STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XVII.: DECEREBRATE PREPARATIONS, SUCCESSIVE PHENOMENA IN COMPOUND REACTIONS—STIMULI OF ASYNCHRONOUS TERMINATION WHERE THE FLEXION‐REFLEX IS LEFT IN ACTION
Authors: Brown T. Graham 
Published: December 2014

1. In this paper the successive reflex phenomena in compound stimulation are examined when the reflex “background” is one of ipsilateral flexion. The “background” stimulus is allowed to continue after the withdrawal of the interrupting contralateral stimulus, and the successive phenomena—as before—are examined in two phases of the compound reaction. These are the “third phase” (in which the “background” stimulus is still in being) and the “fourth phase” (which is that after the withdrawal of the “background”).

2. Where the “pure” contralateral extension‐reflex is followed by a maintained extensor after‐discharge only there may occur in the third phase of the compound reaction a continuation of the flexor depression which obtains in the immediately preceding phase of double stimulation. Where the temporal relations of the two stimuli are varied this depression of the flexor appears to be greater the later in the period of the “background” the contralateral interrupting stimulus falls. This phenomenon occurs when the extensor after‐discharge of the “pure” extension‐reflex is well marked. Gradual extensor relaxation may occur during the third phase, and it seems probable that the phenomena in that phase are conditioned by an algebraic summation of the flexion of the flexion‐reflex and the extensor after‐discharge of the extension‐reflex. The occasional slight restitution of flexion (followed again by relaxation) at the commencement of the third phase in these instances may possibly evidence the presence of a slight factor of flexion at the commencement of the extensor after‐discharge—one not directly evidenced in the “pure” extension‐reflex.

3. Where the “pure” extension‐reflex is followed by an extensor after‐discharge there may yet occur in the third phase of the compound reaction a partial or a complete restitution of flexor contraction accompanied by extensor relaxation. This restitution may be preceded by an augmentation of flexor relaxation at the commencement of the third phase. This perhaps corresponds to a “flexor rebound relaxation after inhibition” in the “pure” extension‐reflex. If the temporal relations of the two stimuli are changed it is found that the flexor restitution of contraction is less the later the contralateral stimulus falls in the period of the ipsilateral “background.”

4. Even where the “pure” extension‐reflex is followed by an extensor after‐discharge, augmentation of flexor restitution of contraction may occur in the third phase of the compound reaction; that is to say, that the level of flexor contraction attained may be greater than that which obtains in the “pure” flexion‐reflex. This augmentation seems to be conditioned by the same factors which condition a flexor rebound contraction in the terminal phenomena of the “pure” extension‐reflex—although it may occur when there is only extensor after‐discharge in that reaction. It looks as if the presence of the flexion “background” could activate a flexion factor in the terminal phenomena of the extension‐reflex. The flexor augmentation increases in extent with increase in the strength of the contralateral extension‐producing stimulus [but in these circumstances a flexor rebound may occur in the “pure” extension‐reflex with increase in the value of the evoking stimulus]. Where flexor augmentation (and not depression) occurs during the phase of double stimulation there is often an additional augmentation in the third phase. The augmentation during double stimulation is often seen where there is a preliminary factor of flexion in the extension‐reflex, and the augmentation in the third phase may occur where there is only extensor after‐discharge in the “pure” extension‐reflex, although with stronger “pure” contralateral stimuli a flexor rebound contraction may appear. In one experiment it was found that the relative value of the flexor augmentation of the third phase (that is, its extent as measured from the level of flexor contraction in the “pure” flexion‐reflex) decreased with increase in the value of the “background” flexion‐producing stimulus.

5. Where there is only “flexor rebound relaxation after excitation” in the “pure” flexion‐reflex no noticeable variation of this has been observed to occur in the “fourth phase” of the compound reaction.

