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488

Recruitment and some Other Features o f Reflex Inhibition.

By E. G-. T. Liddell, M.A., and Sir Charles S. Sherrington, O.M., Pres. R.S.

(Received January 23, 1925.) (Physiology Laboratory, Oxford.)

[Plate 37.]

The present paper deals with some features of inhibitory relaxation of muscular contraction as observable in the knee extensor (decerebrate cat) when the crossed extensor reflex is subjected to inhibition by stimulation of an afferent nerve of the ipsilateral limb. The myograms have been obtained with the isometric optical myograph described in a previous paper, and the method of preparation, etc., has been as before.

I. Relaxation-speed under Strong Inhibition.

The speed of the muscular relaxation produced by the inhibition is often notably high. It then compares in rate with that of the lapse of a peripheral tetanus at cessation of the faradic stimulation of the muscle’s motor nerve (19).

In a number of instances we have compared the speed of these two forms of relaxation. The starting point of the decline of contraction-tension is not difficult to identify closely ; but the exact ending of the relaxation is not easy to assess, because the decline terminates with progressively decreasing gradient as it merges into the passive-tension plateau of the quiescent muscle. As index of the duration of the relaxation we have taken, therefore, the time occupied by the relaxation in the fall from the plateau-contraction tension, whence the relaxation sets out, to a tension half that value. This relaxation time has been compared in inhibitory and mn. tetanus terminal relaxations from one and the same experiment and where the plateau-heights (tensions) of the reflex and mn. tetani were closely similar. In observations taken from one and the same experiment the reflex observations must perforce precede the mn. observations, since the latter necessitate preliminary severance of the muscle’s nerve. Although the interval between the two sets of observations was sometimes not more than a few minutes, it was often longer, and the condition of the muscle must be somewhat the less fresh for the latter.

The observations 'which have yielded the measurements were not designed

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Recruitment and some Other Features o f Reflex Inhibition. 489

for this particular comparison. They give almost invariably relaxation-speeds somewhat higher for the inhibitory relaxation than for the mn. tetanus relaxa­

tion (fig. 1, and Plate 37, fig. 1).* Since inhibitory relaxation of a given

1 sec.

Fig. 1.— Crureus and Vastus medialis. Relaxation from inhibition of reflex tetanus rf and

from motor-nerve tetanus mn compared in same preparation. Bk. sks. at 50 per sec. used for both ; at 16 cm. coil distance for former, at 26-5 cm. for latter. Inhib.

stim. double sks. at 95 per sec. at 16 cm. coil distance. E. to E / excitat. stim. for mn.

tetanus ; E. to E." excitat. stim. reflex. X. to inhib. stim. Tendon movement multiplied 35 times. Time in tenths second.

y

amplitude cannot presumably exceed in speed under the same myographic conditions a mn. tetanus relaxation of the same amplitude, we turned from observations taken in one and the same experiment to comparison of observa­

tions taken (from vastocrureus always, as before) in different experiments, under similar myographic conditions, but in some of which the mn. tetani were recorded without prior use of the preparation for reflexes at all. Com­

parison of relaxation taken thus from different individual experiments introduces, of course, sources of inequality which those taken from one and the same experiment avoid.f In the subjoined list of measurements those

* In all these figures the beginning and ending of tetanic stimuli are signalled by photo­

graphed movement of the actual shorting and unshorting keys in the secondary stimulation circuit; this though precise as to the movement of shorting and unshorting, gives beginning and ending of the tetanic stimulus to as near only as the length of interval between individual stimuli, e.g.,with 50 bk. sks. per second to within 0*02 second only. With the single sk.

stimulus the signal photographed being also the actual key in the circuit, here the primary circuit, the signal records precisely the moment of delivery of the stimulus. The induction coils both for inhibition and for excitation had no core in primaries ; primaries fed by about 0-25 amperes.

f But their fair reliability is shown by the speed of relaxation of the mn. tetanus of tibialis anticus coming out regularly as greater than that of vastocrureus or rectus femoris for comparable amplitudes, e.g., 0-014 second for half relaxation from a tetanus of 38 mm.

2 0 2

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where the mn. tetani were taken at outset without previous reflex observa­

tions are marked by an asterisk.

490 E. G. T. Liddell and Sir C. S. Sherrington.

Mn. tetani. Reflex tetani.

Fall in millimetres of myogram from plateau to base line.

Duration of terminal fall from plateau tension to half of plateau tension.

Fall in millimetres of myogram from plateau to base line.

Duration of inhibitory fall from plateau tension to half of plateau tension.

Sec. Sec.

12 0 029 i i 0-027

20 0 020 13 0-023

25 0 040 25 0-035

29 0 041 31 0-037

36 0 039 34 0-035

39 0 042 34 0-037

41 0.046 36 0-035

43* 0.028 43 0-046

46* 0 030 46 0-058

47* 0-030 50 0-053

54. 0-060 58 0-041

Although where taken from the same experiment the inhibitory relaxations are almost uniformly of greater speed than the mn. tetanus relaxations of comparable extent, some of the mn. tetani relaxations from experiments in which no reflexes were taken are of distinctly higher speed than any of the inhibitory relaxations. Since slight progressive cooling of the partly exposed muscle, though its circulation was to inspection intact and its tetanic contraction showed wavelets corresponding with the stimulus-rhythm at 50 per second, is the attributable source of impairment of the relaxation-speed, the lower speed of the mn. tetanus relaxations in the experiments which had given reflex relaxations of higher speed somewhat earlier, implies that the limit of speed of these reflex relaxations was that of the relaxation process of the muscle at the time, and that the reflex relaxation speed was as quick as this latter would allow. This implies that the reflex inhibition in these cases of strong inhibition quells the discharging activity of all the motoneurones, which are engaged in delivering the impulse-efflux of the reflex tetanus, with a complete­

ness and simultaneity approximately equal to those of the lapse of activity in the motor nerve-fibres of the muscle’s nerve when faradic stimulation of that nerve ceases. This time-relation of the inhibitory effect contrasts strongly with that of the subsidence of activity of the same discharging motoneurones which follows mere cessation of the excitation of the excitatory afferent nerve sup­

porting the reflex contraction. In the latter case the motoneurones not uncommonly persist (32) in their discharge of impulses for a full second or more

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Recruitment and some Other Features o f Reflex Inhibition. 491

after withdrawal of the excitatory afferent’s stimulation, and in some of them the post-stimulatory discharge not rarely persists for three and four seconds.

