Exp. Brain ges. 24, 273 283 (1976)
Experimental Brain Research 9 by S]pringer-Verlag1976
Postsynaptic Inhibition of Oculomotor Neurons Involved in Vestibulo-Oeular Reflexes Arising from Semicircular Canals of Rabbits* M. Ito, N. Nisimaru and M. Yamamoto Department of Physiology, Faculty of Medicine, University of Tokyo, Bunkyo-ku, Tokyo (Japan)
Summary. I n anesthetized albino rabbits, electric stimulation of vestibular nerve branches innervating semicircular canals produced not only reflex contraction in certain extraocular muse]es, but also a transient relaxation in others. From relaxing muscles was recorded a slow muscle potential that reflected depression of spontaneous spike discharges in muscle fibers. When recorded monophasically, spontaneous spikes of muscle fibers were superposed to form a direct current potential, and depression of the spikes resulted in a transient reduction of this direct current potential, i. e., the slow muscle potential. The slow muscle potential was correlated to the postsynaptic inhibition induced in oculomotor neurons through the vestibulo-oeular reflex arc for the fo]lowing reasons ; its latency was compatible with that of the IPSPs recorded from oculomotor neurons; it was removed by severing axons of the inhibitory second-order vestibular neurons; it was blocked by intravenous injection of picrotoxin as were the I P S P s in oculomotor neurons. By recording slow muscle potentials, a specific canal-muscle relationship for the vestibulo-oeular reflex inhibition of oculomotor neurons was shown to be complementary to that obtained for the vestibulo-ocular reflex excitation. Key words : Semicircular canal - - Vestibular-ocular inhibition - - Rabbit. Introduction Labyrinthine impulses induce not only contraction but also relaxation of extraocular muscles (Szents 1950; Cohen, Suzuki and Bender, 1964). Accordingly, recent investigations have revealed that stimulation of the vestibular nerve produces postsynaptie inhibition of oeu]omotor neurons (Sasaki, 1963 ; Richter and Precht, 1968 ; Baker et al., 1969 ; Highstein et al., 1971). The inhibition is mediated by a specialized group of second-order vestibular neurons which are located in the superior vestibular nucleus (Highstein et al., 1971 ; Precht and Baker, 1972; Highstein, 1973a) with the possible exception that abducens motoneurons m a y be inhibited from the rostral portion of the medial vestibular nucleus (Baker et al., 1969; Itighstein, 1973b). *
This work was supported by a grant from Educational Ministry of Japan (844021).
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The aim of the present i n v e s t i g a t i o n was to specify the reflex p a t h w a y s which convey i n h i b i t o r y action from i n d i v i d u a l semicircular canals to extraocular muscles. Together with the excitatory reflex p a t h w a y s dealt with in the preceding paper (Ito et al., 1976), principal c o m p o n e n t s of the vestibulo-ocular reflex arc were identified in terms of receptor canal, relay ~ucleus, effector muscle and reflex action, thus providing the basis for a further analysis of the cerebellar control of vestibulo-ocular reflexes (Ito et al., 1973b). A part of the present results has been published in brief (Ito et al., 1973 a).
Methods The general experimental procedures were identical with those described in the preceding paper (Ito et al., 1976), and the twenty-eight rabbits used in the present experiments were also involved in the preceding experiments. Same abbreviations as those in the preceding paper (Ito et al., 1976) will be used.
Results Relaxation o/ Extraocular Muscle8 Induced by Canal Stimulation Application of c u r r e n t pulses to a semicircular canal i n d u c e d a t r a n s i e n t t e n s i o n decrease i n certain extraocular muscles, as illustrated i n Fig. 1A a n d C for com-
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10msec Fig. 1. Transient tension decrease and the eoneommitant appearance of the slow muscle potential in an extraoeular muscle following stimulation of a semicircular canal, recorded from i-IR muscle while the anterior canal was stimulated. (A and C) Isometric tension of the i-Ig muscle. (B and D) Potentials developed along the i-IR muscle between the two electrodes, one inserted into the muscle belly and the other into the cut end. A and C were recorded simultaneously with B and D, respectively. In A and B, the stimulating pulses were of 5 msee duration and their amplitudes varied as indicated. In C and D the stimulating pulses were of 22 V amplitude and their width varied as shown. In this as well as in succeeding figures, recording was ac-coupled with time constant of 300 msec, and 10--20 sweeps were superimposed at a repetition rate of 1 or 2/see, unless otherwise stated
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b i n a t i o n of t h e a n t e r i o r canal a n d i - I R muscle. This tension decrease s t a r t e d w i t h latencies of 4 - - 6 msee a f t e r t h e onset of s t i m u l a t i n g pulses a n d l a s t e d for 5 0 - - 1 0 0 msee. A m p l i t u d e of t h e t r a n s i e n t t e n s i o n change increased n o t o n l y w i t h t h e increase of pulse i n t e n s i t y as in A, b u t also w i t h t h e increase of pulse d u r a t i o n as in C. I t is n o t e d in C t h a t t h e r e l a x a t i o n was a p p a r e n t even when t h e pulse d u r a t i o n was as s h o r t as 0.1 msec a n d t h a t i t was increased g r a d u a l l y as t h e pulse d u r a t i o n i n c r e a s e d u p to 10 msec. This c o n t r a s t s to t h e fact t h a t c u r r e n t pulses h a v i n g a d u r a t i o n s h o r t e r t h a n 2 - - 3 msec were h a r d l y effective in evoking reflex c o n t r a c t i o n of e x t r a o c u l a r muscles (Ito et al., 1976). I n o t h e r words, in the reflex r e l a x a t i o n t h e r e was no a p p a r e n t t h r e s h o l d value for pulse d u r a t i o n such as t h a t o b t a i n e d in t h e reflex e x c i t a t i o n (Discussion).
