STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XX.: RHYTHMIC MOVEMENTS AS IMMEDIATE REFLEX PHENOMENA OF COMPOUND STIMULATION [PROGRESSION CONDITIONED BY ANTAGONISTIC REFLEX STIMULI]
Authors: Brown T. Graham 
Published: December 2014

1. The results described in this paper have previously been published in a more condensed form. They are now given in greater detail, and shew that in the low spinal, high spinal, and decerebrate preparations there may be evoked a rhythmic phenomenon during a period in which two afferent nerves—which when stimulated separately give antagonistic reflexes—are simultaneously stimulated.2. This rhythmic phenomenon—which is contended to be similar to the activity of rhythmic progression—is examined as it occurs in a pair of isolated antagonistic muscles which act on the ankle‐joint in the cat. It is found that the rhythmic movements occur in the two muscles and are reciprocal.3. It has been obtained in spinal preparations.4. Where the extension‐reflex forms the background in the compound reaction the rhythmic phenomenon during the period of double stimulation opens in the extensor muscle with a movement of inhibitory relaxation. There then follows a reconstitution of extensor contraction, but when this reaches its summit there again ensues inhibitory relaxation—and so the rhythmic phenomenon is constituted.5. The relaxations in the beats of the extensor muscle may be “complete”—that is, down to the level of “rest” of the muscle. Or they may be incomplete. In the latter case there is a foundation of maintained contraction underlying the rhythmic phenomenon in the extensor muscle. This foundation is greater the stronger the factor of extension in compound stimulation is.6. When the period of double stimulation is made to fall ever later in the period of the extension background in a series of compound reactions it is found that the latency of the first extensor beat is increased and the foundation of maintained extensor contraction underlying the rhythmic phenomenon in the extensor muscle is decreased. This points to the conclusion drawn in a previous paper that the extension‐reflex gradually “fatigues” and therefore that the factor of extension in double stimulation is gradually diminished.7. Where the relative strengths of the two stimuli are changed there occur changes in the rhythmic phenomenon. Thus if the background is kept constant and the value of the interruptor changed in strength gradually from very strong to very weak in a series of reactions, no rhythmic phenomena may be obtained at the two ends of the series. In the middle members it may appear. With increase in the strength of the interruptor the rhythmic beats appear; become greater in extent and faster in rate; reach a maximum; then decrease in extent and rate; and finally disappear again.8. Occasionally, when a subliminal or a liminal extension background is used, there may occur a rhythmic phenomenon if it is interrupted by a flexion‐reflex. In such cases the flexion‐reflex is of “decerebrate type,”—that is to say, that the flexor contraction of the “pure” flexion‐reflex may show rapid “fatigue,” although this may not be accompanied by actual late extensor contraction during stimulation. This may be explained as due to a late extension factor in the flexion‐reflex, and when this is augmented by the background extension‐producing stimulus the rhythmic phenomenon may be conditioned.9. When the background is one of ipsilateral flexion the phenomena in the phase of double stimulation open with maintained extensor contraction. This is then broken by rhythmic relaxations and reconstitutions of contraction as before.10. If the temporal relations of the two stimuli are changed it is found that the foundation of maintained extensor contraction becomes greater the later the interrupting extension‐reflex falls. In other words, the rhythmic beats become of less extent by a reduction of the extents of their relaxation phases.11. If the strengths of the stimuli are changed it is found that the phenomena are similar to those described in paragraph 7.12. On comparison of the phenomena where the two different backgrounds (flexion and extension) are used it is found that where flexion is the background the period of double stimulation opens with maintained extensor contraction; and that upon this foundation the rhythmic beats in the extensor muscle appear as relaxations followed by reconstitutions of extensor contraction—and so on. Where the background is one of extension the period of double stimulation opens with a maintained relaxation of the extensor; the rhythmic beats break through this as contractions followed by relaxations in the extensor muscle. In the former case there is a greater foundation of maintained extensor contraction underlying the rhythmic phenomenon in the extensor muscle than in the latter. These phenomena may be explained on the basis that as the background stimulus continues its “capture of the field” deteriorates, and thus reduces the value of it in the period of double stimulation.13. Rhythmic phenomena during double stimulation have been observed in decerebrate preparations.14. This may be the case, with extension backgrounds, even where the flexion‐reflex is weak and there occurs no flexor contraction during double stimulation. In such a case there may be slight rhythmic irregularities on the curve of the extensor muscle. In other cases flexor contraction may occur, and the rhythmic movements are then reciprocal in the two muscles. The beats may be “incomplete” and may become more complete as the period of double stimulation continues. The beats may be complete in other cases. It is, then, by no means rare for pauses at the level of complete relaxation to be exhibited in the curve of the flexor muscle.15. When the temporal relations of the two stimuli are varied it is found that the rhythmic phenomenon may occur when the interrupting flexion‐producing stimulus is applied at a certain duration of time after the commencement of the background extension‐producing stimulus, and that with shorter or longer durations of preceding background it may be absent.16. When the relative strengths of the two stimuli are changed it is found that with a constant background stimulus there is one strength of interrupting flexion‐reflex at which the rhythmic phenomenon occurs; with stronger or weaker flexion‐reflexes the rhythmic phenomenon is not seen. In some cases the rhythmic phenomenon may persist over an unusually large range of change