680

Electroencephalography and Clinical Neurophysiology, 1979, 47:680---692

© Elsevier/North-Holland Scientific Publishers, Ltd.

HYPERBARIC HYPERREFLEXIA: TENDON-JERK AND HOFFMANN REFLEXES IN MAN AT 43 BARS

DAVID J. HARRIS 1 AMTE Physiological Laboratory, Fort Road, Alverstoke, Gosport, Hampshire P012 2DU (England)

(Accepted for publication: April 4, 1979)

The study of the High Pressure Nervous Syndrome (HPNS), a general title for a multiplicity of pressure-related neuromuscular, EEG and 'vestibular' disturbances, has been a feature of hyperbaric research for m a n y years (Bennett 1965; Rostain et al. 1970, 1977; Hempleman et al. 1971, 1978; Hunter and Bennett 1974; Brauer et al. 1975; Fructus et al. 1976). Investigations into h u m a n and animal responses during exposure to high pressures of oxygen/helium and other gas mixtures have become increasingly sophisticated, and it has been shown t h a t the majority of symptoms observed appear to result from upper CNS dysfunction rather than a breakdown of any peripheral mechanism(s) (Kylstra et al. 1967; Brauer 1972; Bachrach and Bennett 1973; Kaufmann et al. 1978; Roll et al. 1978). Among the techniques available to the neurophysiologist to investigate upper CNS function are those of the reflexologist, a practice which is now well established (e.g. Paillard 1955; Gassel and Diamantopoulos 1964, 1966; Rushworth 1964; Delwaide 1971, 1976; Hugon 1974; and many others}. Following pioneer experiments on the tendon-jerk (TJR) and H o f f m a n n (H) reflexes of the baboon Papio p a p i o at pressure up to 111 bars, Roll et al. (1978) and Lacour et al. (1978) reported studies on h u m a n spinal and vestibulo-spinal reactivity at pressures up i Present address and address for reprints: F.G. Hall Environmental Lab., Duke University Medical Center, Durham, N.C. 27710, U.S.A.

to 62 bars. Increased soleus recruitment ratios, reduced facilitation during Jendrassik's manoeuvre and modifications to recovery cycles and vestibulo-spinal relations at pressures in excess of 40 bars demonstrated the productivity of this line of approach. In two later studies, Harris (1977, 1 9 7 9 ) s t i m u l a t e d the TJR of a different muscle, the quadriceps femoris, and also reported significant increases in excitability and the loss of reinforcement during Jendrassik's manoeuvre. However, the depth of onset of these signs was much shallower (26 bars}. Further studies on soleus T and H reflexes were planned to independently verify the earlier findings and to show that, in unselected subjects, reflex modifications can be expected at relatively shallow depths, even during very slow compression. The results reported here were obtained from two subjects during a simulated dive to 420 m (43 bars) at the AMTE Physiological Laboratory (formerly RNPL). Methods Data acquisition

Tendon-jerk and ' H o f f m a n n ' reflexes of the soleus muscle were examined in 2 male subjects at various pressures during an experimental simulated saturation dive into 420 m of sea water. In general, experimental techniques were based on the recommendations of Desmedt et al. (1973) and Hugon (1973}. The subjects were tested in the sitting position with fixation points at the foot and thigh.