Slow Muscle Potential Accompanying the Musde Relaxation As r e c o r d e d w i t h t w o needle electrodes i n s e r t e d into a n e x t r a o c u l a r muscle, one into t h e muscle belly a n d t h e o t h e r into t h e distal cut end (Fig. 3G), t h e muscle r e l a x a t i o n was r e g u l a r l y a c c o m p a n i e d b y a r e l a t i v e l y slow p o t e n t i a l of opposite sign to t h e muscle spike p o t e n t i a l (i. e., t h e p r o x i m a l electrode positive r e l a t i v e to t h e d i s t a l electrode). This p o t e n t i a l , t h e slow muscle p o t e n t i a l as it m a y be called, l a s t e d for 1 0 - - 2 0 msec a n d u s u a l l y was followed b y a r e b o u n d . As seen in Fig. 1 a n d p l o t t e d in Fig. 2, m a g n i t u d e of t h e slow muscle p o t e n t i a l increased m o n o t o n i c a l l y w i t h t h a t of t h e t r a n s i e n t tension change, as t h e pulse i n t e n s i t y or d u r a t i o n was increased. T h a t t h e slow muscle p o t e n t i a l was n o t an a r t i f a c t due to m o v e m e n t of t h e electrodes d u r i n g muscle r e l a x a t i o n was i n d i c a t e d b y t h e f a c t t h a t it 0/4
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Fig. 2. gelationship between the muscle relaxation and the slow muscle potential. Inset diagram indicates how to measure peak amplitudes of the transient decrease of the muscle tension (rot) and the slow muscle potential (stop). Arrows mark the moment of onset o2 stimulating currents. The slow muscle potential is plotted in ordinate and the muscle tension in abscissa. Taken from the series partly illustrated in Fig. 1
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a p p e a r e d even when t h e m o v e m e n t was negligible u n d e r e x t e n s i v e st r et ch of t h e muscle and t h a t e v e n a large m o v e m e n t due to passive s t r e t c h of t h e muscle produced only a p o t e n t i a l change considerably smaller t h a n t h e actual slow muse]e potential. T h e following e x p e r i m e n t s i n d i c a t e d t h a t t h e slow muscle p o t e n t i a l is due to depression of s p o n t a n e o u s spike discharges from o c u l o m o t o r neurons.
Origin o/ the Slow Muscle Potential Th e slow muscle p o t e n t i a l could n o t be recorded when t h e t w o recording needle electrodes were paired w i t h a small ti p s e p a r a t i o n of a b o u t 1 m m and were inserted into t h e muscle belly (Fig. 3G) ; silencing of sp o n t an eo u s spike discharges
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0 -5 0 10 20 msec Fig. 3. Relationship between the slow muscle potential and spike potentials of muscle fibers. (A--D) Single sweep records from i-IR muscle. In A and B, tip separation of the recording electrodes was 1 mm (bi in G), and in C and D about 4 mm (too). In A and C the anterior canal was stimulated. Horizontal bars attached in these as well in succeeding figures indicate the period of passage of stimulating currents. In B and D the IIIrd nucleus was directly stimulated with brief pulses (0.1 msee duration). (E) Upper traces, similar to C, but recorded from i-SR muscle with stimulation of the posterior canal. Lower traces, unitary spikes recorded simultaneously with a glass microelectrode inserted in that i-SR muscle. (F) Poststimulus time histogram obtained for unitary spike discharges similar to those in the lower traces of E. 1 bin width, 0.2 msec. 400 sweeps repeated at a rate of 5 sweeps per sec. Downward and upward arrows indicate the onset and cessation of the stimulating current. (G) Arrangement of the recording electrodes, m microelectrode. Voltage scale; l0 mV for B, D, 0.5 mV for the lower traces of E, 0.25 mV for the upper traces of E, and 70/~V for A and C. Time scale of 5 msec ~pplies to A, C and E, and that of 2 msec to B and D
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was the only visible effect accompanying the reflex relaxation (Fig. 3A). With this recording arrangement, spike potentials evoked by direct stimulation of I I I r d nucleus assumed a biphasic form (B). When one of the two needle electrodes was shifted to the cut end of the muscle, the spike potential became nearly monophasic (D) and the slow muscle potential emerged (C). The occurrence of the slow muscle potential in the monophasic recording can be explained satisfactorily by assuming that spontaneous spike potentials of many muscle fibers as recorded monophasieMly are summated to produce a direct current potential between the two recording electrodes; depression of the spontaneous spike discharges would result in reduction of this direct current potential, thereby yielding the slow muscle potential with the sign opposite to that of spike potentials. With the biphasic recording, there would be no direct current potential developed during spontaneous spike discharges and consequently no initiation of the slow potential during depression of the discharges, for the time integral over the two phases of spike potentials is approximately zero (Fig. 3B). I n agreement with this view, recording with a micro-electrode inserted into a relaxing muscle revealed that individual muscle fibers indeed stopped their spontaneous discharges during the slow muscle potential (Fig. 3E, G). The poststimulus histogram for individual muscle fibers exhibited a time course closely resembling that of the slow muscle potential (Fig. 3F). Development of a direct current potential of a millivolt along extraocular muscles during spontaneous discharges in individual muscle fibers can be indicated by the following calculation. The time integral of the full-sized spike potential induced by direct stimulation of IIIrd nucleus was of the order of 10 mV.msee (see Fig. 3D). Since each extraoeular muscle of rabbits contains about ten thousand muscle fibers (Kato, 1938), individual fibers may contribute to this spike potential by 10-3 mV-msec per impulse on the average. If these fibers discharge spontaneously at a rate around 100 impulses per second (as in Fig. 3F), 103 spikes will appear in a muscle per msec. These spikes will be integrated to yield a direct current potential of 1 mV in amplitude.
