Sherrington and Sowton(1) in particular have shown that there is a

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1 THE REFRACTORY PHASE IN A REFLEX ARC. BY E. D. ADRIAN AND J. M. D. OLMSTED. (From the Physiological Laboratory, Cambridge.) IN the muscle-nerve preparation one of the phenomena which can be most accurately investigated is the refractory state following the passage of an impulse and the process of recovery therefrom. In the reflex arc many indications of a similar refractory state have been recognised; Sherrington and Sowton(1) in particular have shown that there is a minimal "interval for muscular summation" in the reflex arc just as there is in a muscle nerve preparation, since two stimuli applied to the afferent nerve will not give a summated contraction unless there is a definite time interval between them. It appeared to us that a general investigation of this phenomenon might be of value, and as a first step we were anxious to see whether the blocking of the reflex arc by anaesthetics would be accompanied by any increase in the interval for muscular summation. In view of what happens in a frog's muscle nerve preparation a large increase might have been expected. In the sciatic gastrocnemius preparation the interval rises from about 003 sec. to *02 sec. when a narcotic such as alcohol is applied to a portion of the nerve between the electrodes and the muscle (2). This increase is not due to any slowing of the recovery process; with alcohol at any rate Lucas(3) has shown that the rate of recovery is unaltered. It depends solely on the fact that recovery is a gradual affair and that in the early stages of recovery the nerve is capable of transmitting only a very small impulse. Under ordinary conditions such an impulse can reach the muscle and give a summated contraction, but during the narcosis the affected region will conduct with a decrement and all impulses below a certain size will be extinguished. The interval for muscular summation must be increased, therefore, so that the second impulse may be large enough to pass the block. Eventually the block will become so great that only impulses of full size will be able to pass through and the interval for muscular summation will then be about *02 sec., since at this time the relative refractory phase is over and the second impulse is equal in size to the first. Any further increase in the narcosis will cause the first impulse to fail as well

2 REa"FLEX REFRACTORY PHASE as the second, and therefore the interval for summation will never rise above *02 sec. unless the anaesthetic produces an actual slowing in the rate of recovery. In the motor nerve of a mammal the recovery process takes place at body temperature more rapidly than in the frog's nerves, but the interval for muscular summation is increased in the same way by local narcosis. This may be seen from Fig. 1 which records the effect in the muscle nerve preparation of a spinal cat. The motor nerve (peroneal) was cut through high up in the thigh and shielded stimulating electrodes were placed on it. Between the electrodes and the muscle (tibialis anticus) SOO o~~~~~~~~~~~~~ Q ~~~~~~~~~~z.001i 5 10 Duration of Narcosis (minutes) Fig. 1. Peroneal nerve and tibialis anticus muscle of spinal cat. Effect 9f local narcosis on the least interval for muscular summation. the nerve passed through a vulcanite chamber 11 mm. in diameter in which 7 p.c. alcohol solution could be placed. The interval for muscular summation was recorded in the usual way on a stationary drum and its gradual increase is shown in the figure. It will be seen that just before conduction fails the interval is 0095 sec. At this stage the first impulse must be nearly extinguished in the narcotised area, so that the second impulse must be nearly as large as the first if it too is to escape extinction. We should therefore expect that the nerve takes just over 0095 sec. to return to its normal resting state after the passage of an impulse. This estimate agrees very well with the "recovery curve" which gives the rate of recovery of excitability in the cat's peroneal after the passage of