HYPERBARIC HYPERREFLEXIA The angles maintained by the limb at the ankle and knee were 90 ° and 110 ° respectively. The right leg only was examined. The subject was asked to remain relaxed and immobile for the duration of each session which lasted 1 h approximately. The response of the soleus myotatic reflex arc to mechanical and electrical stimulation was explored b y measuring the amplitude of the corresponding EMG potentials. A mechanical hammer built into the footplate of the apparatus, and operated b y the attendant, was used to elicit tendon-jerk reflexes. The impact force of the hammer on the Achilles tendon was originally adjusted to a level giving submaximal responses for b o t h subjects, and then fixed at that level. Therefore for all experimental sessions, the same reproducible intensity of tendon tap was administered. TJRs were elicited at a rate of 0.1/sec. H reflexes were produced b y I msec cathodal stimuli applied to the medial popliteal nerve at the popliteal fossa using an electrode based on the model described b y Simon (1962). Pulses were delivered at a rate of 0.125/sec from a Grass $88 stimulator via a Grass SIU 5A stimulus isolation unit. Electromyographic activity in the soleus was recorded through bipolar surface electrodes placed 3 cm apart on the dorsal midline a b o u t 4 cm below the bulge of the gastrocnemius muscles. Signals were processed b y a Tektronix AM 502 differential amplifier, stored on magnetic tape, and subsequently analysed from U/V chart recordings or b y a Nicolet 660 F F T analyser. The quasi-isometric reflex contraction was monitored using strain gauges in the footplate arranged in a bridge, and recorded and analysed as above. During experiments, all signals were also displayed on a storage oscilloscope (Tektronix 5013) or X-Y display unit (Tektronix 506) linked to a transient recorder (Datalab 9 DL 905). Measuremen ts In each session the following measurements were taken:

681 (a) Tendon-jerk reflex: 10 stimulations at preset intensity. Repeated using Jendrassik's (arm-pull) manoeuvre. (b) H reflex: 10 stimulations at a level giving submaximal H reflexes (approximately Hmax/2). Repeated using Jendrassik's manoeuvre. (c) H reflex recruitment curve: 12--15 stimulation intensities ranging from that eliciting a maximum direct m o t o r response (Mr, ax) down to a level subthreshold for the H reflex. Eight responses were obtained at each level. (d) H reflex excitability cycle (twin pulses method): using a submaximal H reflex (approximately Hmax/2), twin pulses (i.e. conditioning and test stimuli) with interpulse intervals progressively increasing from 1 msec to 5 sec, were delivered with at least 10 sec between successive pairs. Analysis Recruitment ratios

T

1 ° ° ° r Hmax " 100)

were obtained for tendon-jerk and Hoffmann reflexes. The ratio indicates the percentage of the m o t o r pool activated b y the afferent volley and gives an index of excitability which is not affected by variations in electrode resistance or position etc. between sessions. H reflex recruitment curves were prepared in the manner described b y Hugon {1973). To interpret excitability cycles, the interpulse interval (abscissa) is plotted against Test response • 100(ordinate). Conditioning response The characteristic curve so produced describes the changes in the excitability of the myotatic reflex arc following a conditioning reflex discharge (Paillard 1955}. Full excitability may not be restored for up to 5 sec after the conditioning stimulus. Further details are given in the Results section.

682

D.J. HARRIS

The 420 m dive (Dive 8 in AMTE(PL) series) The compression rate for this dive, designed to be as slow as conveniently possible, was 60 m in 12 h with 12 h overnight stops. The chamber pO2 was maintained at 0.4 bar and the atmosphere was essentially nitrogen-free (average pN2 = 0.01 bar). The subjects RM and DH (the author) were fit men aged 30 and 28 years respectively. RM is a regular smoker who necessarily abstained for the period of confinement (26 days). Control sessions took place 6 days (DH) and 3 days (RM) before the dive, and 8 days (DH and RM) after the dive. Three more sessions were completed with each subject in the twelfth week after the end of the dive. Results

Tendon-jerk reflex The force produced by a reflex contraction resulting from percussion of the Achilles tendon was markedly potentiated by the exposure to hyperbaric oxyhelium. Force values increased by up to 90 per 100 in subject DH and by 50 per 100 in RM. Both divers showed a biphasic pattern of enhancement for the dive, there being a large increase after compression but also a notable increase towards the end of decompression (Fig. 1). As might be expected, the muscle potentials recorded during a TJR were similarly enhanced after compression. Recruitment ratios

100) increased from an average control value of 45.0 to a m a x i m u m of 76.0 in DH (a rise of 69 per 100), and from 63.0 to a m a x i m u m of 99.0 in RM (a rise of 57 per 100) (Fig. 1). The average of all controls was 45.6 in DH and 78.2 in RM. During early decompression, recruitment ratios were observed to return towards control values but a further phase of enhancement was evident during late decompression.