Relationship o/the Slow Muscle Potential with the Postsynaptic Inhibition in Oculomotor Neurons The depression of spontaneous discharges in oculomotor neurons as represented by the slow muscle potential can be related to the postsynaptie inhibition of oculomotor neurons for the following reasons. First, the latency of the slow muscle potential was determined accurately on the averaged curves (Fig. IA) and was 3--6 msee. This was approximately the sum of the latency for postsynaptic inhibition in oculomotor neurons (range, 1.I--2.9 reset; mean, 1.72 msec; Highstein et al., 1971) plus that for impulses to propagate from oeulomotor neurons to muscle electrodes (1--2 msec, Ito et al., 1976; see Fig. 3B, D). Second, the slow muscle potential in those muscles innervated by the I I I r d and IVth nuclei motoneurons was removed by cutting the medulla rostromedially to the superior vestibular nucleus (Fig. 4 B - - D ) which mediates the postsynaptic inhibition to I I I r d and IVth nuclei (Highstein et al., 1971 ; ttighstein, 1973 a ; Preeht and Baker, 1972). The slow muscle potential was influenced by severance of neither the braehium eonjunetivum nor the contralateral fasciculus longitudinalis medialis (not illustrated) which are pathways for the vestibulo-ocular excitation arising from the
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A
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Fig. 4. Features of the slow muscle potentials. (A) Upper traces, the slow muscle potential evoked in i-IR muscle by stimulation of the anterior canal. Lower traces, same as the upper traces but averaged for 40 successive sweeps. Downward arrow indicates the onset of the slow muscle potential. (C) Similar to the upper traces of A. Upper traces, i-It~ muscle. Lower traces c-SO muscle. Stimulation of the anterior canal. (D) Same as C but after lesion had been placed in the ipsilateral faseiculus longitudinalis medialis at the level rostral to the superior vestibular nucleus, as shown in B. (E) Monopolar stimulation of the labyrinth. Upper traces, slow muscle potentials recorded from c-SP~ muscle. Lower traces, spike discharges simultaneously recorded from i-SR muscle. (F) Same as in E, but after picrotoxin (10 mg/kg) had been injected intravenously. Voltage scale is 140/IV for upper traces of A, E, F and both upper and lower traces of C and D, and 2 mV for lower traces of E and F. Time scale of 5 msec applies to A, and that of 10 msee to others a n t e r i o r a n d posterior canals (Ito et al., 1976). Third, t h e slow muscle p o t e n t i a l was blocked b y i n t r a v e n o u s i n j e c t i o n of p i c r o t o x i n (10 mg/kg) j u s t as were t h e i n h i b i t o r y p o s t s y n a p t i c p o t e n t i a l s (IPSPs) in o e u l o m o t o r neurons (Highstcin et al., 1971). E v e n t h o u g h t h e o b s e r v a t i o n was l i m i t e d to a r e l a t i v e l y short p e r i o d before i t was i n t e r r u p t e d b y convulsion, t h e r e was a l m o s t c o m p l e t e e l i m i n a t i o n of t h e slow muscle p o t e n t i a l (Fig. 4E, F), while t h e reflex discharges e v o k e d in o t h e r muscles were n o t significantly affected (E, F). F o u r t h , t h e possible c o n t r i b u t i o n of t h e p o s t s y n a p t i e i n h i b i t i o n of second-order v e s t i b u l a r neurons (Mano et al., 1968 ; W i l s o n et al., 1968) to t h e slow muscle p o t e n t i a l was e x c l u d e d b y c u t t i n g t h e commissural fibers. This inhibition, if i t a c t e d u p o n those second-order neurons e x e r t i n g e x c i t a t o r y influences u p o n o c u l o m o t o r neurons, would result in withd r a w a l of b a c k g r o u n d facilitation f r o m o e u l o m o t o r neurons, i.e., disfacilitation, which m a y cooperate w i t h t h e p o s t s y n a p t i c i n h i b i t i o n in o c u l o m o t o r neurons~ I n two p r e p a r a t i o n s , a l o n g i t u d i n a l cut was m a d e along t h e midline of t h e medulla, 3 m m deep from t h e dorsal surface of t h e medulla. A n t e r i o r l y , t h e cut r e a c h e d t h e level of t h e locus coeruleus a n d p o s t e r i o l y covered t h e r o s t r a l a r e a in one p r e p a r a t i o n a n d t h e entire s t r e t c h in t h e other, of t h e m e d i a l v e s t i b u l a r nucleus. Slow muscle p o t e n t i a l s o b s e r v e d in i-II~ a n d c-SO muscles of these p r e p a r a t i o n s d i d n o t show a n y a p p r e c i a b l e difference before a n d after t h e cut.