The arrest of the motoneurone discharge is obtained, therefore, some hundreds of times quicker by inhibition than by cessation of the stimulus exciting the discharge, and with some hundreds of times greater approximation to simultaneity of arrest in the discharging motoneurones as a group.

The reflex inhibition confers, therefore, upon the mechanism of the crossed extensor reflex a degree of dead-beatness of termination which is otherwise wanting to it. The latent interval between delivery of the stimulus to the inhibitory afferent and the onset of the inhibitory relaxation is in these strong inhibitions relatively short, e.g., 30 a for instance in Plate 37, fig. 1, A ; 25 a and 35 a respectively in Plate 37, fig. 2, B and A ; and we have met with latencies of less than 20 a (cf. 5). The dead-beat character of the arrest is thus enhanced further.

For an inhibitory relaxation curve to resemble the relaxation curve at termination of a neuromyal tetanus the inhibition must (1) be “ total,”

i.e., quell completely the discharge of all the muscle’s motoneurones active in the reflex against which the inhibition acts ; (2) quell the discharge from all the individual motoneurones with approximate sim ultaneity; (3) maintain its action long enough to allow the relatively slow process of mechanical relaxation of the muscle fibres to run to completion. The instances of inhibitory relaxation given above fulfilled these requirements. But in some of the instances the inhibitory stimulation was tetanic {e.g., fig. 1), in others it was by a single break- shock {e.g., Plate 37, fig. 1). That the inhibitory effect of a single-shock stimulus should last long enough for fulfilment of the third requirement above is explicable by central after-action prolonging the effect of the stimulus.

Similarly, the central effect induced by a single-shock stimulus of the excitatory afferent lasts long enough, as judged by the reflex contraction resulting, to generate a short stream of repetitive impulse-discharge (28, 36). The brevity of this impulse-efflux is greater with a weak single-shock stimulus than with a strong, and is sometimes reduced to a single impulse-volley (Forbes and Gregg (14), Adrian and Forbes (3) ), but even with a weak single shock the efflux is often repetitive (36). So, similarly, with the inhibitory effect of a single-shock stimulus of the inhibitory afferent its duration is longer when the single shock is strong {e.g., 11 cm. coil distance in instance fig. 3) than when weak, but also it may be considerable when the single shock is of quite moderate intensity {e.g., 15-5 cm. (coreless coil) in instance fig. 7, A).

With tetanic stimuli of the duration illustrated in fig. 1, the fulfilment of

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492 E. G. T. Liddell and Sir C. S. Sherrington.

requirement (3) above is obvious ; and with such stimuli the requirement (1) above is fulfilled by an intensity of the break shocks less than that needed if the stimulus be one single shock. Evidently, therefore, temporal summation is a factor in the inhibitory result of tetanic stimuli. In fig. 1 the frequency-rate of the tetanic stimulus causing the inhibition was high, 190 per second.

In such a case there is nothing in the myogram-curve which disproves summa­

tion of the central effects of the first few stimuli opening the tetanic series as a factor in producing the “ totality ” of the inhibition, and in producing the inhibition’s approximate simultaneity of onset throughout the motoneurone aggregate. And in regard to the latter point it is noticeable that even with single-shock total inhibition the onset of inhibitory relaxation though abrupt does not give an angle quite so sharp as th at often characterising the onset of the relaxation fall at cessation of a motoneurone tetanus, an abruptness of angle the significance of which has been pointed out recently by Fulton (17).

Hence, in saying above that the reflex extinction of discharge is in some cases executed with approximate simultaneity throughout the total aggregate of discharging motoneurones, we mean no more than that its extension throughout that aggregate is in such cases complete within the course of about 1 /50th second. Such a period, in view of the motoneurone aggregate comprising for quadriceps extensor muscle alone some 2,000 motoneurones (29), is notably short.

II. Reflex-contraction Plateau and Inhibition.

As shown above, a single break-shock stimulus delivered to the inhibitory afferent may cause, even when the reflex contraction has reached its plateau - height, total relaxation of the contraction. But the plateau-contraction (fig. 2) of the crossed extension reflex is divisible into two portions, which, although, since they are of the same height, undistinguishable in the myogram curve, are yet separable as regards their time-relation to the stimulation. In this crossed reflex it is common for the contraction after attaining its full plateau- height, and maintaining it under further equable continuance of the tetanic stimulation of the afferent nerve, then on withdrawal of the tetanic stimulation to maintain the plateau-contraction undiminished, even for long periods, e.g., a couple of seconds. The plateau-contraction is therefore separable into what may be termed the “ stimulation plateau ” and the “ after-discharge plateau.” Commonly the plateau in both periods is quite smooth ; in that case visible distinction between them fails. Often, however, especially when the rate of frequence of the tetanic stimulus is relatively low, e.g., below 35 per

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second, inspection of the ^“ stimulation-plateau’s ” line can detect in it slight undulations corresponding with the rate of rhythm of the tetanic stimulus.

Recruitment and some Other Features of Reflex Inhibition. 493

JF i 1 j t A

sec. -2 ^

Fig. 2.—A. Vastocrureus. Inhibitory distinction between “ stimulation plateau ” and

“ after-discharge plateau ” of reflex contraction. E. to E/ stim. of excitat. afferent at 39 bk. sks. per sec. and 14-5 cm. coil distance. I., delivery of single bk. sk. to inbib. afferent, at 12 cm. coil distance. A., debvery of inhibition in “ stimulation plateau ” ; B., delivery in “ after-discharge ” plateau ; C., repetition of reflex B with no inbib. sk. delivered. Tendon movement multiplied 45 times. Time in fifths second.

B. Semitendinosus, decerebrate preparation ; see text. The broken line gives the course pursued by the after-contraction in the control (uninhibited) reflex for reflex B.

[The absence of “ smothering ” of the stimulus-rhythm of a rate of 45 per sec. charac­

terises contractions of the ipsil. flexion type ( X 19).]