Hoffmann reflex From the recruitment curves, it was manifest t h a t RM has a relatively high H recruitment ratio (averaging 92.0). Such a value is fairly u n c o m m o n although H reflexes activating virtually 100 per 100 of the m o t o r pool have been recorded (Delwaide 1971). Ratios of this order allow little scope for the indication of enhancement of H reflex excitability (T~boHkov~ 1973). In this case the careful inspection of the whole recruitment curve for movement of the H curve to the left was particularly important. Notwithstanding this, the recruitment ratios were calculated from the curves and plotted on the same axes as the T reflex data. It is apparent from Fig. 1 that the large increases in T reflex excitability are not prima facie matched by comparable increases in H reflex excitability. There are variations in the H recruitment ratios and in both subjects the largest ratios occurred after compression with a second peak towards the end or at the end of decompression. However, the magnitude of the changes was relatively smaller. The pre- and post-dive control ratios of DH in fact show more variation than those measured during the dive. Careful examination of all the recruitment curves revealed remarkable uniformity throughout the experimental series. At no time did the plot of H reflex amplitude against stimulus intensity of either subject shift to the left to any great extent, thus confirming that the H reflex was not grossly affected by the exposure to pressure. However, it would be appropriate to conclude that the large variations in TJR excitability are reflected by synchronised but much smaller variations in excitability of the H reflex.

Reinforcement o f reflexes The facilitatory effect of Jendrassik's manoeuvre on reflex responses is shown in Table I. It is clear that in both subjects, even under normobaric conditions, a reinforcement manoeuvre did n o t always enhance the reflex response. Indeed, except in RM's T reflex, the general pattern was either of no change or

HYPERBARIC H Y P E R R E F L E X I A

100 -

683

RM

-10 -9 -8

8C Reflex

%

..

Mmax

-7 KGS

o.

FORCE -6 -5

.o- . . . .

"b"

M

o -4

O"

we,dive - c o m p r e s s i o n

- saturation

6

~ 2,50 DEPTH (metres)

decompression

~

420

3~

237

DH

6

40

•

100-

post-dive

6

6

6

• TJR

~------o H max o . . . . . -o Force of TJR

-11 -10

[3--... ~

~

~ ~

80

~-~,~

~

/~

~

.~

-- -

- O .~.

~~

o.........~

,;

"--~

Re,,ex %

"~" D \

o~

\.~_I~

1- I-0

•8 KGS FORCE

Mmax 60

/

-9

-7

\,,,~.- ,0,

.6

40

o pre,dive

6

g

- compression,

~:}

2.~

saturation ~

420

post-dive

decompression

3F,6

237

-5

4'G

6

6

6

6

DEPTH (metres)

Fig. 1. Recruitment ratio values for subject RM (top) and DH (bottom). The force of the ankle jerk during the TJR has been included for comparison.

even a clear inhibition. This was unexpected and difficult to understand in view of the fact that both subjects usually show considerable positive facilitation of the knee-jerk reflex during Jendrassik's manoeuvre, albeit in a different posture. (In a later series of experiments, a fist-clinching technique as opposed

to an arm-pull, invariably potentiated RM's T and H reflex.) In spite of this situation, a trend is nevertheless apparent. Thus, where reinforcement normally produced positive facilitation, this effect was reduced or absent during the dive; and in those cases where Jendrassik's

684

D.J. HARRIS

TABLE I Facilitation (per 100) of reflexes during Jendrassiks manoeuvre. Subject

Reflex

Pre-dive (1)

Compression (2)

Saturat ion (1)

Decompression (3)

DH

T H T H

--5.7 --7.5 +15.0 +1.1

--5.4 --13.9 +1.4 --3.5

--3.2 --19.8

--8.2 --48.2

+0.2 --8.2

--3.5 --75.3

--1.0 --57.9

+4.0 --14.5

RM

manoeuvre was effectively inhibitory during controls, inhibition was maintained or increased during hyperbaric exposure. Furthermore, in both subjects the H-reflex was apparently more affected than the T reflex and the greatest changes were observed at saturation (RM) or during decompression (DH).

