Studies in the Physiology of the Nervous System. Xi.: Immediate Reflex Phenomena in the Simple Reflex

In this paper the mutual relations of two antagonists at the ankle‐joint‐tibialis anticus and gastrocnemius—are examined during the occurrence of simple reflex stimuli and in different reflex preparations and conditions. 2. The two conditions used are the normal—that is, the condition with intact muscle afferents—and the de‐afferented, in which the muscle afferents are divided. 3. The preparations used are the low spinal (the preparation otherwise being decerebrate), the high spinal (decapitate), and the decerebrate. 4. These conditions and preparations were “recent” in all cases. That is to say, that all operative procedure was performed within a few hours of the recording of the reflex reactions. 5. In general two types of the flexion‐reflex and of the extension‐reflex may be distinguished. In either case these may be termed the “spinal” type and the “decerebrate” type. Either type of either reflex may occur in any of the preparations and in either condition (intact and de‐afferented). The “spinal” types, however, characterise the low and high spinal preparations, and the “decerebrate” types the decerebrate preparation. 6. The “spinal” type of flexion‐reflex is characterised by a phase of rapidly increasing contraction in the flexor muscle. This is followed by a phase of regular maintained contraction, and finally by a rapid decrease of the maintained contraction at the cessation of stimulation. The latency of the phases of increasing and decreasing contraction are short. The latency of the phase of increasing contraction decreases with increase of strength of stimulus, but that of decreasing contraction at cessation of stimulation increases. The extensor muscle shews evidence of relaxation or of no trace of movement. In the flexor phase of maintained contraction there may sometimes be a slow fall in the level of maintained contraction as the stimulus is continued. This is best seen in minimal reactions. In the decerebrate preparation the spinal type of flexion‐reflex usually has a longer latency, a slower phase of increasing contraction, and a greater latency of decreasing contraction at cessation of stimulation. 7. The “decerebrate” type of flexion‐reflex is characterised in the flexor muscle by a longer latency of the commencement of contraction and by a slower phase of increasing contraction. At the same time the maintenance of contraction is less perfect. The maximum extent is soon reached, and thereafter the curve falls gradually and in some cases during stimulation may even reach the level which obtained before the commencement of stimulation. The extensor muscle in this type of flexion‐reflex shews a phase of contraction. This commences late in the period of stimulation and is of a very gradually increasing contraction. This phase of slowly increasing contraction commences after the commencement of the flexor relaxation but otherwise corresponds to it in time. In form this contraction is quite unlike that of the extensor muscle in the extension‐reflex. It is seen best with minimal stimuli. If the strength of stimulation be increased it tends to disappear, the record approaching more nearly to the spinal type. 8. The “spinal” type of extension‐reflex is characterised in the flexor muscle by a state of slight relaxation or by no movement at all. In the case of the extensor muscle there is, in the records, a curve of contraction which has a longer latency than has that of the flexor contraction in the flexion‐reflex. The phase of increasing contraction is slower and is in form like the segment of a circle arranged convexly to the abscissa, whereas that of the flexor contraction is almost a straight line. The phase of maintained contraction is regular, and on cessation of stimulation there is a phase of decreasing contraction which has a short latency and in form is usually the reverse of that of the phase of increasing contraction, i.e. like the segment of a circle concave to the abscissa. In the decerebrate preparation there is often a marked continuation of the state of maintained contraction after the cessation of stimulation—a tonic after‐discharge. 