3 428 E. D. ADRIAN AND J. M. D. OLMSTED. 4).( 100 4N_ 50 an impulse. Fig. 2 is a typical recovery curve from a similar preparation and it shows that the excitability has returned to 90 p.c. of its normal value in *0085 sec. Thus the two figures taken together show the effect of local narcosis and its explanation in terms of the gradual return of function after the passage of an impulse. The same reasoning should apply to the passage of two impulses through a reflex arc. If a small second impulse travelling in incompletely recovered tissue is able to pass through the arc under normal conditions, the effect of an anesthetic applied to the central part of the arc should be to prevent such an impulse from passing before it would one of full size; the interval for muscular summation should in Interval between Stimuli (seconds) Fig. 2. Recovery of excitability in peroneal nerve of spinal cat after the passage of an impulse. crease therefore and the final value just before conduction fails should be such that the second impulse when it enters the decremental region is travelling in tissue which has had time to recover completely from the passage of the first. In the flexion reflex arc of the spinal cat Sherrington and Sowton found that if two stimuli were applied to the afferent side of the arc the least interval for muscular summation was about *001 sec. This is only slightly greater than the interval required when the stimuli were applied to the motor nerve and, unless the sensory nerve differs considerably from the motor, it is not at all likely that the sensory nerve can have recovered completely in so short a time. Thus we might well expect to find a considerable increase in the interval for reflex muscular summation when the central nervous system is anaesthetised and the final value should give us the time of recovery of the central part of the arc. The interval for muscular summation in the reflex arc. The point was

4 REFLEX REFRACTORY PHASE tested in five experiments on spinal cats. The animals were decapitated under deep anoesthesia; the ansesthetic was then discontinued and the preparation ventilated artificially. The nerves to the hamstrings and the anterior crural nerve were cut and the popliteal branch of the sciatic was separated from the peroneal and divided at the level of the knee joint. Shielded stimulating electrodes were applied to the central end of the popliteal and the reflex contraction of the tibialis anticus muscle was recorded on a stationary drum. The stimuli were break shocks from two induction coils without iron cores. The interval between the two stimuli was controlled by connecting their primary circuits with the two keys of a Lucas pendulum and their strength was adjusted by changing the resistance in the primary circuit. Both coils were connected to the same pair of electrodes. After the experiments on the reflex arc the peroneal division of the sciatic was cut through high up in the thigh and the stimulating electrodes were applied to it in order to determine the interval for summation in the efferent part of the arc. Aks Sherrington and Sowton have pointed out, it is more difficult to obtain an exact value for the interval when the stimuli are on the afferent side of the arc, since the response is more variable than the myoneural twitch and. more dependent on the strength of the stimulus. We found, however, that a fairly constant summation interval could be obtained by using both stimuli about 5-6 times the threshold strength, and this interval was not appreciably reduced by increasing the strength of the second stimulus. The values obtained in eight preparations (including the five on which the anaesthetic effect was tested) are given in Table I: they vary from *0012 sec. to *0024 sec. The intervals obtained by stimulating the motor nerve are usually shorter though the difference may be very slight. The values we have obtained for both reflex and motor nerve stimulation are appreciably longer than those of Sherrington and Sowton, who give *001 sec. for the reflex and *0007 sec. for the motor nerve. Possibly the limb was at a lower temperature in our experiments though the. usual precautions were taken to prevent cooling. The second column gives the least interval for muscular summation during the period of anaesthesia just before the reflex response became too small to give any visible record on the drum. The anaesthetic was a chloroform and ether mixture the vapour of which was allowed to enter the artificial respiration system. The percentage was adjusted so that the reflex response to strong stimuli disappeared in about ten minutes; at this stage the anaesthetic effect was confined to the central nervous system, for in a control experiment the threshold of PH. LVI. 28

5 430 E. D. ADRIAN AND J. M. D. OLMSTED the motor nerve fibres in the other leg remained constant throughout the process. TABLE I. Least interval for muscular summation. Stimuli applied to sensory nerve 11 I Exp. Before anaesthetic 1. sec. * (a) (b). * * * * * * * * During anoesthetic just before failure sec. * * * * * * * Stimuli to motor nerve sec. * * * * * * * * * * * *oorm - Fig. 3. Reflex contractions of tibialis anticus of spinal cat in response to stimulation of popliteal nerve. B. During ansesthesia. A. Before anmesthesia. Contractions (from right to left) due to Coil 1 alone Coil 1 alone Coil 2 alone Coil 2 alone Coils 1 and 2 at *0016 sec. Coils 1 and 2 at *0016 sec.,, 0024,,,,, *0024,,, 0020,,,, 0020, Coil 2 alone Summation at 0024 sec., not at *0020. Summation at *0024 sec., not at *0020. The table shows clearly enough that there is no appreciable change in the interval for muscular summation when conduction in the arc is impaired by anesthetics. The absence of effect may be seen in Fig. 3