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The Canal-Muscle Relationship/or the Slow Muscle Potential B i p o l a r s t i m u l a t i o n o f t h e a n t e r i o r c a n a l r e g u l a r l y p r o d u c e d p r o m i n e n t slow m u s c l e p o t e n t i a l s in t w o p a r t i c u l a r m u s c l e s , i.e., i-It~ a n d c-SO, as i l l u s t r a t e d in F i g . 5 ; in t h e s e m u s c l e s , t h e slow m u s c l e p o t e n t i a l was p r o d u c e d w i t h a s i m i l a r l y i-IR
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Fig. 5. Slow muscle potentials evoked by stimulation of the anterior canal. Records taken simultaneously from 5 muscles with various stimulus intensities indicated (as inultiple of the threshold T). Note that reflex discharges instead of slow muscle potentials were induced in c-IO muscle. Voltage scale of 2 mV applies to c-IO and that of 0.5 mV to others A
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I0,2mV 10msec Fig. g. Slow muscle potentials evoked by stimulation of the posterior and horizontal canals. (A) Selective stimulation of the posterior canal. (B) That of the horizontal canal. (C) ~onopolar stimulation with an electrode placed in the horizontal canal. Muscles from which potentials were recorded are indicated. In A and B stimulus intensities are given as the multiple of the threshold for i-IO and i-LR, respectively. In C the strength of stimulating currents is indicated in #A
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low threshold and developed with a similar stimulus-response relationship. I n i-IO and e-SR muscles illustrated for comparison, stimulation of the anterior canal produced virtually no potential change. The slow muscle potential in i-SR in some cases was of significant amplitude, yet it was much smaller than those in i - I g and c-SO muscles. Figure 5 also shows the reflex discharges evoked in e-IO muscle, as described in the preceding paper (Ito et al., 1976). I t was regularly seen that the threshold for evoking the slow muscle potential was considerably lower than that for initiating reflex excitation (Discussion). Similar stimulation of the posterior canal revealed that two muscles, i-IO and c-SI~, exhibited prominent slow muscle potentials as shown in Fig. 6A, with equally low thresholds. The posterior canal stimulation sometimes induced a small slow muscle potential also in i-IR muscle with the threshold similar to those for i-IO and c-SR muscles. This effect for i-It~ muscle, however, was not regularly observed. The stimulation of the horizontal canal always evoked a prominent slow muscle potential in i-LR muscle, as illustrated in Fig. 6B. A prominent slow muscle potential was induced also in e-MR muscle but with a threshold about twice as high as that for i-LR muscles. Two to five-fold differences of the threshold between i-LR and c-MR were consistently observed in 4 experiments where the stimulation of the horizontal canal was performed with great care. Figure 6C illustrates the effects of monopolar stimulation in the horizontal canal which is presumed to activate all branches of the vestibular nerve non-selectively (Ito et al., 1976). The slow muscle potentials were in fact induced in all of the i-IR, c-SO, i-IO, c-SI~ and i-LR muscles with comparable threshold values as exemplified by i-LR and i-IR. Yet, the threshold for e-Mt~ muscle was about three times as high as those for other muscles. Similar observation was made in 3 preparation. Implication of these observations will be discussed later (Discussion).
Discussion I n the present investigation, the reflex inhibition was monitored by depression of spontaneous discharges from oeulomotor neurons as reflected in transient decrease of the muscle tension as well as in appearance of the slow muscle potential, while the reflex excitation was evaluated by synchronized discharges from oculomotor neurons. Spontaneous discharges from oculomotor neurons may readily be influenced by their postsynaptic inhibition, in a more or less proportionate manner, whereas synchronized reflex discharges may be evoked only when the postsynaptic excitation rises over a certain critical value. This may explain, at least partly, why the reflex inhibition was induced with a lower threshold intensity of stimulating currents than the reflex excitation and without apparent threshold for current duration such as obtained for the reflex excitation (Ito et al., 1976). The possibility, however, still remains that the reflex inhibition and excitation differ from each other in the ease of impulse transmission from the primary to second-order vestibular neurons. The present investigation demonstrated that with stimulation of each canal postsynaptic inhibition occurred in oeulomotor neurons of two muscles antagonis-
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c-SO i-LR
9 HC
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Fig. 7. Schematic drawing of the inhibitory vestibulo-ocular reflex pathways. III IIIrd nucleus. IV Trochlear nucleus. VI Abducens nucleus. VN Vestibular nuclear complex, M V Medial vestibular nucleus, S V Superior vestibular nucleus. Other symbols are defined in the text. Vertical interrupted lines indicate the mid sagittM line. Inhibitory second-order neurons were located according to Baker et al. (1969), Highstein et al. (1971), Precht and Baker (1972), Highstein (1973a, b). In A and C it is assumed that pathways to the two muscles influenced by a canal are relayed by the same inhibitory second-order neuron. This is simply because there is no special reason to postulate separate groups of inhibitory neurons bound to different muscles, except for i-LR and c-MR muscles in B. Note that those muscles excited by canal signals are drawn by dotted lines