in the strength of the interrupting ipsilateral stimuli, and it is then found that increase of the flexion factor in the period of double stimulation leads to an increase of the element of flexion in the rhythmic phenomenon—the flexor beats become less complete. In other words, the underlying foundation of maintained flexor contraction increases.17. Where the flexion‐reflex is used as background a rhythmic phenomenon may occur during double stimulation. The flexor beats may be complete and with pauses at the level of maximum relaxation; there are then corresponding extensor pauses at the level of maximum contraction. The rhythmic phenomenon may be at first incomplete, and may then become complete towards the end of the period of double stimulation. Or it may at first not be present, but may make its appearance later in the period. Occasionally the rhythmic phenomenon may appear even where there is augmentation of flexor contraction during the period of double stimulation.18. Where the temporal relations of the two stimuli are varied and the interrupting extension‐reflex applied ever later in the period of the background flexion‐reflex, it may be found that at first—where the two stimuli are synchronously commenced—there is no rhythmic phenomenon. As the extension‐reflex is applied ever later a rhythmic phenomenon appears, becomes more pronounced, and finally disappears again when the extension‐reflex is applied too late in the period of the background. As in these cases there is a great preponderance of flexion in the period of double stimulation when that falls early in the period of the background, and a great preponderance of extension at the end of the series where the extension‐reflex falls very late, it is probable that the rhythmic phenomenon appears at an intermediate point where the balance between the two factors is more or less equal.19. Where the strengths of the two stimuli are varied it is found that at a certain relationship of the two strengths a rhythmic reaction during double stimulation may occur, but that variation of the strength of one stimulus may serve to abolish it. Thus if the strength of the ipsilateral flexion background be gradually increased from zero to a maximum in a series of reactions, it is found that at the commencement of the series extension is at its maximum during double stimulation. At the end of the series maintained flexor contraction alone occurs. But in an intermediate reaction a rhythmic phenomenon is presented. Where the rhythm can be obtained over a considerable range of stimuli it may be found that increase in the strength of the flexion factor may change the rate of the beats. In one case they were found to be slowed.20. When reactions in which the same strengths of stimuli are used, but in which the background preceding double stimulation is varied are compared together, differences are found to occur in the rhythmic phenomena. These differences can be explained on the assumption that the value of a background changes with its temporal duration, and that therefore in the balance of double stimulation the background factor is relatively less than when its evocative stimulus is used as the interruptor and the previous interruptor as the background.21. When the factor of flexion is increased there appear certain changes in the rhythmic phenomenon. The flexor beats tend to become less complete. Thus the pause in complete relaxation may disappear; the relaxation phase becomes a slower movement, its lowest point fails to reach down to the level of “rest” of the muscle; sometimes the rate of the beats appears to increase, in other cases to decrease. In the extensor muscle the beats tend to become more complete; pauses in complete relaxation may appear; the beats decrease in extent.22. When the factor of extension is increased in the rhythmic phenomenon the flexor beats tend to become more complete; pauses appear at the level of complete relaxation; the beats appear usually to become slower in rate, but their relaxation movement becomes faster. In the extensor beats the relaxations become less complete; pauses appear at the level of maximum contraction.23. In either case, if one factor be too much increased in value, the rhythmic phenomenon gradually disappears and is replaced by the arrhythmic maintained activity conditioned by that factor which is made to preponderate in the balance.24. Where the “pure” reactions are rhythmic the addition of an interrupting reflex may change the rhythmic phenomenon.25. Thus where the contralateral stimulus evokes a rhythmic reaction the compounding against it of a flexion‐reflex may serve to abolish or depress the rhythmic beats and replace them by a state of maintained flexion. In other cases the compounding against it of an ipsilateral extension‐reflex may serve to abolish the rhythmic phenomenon and replace it by a state of maintained extension. In the former case it is probable that the balance is overset in the direction of flexion, and in the latter case in that of extension.26. Where the ipsilateral flexion‐reflex is rhythmic, if it is compounded against a strong extension‐reflex the rhythm may be entirely suppressed. In such a case there appears to be abolition of the rhythmic phenomenon by the oversetting of the balance of the centres in the direction of extension. In other cases the rhythm may be abolished and its place taken by an augmented flexor contraction; or the factor of maintained flexor contraction in the rhythmic phenomenon may be increased. In such an instance it is possible that the contralateral “capture of the field” has so far reversed as to condition an oversetting of the balance to the side of flexion.27. In all these cases, which this summary cannot completely deal with, there appears to be evidence which supports the conclusion already drawn that these rhythmic phenomena—which resemble various types of rhythmic progression—are conditioned by a nice balance of equal and opposite activities (flexion and extension) in antagonistic and linked spinal centres (or “half‐centres”). But they shew, as has already been pointed out, that the rhythm is not one conditioned by a natural disposition of the single “half‐centre” or efferent neurone. Rather they shew that the rhythm is conditioned during a state of nice “neural balance” by the mechanism which links the “half‐centres” reciprocally; and that the oversetting of this balance by the increase of one of its components serves to abolish the rhythm and to replace it by a maintained activity.