Excitability cycles The technique of paired stimulations is used to investigate the excitability states of a myotatic reflex arc following a conditioning stimulus. The derived curves have a characteristic shape which depends, among other things, on the strength of the conditioning stimulus. Paillard (1955), who used conditioning and test stimuli of similar intensity, was able to define five distinct phases of excitability. Phase 1 lasts 2 msec and is a period of total inhibition. Phase II is characterised by a brief restoration of reduced excitability and lasts 1--10 msec. The test reflex is then completely or partially inhibited for several tens of msec (Phase III). Phase IV shows a fairly rapid return towards a normal or supernormal level of excitability and lasts for up to 300 msec. This is followed by a long period of subnormal excitability lasting for several hundreds of msec (Phase V). Similar observations have been made by Delwaide (1971), Roll et al. (1972) and m a n y others. The H-reflex excitability cycles obtained from the present investigation have been plotted on a 4-cycle logarithmic abscissa to facilitate inspection of Phases II and III (Fig. 2). This inevitably means that Phases IV and

Post ~

J,

r

/%

4my} __ 200

16kg ms

Fig. 3. Abnormal EMG responses recorded from the soleus muscle during the compression and saturation at 43 bars. A--C: Type 1 potentials appearing after tendon jerk (t) and H reflexes (h), and a direct muscle response (m) in subject RM at 26 bars (s, stimulus artefact). The force trace in C shows that Type 1 potentials produce no mechanical output from the muscle. D: Type 1 potentials in DH at 43 bars. E: Types 2 and 3 potentials in RM at 43 bars. The large, low frequency Type 2 potentials also produce no synchronised mechanical output in contrast to the smaller, fixed latency Type 3 late response (lr). F: Type 3 late response in DH at 43 bars.

This became apparent during the twin-pulse stimulation for the excitability cycles. It is logical to assume t hat these late responses are reflex in nature, possibly long-loop reflexes, occurring in the same muscle.

Discussion

Hyperbaric hyperreflexia The essential rationale for the comparative study of H o f f m a n n and tendon-jerk reflexes has been to determine the different e x t e n t to which the two reflexes are modified during certain clinical disorders or experimental situations, and to implicate t h e r e b y either altered fusimotor discharge and thus muscle spindle excitability, or central m o t o r pool excitability as being responsible for any observed change in the state of reflexia (e.g. Paillard 1955; Buller 1957; Gassel and Diamantopoulos 1964; Bishop et al. 1975; Gottlieb and Agarwal 1978). Recently experiments with superimposed tonic vibration reflexes and phasic stretch reflexes have suggested there is a strong inhibitory background on the presynaptic Ia terminals controlling afferent inflow (Delwaide 1971, 1973). It has been inferred that any central influences reducing the level o f inhibition might favour the dispersed T-reflex afferent volley rather than the more synchronised afferent burst of the H-reflex (Delwaide and Delbecq 1973). Therefore the present data on the differential e n h a n c e m e n t of T and H reflexes by hyperbaric exposure cannot of themselves confirm whether the observed pot ent i at i on of the T reflex is mediated through the gamma (fusimotor) control system or by a central effect on alpha m o t o r pool excitability, or indeed both. If corresponding changes had been seen in bot h reflexes, it may have been possible to rule o u t gamma loop involvement, but such was n o t the case and a 'classical' gamma-loop effect cannot be dismissed on these grounds alone. However, for the reasons given above, it would be inappropriate to infer the converse,

HYPERBARIC HYPERREFLEXIA that because the changes in the T reflex are n o t matched by a parallel effect on the H reflex, a central modification of m o t o r pool excitability is not responsible. The results reported here are strikingly similar to those described by Delwaide and Delbecq (1973) after electric/caloric vestibular stimulation (soleus T reflex increased b y 60--80 per 100, H reflex increased by only 10--20 per 100. The authors suggest that the increase in reflectivity could be brought a b o u t b y the de-activation of an interneurone which normally has a tonic presynaptic inhibitory effect on the Ia afferents (Delwaide 1973). The differential effect on the H and T reflexes is explained by the greater susceptibility of the prolonged T reflex afferent discharge to changes in the level of such tonic presynaptic inhibition. Such a release from tonic inhibitory control of reflex loop excitability may well account for the observed hyperbaric hyperreflexia. Further indirect support for this tenet has come from studies on the effects of Jendrassik's manoeuvre on reflectivity.