These are absent from the “ after-discharge plateau.” The exact point where stimulation plateau and after-discharge plateau merge can thus sometimes be detected by noting the place of disappearance of the slight undulations. They show that after termination of the external stimulus for a period closely similar to that of the initial latency of the reflex contraction , the contraction plateau is still “ stimulation plateau,” and that only when that interval is past does pure

“ after-discharge plateau ” begin (30).

“ After discharge is suppressed strikingly easily and competely by reflex inhibition ” (35). Thus, though myographic examination, whether isometric or isotonic, reveals no difference in tension or course between these two portions of the contraction plateau, a clear distinction between them is found by reflex inhibition. A single break-shock stimulus of given intensity, which when applied to the inhibitory afferent during the “ stimulation plateau ” evokes but a small relaxation, evokes when applied during the “ after-discharge plateau ” a large and even total relaxation of the contraction (figs. 2 and Plate 37, fig. 2).

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494 E. G. T. Liddell and Sir C. S. Sherrington.

Moreover, the inhibitory relaxation in the “ stimulation-plateau ” is followed by recovery of contraction ; the inhibitory relaxation in the “ after-discharge plateau ” is very rarely, if ever, followed by any recovery of contraction.

A single break-shock stimulus of the inhibitory afferent can, it is true, exert even in the “ stimulation-plateau ” period an inhibition causing total relaxa­

tion of the contraction (fig. 3). In order to do so, however, the break-shock lcm.

S ec. -2

Fig. 3.—Crureus and vastus medialis. Total relaxation during “ stimulation plateau ” by single bk. sk. delivered at I. 11 cm. coil distance ; E. to E / stim. of excitat. afferent bk. sks. at 40 per sec. and 17-5 cm. coil distance. Broken line gives the course of control (uninhibited) reflex. Tendon movement multiplied 45 times. Time in fifths second.

used has to be much stronger than will suffice during the “ after-discharge plateau.” Throughout the whole period of after-discharge contraction, whether of plateau-height or lower, it is characteristic of the contraction that it can be inhibited much more easily than can the contraction of other parts of the reflex.

And this holds also for the ipsilateral flexor reflex. Thus, in fig. 2 B, where the reflex contraction of the knee flexor, semitendinosus, is produced from ipsilateral afferent by a break-shock series at 40 per cent., and the inhibition from contralateral afferent by a double-shock series at 95 per second. For reflex 1 the excitation is at 16*5 cm. coil distance, the inhibitory stimulus at 14 cm. For reflex 2 the excitation is at 15-5 cm., i.e., stronger than in 1, the inhibitory stimulus at 16 cm., i.e., weaker than in 1. In 2 the inhibitory stimulus is thus not only weaker, but is pitted against a reflex due to a stronger excitation than in 1 ; and an inhibitory stimulus evokes less inhibitory relaxa­

tion against a more strongly excited reflex contraction than against a less strongly excited (30). Also in 2 the application of the inhibitory stimulus is continued less long. Yet the inhibitory relaxation in 2 is considerably greater than in 1 ; the contraction tension is reduced by some 700 gm. in 2, by some 200 gm. only in 1. The greater vulnerability to inhibition of the contraction in 2 is due to the inhibition falling in the after-discharge contraction, whereas in 1 the inhibition falls in the stimulation-period contraction.

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Recruitment and some Other Features o f Re

flex

Inhibition.

495 The motoneurones discharging in the “ after-discharge plateau ” of the reflex must presumably be the same both individually and in the aggregate as those discharging in the “ stimulation plateau.” Hence it follows that in such a case the same inhibitory stimulus finds one and the same motoneurone aggregate more difficult to arrest discharge in at one time than at another.

Hence, further, one and the same individual motoneurone offers at different times resistance of different degree to the extinguishing of its discharge by inhibition of the same given intensity.

I t is noteworthy that in the facility with which inhibition of the contraction is produced and in the relative inability of recovery after inhibitory relaxation the postural contraction of decerebrate rigidity much resembles that of the after-discharge contraction. This resemblance is one of several features which support the view that the after-discharge contraction is of the nature of a postural contraction appended to a “ movement ” contraction. Post-inhibitory

“ rebound contractions ” also in the same respects resemble “ after-discharge contractions ” and the postural contractions of decerebrate rigidity (35).

The difference which reflex inhibition reveals between the seeming parity of the plateau-contraction’s two portions, “ stimulation plateau ” (S.P.) and

“ after-discharge plateau ” (A.P.), allows certain inferences. For both portions the muscle-fibres engaged must be individually and in the aggregate the same and contracting to like extent. The inhibitory stimulus used as a test is the same for both, e.g., fig. 2, Plate 37, fig. 2. Pitted against the internal “ central ” excitation, i.e., the central “ drive,” causing the contraction, it is scarcely effective in S.P. and greatly effective in A.P. Central excitation (E.) and central inhibition (I.) are states which oppose each other quantitatively, and can in so far be treated as -f- and — amounts. Two samples of E. can be compared for quantity by comparing the extents of neutralisation of each produced by a given sample of I. The sample of I., which wholly neutralises the E. of A.P., neutralises but a small fraction of the E. of S.P. Therefore the quantity (intensity) of E. is greater in S.P. than in A.P. ; that is, the central “ drive ” present for the muscular contraction of S.P. is greater than that for the muscular contraction of A.P. Yet the muscular contraction is the same in amount and in every other way in A.P. as in S.P. The contraction output is in the two cases the same, although the central “ drive ” or pressure head is in the two cases different. This is explicable in light of the “ all-or- none ” feature of skeletal muscle-fibre reaction (Keith Lucas (23, 24), Adrian (2), Pratt (26, 27)). But the nerve-fibres innervating the muscle-fibres are likewise all-or-none reagents (Adrian (1), Adrian and Forbes (3), Kato (18) ;

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496 E. G. T. Liddell and Sir C. S. Sherrington.

and the greater central “ drive ” in S.P. is applied to the mnscle-fibres through the nerve-fibres. If greater central “ drive ” in S.P. gives greater frequency of impulses per motor fibre than in A.P., the myogram shows that such increase is unaccompanied by greater contraction of the muscle-fibre in S.P. than in A.P. Buchanan (7), Forbes (16, 15), and others have concluded that the muscle-fibre on receipt of serial nerve-impulses, e.g. at beyond a certain frequency, responds to some impulses only of the series. This goes to meet the observed want of difference between the contraction in S.P. and A.P. But the difference revealed by inhibition between the amount (intensity) of E. in S.P. and A.P. suggests the motoneurone itself, or at least that part of it (the axon-hillock) which forms the actual commencement of the motor nerve-fibre, as likewise a place of mechanism where a maximum response-reaction is obtained by application of a certain degree of incitement, application of further incitement beyond that required degree producing no further increase in the response. The excess of E. distinguishing S.P. from A.P. must therefore lie central to the axon of the motoneurone.