6. Where “extensor rebound contraction after excitation” occurs in the “pure” extension‐reflex it may also appear in the “third phase” of the compound reaction. There is then often an increase of the flexor relaxation of the phase of double stimulation at the commencement of the third phase. The augmentation of extensor contraction at the commencement of that phase is not of so great an extent as that of the extensor rebound contraction in the “pure” extension‐reflex. The reciprocal phenomena of extensor augmentation of contraction and flexor augmentation of relaxation in the third phase may be greater the later the contralateral stimulus falls in the period of the ipsilateral flexion “background” in series of compound reactions in which the temporal relations of the two stimuli are varied.

7. Where “extensor rebound relaxation after excitation” occurs in the “pure” extension‐reflex the phenomenon may be either partial or complete. In cases where the terminal relaxation in the “pure” extension‐reflex is partial it may be found that in the third phase of the compound reaction there persists an extensor after‐discharge—which, however, is less than that conditioned by the partial extensor relaxation in the “pure” reflex. At the same time there may be flexor depression. Where the terminal relaxation of the extensor in the “pure” extension‐reflex is greater there may yet be only complete restitution of flexor contraction in the third phase, and no augmentation of flexor contraction above the level which obtains in the “pure” flexion‐reflex. But more usually this augmentation occurs, and this is especially the case where the “pure” extension‐reflex is of “decerebrate” type. Where the ipsilateral “background” is subliminal flexor contraction may appear in the third phase of the compound reaction.

8. If, in these circumstances, the temporal relations of the compounded stimuli are varied, it may be found that the flexor augmentation of the third phase is greater the later the interrupting contralateral stimulus falls in the period of the flexion “background.” As then the extensor contraction during double stimulation is found to be greater the later the contralateral stimulus falls, it is possible that this increase of augmentation is conditioned by an increase in the value of the interrupting extension‐reflex.

9. If, again in these circumstances, the relative values of the compounded stimuli are varied, it is found that the flexor augmentation is greater the stronger the interrupting contralateral extension‐producing stimulus is. The same result seems to occur when the strength of the flexion “background” is increased. Where the flexion “background” is of “decerebrate” type with marked late extensor contraction, in the third phase of the compound reaction there may occur a summation of this extensor contraction with the extensor after‐discharge qualified by a partial terminal relaxation in the “pure” extension‐reflex.

10. Where there is “extensor terminal relaxation” accompanied by “flexor rebound contraction” in the “pure” extension‐reflex, a great flexor augmentation may occur in the third phase of the compound reaction. This may resemble the flexor rebound contraction of the “pure” extension‐reflex in form—but is of much greater extent. It may appear—and be of greater extent than the flexor rebound—where the ipsilateral “background” is subliminal. The extensor shews reciprocal relaxation. In these cases in which the flexor contraction phenomenon of the third phase is of the form of the flexor rebound contraction in the extension‐reflex there follows, upon the attainment of its maximum, a partial or complete flexor relaxation. This again may be followed by a secondary and slower flexor contraction during the remainder of the third phase. In other words, the primary flexor movement in the third phase is a di‐phasic one and resembles the “beat” of the flexor in rhythmic acts. It looks here as if the transient “beat”‐like flexor rebound of the extension‐reflex is conditioned by a flexor inhibitory relaxation which cuts short the flexor contraction, and that this inhibitory relaxation may still appear in the complicated phenomena of the third phase—that is, where there is a flexion “background.” In these circumstances the relaxation is a later phenomnenon (that is, the “beat” is of greater duration), and it is less complete than in the rebound phenomenon of the “pure” extension‐reflex. Where the flexion “background” is strong the relaxation may not appear, and the flexor augmented contraction then persists throughout the third phase.

11. In these circumstances, if the values of the two stimuli are varied the flexor augmentation may appear to be greater the stronger the interrupting contralateral extension‐producing stimulus. This may be the case where the ipsilateral “background” is subliminal. Where the value of the ipsilateral “background” stimulus is varied it is usual for the flexor augmentation to be greater the stronger the ipsilateral stimulus. In one instance the reverse was observed, and here there was a well‐marked element of late extension in the “pure” flexion‐reflex of “decerebrate” type.