6 REFLEX REFRACTORY PHASE. 431 which gives the contraction heights in Exp. 2 to single and double stimuli before the anaesthetic was started and just before the arc ceased to respond. This result can only mean that the second of two impulses set up by a stimulus which occurs only about *002 sec. after the first is nevertheless as large as the first impulse (or as capable of withstanding a decrement) at the moment when it arrives at the decremental region; for there is no stage of ansesthesia in which the first impulse can pass through and the second cannot. In other words, the second impulse must reach the affected area at a time when the refractory state is over and normal conduction has returned'. It is very unlikely that the sensory nerve itself can have recovered completely in so short a time. Forbes and Adrian(4) working with the cat's internal saphenous nerve found an absolute refractory period, judged by the absence of a second electric response, of -003 sec. at a temperature of 200 C. At 350 C. this would be about 0007 sec. and if the absolute refractory period lasts as long as this the relative refractory period will not be over in as short a time as *002 sec. Yet the impulse set up by a stimulus *002 sec. after the first seems to be travelling in tissue which has recovered completely when it enters on the decremental region in the central nervous system. The simplest explanation of this would be to suppose that the synapse, or that part of the reflex arc which is affected by the anaesthetic, has an extremely short recovery time. There is, however, another possibility, namely that the second impulse is somehow delayed so that it does not arrive at the decremental area until considerably later than *002 sec. after the first. Keith Luca s(5) and Samo j lof f (6) showed that in the nerve and the cardiac and skeletal muscle of the frog the response to the second of two stimuli may be delayed relatively to the first if the interval between the stimuli is short enough, and Lewis(7) has shown that the same thing occurs in mammalian cardiac muscle. In the latter the delay seems to be due to a conduction of the second impulse by a more devious path than the first. It is doubtful whether this explanation will apply to other tissues than the heart, but in any case the phenomenon appears to be general to most conducting tissues and we might well expect to find it in the reflex arc. Until it is known what interval separates the arrival of the two impulses at the synaptic region it is clearly impossible to say how long this region takes for full recovery. We have therefore investigated the possibility 1 In one experiment we tested the effect of fatigue on the interval for muscular summation, using the internal saphenous nerve as afferent. As in the experiments with ansesthetics, the interval remained unaltered although the response had nearly disappeared. 28-2

7 432 B. D. ADRIAN AND J. M. D. OLMSTED. of a delayed arrival of the second response when the two stimuli are separated by a very short time interval. The maximumfrequency of response in the reflex arc. Any delay in the conduction of an impulse which travels in the wake of another will have a noticeable effect when we determine the response of the arc to a series of stimuli instead of to two only. Suppose, for instance, that two impulses can just be set up in the sensory nerve at an interval of *002 sec. apart and that, owing to the delay of the second impulse they are *004 sec. apart when they reach the central part of the arc. Probably, then, we cannot make impulses arrive at the central part of the arc at intervals of less than 004 sec., and therefore we cannot produce a series of reflex responses less than -004 sec. apart however rapidly we may stimulate the afferent nerve. The maximum frequency of response is therefore 250 a second although the least interval for muscular summation is only of a second. It is true that many other causes besides delay might produce such a discrepancy; for instance, the recovery process might take place more slowly during the repeated stimulation; but if the discrepancy is not found we may safely conclude that the delay is negligible. In the frog's muscle-nerve preparation, where a delay of the second impulse is known to occur, the maximum frequency of response is considerably slower than would be expected from the published figures for the least interval for muscular summation. This has been pointed out by Hoffmann(s) and byberitoff(9); Hoffmannattributesitto thedelay and Beritoff considers this a possible cause, though not the most important. In the case of the reflex arc, however, extensive investigations have been made by Beritoff, Foa (10) and others on the maximum frequency of reflex response in the frog's muscles, but their results do not help us since there are no measurements of the least interval for reflex muscular summation in the frog with which they may be compared. We have therefore determined the maximum frequency of response in the cat's tibialis anticus muscle to reflex and motor nerve stimulation and we have compared this with the least interval for muscular summation in the same animal. For repeated stimulation with different frequencies we have used break shocks from an induction coil connected with a rotating contact breaker of the drum and segment type. This made and broke the primary circuit and short-circuited the make shocks sixteen times in each revolution. The shaft of the contact breaker was coupled direct or through a reduction gear to an electric motor; the speed was controlled by a