The reflex plateau-contraction in its “ stimulation plateau ” period is there­

fore a contraction due to a central excitation of intensity above, and probably much above, that necessary for production of the maximal response of motor units it employs, including under that designation the motor nerve-fibre as well as the muscle-fibre itself. And this is true not only for the “ stimulation- plateau ” contraction but also for the greatest portion of the “ after-discharge plateau ” also. For throughout this latter, produced as it is by central after­

action, that after-action is waning ; explicit evidence of the waning is shown (1) by the gradual decline of contraction height (tension) after the plateau itself comes to an end, a decline which would be merely the continuance of what was already in progress before the subsidence of the after-action actually reaches the threshold of the various muscle-fibres engaged ; (2) by the pro­

gressively greater effect of an inhibitory stimulus of one given strength applied at points progressively later in the course of the “ after-discharge plateau.”

And if the reflex contraction both in the “ stimulation plateau ” and in the

“after-discharge plateau” is produced by the reflex exerting an internal (central) stimulation of an intensity-grade above that necessary to evoke in the motor- units engaged their maximal response, the inference is that the same supra­

maximal relation of central excitation holds also during the ascent-contraction of the reflex, in regard to all the motor-units so soon as the point of initial excitation of each of them is passed. And if this holds in the ascent-contrac­

tion period it holds in the descent-period returning from the ascent and following

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Recruitment and some Other Features o f Reflex Inhibition. 497

the “ after-discharge plateau,” only with the converse provision that it ceases for each motor unit when the excitation in its decline reaches threshold value for that unit.

Hence the conclusion arrived at is that the reflex mode of employing the motor units is to subject them to an incitement which is “ supramaximal ” in the sense that it is in excess, and sometimes much in excess, of that which is necessary to evoke in them individually their “ maximal ” response. The loci of application of this supramaximal concentration of the excitatory agent must be confined to that fraction of the motor units which is engaged in the particular reflex excited ; were there general spread of the agent, the reflex contraction would not confine itself to fractions of the aggregate, and the reflex grading of the contraction would be lost. To picture the central dis­

tribution of the supramaximal concentration of the excitatory agent as puncti- form and its points as minute discrete foci would be in accord with histological data of neurone junctions and synapses in the grey matter.

III. Inhibitory Recruitment.

Contrasting with the course of the strong inhibitory relaxations dealt with in § I are the courses run by weaker degrees of inhibition. Thus, in fig. 4

3 sec.

Fig. 4.—Quadriceps extensor. E. to E / stim. of excitat. afferent by double shocks at 95 per sec. and 15'5 cm. coil distance ; I. to 1 / stim. of inhib. afferent by bk. sks.

at 35 per sec. and 14-5 cm. coil distance. Tendon movement multiplied 67 times.

Time in tenths second.

with faradisation of the inhibitory afferent by a break-shock series at 36 per sec., the subsidence of the active tension of the muscle, although ultimately attaining total suppression of the whole reflex contraction, requires 0-28 sec. to arrive at 50 per cent, reduction of the tension. Moreover, the duration of subsidence of the remaining half of the tension is, instead of being several

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498

times lengthier than that of the fall from plateau to halfway, only twice this latter. These tetanic inhibitory relaxations run, therefore, a course very different from that of the terminal relaxation of a mn. tetanus. Obvious further discrepancies between the two are (1) the rounded curve instead of angle with which the inhibitory fall starts from the contraction plateau, and (2) the stepped character of the greater part of the descent of the inhibitory relaxation, the steps corresponding with break shocks applied to the inhibitory afferent. An inhibitory relaxation of such character indicates that the arrest of the discharging activity of the motoneurones is in the earlier period of the relaxation only partial, although by successive increase it finally becomes

“ total,” i.e., ultimately extinguishes the discharge from all the motoneurones engaged in emitting motor impulses to the muscle.

In fig. 5, with stimuli of weaker intensity but otherwise like those of fig. 4,

kg-

E. G. T. Liddell and Sir C. S. Sherrington.

3 sec.

Fig. 5.—-As in fig. 4, but inhib. stim. at 15 cm. coil distance.

and applied to the inhibitory afferent for a like period, against a similar reflex contraction, the inhibition does not become “ total.” In this case the course of the inhibitory relaxation differs even more obviously from that of a mn.

tetanus relaxation of similar extent. The general course of the inhibitory relaxation is almost rectilinear. The earlier 50 per cent, of the decline occupies 0-54 sec., the remaining half 0-6 sec. The descent shows small steps corre­

sponding with the rhythm of the inhibitory stimulus. In fig. 6, with inhibitory stimuli weaker still but otherwise as before, the gradient of relaxation is less steep still, approximately rectilinear, and the earlier 50 per cent, of the relaxa­

tion occupies 0-52 sec. as against 0*66 sec. for the remainder.

The inhibition clearly does not in such instances as these accomplish the abolition of the motor discharge even approximately simultaneously throughout

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Recruitment and some Other Features o f Reflex Inhibition. 499

the aggregate of the motoneurones which it invades. The quelling of the motoneurone discharge, instead of occurring with approximate simultaneity

3 sec

- i ~ t t v i 1 t t t i t » i t

Fig. 6.—As in figs. 4 and 5, but inhib. stim. at 15*7 cm. coil distance.

throughout the motoneurone aggregate attacked shows under mere unaltered continuance of the inhibitory stimulus a progressive increase, which proceeds quite gradually, more slowly with weaker than with stronger stimuli.