tie to those which were excited with the same stimulation (Ito et al., 1975). Figure 7 illustrates the pathways and the specific pattern of distribution of the vestibuloocular reflex inhibition thus determined. The pattern for c-SO and i-LR muscles is in good agreement with microelectrode studies on IVth (Baker et al., 1973) and VIth (Baker et al., 1969; Highstein, 1973b) nucleus motoneurons. Concerning the inhibitory pathway to c-MR motoneurons, the threshold for evoking the slow muscle potential in this muscle from the horizontal canal is exceptionally high (Fig. 6B), and therefore it is necessary to exclude the possibility that this effect is derived by current spread out of the horizontal canal to other endorgans. Spread to the anterior and posterior canals does not account for this effect, because selective stimulation of these canals did not produce the slow muscle potentials in c-MR muscle. Direct stimulation of the utricle produces contraction in c-MR muscle instead of its relaxation (Suzuki et al., 1969). And, there is no evidence indicating that the c-MR muscle is influenced by stimulation of the saccule (Owada and Shiizu, 1960; Fluur and MellstrSm, 1970). The present observation that the threshold for evoking the slow muscle potential in c-MR was high even with monopolar stimulation of all vestibular nerve branches (Fig. 6C) indicates that the high threshold is not due to remoteness of the responsible endorgan to the stimulating electrode, but that the reflex pathway leading to the postsynaptic inhibition of c-Ml• motoneurons from the horizontal canal has a high threshold locus in its course. Weak effects obtained besides the prominent inhibition of Fig. 7 would also be a matter of argument, just as with the weak reflex excitation dealt with in the preceding paper (Ito et al., 1976) ; whether it is due to spread of currents from the stimulated canal to its surround or alternatively due to the existence of a pathway
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m e d i a t i n g a weak, b u t genuine, effect from t h a t canal. W e a k i n h i b i t i o n was indueed from the anterior canal to i-SR a n d from the posterior canal to i-It~ muscle. Since i-SR muscle receives strong i n h i b i t i o n from the posterior canal a n d i-It~ muscle from the anterior canal, it is possible t h a t spread of a small fraction of currents between these two canals causes the weak inhibition. T h a t the weak i n h i b i t i o n often occurred with a threshold comparable to t h a t for the p r o m i n e n t ones does n o t necessarily exclude the possibility of the c u r r e n t spread, as the threshold does n o t d e p e n d only on the accessibility of n e r v e branches to the stimulating electrode, b u t also on the ease for impulse transfer t h r o u g h the reflex pathways. Hence, it seems wise to limit out present conclusion only to the prom i n e n t , u n q u e s t i o n a b l e effects of canal stimulation, u n t i l a f u r t h e r i n v e s t i g a t i o n is made to elucidate the cause of the weak inhibition. To summarize, the present i n v e s t i g a t i o n d e m o n s t r a t e s t h a t the t r a n s i e n t tension decrease a n d the slow muscle p o t e n t i a l c o n j o i n t l y provide a c o n v e n i e n t a n d reliable measure of the p o s t s y n a p t i e i n h i b i t i o n of oculomotor n e u r o n s produced b y l a b y r i n t h i n e impulses. Usefulness of these indices was obvious n o t only in revealing the specific canal-muscle relationship, as reported i n this paper, b u t also i n d e t e r m i n i n g the specific projection from the cerebellum to vestibulo-ocular reflex arcs (Ito et at., 1973b).
References Baker, R. G., Mano, N., Shimazu, H.- Postsynaptic potentials in abducens motoneurons induced by vestibular stimulation. Brain Res. 15, 577--580 (1969) Baker, I~., Precht, W., Berthoz, A. : Synaptic connections to trochlear motoneurons determined by individual vestibular nerve branch stimulation in the cat. Brain Res. 64, 402--406 (1973) Cohen, B., Suzuki, J., Bender, M. B. : Eye movement from semicircular canal nerve stimulation in the cat. Ann. oto-rhino-laryng. 73, 153--170 (1964) Fluur, E., MellstrSm, A. : Saecular stimulation and oculomotor reactions. Laryngoscope (St. Louis) 80, 1713--1721 (1970) Highstein, S.M.: The organization of the vestibulo-ocular and trochlear reflex pathways in the rabbit. Exp. Brain Res. 17, 285--300 (1973a) Higtlstein, S.M. : Synaptic linkage in the vestibulo-ocular and cerebello-vestibular pathways to the VIth nucleus in the rabbit. Exp. Brain Res. 17, 301--314 (1973b) Highstein, S.M., Ito, M., Tsuchiya, T.." Synaptic linkage in the vestibulo-ocular reflex pathway of rabbit. Exp. Brain Res. 13, 306--326 (1971) Ito, M., Nisimaru, N., Yamamoto, M. : The neural pathways relaying reflex inhibition from semicircular canals to extraocular muscles of rabbits. Brain Res. 55, 189--193 (1973a) Ito, M., Nisimaru, N., u M. : Specific neural connections for the cerebellar control of vestibulo-ocular reflexes. Brain Res. 66, 238--247 (1973b) Ito, M., Nisimaru, N., u M. : Pathways for the vestibulo-ocular reflex excitation arising from semicircular canals of rabbits. Exp. Brain Res. 24, 257--271 (1976) Kato, T. : [~ber histologische Untersuchungen der Augenmuskeln yon Menschen und SiiugeR tieren. Okajimas Folia anat. jap. 16, 131--145 (1938) Mano, N., Oshima, T., Shimazu, H. - Inhibitory commissural fibers interconnectingthe bilateral vestibular nuclei. Brain Res. 8, 378--382 (1968) Owada, K., Shiizu, S. : The eye movement as a saccular function. Acta oto-laryng. (Stockh.) 52, 63--71 (1960) Precht, W., Baker, R." Synaptic organization of the vestibulo-trochlear pathway. Exp. Brain Res. 14, 158--184 (1972)
