In these circumstances, if series of compound reactions in which the strength of ipsilateral stimulation is increased progressively are recorded, it is found that with weak ipsilateral stimuli the extensor rebound is augmented, but that with strong ipsilateral stimuli it is depressed or even abolished‐although there is a good extensor after‐discharge after cessation of the “pure” contralateral stimulus of the constant value used throughout the series. Again, in such cases the extensor rebound after cessation of double stimulation may be found to be greater the greater the duration of a preceding contralateral extension “background” when the contralateral stimulus is commenced in time before the ipsilateral, but the two are synchronously withdrawn. Here it may be noted that the extensor after‐discharge may in these cases be greater the longer the duration of the “pure” contralateral stimulus. 7. In compound reactions in which the ipsilateral interrupting stimulus is withdrawn before the termination of the contralateral extension “background” there occur two phases in which the terminal phenomena of compound stimulation may be examined. The first of these is the “third phase” of the compound phenomenon; in it the “background” extension stimulus is still running—it commences at the point at which the ipsilateral interrupting stimulus is withdrawn, and terminates with the withdrawal of the contralateral “background” stimulus. The second is the “fourth phase” of the compound reaction; in it there is no stimulus in being—it commences with the withdrawal of the contralateral “background” stimulus, and may be held to terminate when the centres again assume the condition of “rest.” 8. Where the “pure” ipsilateral reflex is not followed by any rebound contraction—but only by terminal flexor relaxation—the “third phase” of the compound reaction (extension “background”) is usually characterised by a flexor relaxation more rapid than in the “pure” flexion‐reflex, and by a reconstitution of extensor contraction which is more slow, and less regular, a movement than that at the commencement of the “pure” extension‐reflex. The extent of extensor contraction attained is in some cases greater than that in the “pure” extension‐reflex, but it is usually less. 9. When the time relations of the two stimuli are changed it is found in these cases that the extent of extensor restitution in the third phase is usually greater the later the ipsilateral stimulus falls in the period of the contralateral “background.” At the same time the curve of extensor restitution appears as a sharper movement the later the ipsilateral stimulus falls. 10. When the strengths of the two stimuli are relatively varied in compound reactions in which the time relations of the stimuli are kept constant, it is found that in some cases the extensor restitution of the third phase is greater the stronger the ipsilateral flexion‐producing stimulus. In other cases it is weaker. In one case it became stronger, reached a maximum, and then declined as the ipsilateral stimulus was progressively increased in value. In the other cases the series examined may have been “partial” ones only—either above or below the optimum. 11. In the “fourth phase” of compound reactions, where the “pure” ipsilateral stimulus is followed by flexor relaxation alone, there may continue an extensor after‐discharge if that be present in the “pure” extension‐reflex. This carries on the extensor at the level of contraction attained in the third phase. But sometimes extensor relaxation occurs. When this is the case and the temporal relations of the two stimuli are varied, it is found that the depression of extensor after‐discharge is greater the later the ipsilateral stimulus falls (that is, in these cases, the shorter the period of the third phase). 12. Where "flexor rebound contraction after excitation" is present in the “pure” ipsilateral flexion‐reflex, it may also appear in the third phase of compound reactions. Here there then appears to be a summation of the flexion factors of the ipsilateral terminal phenomenon and the extension factors of the continued “background” extension‐reflex—for the flexion rebound in the third phase may be smaller than that in the “pure” flexion‐reflex, and the extensor contraction in the third phase may be depressed. But it occasionally happens that a flexor rebound may appear in the third phase when not present after the same duration of stimulation in the “pure” flexion‐reflex. In an instance in which this occurred it was found that the flexor rebound was smaller the later the ipsilateral stimulus fell in the period of the extension “background.” Where extensor restitution begins to occur during double stimulation there is a marked extensor relaxation at the commencement of the third phase reciprocal to the flexor rebound. 13. Where “extensor rebound contraction after inhibition” is present in the “pure” flexion‐reflex the phenomena in the third phases of compound reactions are of interest. Thus in some cases it may occur that there is a depression of extensor restitution in the third phases even in these circumstances. But more usually there appears an augmentation. The level of extensor contraction then attained in the third phase may be greater than the summed extents of extensor contraction in the “pure” extension‐reflex and in the extensor rebound of the “pure” flexion‐reflex. A point of interest is the appearance of a notch‐like relaxation of extensor contraction during the third phase. This has appeared in instances in which there appeared in the terminal phenomena of the flexion‐reflex an “extensor rebound relaxation after inhibition” followed by an extensor rebound contraction. The phenomenon has appeared in the compound reaction when not present in the flexion‐reflex at that strength of stimulus. 