8 REFLEX REFRACTORY PHASE rheostat and a friction break and could be read at any moment by a speedometer of the magnetic type. String galvanometer records of the currents produced at different speeds showed that the apparatus would deliver a regular series of stimuli at any frequency from 16 to about 500 a second and that the speedometer gave a value for the frequency which was accurate to about 5 p.c. At higher speeds than 500 a second the galvanometer excursions often showed periodic variations in height though none of the stimuli were actually missed at the maximum frequency of which the machine was capable (800 a second). Erlanger and Garrey(11) have pointed out the danger of an interference of make and break induction effects with very rapid series of stimuli. To some extent the use of a coreless induction coil will minimise this difficulty since the duration of the make shocks will be very short, but the difficulty does not really arise in the present experiments as there was no need to keep the strength of the stimuli constant provided that all were strong enough to excite. The frequency of response in the muscle was determined by photographic records of the action current made with the string galvanometer on cinematograph film. Arrangements were made for connecting the galvanometer through appropriate shunts with the stimulating circuit so that a record of the stimuli could be taken immediately after the record of the electric response without altering the string tension or the speed of the film. This made it possible to check the speedometer readings and to see at once whether the frequency of response was equal to or less than the frequency of stimulation. The speed of the film was recorded by a "phonic wheel" time marker which produced short lines *02 sec. apart at the edge of the film where they would not interfere with the action current records. Five experiments were made on these lines. The spinal preparation was made as before and non-polarisable electrodes were attached to the tibialis anticus for connection with the string galvanometer. The stimulating electrodes were attached first to the afferent nerve (the popliteal branch of the sciatic) and the least interval for muscular summation was determined with two stimuli delivered by a Lucas pendulum. The electrodes were then connected with the rotating contact-breaker and a series of stimuli were delivered with frequencies ranging from 50 to 500 a second. The strength of stimulus was varied from about 2 to 20 times the threshold strength for a single shock. Several records were usually made with frequencies in the neighbourhood of 160 a second as this appeared to be near the critical value for the reflex arc, and these fre-

9 434 E. D. ADRIAN AND J. M. D. OLMSTED. quencies were tested with several strengths of stimulus. In four out of the five experiments after the measurements on the reflex arc the efferent nerve (peroneal) was cut through high up in the thigh and the stimulating electrodes were transferred to it; the least interval for muscular summation was determined again and the stimulation with different frequencies was repeated. Typical records of the response to various frequencies are shown in Fig. 4. At the slower rates the frequencies of stimulus and response Reflex e ex ~Motor ~Nerve. A F 240 A'r1(w t 91 D G 320 B 40 E 320 H 480.Fig. 4. Tracings from photographs of electric response of tibialis anticus stimulated by repeated shocks to sensory or motor nerve (Exp. 5). A-E. Reflex. A. 19 stim. per sec. 3-5 times threshold strength. Single response to each stimulus. A'. 19,,,, 68 times threshold strength. Multiple responses. B. 140,,,, Responses at same frequency. C. 170,,,, Responses at same frequency. D. 240,,,, Responses irregular, about 160 per sec. E. 320,,,, Responses irregular, about 140 per sec. F-H. Motor nerve stimullation. F. 240 stim. per sec. Responses at same frequency. G. 320,,,, Responses at same frequency. H. 480,,,, Responses irregular, about 220 per sec. Time marker gives *02 sec.