For discussion of this progressive increase due to mere prolongation of other­

wise unaltered stimulation of the inhibitory afferent, account must be taken of the distinction which inhibition finds (v.s. § II) between “ after-discharge ” contraction and the contraction supported by contemporaneous stimulation of the afferent nerve inducing the contraction.

The term “ after-discharge ” was applied originally to post-stimulation persistence of action of a reflex centre after withdrawal of a tetanic stimulus.

“ There is no feature of the conduction of a reflex-arc which distinguishes its mechanism more universally from that of a mere nerve-fibre tract or trunk than lengthy after-discharge ” (30). After-discharge is certainly eminently obvious at end of a reflex induced by tetanic stimulation. But it is evidenced also by the reflex contraction excited by a single-shock stimulus (36), and that is so even in the spinal ipsilateral flexion-reflex, a reflex exhibiting relatively less after-discharge than do most. After-discharge is, therefore, a sequel ensuent not only at end of a tetanic series of stimuli, but also on each single stimulus comprising the tetanic series. I t takes part, therefore, in the tetanic response from quite early (20) in the course of that response. Now, the crossed extensor reflex is characterised by conspicuously ample and prolonged “ after-discharge.”

Even when evoked by a single-shock stimulus the duration of the contraction- response indicates especially prolonged after-discharge. The crossed extensor

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500 E. G. T. Liddell and Sir C. S. Sherrington.

reflex contraction may show a fairly fused contraction under a sequence of excitatory shocks as low as 3 per sec. (32, 33), and an almost smooth plateau at 7 per sec. (21), rates which give little or no fusion of contraction in the ipsilateral flexor reflex, in which reflex after-discharge is relatively slight, and none at all in the mn. tetanus. Hence, th at rates of rhythm of the exciting stimulus easily visible in the contractions of some reflexes are characteristically obliterated and smothered in the contractions of the crossed extensor-reflex obtained explanation (20) as due largely to the circumstance that the after­

discharge ensuent on each successive stimulus of the tetanic series suffices to maintain the contraction effectively in and throughout the intervals between

the successive individual stimuli ( cf. 8 and 13, in regard to action-currents).

But the after-discharge contraction as examined after cessation of the tetanic stimulation is peculiarly vulnerable to reflex inhibition (v.s. § II).

And this character of easier extinction by inhibition is present in it, not only during its later course, but also very early after cessation of the stimulation (figs. 2 and Plate 37, figs. 1 and 2). This marked vulnerability of after-discharge to inhibition raises expectation that inhibitions of weak intensity, such as those under consideration in this section, will during the reflex tetanus’ “ stimu­

lation plateau ” produce more relaxatory effect at recurrent intervals along the plateau, corresponding with the intervals between the individual stimuli composing the tetanic stimulation, since at those intervals the plateau con­

traction is • being maintained to a larger extent by after-discharge. The proviso “ to a larger extent ” is required, because we must suppose some asynchronism of response of the motoneurones as a group to the successive rhythmic repetitions of the external stimulus. This inhibitory effect would be revealed as a rhythmic notching of the contraction height (tension), the rhythm having a rate corresponding with that of the stimulus employed for exciting the reflex contraction. As was pointed out by us in a previous paper (19), rhythmic notching of this kind does, in fact, occur during the relaxations

produced by partial inhibition (cf. also Beritoff (5)); fig. 7 here instances it also.

The partial inhibition dissects out, so to say, the ribs of the reflex contraction, so that against the ill-supported remainder they become prominent like ribs of an ill-nourished child. Inhibition exerted during the “ stimulation plateau ” does, therefore, find a more vulnerable part of the discharge occurring recur­

rently at a rate corresponding with that of the recurrence of the stimuli com­

posing the tetanic stimulation of the excitatory afferent.

Proceeding now to consideration of the progressive increase of inhibitory relaxation obtained by merely prolonging an unaltered stimulation of the

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Recruitment and some Other Features o f Reflex Inhibition. 501

inhibitory afferent :—In the powerful inhibitions of § I the rapid, smooth course of the inhibitory relaxation sweeps away! all contraction without

1-2 Sec.

Fro. 7.— Crureus and vastus medialis. Deafferented some months previously. E. to E.' stim. of excitat. afferent by bk. sks. at 38 per sec. and 14 • 5 cm. coil distance. I. delivery

of single bk. sk. to inkib. afferent, at 15-5 cm. coil distance in A., at 12-5 cm. coil distance in B. Broken line gives the course of control (uninhibited) reflex. Tendon movement multiplied 50 times. Time in fifths second.

giving any sign of distinction between more or less vulnerable elements in it. Inhibition can therefore extinguish, and speedily, discharge of all kind in the motoneurones. And with the inhibitions, instanced in the present section, produced by weaker intensities of inhibitory stimulus, it is clear that where under prolongation of the stimulation they proceed to ultimate “ total ” relaxation of contraction, they then accomplish total extinction of discharge throughout the active motoneurone aggregate, and have done more than suppress merely the more vulnerable portions only. Moreover, such periodic inhibitory abbreviation of “ after-discharge,” as was considered above, even if it went so far as to reduce the whole motoneurone group’s discharge to one single motor-impulse volley in response to each arrival of the external stimulus, would not, even though the arrivals were at a rate as low as 50 per second, reduce the tension of the contraction plateau by any large fraction. Still less would be the reduction where, as in the instances given by figs. 4, 5 and 6, the stimuli of the excitatory afferent were at the rate of 190 per second.

It has been shown (9) that halving an excitation frequency of 100 a second does not reduce the plateau tension by one-tenth, even in a reflex contraction (ipsilateral-flexion reflex) with relatively little after-discharge ; and that halving an excitation frequency of 160 per second reduces the reflex contraction tension hardly perceptibly at all.

Further, if the progressively cumulative action of the inhibition lay wholly in curtailing more and more the mere after-duration of the motor-impulse train ensuent on each stimulus imparted to the excitatory afferent the

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502 E. G. T. Liddell and Sir C. S. Sherrington.

inhibitory relaxation curve should as it proceeded show deeper and deeper notches in the contraction, the notches revealing more and more clearly the rate of stimulation of excitatory afferent. But this progressive deepening of the inhibitory notching is, in fact, not evidenced by the myograms, though in many instances rates of excitatory stimuli were used which the myograph was well competent to show. Besides inhibitory curtailment of the after-discharge response to each stimulus of the excitatory series there must, in order to account fully for the extent and course of the inhibition observed, be some further and progressive factor in the inhibitory effect. This seems cleatly a progressive total extinction of discharge in more and more of the motoneurones as the inhibition proceeds.