14. Where the temporal relations of the stimuli are varied and such a “notch” appears, it has been found that it was larger the later the ipsilateral stimulus fell in the period of the contralateral “background” stimulus. The primary movement of extensor restitution in the third phase was found to be sharper the later that stimulus fell, but little variation was observed in the secondary extensor contraction which followed the “notch.” 15. Where the temporal relations of the stimuli are constant but their values changed, the level of extensor contraction in the third phase may be found to augment with increase in the value of the ipsilateral stimulus. In such cases it is found that the extensor rebound contraction of the “pure” flexion‐reflex increases with increase in strength of stimulus. The extensor contraction in the third phases of the compound reactions is greater than that in the “pure” extension‐reflex. With weak ipsilateral stimuli the difference between the levels of extensor contraction in the extension‐reflex and in the third phase of the compound reaction may be equal to, or even greater than, the extent of extensor contraction in the extensor rebound contraction of the flexion‐reflex. But with stronger ipsilateral stimuli this relative difference of extent becomes smaller than the extent of the extensor rebound in the corresponding flexion‐reflex—although the absolute extent of extensor contraction in the third phase continues to increase. The “notch”—if present—may be greater the stronger the ipsilateral stimulus. Where the ipsilateral stimulus is of constant strength and the contralateral “background” stimulus is increased in strength, the extensor contraction in the third phase of the compound reaction increases with increase in strength of the contralateral stimulus. 16. In some cases the extensor rebound of the “pure” flexion‐reflex decreases in value with increase in the strength of stimulus. This may take the form of a reduction of the maintenance of the rebound—the reduction being of the nature of a sharp relaxation. When this occurs it may also be seen in the third phase of the compound reaction. The contraction phase in the third phase of the compound reaction is augmented as compared with the extensor rebound of the flexion‐reflex. The relaxation phase increases with increase of the strength of ipsilateral stimulus (a reciprocal flexor contraction may even appear), and decreases with increase of strength of the contralateral “background” stimulus. Its occurrence in the third phase of the compound reaction seems to shew that the relaxation of the extensor rebound is, as it were, due to “active inhibition.” 17. In the “fourth” phase of compound reactions the phenomena where there is extensor after‐discharge in the “pure” extension‐reflex have already been summarised (see section 11 of this summary). 18. Where “extensor rebound relaxation after excitation” is present in the “pure” extension‐reflex it may also occur in the “fourth phase” of compound reactions. If the relaxation is incomplete in the “pure” reflex it becomes greater—and sometimes complete—in the compound reaction. If complete in the “pure” reflex it may become accompanied by a flexor rebound contraction in the compound one—although that is absent in the “pure” contralateral reaction. 19. If the temporal relations of stimuli of constant strength are varied in a series of compound reactions in which this phenomenon occurs, it is found that the extensor terminal relaxation becomes greater the later the ipsilateral stimulus falls in the period of the extension “background” (that is, also, the shorter the period of the third phase—or the shorter the interval which separates the fourth phase from the phase of double stimulation). Where reciprocal flexor rebound contraction appears, that also becomes greater under these conditions. 20. Where the strengths of the stimuli are varied it is found that, the extensor terminal relaxation—and the flexor rebound contraction if that appears—are greater the stronger the ipsilateral flexion‐producing stimulus is. 21. Where “flexor rebound contraction after inhibition” accompanies “extensor rebound relaxation after excitation” in the “pure” contralateral extension‐reflex, the fourth phase of the compound reaction in which extension is the “background” is almost invariably characterised by an increase in the flexor rebound contraction. This increase is augmented pari passu with augmentation of the strength of the interrupting flexion‐producing ipsilateral stimulus, and it seems also to be augmented when the ipsilateral stimulus is made to fall late in the period of the extension “background.” These phenomena in the fourth phase seem to be conditioned chiefly by the ipsilateral interrupting stimulus; and it appears as if the influence of this, as it were, can persist over a period (the third phase of the compound reaction) in which the antagonistic activity (extension) becomes reëstablished—seemingly in full force.