10 REFLEX REFRACTORY PHASE. 435 agree, each stimulus giving rise to one response and one only. In this our results differ from those of B erit off who found that in the frog each stimulus to the sensory nerve would give rise to several responses in the muscle unless the frequency of stimulation exceeded 100 a second. With the spinal cat we have found that each stimulus produces multiple responses when a very strong current is used but not otherwise. This agrees with the work of Forbes and Adrian(12) on the response to single break shocks in the flexion reflex arc of the spinal cat. With frequencies above 100 a second the height of the responses varies irregularly but the rhythm is still followed up to about 140 a second for reflex and 320 for motor nerve stimulation. Rhythms of 200 a second for the reflex and 400 for the motor nerve are sometimes followed for a few hundredths of a second at the beginning of a series but such rates are never maintained. The results of all the experiments agree closely. Their essential features are shown in Table II which gives the maximum frequency of TABLE II. A. Stimuli to sensory nerve. Maximum Least interval Interval for frequency of between succes- muscular summa- Exp. response sive responses tion per sec. sec. sec * * * *0065 * *00625 * * *0080 * Average B. Stimuli to motor nerve * * * Average response with stimuli applied to the sensory and to the motor nerve together with the corresponding intervals for muscular summation. The

11 436 B. D. ADRIAN AND J. M. D. OLMSTED. "maximum frequency" referred to in the table is the maximum frequency which can be maintained for half a second at a time. From this it is easy to calculate the least interval between successive responses and these are given for comparison with the least interval between two stimuli necessary for muscular summation. It is clear that in the reflex arc there is a very great difference between these two intervals. The interval between the successive responses to a series of stimuli cannot be made smaller than second whereas two stimuli alone will give summation (i.e. will give two responses) although they are only separated by second. The difference is still present but much less in amount when the stimuli are applied to the motor nerve. These results might well be due to a delayed appearance of the second of two impulses in the effector side of the reflex arc, as we have suggested above. But they are not conclusive evidence on this point since the limiting frequency with serial stimulation might be determined by some other factor which does not come into play when only two stimuli are used. It appeared to us that the question could not be settled conclusively without a direct comparison of the interval separating two responses and the interval separating the stimuli which gave rise to them. We have therefore made records of the electric responses to two stimuli for comparison with the records of serial stimulation. Measurement of the delay of the second response. In these experiments we have used the capillary electrometer to record the action currents of the muscle in response to two stimuli on the afferent or efferent side of the arc. A recent investigation (13) of the accuracy to be expected from analysed electrometer records showed that the probable error in time measurements was much less than the error we should expect from uncorrected string galvanometer records. The capillary electrometer is, of course, not nearly so sensitive as the string galvanometer, but this is no drawback when the response of a large muscle is to be measured. The excursions of the mercury were photographed on plates travelling at about one metre per second and we found no difficulty in obtaining records suitable for analysis. The experiments were carried out as follows. The reflex preparation was set up as before with stimulating electrodes on the afferent nerve (popliteal) and leads from the tibialis anticus muscle. The least interval for muscular summation was determined and electrometer records were made of the responses to two stimuli separated by intervals ranging from 002 to *01 second. Records of the response to repeated stimulation at different frequencies were then made with the string galvanometer, this