Effect o f Jendrassik's manoeuvre In the present study, the usual facilitation of reflexes during a classical Jendrassik's armpull manoeuvre was n o t generally evident in control experiments, and this may have been due to the presence of some other countereffective inhibitory mechanism also induced b y the m e t h o d of reinforcement. It is possible t h a t - t h e subject's posture may have been unsuitable for such a manoeuvre since subsequent tests with first clenching have invariably produced reflex potentiation (Harris, in preparation). While the reasons for the unusual results from the arm-pull technique remain obscure, inhibition of H reflexes by Jendrassik's manoeuvre has, in fact, been noted previously (Isaacs et al. 1968). Other investigations have shown that the normal facilitatory effect of Jendrassik's manoeuvre may be considerably reduced during hyperbaric exposure (Roll et al. 1978; Harris 1977, 1979), and in spite of the comments above,

687 the results reported here (decreased potentiation or increased inhibition during saturation and decompression) may also be said to fall into the same pattern. Most recent explanations of the causes of the Jendrassik effect invoke a centrallymediated change in alpha-motor pool excitability (Landau and Clare 1964; Gassel and Diamantopoulos 1964; Gassel 1969; Delwaide 1973; T~boHkov~i 1973; Delwaide and Delbecq 1973; Hagbarth et al. 1975). It is well known that if the effects of t w o excitatory influences fail to summate when acting simultaneously, the resultant 'occlusion' may indicate that b o t h mechanisms are operating along the same circuit or pathway (Delwaide 1976). It is quite conceivable that the enhancement of a reflex response by Jendrassik's manoeuvre utilises at least part of the same gross mechanism proposed for vestibular potentiation by Delwaide and Delbecq (1973), namely a reduction in the level of presynaptic inhibitory action on the Ia terminals. If this is the case, the apparent occlusion between hyperbaric hyperreflexia and Jendrassik's reinforcement suggests that the aetiology of the former is concerned with supraspinal influences and may be closely linked with the release of tonic presynaptic inhibitory control of m o t o r excitability. The intervention of presynaptic inhibition release in hyperbaric environments has previously been postulated b y Roll et al. (1978) following studies on the spontaneous variability of monosynaptic reflex responses. The suggestion that other spinal inhibitory control systems (e.g. Ib and Renshaw circuits) are also released under hyperbaric conditions by central mechanisms (Roll et al. 1978) is not contraindicated b y the present observations.

Excitability cycles Many investigators have studied the excitability of the soleus myotatic reflex arc with twin pulses of equal amplitude and suprathreshold for an H reflex (Hoffmann 1924; Magladery et al. 1952; Paillard 1955; TfiboHkov~ and Sax 1969; Delwaide 1971;

688 Gassel 1973; Roll et al. 1972, 1978). Others have preferred to use a conditioning stimulus subliminal for an H reflex and, although the shapes of the curves differ in some respects, the five phases defined b y Paillard (1955) are nevertheless obtainable (T~ibo~ikov~ and Sax 1969; Delwaide 1971, 1976). In the interpretation of excitability cycles, there are many factors to be taken into account (reviewed by Delwaide 1971). At present, the extensive experimental data on the subject have defied satisfactory explanation. Authors tend to accept that there are many mechanisms involved, some facilitatory, some inhibitory, with various time courses, and it is the net effect of these simultaneous and sequential variables which determines the shapes of the curves (Roll et al. 1972; Delwaide et al. 1976). Notwithstanding the lack of an adequate unified interpretation of the excitability cycle, the technique has nevertheless been used clinically in the study of patients with various neurological syndromes such as spasticity, Parkinson's disease, chorea and dystonia musculorum deformans. In these cases Phase III is usually shortened and Phase IV enhanced, and the curve can act as an index of disease states (Diamantopoulos and Zander Olsen 1966; Zander Olsen and Diamantopoulos 1967; Gassel 1973; Delwaide et al. 1976). Excitability cycles, particularly Phase IV, can also be a sensitive indicator of the action of some drugs such as nicotine, pentobarbital sodium and Nefopam (Gassel 1973). In an earlier hyperbaric study, Roll and his colleagues described the excitability cycles of divers during a deep oxyhelium dive to 62 bars (Sagittaire IV). They noted that Phase III was prolonged and Phase IV was markedly depressed at saturation {Roll et al. 1978). Similar observations have been noted in one subject in the present study and these convergent findings are interesting in that they contrast with the general picture in clinical hyperreflexic patients mentioned earlier. On the other hand, subject RM appeared to