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Richter, A., Precht, W. : Inhibition of abducens by vestibular nerve stimulation. Brain Res. l l , 701--705 (1968) Sasaki, K. : Electrophysiological studies on oculomotor neurons of tile cat. Jap. J. Physiol. 13, 287--302 (1963) Suzuki, J., Tokumasu, K., Goto, 14. : Eye movements from single utricular nerve stimulation in the cat. Acta oto-taryng. (Stockh.) 68, 350--363 (1969) Szent~gothai, J. : The elementary vestibnlo-ocular reflex arc. J. Neurophysiol. 13, 395--407 (1950) Wilson, V.J., Wylie, R.M., Marco, L.A.: Synaptic inputs to cells in the medial vestibular nucleus. J. Neurophysiol. 31,176--185 (1968)
Received July 14, 1975 Prof. M. Ito Department of Physiology Faculty of Medicine University of Tokyo Bunkyo-ku, Tokyo
Japan
20 Exp. Brain l~es~Vol~24
Experimental Brain Research 9 by S]pringer-Verlag1976
Postsynaptic Inhibition of Oculomotor Neurons Involved in Vestibulo-Oeular Reflexes Arising from Semicircular Canals of Rabbits* M. Ito, N. Nisimaru and M. Yamamoto Department of Physiology, Faculty of Medicine, University of Tokyo, Bunkyo-ku, Tokyo (Japan)
Summary. I n anesthetized albino rabbits, electric stimulation of vestibular nerve branches innervating semicircular canals produced not only reflex contraction in certain extraocular muse]es, but also a transient relaxation in others. From relaxing muscles was recorded a slow muscle potential that reflected depression of spontaneous spike discharges in muscle fibers. When recorded monophasically, spontaneous spikes of muscle fibers were superposed to form a direct current potential, and depression of the spikes resulted in a transient reduction of this direct current potential, i. e., the slow muscle potential. The slow muscle potential was correlated to the postsynaptic inhibition induced in oculomotor neurons through the vestibulo-oeular reflex arc for the fo]lowing reasons ; its latency was compatible with that of the IPSPs recorded from oculomotor neurons; it was removed by severing axons of the inhibitory second-order vestibular neurons; it was blocked by intravenous injection of picrotoxin as were the I P S P s in oculomotor neurons. By recording slow muscle potentials, a specific canal-muscle relationship for the vestibulo-oeular reflex inhibition of oculomotor neurons was shown to be complementary to that obtained for the vestibulo-ocular reflex excitation. Key words : Semicircular canal - - Vestibular-ocular inhibition - - Rabbit. Introduction Labyrinthine impulses induce not only contraction but also relaxation of extraocular muscles (Szents 1950; Cohen, Suzuki and Bender, 1964). Accordingly, recent investigations have revealed that stimulation of the vestibular nerve produces postsynaptie inhibition of oeu]omotor neurons (Sasaki, 1963 ; Richter and Precht, 1968 ; Baker et al., 1969 ; Highstein et al., 1971). The inhibition is mediated by a specialized group of second-order vestibular neurons which are located in the superior vestibular nucleus (Highstein et al., 1971 ; Precht and Baker, 1972; Highstein, 1973a) with the possible exception that abducens motoneurons m a y be inhibited from the rostral portion of the medial vestibular nucleus (Baker et al., 1969; Itighstein, 1973b). *
This work was supported by a grant from Educational Ministry of Japan (844021).
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The aim of the present i n v e s t i g a t i o n was to specify the reflex p a t h w a y s which convey i n h i b i t o r y action from i n d i v i d u a l semicircular canals to extraocular muscles. Together with the excitatory reflex p a t h w a y s dealt with in the preceding paper (Ito et al., 1976), principal c o m p o n e n t s of the vestibulo-ocular reflex arc were identified in terms of receptor canal, relay ~ucleus, effector muscle and reflex action, thus providing the basis for a further analysis of the cerebellar control of vestibulo-ocular reflexes (Ito et al., 1973b). A part of the present results has been published in brief (Ito et al., 1973 a).
Methods The general experimental procedures were identical with those described in the preceding paper (Ito et al., 1976), and the twenty-eight rabbits used in the present experiments were also involved in the preceding experiments. Same abbreviations as those in the preceding paper (Ito et al., 1976) will be used.
Results Relaxation o/ Extraocular Muscle8 Induced by Canal Stimulation Application of c u r r e n t pulses to a semicircular canal i n d u c e d a t r a n s i e n t t e n s i o n decrease i n certain extraocular muscles, as illustrated i n Fig. 1A a n d C for com-
A 1 , 2 ~
B ~ : ~~, . .