12 REFLEX REFRACTORY PHASE instrument being used because it had a more convenient recording apparatus for the purpose. After these measurements on the reflex arc, the sciatic was cut high up in the thigh, the electrodes were transferred to the efferent (peroneal) nerve and a similar series of records were taken with double and repeated stimulation. The full procedure was carried out in two experiments (Exps. 7 and 8) and the reflex measurements only in a third (Exp. 6). The results of these have already been stated in part in Table II. The results as regards the delay of the second impulse are strikingly concordant. In all of them with reflex stimulation the least interval between the two electric responses is very much greater than the least interval between two successful stimuli; in other words, there is a considerable delay in the appearance of the second electric response in the muscle when the two stimuli are separated by a short time interval. The delay is still present but very much smaller when the stimuli are Exp. 7. Reflex Exp. 8. Reflex Exp. 7. Exp. A Motor Nerve Motor Nerve Fig * Seconds Corrected capillary electrometer records of tibialis anticus response to two stimuli separated by a very short time interval. With reflex stimulation the delav of the second response is much greater than with stimulation of the motor nerve. sent into the motor nerve. The existence of the delay and the difference between the reflex and motor nerve effect may be seen from Fig. 5 which gives some analysed electrometer curves from two experiments. In

13 438 E. D. ADRIAN AND J. M. D. OLMSTED. these curves the dotted line represents the continuation of the first electric response as determined from records with one stimulus only. The arrows show the moments at which the two responses reach their maximum and this probably gives the best measure of the interval between them, since the beginning of the responses is not so easily determined. The figures show the characteristic differences in the latent periods with reflex and motor nerve stimulation as well as the much greater delay of the second response in the reflex. The extent of the delay in relation to the interval between the two stimuli may be seen from Fig. 6 which records the most complete series of measurements on the reflex arc (A) and on the muscle nerve preparation (B). The two series are from different animals since the corresponding measurements from one and the same animal would not cover such a wide range. The figure is constructed on much the same lines as are those given by Lucas(5) in his paper on the delay phenomena in the tissues of the frog. A comparison with his curves shows no essential A (Reflex) Interval between Stimuli > --Interval between Responses 6 B (Motor Nerve) No second response No second response in muscle ~ ~ in musclq '001 '005 ' Seconds Fig. 6. Relation between the delay of the second response and the interval between the two stimuli. difference in the phenomenon though the delay is considerably smaller in the mammalian muscle nerve preparation than in that of the frog. These results show that if two stimuli sepat.ted by a very short time interval are applied to the afferent nerve of the arc, the second impulse appears in the muscle much later than it would have done if the stimuli had been applied to the efferent nerve instead. Thus a considerable delay occurs somewhere in the afferent side of the arc and this delay must be

14 REFLEX REFRACTORY PHASE. 439 taken into account before we can make an estimate of the rate of recovery of the synapse. Before we attempt this, however, it is interesting to consider whether the delay we have found is enough to account for the great difference in the least interval for reflex muscular summation and the least interval between successive reflex responses. The essential figures are set out in Table III. This gives the capillary electrometer TABLE III. A. Stimuli to sensory nerve. Least interval for muscular Interval between Least interval be- Exp (two stimuli) response respionses sec. sec. 6Bec. 6. *0018 * summation lst and 2nd tween successive 7. '0016 ' *0020 '0057 *0 080 B Stimuli to motor nerve *0024 * *0032 measurements of the least interval between two isolated responses and also the least interval between the successive responses in a series. In two out of three experiments the interval is longer when the responses form part of a series, but the difference is quite small. The state of affairs Response Stimulus I -- Seconds'005 1 '01 5 '02 Fig. 7. Intervals between stimuli and between responses with reflex stimulation (Exp. 6). Upper half shows repeated stimulation at rhythm giving maximum frequency of response. Lower half shows two stimuli at least interval for muscular summation. revealed by this table is shown more clearly by Fig. 7 which gives the intervals between stimuli and responses in Exp. 6. In the upper half of