Conformably with this the inhibitory relaxation shows commonly a stepped character, from temporary accelerations of descent recurring at a rate corre­

sponding with that of the accessions of the inhibitory stimuli (33) or with simple sub-multiples of th at (figs. 4, 5, 8). Each of these recurrent increases of relaxation-gradient would be explicable by extinction of discharge in an additional number of motoneurones as a fresh inhibitory stimulus added its effect to that of the previous ones. Again, the relaxation opens not infrequently with an initial plunge of a steepness much greater than that of the later course of the decline, and very unlike that of a commencement by nibbling away merely a rhythmically recurrent more vulnerable phase of the motoneurone group’s discharge. The initial plunge (fig. 8) recalls that of the relaxations dealt with in § I, where, as there shown, complete extinction of all discharge occurs quickly in a certain number of the motoneurones. The difference between the two would be that with the weaker inhibitory stimulus the extinction is limited to a smaller number of the discharging motoneurones.

The relaxation curves produced by these weaker inhibitions seem compounded of the relaxation of muscle fibres from which motoneurone discharge has been wholly withdrawn and of muscle fibres from which motoneurone discharge has been partially withdrawn, owing to intervals of remission corresponding in rate with the successive intervals between the excitatory stimuli and with sub­

multiples of that rate.

The inhibition in its ingravescence under mere continuance of the unaltered stimulation of the inhibitory afferent clearly proceeds to extinction of more and more of the discharging motoneurones, and can, under suitable conditions, gradually accomplish total extinction of them all. Such central spreading of the reflex inhibitory effect so as to arrest discharge in more and more moto­

neurones under unaltered prolongation of stimulation of the afferent nerve

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resembles, though of converse effect, the central “ recruitment ” (20) which brings more and more motoneurones into play under prolongation of stimulation

Recruitment and some Other Features o f Reflex Inhibition. 503

i cm.

3 sec.

Fig. 8.—Quadriceps extensor. E. to E.' stim. of excitat. afferent by double sks. at 95 per sec. at 17 • 5 cm. coil distance. I. to X.' stim. of inhib. afferent by bk. sks. at 35 per sec.

and 19-2 cm. coil distance. Tendon movement multiplied 116 times. Time in tenths second.

of the excitatory afferent in the crossed extensor reflex. In the case of the inhibitory reflex it may therefore be termed “ inhibitory recruitment.”

The stronger inhibitory stimulus reduces somewhat the duration of the initial latent interval of the inhibitory reflex (30). In other words, with the stronger stimulus the time taken for arrest of those motoneurones which are earliest arrested is somewhat less than with a weaker stimulus. A more prominent difference between the effect of the stronger stimulus and that of the weaker lies in the much greater lateness of arrest by the weaker stimulus than by the stronger of those motoneurones which are arrested latest. The time occupied by the progress of the inhibitory recruitment is significant not mainly of spatial tra v e l; the time differences observable, e.g., that between 0*01 second and 1*5 seconds, are too great to be explicable as alterations of speed of conduction in covering one and the same distance according as the stimulus is weaker or stronger. The differences signify mainly differences of time spent in overcoming opposition to the process of arrest of discharge from the individual motoneurone, and the time taken is less or more according as the inhibitory stimulus is strong or weak. The inhibitory recruitment when pushed, against similar excitatory reflexes, to total extinction of discharge

2 p

VOL. x c v ii.— B.

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504 E. G. T. Liddell and Sir C. S. Sherrington.

from the motoneurone aggregate arrives at that totality much earlier under strong inhibitory stimuli than under weak. Therefore, not only do different individual motoneurones engaged in one and the same excitor reflex require, under the same inhibitory stimulus, very different periods for extinction of their discharge, but, further, for one and the same motoneurone the time required to extinguish its discharging activity under similar excitor-reflexes differs greatly according as, other conditions equal, the stimuli of the inhibitory afferent are weak or strong, the time being less with the latter. Differences in this respect are observable of the order of a hundred-fold.

So also in excitatory recruitment the time occupied in the spread of activation throughout the motoneurone aggregate ultimately activated is spent mainly not in simple travel over spatial distances, but in the process of inducing discharging activity in the less readily activated motoneurones. In the ipsilateral flexor reflex the spread of activation is practically simultaneous (20) to all the motoneurones which the particular intensity of stimulus of the excitatory afferent can activate. Hence in this latter type of reflex recruit­

ment is hardly observable.

IV. Comparison of Inhibitory Recruitment with Excitatory Recruitment.

1. The speed of progress of inhibitory recruitment has a wide range. With tetanic stimuli, of the same frequency, applied to the inhibitory afferent, it is greater when the stimuli are stronger than when they are weaker (figs.

4, 5, 6). With strong stimuli the extinction of the efflux of motor impulses from the motoneurones may be, e.g., fig. 1 and Plate 37, fig. 1, practically simultaneous throughout the whole group of motoneurones engaged in the reflex contraction. The influence of intensity of the stimuli is well seen when the reflex inhibition is exerted against the postural contraction of decerebrate rigidity. In an instance figured in an earlier paper (32), the postural contraction undergoes total abolition under stimuli of the inhibitory afferent of 100 Berne-units intensity at a rate of four a second, in 2*25 seconds, namely, with nine successive stim uli; with stimuli of 20 Berne-units intensity at the same rate 86 successive stimuli were required, the time taken being 21-5 seconds.

In this respect the inhibitory recruitment resembles excitatory recruitment, increase of stimulus intensity increasing the speed of progress of that recruit­

ment also (20).

2. The speed of progress of the recruitment is influenced by rate of frequence of repetition of the stimuli. Instances illustrating this were figured in previous papers (32, 33), where the rate of frequence of the inhibitory stimuli was

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Recruitment and some Other Features Reflex Inhibition. 505

changed during progress of tetanic stimulation of the inhibitory afferent against the background of postural contraction in decerebrate rigidity ; with each increase of frequency there was increase of speed of relaxation. Again in this respect the inhibitory recruitment resembles excitatory recruitment (20).