C ~ 0 ' 1
D 'v"~r ' l ~~lr" ~ v~m~r,"'
10msec Fig. 1. Transient tension decrease and the eoneommitant appearance of the slow muscle potential in an extraoeular muscle following stimulation of a semicircular canal, recorded from i-IR muscle while the anterior canal was stimulated. (A and C) Isometric tension of the i-Ig muscle. (B and D) Potentials developed along the i-IR muscle between the two electrodes, one inserted into the muscle belly and the other into the cut end. A and C were recorded simultaneously with B and D, respectively. In A and B, the stimulating pulses were of 5 msee duration and their amplitudes varied as indicated. In C and D the stimulating pulses were of 22 V amplitude and their width varied as shown. In this as well as in succeeding figures, recording was ac-coupled with time constant of 300 msec, and 10--20 sweeps were superimposed at a repetition rate of 1 or 2/see, unless otherwise stated
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b i n a t i o n of t h e a n t e r i o r canal a n d i - I R muscle. This tension decrease s t a r t e d w i t h latencies of 4 - - 6 msee a f t e r t h e onset of s t i m u l a t i n g pulses a n d l a s t e d for 5 0 - - 1 0 0 msee. A m p l i t u d e of t h e t r a n s i e n t t e n s i o n change increased n o t o n l y w i t h t h e increase of pulse i n t e n s i t y as in A, b u t also w i t h t h e increase of pulse d u r a t i o n as in C. I t is n o t e d in C t h a t t h e r e l a x a t i o n was a p p a r e n t even when t h e pulse d u r a t i o n was as s h o r t as 0.1 msec a n d t h a t i t was increased g r a d u a l l y as t h e pulse d u r a t i o n i n c r e a s e d u p to 10 msec. This c o n t r a s t s to t h e fact t h a t c u r r e n t pulses h a v i n g a d u r a t i o n s h o r t e r t h a n 2 - - 3 msec were h a r d l y effective in evoking reflex c o n t r a c t i o n of e x t r a o c u l a r muscles (Ito et al., 1976). I n o t h e r words, in the reflex r e l a x a t i o n t h e r e was no a p p a r e n t t h r e s h o l d value for pulse d u r a t i o n such as t h a t o b t a i n e d in t h e reflex e x c i t a t i o n (Discussion).
Slow Muscle Potential Accompanying the Musde Relaxation As r e c o r d e d w i t h t w o needle electrodes i n s e r t e d into a n e x t r a o c u l a r muscle, one into t h e muscle belly a n d t h e o t h e r into t h e distal cut end (Fig. 3G), t h e muscle r e l a x a t i o n was r e g u l a r l y a c c o m p a n i e d b y a r e l a t i v e l y slow p o t e n t i a l of opposite sign to t h e muscle spike p o t e n t i a l (i. e., t h e p r o x i m a l electrode positive r e l a t i v e to t h e d i s t a l electrode). This p o t e n t i a l , t h e slow muscle p o t e n t i a l as it m a y be called, l a s t e d for 1 0 - - 2 0 msec a n d u s u a l l y was followed b y a r e b o u n d . As seen in Fig. 1 a n d p l o t t e d in Fig. 2, m a g n i t u d e of t h e slow muscle p o t e n t i a l increased m o n o t o n i c a l l y w i t h t h a t of t h e t r a n s i e n t tension change, as t h e pulse i n t e n s i t y or d u r a t i o n was increased. T h a t t h e slow muscle p o t e n t i a l was n o t an a r t i f a c t due to m o v e m e n t of t h e electrodes d u r i n g muscle r e l a x a t i o n was i n d i c a t e d b y t h e f a c t t h a t it 0/4
m
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Fig. 2. gelationship between the muscle relaxation and the slow muscle potential. Inset diagram indicates how to measure peak amplitudes of the transient decrease of the muscle tension (rot) and the slow muscle potential (stop). Arrows mark the moment of onset o2 stimulating currents. The slow muscle potential is plotted in ordinate and the muscle tension in abscissa. Taken from the series partly illustrated in Fig. 1
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a p p e a r e d even when t h e m o v e m e n t was negligible u n d e r e x t e n s i v e st r et ch of t h e muscle and t h a t e v e n a large m o v e m e n t due to passive s t r e t c h of t h e muscle produced only a p o t e n t i a l change considerably smaller t h a n t h e actual slow muse]e potential. T h e following e x p e r i m e n t s i n d i c a t e d t h a t t h e slow muscle p o t e n t i a l is due to depression of s p o n t a n e o u s spike discharges from o c u l o m o t o r neurons.
Origin o/ the Slow Muscle Potential Th e slow muscle p o t e n t i a l could n o t be recorded when t h e t w o recording needle electrodes were paired w i t h a small ti p s e p a r a t i o n of a b o u t 1 m m and were inserted into t h e muscle belly (Fig. 3G) ; silencing of sp o n t an eo u s spike discharges
] D
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0 -5 0 10 20 msec Fig. 3. Relationship between the slow muscle potential and spike potentials of muscle fibers. (A--D) Single sweep records from i-IR muscle. In A and B, tip separation of the recording electrodes was 1 mm (bi in G), and in C and D about 4 mm (too). In A and C the anterior canal was stimulated. Horizontal bars attached in these as well in succeeding figures indicate the period of passage of stimulating currents. In B and D the IIIrd nucleus was directly stimulated with brief pulses (0.1 msee duration). (E) Upper traces, similar to C, but recorded from i-SR muscle with stimulation of the posterior canal. Lower traces, unitary spikes recorded simultaneously with a glass microelectrode inserted in that i-SR muscle. (F) Poststimulus time histogram obtained for unitary spike discharges similar to those in the lower traces of E. 1 bin width, 0.2 msec. 400 sweeps repeated at a rate of 5 sweeps per sec. Downward and upward arrows indicate the onset and cessation of the stimulating current. (G) Arrangement of the recording electrodes, m microelectrode. Voltage scale; l0 mV for B, D, 0.5 mV for the lower traces of E, 0.25 mV for the upper traces of E, and 70/~V for A and C. Time scale of 5 msec ~pplies to A, C and E, and that of 2 msec to B and D