15 440 E. D. ADRIAN AND J. M. D. OLMSTED. the figure a series of stimuli and responses are shown occurring at the maximum rate which the reflex arc can follow, i.e. 155 a second; for simplicity the responses are drawn as though they occurred immediately after each stimulus. The lower half shows two stimuli separated by the least interval for muscular summation (Q0018 sec.) and the two responses corresponding to these stimuli separated by a much longer interval (-0046 sec.). It will be evident from this part of the diagram that although two isolated stimuli *0018 sec. apart can give two responses in the muscle, yet, owing to the tardy appearance of the second response, the least interval between the successive responses in a series cannot well be less than *0046 sec. Actually it turns out to be *0065 sec. when the series lasts a second or more. This may be due to an increase in the delay with repeated stimulation or to some other cause, but in any case the delay seems to be much the most important factor in determining the maximum frequency of response in the arc. The same holds good for the frequency of response in the muscle nerve preparation. The second part of Table III gives the figures for the two experiments in which the stimuli were sent into the motor nerve and it will be seen that the least interval between two isolated responses agrees very closely with the least interval between the successive responses in a series, although the interval for muscular summation is shorter. These results give a satisfactory explanation of the great discrepancy between the least interval for muscular summation and the maximum frequency of response in tjhe reflex arc. The cause of the delay is still unexplained. We have not attempted to deal with it in the present research and there are not enough data even to say whether it occurs in the setting up of the impulse or in its conduction. Rate of recovery in the reftex arc. We are now in a position to estimate the maximum time which the central part of the arc must take to recover its normal conductivity after the passage of an impulse. The experiments in the last section show that the least interval which can elapse between the arrival of two impulses in the central part of the arc is about *006 sec. The experiments with anaesthetics showed that the second of two impulses separated by the least possible interval has just as much chance of passing through the anmesthetised region as had the first impulse. This means that the region must have recovered its normal conductivity by the time the second impulse arrives, and the maximum time which it can take for the recovery to normal is therefore *006 sec. An objection may be brought against this conclusion on the ground that the figure *006 sec. gives the least possible interval between two

16 REFLEX REFRACTORY PHASE. 441 impulses in the arc under normal conditions. Our argument assumes that this interval does not change when conduction in the arc has been nearly abolished by an anesthetic. It is quite conceivable that the interval would then become longer, and if it did so our estimate for the time for recovery would have to be increased. To settle this point we have determined the maximum frequency of response in the arc before and during a period of anesthesia. This was done in two experiments and in neither was there any appreciable change in the maximum frequency although the response was nearly abolished by the ansesthetic. Thus the least interval between successive re ponses is not altered by an,esthetising the central part of the arc and our estimate of '006 sec. for the maximum recovery time may be allowed to stand. In considering this result it must be remembered that we cannot be sure that the synaptic region, or region affected by anaesthetics, takes any time at all to recover. We have no direct evidence that it enters into a refractory state after the passage of an impulse, for the existence of a minimal interval for reflex muscular summation may be due to a refractory state either in the sensory nerve or in some part of the central nervous system other than the synaptic region. The latter may be able to remain continuously in the excited state without the periodic intermissions which must occur in the peripheral parts of the arc. This view is not in harmony with the idea that the process of conduction is much the same in the central parts of the arc as in the peripheral, nor does it agree with the conclusions of Fr6hlich(14), Veszi(15), Tiedermann(16) and others who have worked on the refractory phase in the spinal frog. In the present case the delay of the second impulse indicates some alteration in conduction which it is natural to associate with imperfect recovery, though it is quite conceivable that the delay occurs in a different region from that in which the aneesthetic block takes place. The absence of any refractory phase in the synaptic area is therefore a possibility which must be taken into consideration, though it need not deter us from investigating the consequences which would follow if the area passed through the usual absolute and relative refractory stages after the passage of an impulse. The estimate of *006 sec. for the maximum time for recovery applies only to that part of the arc which is affected by the anesthetic; parts which are not affected may recover more slowly, and indeed we have already seen that the motor nerve takes *01 sec. for recovery to normal. The synaptic area may recover in less than *006 sec. but it is unnecessary to suppose that it takes much less than this. Forbes and Gregg(l7)