12. The presence of a flexion “background” therefore appears to augment the flexion factor in the terminal phenomena of the extension‐reflex—summation of the two flexions occurring.

13. Where “extensor rebound contraction after excitation” occurs in the “pure” extension‐reflex, but no extensor rebound occurs on termination of the “pure” flexion‐reflex, there may yet appear in the fourth phase of the compound reaction (that is, on termination of the flexion “background”) a marked extensor rebound contraction. Where the temporal relations of the two stimuli are varied this extensor rebound in the fourth phase is found to be greater the later the interrupting contralateral stimulus falls in the period of the flexion “background”—that is, the shorter the duration of the third phase after double stimulation has ceased. As here complete extensor relaxation may occur in the third phase, it looks as if the effect of the interrupting extension‐reflex is carried over in such a manner as to influence the phenomena of the fourth phase.

14. Where “extensor rebound relaxation after excitation” is present in the “pure” extension‐reflex, and where there is no extensor rebound in the “pure” flexion‐reflex, that may yet occur in the fourth phases of compound reactions if the extensor rebound relaxation of the extension‐reflex is incomplete. This is greater the later the contralateral stimulus falls in the period of the ipsilateral “background” when the temporal relations of the two stimuli are varied, and also greater the stronger the interrupting contralateral stimulus where their temporal relations are constant but their relative strengths varied. Where the “extensor rebound relaxation” of the extension‐reflex is complete a flexor rebound contraction may appear in the fourth phase of the compound reaction when absent in the “pure” flexion‐reflex.

15. Where “extensor rebound contraction after inhibition” occurs in the “pure” flexion‐reflex it is found that in the fourth phase of the compound reaction there is also an extensor rebound contraction, and that this is of greater extent than that of the “pure” flexion‐reflex of corresponding strength and duration of stimulation. Where the strength of the two stimuli are varied in series of reactions it is found that this extensor rebound contraction is greater the stronger the interrupting contralateral extension‐producing stimulus where the ipsilateral flexion “background” is of constant value, and that it is also greater the stronger the “background” stimulus is where the strength of the interrupting contralateral stimulus is of constant value. Where the temporal relations of the two stimuli are varied, but their values kept constant, it is found that the extensor rebound contraction of the fourth phase is greater the later the contralateral stimulus falls in the period of the ipsilateral “background” stimulus. In these experiments this is equivalent to saying that it is greater the shorter the duration of the third phase after the termination of double stimulation.

16. Where “flexor rebound contraction after excitation” occurs in the “pure” flexion‐reflex the terminal phenomena of the fourth phases of compound reactions may shew either exaltation or depression of the flexor rebound. These different results are probably conditioned by differences which obtain in the terminal phenomena of the interrupting extension‐reflex. Thus it has been noticed that where there is a marked extensor after‐discharge in the “pure” extension‐reflex there is depression of the flexor rebound contraction in the fourth phase of the compound reaction in which it is used as the interrupting stimulus against a flexion “background.” On the other hand, where there is extensor terminal relaxation in the “pure” extension‐reflex there is usually exaltation of the flexor rebound in the fourth phase of the compound reaction. It may therefore appear that the terminal effect of the extension component of the period of double stimulation may persist across the third phase of the compound reaction and affect the terminal phenomena conditioned by the withdrawal of the “background” flexion‐reflex. The fact that exaltation of the flexor rebound occurs when there is an extensor terminal relaxation in the “pure” extension‐reflex seems to point to the conclusion that this terminal phenomenon is a positive one and due to an activity akin to the flexion‐reflex which cuts short the extensor after‐discharge. Were it due merely to the sudden death of the extensor discharge there would not be expected to occur a flexor exaltation of the rebound in the fourth phase of the compound reaction.