3. As to the amplitude to which, and length of period during which, inhibitory recruitment may still continue to progress under prolongation of a given in­

hibitory stimulation, the recruitment may reach its term without accomplishing arrest of the whole aggregate of the motoneurones discharging in the excitatory reflex against which the inhibition is pitted (fig. 9). In that case the further

lsec.

F1 0. 9.—Rectus femorisand part of crureus.Stim. of excitat. afferent by magneto at 46 per sec. and 10 cm. coil distance. This stimulation begins at E. and is continued

beyond the time limit of the figure. Stim. of inhib. afferent by double sks. of 16-5 cm.

at 95 per sec. This stim. begins at I. and is continued beyond the time limit of the figure. Tendon movement multiplied 50 times. Time in tenths second.

prolongation of the inhibitory stimulation merely maintains, without increasing further, the degree of inhibitory relaxation already reached. In this again the inhibitory recruitment resembles excitatory recruitment (20).

4. Immediately after cessation of the external stimulus, allowance being made of a time-interval equal in length to th at of the initial latent period of the inhibitory reflex, the progress of the inhibitory recruitment is found (figs. 5, 0, 8,10) to cease. In this feature also inhibitor}" recruitment resembles excitatory recruitment (20). Both forms of recruitment exhibit a fair degree of freedom from “ momentum " (30). In both the excitatory and the inhibitory reflex the

“momentum ” which impairs deadbeatness is given not by the recruitment process but by the after-discharge process.

In judging this feature of the inhibitory recruitment avoidance has to be made of the subsidence of the small ipsilateral contraction which not uncommonly complicates the effect of strong stimulation of the ipsilateral afferent (22). And conversely in judging the termination of excitatory recruitment avoidance has to be made of the rebound contraction which not infrequently complicates the effect of withdrawal of stimulation of the contralateral afferent (31).

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506 E. G. T. Liddell and Sir C. S. Sherrington.

5. With moderate intensity of faradic stimulation of the inhibitory afferent applied during the stimulation-plateau contraction for periods shorter than would allow full relaxation of the muscle-fibres whose motoneurone impulse- stream is cut off by the inhibition, the progress of the inhibitory recruitment is still confined to a duration corresponding with that of the external stimulus (fig. 10). The relaxation-descent continues for that period only ; on it abruptly

I J ,

is e c .

Fig. 10.— Rectus femorisand part of crureus. E. to E / stim. of excitat. afferent by magneto at 46 per sec. and 11 cm. coil distance. I. to 1 .'stim. of inbib. afferent by double sks.

at 95 per sec. and 16-5 cm. coil distance. Duration of tetanic stim. of inbib. afferent longer in B. than in A., in C. than in B. Tendon movement multiplied 50 tinjes.

Time in tenths second.

succeeds for a brief period a horizontal plateau in which the contraction remains at the level reached by it at end of the relaxation descent. This relaxation- plateau lasts about 0-05 second, i.e., is longer than the interval between the successive stimuli employed for the excitatory afferent, e.g., in fig. 10.

In regard to this plateau the following considerations are relevant. When during a mn. tetanus a brief gap in the series of break shocks composing the tetanising series is made, e.g., by short-circuiting one member of the series, e.g.,fig. 5 of (19), during the tetanus plateau, the result is a slight and abrupt drop in the contraction-height (tension) and then at end of that descent the

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Recruitment and some Other Features o f Reflex Inhibition. 507

reascent of the contraction sets in at once. No horizontal plateau occurs between the end of the descent and the commencement of the ascent. The intervention of a horizontal plateau between the end of the descent and the beginning of the reascent distinguishes the inhibitory result from that in the mn. tetanus. The inhibitory effect comprises more, therefore, than the period of actual descent; the effect changes (and with brief tetanic inhibitory stimuli the change corresponds with the end of th at stimulus) from the actual undoing of existent contraction to the preclusion of further increase of contraction.

Light on this effect of inhibition upon the reascent of the reflex contraction can be obtained by study of the result of a single-shock delivered to the inhibitory afferent during the initial ascent of the reflex contraction under the early course of the tetanic stimulation of the excitatory afferent. Delivered during that period of the reflex contraction a single shock of appropriate strength causes no actual fall of the contraction but merely changes the up-gradient of con­

traction to a short horizontal plateau followed by reascent (fig. 11); although

•2 -4 sec. 2 -4 -6sec

'•i-Sec.

E 1 *V

Fig. 11.— Vastocrureus. Effect of single-shock inhibitory stimulus delivered during the ascent period of the reflex contraction. 1 and 2 from same experiment : E. beginning of excitat. stimulation, 38 bk. sks. per sec. at 14-5 cm. coil distance. I., single bk.

sk. to inhibit, afferent, in 1 at 11 cm., in 2 at 11-5 cm. Time above in fifths second.

3: E. beginning of excit. stimulus 38 bk. sks. per sec. at 14’5 cm. coil distance.

I., inhibit, sk. at 12 cm. to afferent. 4 : E., beginning of excit. stimulation, 38 bk.

sks. per sec. at 14-5 cm. c o il; I., single bk. sk. to inhib. afferent at 12 cm. coil. Time below in fifths second. Tendon movement multiplied 68 times.

a stronger shock will undo some of the contraction already attained, and then for a short period, while not deepening the relaxation further, arrest further increase of the existent contraction.

The horizontal course taken by the checked ascent-curve might mean that the inhibition kept putting out of action throughout the period of that horizontal course a number of discharging motoneurones at exactly the rate and measure sufficient to counterpoise the muscular effect which the excitatory

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508

recruitment of additional motoneurones by the reflex excitation keeps, during that period, producing. A simpler explanation is, however, that the inhibition evoked by the weak break shock in the ascent-period of the reflex contraction, although unable to arrest discharge in the motoneurones already activated, is able to prevent the excitatory process from recruiting additional ones. On this latter view the reaction should be imitable as regards its myogram by suppressing a single or a couple of the stimuli of the excitatory tetanising series during the production by th at series of the ascent contraction of the reflex. I t is, in fact, found to be exactly thus imitable. Such a lapse in the exciting series causes ( (19), e.g., fig. 9), in some cases even when the lapse includes four successive stimuli at 35 per sec., no fall in the contraction already attained, but merely an arrest of further increase of it for the time being. The inhibition thus gives a “ deviation of the ascent,” a postponement of further climbing, indicating a temporary check in the excitatory recruit­

ment.

