STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. IX.: REFLEX TERMINAL PHENOMENA—REBOUND—RHYTHMIC REBOUND AND MOVEMENTS OF PROGRESSION

1. In the experiments here described reflex terminal phenomena have been examined in individual antagonistic muscles during the flexion- and extension-reflexes of the hind limb. 2. The phenomena have been investigated in the “normal” and in the “de-afferented” low spinal, high spinal, and decerebrate preparations of the cat. In these conditions the details of the phenomena differ in degree more than in kind. They are most clearly evidenced in the “normal” and “de-afferented” decerebrate preparations. 3. Reflex terminal phenomena may be defined as the sum total of the events which take place between the point at which the exciting stimulus ceases and the point at which the active muscles attain permanently their posture of rest. 4. In the records the terminal phenomenon may be evidenced as a phase of relaxation only. Or it may be composed of a phase of contraction. Or the phenomenon may be compounded of relaxation and contraction phases. 5. Contraction rebound may appear either after a state of relaxation or of contraction in the muscle which exhibits it. Relaxation rebound has been observed only after a preceding state of contraction. 6. “Rebound relaxation after excitation” (flexor or extensor) is the ordinary relaxation of the contracting muscle seen after every reflex act. It calls for no comment save the observation that its latency (“after-discharge”) increases with increasing strength of stimulus. 7. “Rebound relaxation after inhibition” is a phenomenon which has not been observed to take place. A central process which would otherwise condition it may possibly be present, but the effect of this may be masked, as the inhibited muscle almost always attains its “post-mortem length” during inhibition. No further relaxation is then possible. 8. “Rebound contraction after inhibition” is a common reflex terminal phenomenon. It is seen in the case of the flexor as well as that of the extensor muscle. In these two muscles it exhibits slightly different forms. 9. "Extensor rebound contraction after inhibition" consists most usually of a simple contraction of the muscle immediately succeeded by relaxation. This simple act is of short duration and slight extent. In some cases the extent is comparatively great, while the duration may also be great; thus no real relaxation phase may be observed within the limits of a record—the rebound then being “maintained.” 10. “Flexor rebound contraction after inhibition.” This act differs in extent and duration from the extensor. Both these are greater in the former reaction. Long-continued flexor rebounds are in form very similar to a flexion contraction in the reflex act itself. They often exhibit irregularities in the plateau of contraction. 11. “Rebound contraction after excitation” is a reflex terminal phenomenon which is not infrequently observed. It may occur either in a flexor or in an extensor record, but is most frequently seen in the former. It is most probably conditioned by a central phase of inhibition which simultaneously complicates the excitation which accomplishes the contraction of the muscle which exhibits it. The rebound contraction may succeed directly the foregoing reflex contraction or a phase of relaxation may separate the two. In the case of the flexor it resembles in its different forms the different forms of the rebound contraction after inhibition very closely. 12. The latency, extent, and duration of rebound contraction and rebound relaxation are affected by changing the strength of the exciting stimulus or by altering the duration of its application. 13. The effect of increasing the strength of stimulus upon rebound relaxation is to increase its latency pari passu and to decrease the rapidity of the relaxation when it appears. 14. The effect upon rebound contraction of increasing the strength of stimulus is more complex. In the flexor rebound contraction both of the extension- and the flexion-reflexes the latency of the movement is greatest with weak stimuli and progressively decreases as the strength of stimulus is increased. If the act of rebound remains simple its duration increases and the relaxation phase becomes more slow. But if the rebound is at first simple and then becomes rhythmic, the duration of the act in response to weak stimuli may be greater than the duration of each component of the act in response to strong stimuli. 15. The effect upon rebound contraction of increasing the duration of an otherwise constant stimulus is very similar. Flexor rebound in either the flexion-or the extension-reflex will not occur if the stimulus is weak and of short duration. If the duration of the stimulus is greater the reaction appears. With increasing duration of stimulus the latent time of the rebound decreases and its extent and duration also do. A maximum is then reached, after which increasing duration of stimulus produces a progressive lengthening of the latency and lowering of the extent of the contraction. Finally, the rebound contraction fails. 16. Rebound relaxation of one muscle is associated in the records with rebound contraction of the other when that is present. While the relaxation and the contraction may appear almost synchronously in some cases, it is far more common for the initiation of relaxation to precede contraction by a very distinct interval of time. This “relative latency” of contraction may be as much as 0·75 second or more. 17. Rebound contractions may appear in both the muscles in the same record. When this occurs the contractions are found not to be synchronous but to alternate. Contraction in one muscle then slightly succeeds the initiation of a phase of relaxation in the other. The contraction terminates with the commencement of a following relaxation, and this slightly precedes the contraction phase in the other muscle. 18. “Rhythmic” rebound occurs in certain cases. The rebound contractions are then rhythmically repeated, relaxation following contraction and contraction relaxation in the same muscle. The rhythmic phenomenon may be “complete” or “incomplete.” 19. “Incomplete” rebound occurs when the rebound contraction of the flexor muscle is of long duration and is rhythmically interrupted by relaxations which are immediately succeeded by restitutions of contraction, and when these relaxations do not attain to the position of rest of the muscle at their lowest points. There is no concomitant contraction of the extensor as a rule. 20. In “complete” rhythmic rebound the relaxations attain at their lowest points to the position of rest of the muscle. The termination of relaxation may be immediately succeeded by the contraction which reconstitutes the shortened state of the muscle. But there is often a distinct pause separating the termination of relaxation and the commencement of the succeeding contraction. During these pauses, which may be of a duration as great as a second or even more, the curve remains parallel to the abscissa. There are usually rebound contractions of gastrocnemius synchronous with the relaxation phases of the flexor. 21. In a “complete” rhythmic rebound of another form the contractions of the flexor are separated by long intervals of time and are of short extent. This type is characterised by a maintained contraction of the extensor, and this is relaxed slightly before the appearance of the flexor contractions and reconstituted thereafter. 22. Transition forms between simple rebound of long duration and incompletely rhythmic rebound occur. So do also transition forms between the incompletely rhythmic rebound and complete rhythmic rebound. 23. The forms of the tracings described in simple flexor rebound of long duration, in incomplete rhythmic rebound, and in complete rhythmic rebound of the two varieties correspond very closely with the appearances in the different phases of certain movements of progression described already in another paper. 24. So great is this correspondence that it may be assumed that the phenomena of progression and of rebound are intrinsically similar and conditioned by the same central factors.