D.J. HARRIS respond quite differently, showing a shortening and elevation of Phase III and an enhancement of Phase IV, a picture much more reminiscent of clinical hyperreflexia. In spite of the variation between the two subjects the biphasic pattern of increased recruitment ratios (immediately after compression and during late decompression) mentioned earlier (and also observed previously by Harris (1977) and Roll et al. (1978)), is reflected in the shape of the curves of the excitability cycles. Thus while there is a considerable difference between subjects, the curves for saturation and late decompression in each subject are rather similar and contrast with those for compression and early decompression. Modifications to Phase V consisted largely of a depression of part or all of the phase (with the notable exception of those in RM during compression and early decompression). This phase was also apparently affected during an earlier French 300 m dive (Sagittaire III) b u t n o t during the later 610 m dive (Sagittaire IV) (Roll et al. 1978).

Abnormal electromyographic recordings Of the various types of soleus EMG phenomena recorded during this exposure to hyperbaric oxyhelium, the Type 1 and Type 2 responses are most similar in amplitude and frequency content to the potential fluctuations observed in the quadriceps femoris during an earlier dive (Dive 6 - 300 m (Harris 1977)). Although they are all recognisably different in appearance, the major feature in c o m m o n is the lack of any measurable mechanical o u t p u t from the muscle in question which could be related to the EMG activity. None of these forms of muscle electrical activity are tremorogenic. Whether the Type 1 and 2 potentials in the present dive can be attributed to muscle fasciculations as was believed to be the case in Dive 6 has not been verified. In contrast, the Type 3 potentials appear with a relatively fixed latency and are evidently responsible for a reflex-like contrac-

HYPERBARIC HYPERREFLEXIA tion of the muscle. These late responses, which presumably result from the activation of some polysynaptic pathway, are similar to the polyphasic discharges reported b y Roll and his colleagues. They observed such 'asynchronised' bursts at a latency of 200 msec beginning at 56 bars during the compression and compared them to m o t o r 'startle reactions' (Roll et al. 1978). Apart from the slightly longer latency and the shallower depth of onset, there seems to be no reason to discriminate further between the late responses of these t w o different studies. The explanation that both appeared as a result of increased accessibility of supraspinal pathways (which in turn may be related to the release of inhibitory control mechanisms) seems equally plausible for both experiences. However, one notable difference between Sagittaire IV and the AMTE(PL) Dive 8 is that whereas abnormal EMG responses persisted until after the end of decompression in the case of the deeper French dive, all abnormal activity (Types 1, 2 and 3) has ceased after the beginning of decompression in the 420 m dive reported here.

Conclusions The existence of hyperbaric hyperreflexia is now reasonably well established and may occur during and after compression on oxyhelium to 25 bars or more. The resemblance between aspects of this condition and the hyperreflexia induced b y direct vestibular stimulation has been discussed and the suggestion made that part of the proposed explanatory mechanism may be c o m m o n to both situations. If hyperbaric hyperreflexia is to be explained in a similar way, it is necessary to identify possible sources of altered supraspinal activity which normally influence spinal processes. Prominent among the many different s y m p t o m s of the High Pressure Nervous Syndrome (HPNS) are those highly reminiscent of vestibular system dysfunction,