The inhibitory “ deviation of the contraction ascent ” (fig. 10) explains itself, therefore, by temporary inhibitory arrest of the excitatory recruitment.

When the single-shock inhibitory stimulus is weaker still the brief plateau deviating the ascent may not be horizontal but merely a less steep stretch of up-gradient (fig. 11, 2) interposed between a precurrent and an ensuing steeper course of the ascent. Hence the inhibition can be so graded as to merely diminish without wholly arresting the excitatory recruitment. The inhibition, therefore, evidences itself more readily by check of the recruitment of the excitatory process than by suppression of discharge of motoneurones brought to discharge already by the excitation. A stronger inhibitory stimulus is required, during the “ ascent contraction,” to produce an actual relaxatory descent than to cause a simple horizontal deviation postponing further progress of the ascent, or, still more, a mere lessened rate of ascent. Hence the short plateau ensuent on the actual descent in such cases as fig. 10 reveals an “ after­

action period ” of the inhibitory effect, the inhibition in the after-action period being weaker than the inhibition of the preceding period corresponding with actual delivery of the inhibitory stimulation. If the inhibitory stimulation be stronger, the inhibitory after-action documents itself for longer by producing, subsequently to the horizontal plateau period, a temporary reduction of the steepness of the ascent when this last recommences.

The myograms of these checks to excitatory recruitment effected by weak single-shock stimuli of the inhibitory afferent instance very brief-lasting mechanical effects of reflex inhibition (fig. 11). They give a mechanical effect

E. G. T. Liddell and Sir C. S. Sherrington.

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Recruitment and some Other

Featu Reflex Inhibition. 509 sometimes of only 0-04 sec. duration. This brevity it would doubtless be not

difficult to abbreviate further still by using fast frequencies of the excitatory tetanic stimulation and grading Avith minuteness the strengths of single-shock stimulus employed for the inhibition.

The horizontal plateau of inhibitorily reduced contraction following the inhibitory descents in fig. 10 is therefore evidence of after-action of the inhibitory stimulus, and of a period during which the intensity of the central inhibition process is such as to reduce the intensity of the central excitatory process to maintaining the motoneurone activation it has already affected, while the two recruitments, excitatory and inhibitory, preclude each other mutually.

6. A strong single-shock stimulus of the inhibitory afferent gives often relaxation of contraction evidencing recruitment, and this even when the inhibition is exerted in the stimulation-plateau contraction. A similar result obtains not rarely when the single-shock stimulus is of quite moderate strength (fig. 7). This affords evidence, and there is also much other, that the central process of inhibition evoked by a single stimulus even so short-lasting as a break shock applied to the afferent nerve persists for a time, as “ after-action.”

The result parallels the after-discharge feature of single-shock excitatory reflexes (36).

IV. Some Features of Inhibition of Reflex After-Discharge.

The relaxation produced by a single-shock stimulus applied to the inhibitory afferent during the contraction's “ stimulation-plateau ” period presents commonly an initial fall of the contraction-tension to a level which leaves some residual contraction, less or more according as the shock is stronger or weaker.

This initial fall is abruptly followed by a horizontal plateau of reduced tension at the level reached by the end of the fall (figs. 7, B, 12, A, 14, B). When this break-shock inhibition is exerted not too far prior to end of the stimulation- plateau period of the reflex, the loss of contraction-tension due to the inhibition may not be recovered from, and the plateau of reduced tension runs smoothly on, into and through the after-discharge period of the reflex. This inhibition plateau thus presents two portions : an earlier concurrent with stimulation of the excitatory afferent; a later which lies wholly in the after-discharge period of the reflex contraction. The former portion of course resembles that plateau of inhibition dealt with in § III, 4, and offers no further difficulty.

The latter portion explains itself simply as due to the absence of the after­

discharge of that fraction of the motoneurone aggregate whose discharge was

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510 E. G. T. Liddell and Sir C. S. Sherrington.

arrested in the stimulation-plateau period. This absence accords with the abundantly testified fact of inability of after-discharge when once it has lapsed to recommence spontaneously ; such recommencement is of extremely rare occurrence.

But the records (fig. 12, A, Plate 37, fig. 3) show further that the after-discharge A

icm.

Sec. *2

1*5 sec.

Fig. 12.—A. Vastocrureus. E. to E / stim. of excitat. afferent by bk. sks. at 45 per sec.

and 15 cm. coil distance. I. delivery of single bk. sk. to inhib. afferent at 16 cm. coil distance. Broken line gives the coarse of the control (uninhibited) reflex. Tendon movement multiplied 44 times. Time in fifths second-

Fig. 12—B. Vastocrureus. Inhibitory plateau in reflex contraction followed by “ eclipse ” L. E. to E / excitat. stimulation, bk. sks. at 38 per sec. at 16*5 cm. coil for reflex 1 ; at 15-5 cm. for reflex 2 ; I. to 1 / stimulation of inhib. afferent by double sks. at 95 per sec. and 18 • 5 cm. coil. The course of the control (uninhibited) reflex given by line 1. L., eclipse of after-discharge. Time in tenths second. Tendon move­

ment multiplied 52 times.

which the inhibition leaves intact is that part which, as shown by the control (uninhibited) reflex, lasts the longer. In other words the inhibition, affecting as it does a certain proportion only of the motoneurones engaged in the reflex contraction, extinguishes the discharge in those only of them whose after-discharge would, were the reflex contraction allowed to pursue its course unmodified by inhibition, endure least long. That is, the inhibition quells more easily those motoneurones in which, quite apart from inhibition, discharging activity will, after withdrawal of the excitatory stimulus, lapse sooner than in the rest. Therefore, some same circumstance, which makes after-discharge in a motoneurone long-enduring, also fortifies a motoneurone against inhibition.

And in the group of discharging motoneurones some individuals of the group possess this attribute more than do others. Moreover, those more liberally endowed writh it are so both in the stimulation period and in the after-discharge

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