9. The “decerebrate” form of extension‐reflex differs from the spinal principally in the presence of a phase of extensor relaxation which may occur before the commencement of contraction, or just after that commencement. This is often accompanied by a flexor contraction. The different variations are described in the text. 10. Grading of reaction in response to grading in the strength of stimulation is seen both in the flexion‐reflex and in the extension‐reflex. In the former reaction the extent of maximal flexor contraction and of the level of maintained contraction markedly augments with increase in the strength of stimulation: the latency of contraction decreases, and the reaction tends to become more nearly of the “spinal” type. In the extension‐reflex there is not so great a degree of grading; a maximum contraction of the extensor is soon reached, with increasing strength of stimulus; the latency of contraction tends to increase (but sometimes to decrease); and the reaction tends more nearly to approach the “decerebrate” form. 11. The reactions are also influenced by changes in the resistance against which the muscles are made to contract, and by “fatigue.” In the latter case the repetition of a stimulus which evokes a flexion‐reflex, if sufficiently frequent, tends to reduce the extent of the reaction, to increase its latency, and to produce a more “decerebrate” form of curve, an extensor contraction often then appearing for the first time. The change in form of the reactions may be described briefly as the reverse of that seen when the strength of stimulus is progressively increased. 12. The relative amount of tonus at the time of stimulation also influences the form of reaction. The extensor tonus, if greater than usual, demonstrates more clearly the presence of extensor relaxation in the flexion‐reflex. On the whole the augmentation of extensor tonus, at any rate in the low spinal preparation, tends to make the reaction more nearly of the “decerebrate” type. 13. In the de‐afferented condition the reactions do not markedly differ from those of the intact preparations. On the whole, the decerebrate form of flexion‐reflex is more common in the de‐afferented condition than in the normal condition, and the decerebrate form of the extension‐reflex less common. In the case of the flexion‐reflex there is often a very irregular form of flexor contraction curve. This may be termed an “ataxic” form, and it is seen rarely in the intact conditions, and is then slightly marked. 14. As has previously been stated (21), ipsilateral stimuli tend to evoke flexion reactions, and contralateral stimuli to evoke extension reactions; but sometimes the ipsilateral reaction is the extension‐reflex, and the contralateral reaction the flexion‐reflex. The latter irregular phenomenon is the more common, and is seen both in the low and high spinal and in the decerebrate preparations. The former phenomenon is more rare and seems to be confined to the decerebrate preparation. 15. These irregular reactions of ipsilateral extension and contralateral flexion are rarely seen in the same experiment, and have never been observed to be present at the same period of experiment. This means that when these reactions occur, the reactions to ipsilateral and to contralateral stimulation are set in the same direction, either in the direction of flexion or of extension. In one instance, although a weak ipsilateral stimulus at one period of the experiment evoked a reaction of extension, at a later period in the experiment the contralateral stimulus evoked a flexion‐reflex. 16. Reversal of reaction is a phenomenon which is occasionally seen. In the case of contralateral stimuli the reaction may change from the regular extension to the irregular flexion within a short space of time, and when the conditions of the experiment have not apparently been altered. This is seen in decerebrate preparations both in the intact and in the de‐afferented conditions. 17. Reversal may also occur in response to increase of strength of stimulus. This is seen in decerebrate preparations (intact and de‐afferented) in the case of the flexion‐reflex, and in a lesser degree in the case of the extension‐reflex. The reaction may change from an abnormal extension to the normal flexion as the strength of stimulus is progressively increased. 18. There is to be seen a certain grading in the forms of the reactions between the one extreme of pure flexor contraction and extensor relaxation, and the other extreme of pure extensor contraction and flexor relaxation. The various stages in a series of reactions may be selected from many experiments, but they may also occur in one or in a small number of experiments. In general, as the contralateral stimulus is increased in strength in successive reactions, the form of these passes from the extreme of pure extension to the diphasic reactions which characterise the “decerebrate” type. These finally may completely resemble reactions obtained in response to ipsilateral stimuli. If that form of stimulus is then progressively increased, the succeeding reactions tend to approach more nearly to the extreme of pure flexion. In some cases the whole series may occur in one experiment, and in response to ipsilateral stimuli alone. This grading strongly suggests that in each reaction there are factors of flexion and of extension excitation; that, for instance, a flexion‐producing centripetal impulse does not merely activate the flexor “half‐centre” and inhibit the activity of the extensor “half‐centre;” but that it c