689 although evidence has been accumulating that a disturbance of the vestibular end organ itself is n o t implicated (TSrSk, personal communication; Hempleman et al. 1978). One may speculate that, at least as far as the compression phase is concerned, the apparent central disturbances in the vestibular system may result in the alteration of vestibulo-spinal relations and ultimately in the release of tonic presynaptic inhibitory control. However, it is stressed that the 'vestibular' theory neither explains the hyperreflexia observed at the end of the decompression phase, nor takes into account possible effects on all the other motor control systems. Delwaide (1973) has argued that this arrangement can also explain the increased myotatic reflex arc excitability during pyramidal hyperreflexia. However, further comparisons of the present syndrome with clinical states of hyperreflexia are not entirely encouraging. Only in one subject have excitability cycles of typical clinical hyperreflexic character been observed, and the unusual electromyographic activity observed in deep diving is n o t a notable feature of 'pyramidal' hyperreflexia. It is perhaps reasonable to conclude that the motorneuronal release from presynaptic inhibitory control by supraspinal influences of midbrain origin is probably only one of several mechanisms by which the myotatic reflex arc is affected during hyperbaric exposure, b u t is nevertheless a good starting point for further elucidation of some of the causes of HPNS.

Summary Tendon jerk (TJR) and Hoffmann (H) reflexes of the soleus muscle were studied in t w o men during a 26-day simulated oxygenhelium dive to a maximum pressure of 43 bars. The amplitude of the T J R response was observed to increase markedly after compression and also at the end of decompression. This biphasic pattern of enhanced reflectivity

D.J. HARRIS

690

was reproduced by synchronised but much smaller variations in H reflex response. The positive facilitatory effects of applying Jendrassik's manoeuvre were reduced and the negative effects increased during hyperbaric exposure. Excitability cycles (twin -pulses methods) revealed modifications in one subject similar to those observed in clinical hyperreflexia, namely a shortening of Phase III and an enhancement of Phase IV. The other subject exhibited a notable depression of Phase IV. Abnormal EMG recordings are described including randomly-triggered slow wave potentials with no mechanical effects, and fixed, long latency late responses of a reflex nature with a definite mechanical effect. The contention that alterations in vestibulo-spinal relations may result in the release from tonic presynaptic inhibitory control of myotatic reflex arc excitability is discussed as a partial explanation for these and related findings.

~l~vation de la phase IV. Le deuxi~me sujet a montrd un abaissement notable de la phase IV. On a observ~ des trac~s EMG anormaux, n o t a m m e n t des potentiels lents sans effets m~caniques et des r~ponses ~ longue latence, multiphasiques, probablement de nature r~flexe, e t responsables de contraction musculaires nettes. L'hypoth~se selon laquelle des modifications des activitds vestibulo-spinales auraient relevd l'excitabilit~ du reflexe myotatique par suppression d'une inhibition de type pr~synaptique est discut~e en tant qu'explication partielle de nos r~sultats et de r~sultats comparables obtenus par d'autres auteurs. The author is greatly indebted to the following: to Dr G. Rushworth and Professor M. Hugon for considerable help and advice on matters of reflexology; to Dr Z. T6rSk for his encouragement and critical reading of the manuscript; to the other subject, Mr R. McKenzie; to Mr P. Atherton who operated all the external equipment during the author's confinement, and without whom this study would have been impossible; to Mr R. Belcher and his staff for photographic work; and to Miss V. Rees for typing the manuscript.

R~sumd

Hyperr~flexie hyperbare: r~flexe tendineux et rdflexe de Hoffmann chez l'homme d 43 bars On a ~tudid le rdflexe tendineux et le r~flexe de H o f f m a n n du muscle soleus chez deux hommes pendant 26 jours de plongde fictive en oxyg~ne-hdlium ~ la pression maximale de 43 bars. On a observ~ un accroissement sensible du rdflexe tendineux ~ la fin de la compression et ~ la fin de la d~compression. La rdponse de H o f f m a n n a subi, dans les m~mes conditions, des modifications comparables, quoique plus petites. La facilitation des r~flexes li~e ~ la manoeuvre de Jendrassik a ~td rdduite en atmosphere hyperbare; les effets inhibiteurs, parfois observds, ont ~t~ accrus. Les cycles d'excitabilit~ dtablis par la mdthode des doubles chocs ont rdv~l~, chez un sujet, l'existence de modifications comparabies ~ celles qui sont ddcrites en hyperr~flexie clinique: diminution de la phase III et

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