17. Where this flexor rebound contraction occurs after the “pure” flexion‐reflex, and where there is marked extensor after‐discharge in the “pure” extension‐reflex, it is found that if the temporal relations of the two stimuli are altered in series of compound reactions the depression of the flexor rebound in the fourth phase is greater the later the interrupting contralateral stimulus falls in the period of the flexion “background.” This is just what would be expected if the depression of the flexor rebound is attributable to the continued extensor after‐discharge factor in the terminal phenomena of the interrupting extension‐reflex.

18. On the other hand, where there is extensor terminal relaxation in the “pure” extension‐reflex it is found that the exaltation of the flexor rebound of the fourth phase of compound reactions is greater the later the contralateral stimulus falls in the period of the flexion “background” in series of reactions in which the temporal relations of the two stimuli are changed.

19. Where a “flexor rebound contraction after excitation” occurs in the “pure” flexion‐reflex, series of reactions in which the relative values of the compounded stimuli are varied may be obtained. In such cases the ipsilateral flexion “background” may be kept of constant value, and that of the interrupting extension‐reflex may be progressively increased. This may be done in instances in which weak “pure” contralateral stimuli give extension‐reflexes followed by extensor after‐discharge alone; stronger “pure” stimuli may give reactions which are followed by extensor relaxation which becomes greater the stronger the stimulus is. In these circumstances it is found that there is depression of the flexor rebound of the fourth phase of the compound reaction when the strength of the interrupting contralateral stimulus is such that when applied “pure” it gives extensor terminal after‐discharge. But when its strength is such that extensor terminal relaxation is conditioned in the “pure” extension‐reflex, then exaltation of the flexor rebound of the fourth phase of the compound reaction occurs. This is then greater the stronger is the interrupting contralateral extension‐producing stimulus. Where in series of compound reactions the strength of the interrupting stimulus is kept constant but that of the “background” flexion‐reflex is varied, it is found that the flexor rebound augments with increase in the strength of the ipsilateral “background” stimulus. If the terminal phenomenon of the “pure” extension‐reflex is extensor relaxation, and if strengths of ipsilateral stimulation at which no flexor rebound occurs are used, a flexor rebound contraction may yet appear in the fourth phase of the compound reaction.

20. These results are of interest in that they seem to shew that the terminal phenomena of the “pure” extension‐reflex are, at any rate in part, of central conditioning. Thus the extension after‐effect—which is seen in the extensor after‐discharge—appears to be carried over the third phase of these compound reactions (that is, a phase in which extension is suppressed) in such a manner as to influence the phenomena of the fourth phase. And in a similar way the extensor relaxation (flexion) after‐effect of the extension‐reflex may be carried over also. It looks as if the flexion factor in the terminal phenomena of the extension‐reflex is relatively increased with increase in the strength of the evocative stimulus, and as if the extensor terminal relaxation seen in such cases is due to a positive activity, antagonistic to the extension‐reflex, which tends to cut short the extensor after‐discharge. A similar activity (but one of opposite sign) probably cuts short the flexor after‐discharge of the flexion‐reflex. There is a great temptation to make the generalisation that “the activity of a centre (or of a pair of linked ‘half‐centres’) having been tilted in a certain direction tends to remain in the state into which it passes until again tilted by another external force”—or, in other words, that “the discharge of a centre tends to be maintained until cut short by an opposite activity.” But some such rule of “central inertia” is probably too sweeping a one, and at present all we can say is that at any rate the activity of extension seems to shew some evidence of it.

21. Where “flexor rebound contraction after excitation” has not been present in the “pure” flexion‐reflex, but where “flexor rebound contraction after inhibition” has been present after the “pure” extension‐reflex, there has not been observed to occur a flexor rebound contraction in the fourth phases of compound reactions in these experiments.

22. Where “flexor rebound contraction” occurs in the terminal phenomena both of the “pure” flexion‐reflex and of the “pure” extension‐reflex, it is found that the flexor rebound in the fourth phase of the compound reaction is augmented.