17 442 E. D. ADRIAN AND J. MA DA OLMSTED. have suggested that some part of the synaptic region must recover very rapidly since it is possible for two impulses to produce a reflex effect where one fails, although the two impulses have been set up very close together in the afferent nerve. For this kind of summation we should expect that the second impulse would have to arrive during the " supernormal phase" of recovery in the synaptic region, i.e. after the recovery to normal is over. But we have seen that if two impulses come through the arc they are spaced out to *006 sec. apart however short the interval between the two stimuli which set them up. Consequently the second impulse might arrive during the supernormal phase of recovery although the recovery to normal took nearly *006 sec. It is in fact an interesting consequence of the delay effect that impulses can only come through the synaptic region at intervals of *006 sec. at which time recovery to normal is always complete. This may possibly have some bearing on the difference between inhibitory and excitatory paths in the central nervous system, for it would be impossible to obtain the Wedensky type of inhibition under these conditions. Further deductions would be of little value in the absence of other quantitative measurements and of some further knowledge of the factors which produce the delay of the second response. For this reason we have avoided discussing the present results in connection with the scheme of central connections outlined by F orb e s(1s). The experiments seem to us of interest as offering a method of general applicability for measuring the refractory phase phenomena in different arcs. It is true that the measurements are none too straightforward but the results are quantitative and directly comparable with those obtained from simple conducting tissues. CONCLUSIONS. 1. In the arc for the flexion reflex in the spinal cat we find that the least interval which must separate two stimuli giving a summated contraction is about *0019 sec. when the stimuli are applied to the afferent nerve and *0017 sec. when they are applied to the efferent nerve. These values are slightly longer than those given by Sherrington and Sowton. 2. When the arc is stimulated by repeated shocks at different frequencies the maximum frequency to which the muscle will respond synchronously is on the average 150 a second for stimulation of the afferent nerve and 320 a second for the efferent. The least interval between successive responses is therefore *0067 sec. for afferent nerve stimulation and *0031 sec. for efferent.

18 REFLEX REFRACTORY PHASE With afferent nerve stimulation the discrepancy between the least interval for muscular summation and the least interval between successive responses depends on the fact that if only two stimuli are used the second response does not appear in the muscle until sec. after the first, however close together the stimuli may have been. The delay of the second response occurs in the afferent nerve or in the central part of the arc. With efferent nerve stimulation the second response is delayed until 003 sec. after the first. 4. When conduction in the central part of the arc is abolished by an anaesthetic the interval for reflex muscular summation remains constant until conduction fails. Thus the second impulse, which arrives at the ansesthetised region *006 sec. after the first, has no greater difficulty in passing through than had the first impulse. The region on which the anesthetic takes effect ("the synaptic region") has therefore recovered its normal conductivity within *006 sec. of the passage of an impulse. 5. The recovery process in the synaptic region is shorter than in the motor nerve (which recovers normal conductivity in *01 sec.). There is, however, no reason to suppose that recovery takes much less than *006 sec. in the synaptic region though it may conceivably do so. REFERENCES. (1) Sherrington and Sowton. This Journ. 49, p (2) Lucas and Adrian. Ibid. 44, p (3) Lucas. Ibid. 46, p (4) Forbes and Adrian. Ibid. 56, p (5) Lucas. Ibid. 41, p (6) SamojIoff. Arch. f. d. ges. Physiol. 143, p (7) Lewis, Feil and Stroud. Heart, 7, p (8) Hoffmann. Arch. f. Anat. u. Physiol. p (9) Beritoff. Ztsch. f. Biol. 42, p (10) Foi. Ztsch. f. allgm. Physiol. 13, p (11) Erlanger and Garrey. Amer. Journ. of Physiol. 35, p (12) Forbes and Adrian. Loc. cit. p (13) Adrian. This Journ. 55, p (14) Frohlich. Ztsch. f. allgm. Physiol. 9, p (15) Veszi. Ibid. 11, p. 168, 1910; ibid. 18, p. 58, (16) Tiedermann. Ibid. 10, p (17) Forbes and Gregg. Amer. Journ. of Physiol. 37, p (18) Forbes. Physiol. Reviews, 2, p. 397 et seq