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was the only visible effect accompanying the reflex relaxation (Fig. 3A). With this recording arrangement, spike potentials evoked by direct stimulation of I I I r d nucleus assumed a biphasic form (B). When one of the two needle electrodes was shifted to the cut end of the muscle, the spike potential became nearly monophasic (D) and the slow muscle potential emerged (C). The occurrence of the slow muscle potential in the monophasic recording can be explained satisfactorily by assuming that spontaneous spike potentials of many muscle fibers as recorded monophasieMly are summated to produce a direct current potential between the two recording electrodes; depression of the spontaneous spike discharges would result in reduction of this direct current potential, thereby yielding the slow muscle potential with the sign opposite to that of spike potentials. With the biphasic recording, there would be no direct current potential developed during spontaneous spike discharges and consequently no initiation of the slow potential during depression of the discharges, for the time integral over the two phases of spike potentials is approximately zero (Fig. 3B). I n agreement with this view, recording with a micro-electrode inserted into a relaxing muscle revealed that individual muscle fibers indeed stopped their spontaneous discharges during the slow muscle potential (Fig. 3E, G). The poststimulus histogram for individual muscle fibers exhibited a time course closely resembling that of the slow muscle potential (Fig. 3F). Development of a direct current potential of a millivolt along extraocular muscles during spontaneous discharges in individual muscle fibers can be indicated by the following calculation. The time integral of the full-sized spike potential induced by direct stimulation of IIIrd nucleus was of the order of 10 mV.msee (see Fig. 3D). Since each extraoeular muscle of rabbits contains about ten thousand muscle fibers (Kato, 1938), individual fibers may contribute to this spike potential by 10-3 mV-msec per impulse on the average. If these fibers discharge spontaneously at a rate around 100 impulses per second (as in Fig. 3F), 103 spikes will appear in a muscle per msec. These spikes will be integrated to yield a direct current potential of 1 mV in amplitude.
Relationship o/the Slow Muscle Potential with the Postsynaptic Inhibition in Oculomotor Neurons The depression of spontaneous discharges in oculomotor neurons as represented by the slow muscle potential can be related to the postsynaptie inhibition of oculomotor neurons for the following reasons. First, the latency of the slow muscle potential was determined accurately on the averaged curves (Fig. IA) and was 3--6 msee. This was approximately the sum of the latency for postsynaptic inhibition in oculomotor neurons (range, 1.I--2.9 reset; mean, 1.72 msec; Highstein et al., 1971) plus that for impulses to propagate from oeulomotor neurons to muscle electrodes (1--2 msec, Ito et al., 1976; see Fig. 3B, D). Second, the slow muscle potential in those muscles innervated by the I I I r d and IVth nuclei motoneurons was removed by cutting the medulla rostromedially to the superior vestibular nucleus (Fig. 4 B - - D ) which mediates the postsynaptic inhibition to I I I r d and IVth nuclei (Highstein et al., 1971 ; ttighstein, 1973 a ; Preeht and Baker, 1972). The slow muscle potential was influenced by severance of neither the braehium eonjunetivum nor the contralateral fasciculus longitudinalis medialis (not illustrated) which are pathways for the vestibulo-ocular excitation arising from the
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A
C
,
, msec
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Fig. 4. Features of the slow muscle potentials. (A) Upper traces, the slow muscle potential evoked in i-IR muscle by stimulation of the anterior canal. Lower traces, same as the upper traces but averaged for 40 successive sweeps. Downward arrow indicates the onset of the slow muscle potential. (C) Similar to the upper traces of A. Upper traces, i-It~ muscle. Lower traces c-SO muscle. Stimulation of the anterior canal. (D) Same as C but after lesion had been placed in the ipsilateral faseiculus longitudinalis medialis at the level rostral to the superior vestibular nucleus, as shown in B. (E) Monopolar stimulation of the labyrinth. Upper traces, slow muscle potentials recorded from c-SP~ muscle. Lower traces, spike discharges simultaneously recorded from i-SR muscle. (F) Same as in E, but after picrotoxin (10 mg/kg) had been injected intravenously. Voltage scale is 140/IV for upper traces of A, E, F and both upper and lower traces of C and D, and 2 mV for lower traces of E and F. Time scale of 5 msec applies to A, and that of 10 msee to others a n t e r i o r a n d posterior canals (Ito et al., 1976). Third, t h e slow muscle p o t e n t i a l was blocked b y i n t r a v e n o u s i n j e c t i o n of p i c r o t o x i n (10 mg/kg) j u s t as were t h e i n h i b i t o r y p o s t s y n a p t i c p o t e n t i a l s (IPSPs) in o e u l o m o t o r neurons (Highstcin et al., 1971). E v e n t h o u g h t h e o b s e r v a t i o n was l i m i t e d to a r e l a t i v e l y short p e r i o d before i t was i n t e r r u p t e d b y convulsion, t h e r e was a l m o s t c o m p l e t e e l i m i n a t i o n of t h e slow muscle p o t e n t i a l (Fig. 4E, F), while t h e reflex discharges e v o k e d in o t h e r muscles were n o t significantly affected (E, F). F o u r t h , t h e possible c o n t r i b u t i o n of t h e p o s t s y n a p t i e i n h i b i t i o n of second-order v e s t i b u l a r neurons (Mano et al., 1968 ; W i l s o n et al., 1968) to t h e slow muscle p o t e n t i a l was e x c l u d e d b y c u t t i n g t h e commissural fibers. This inhibition, if i t a c t e d u p o n those second-order neurons e x e r t i n g e x c i t a t o r y influences u p o n o c u l o m o t o r neurons, would result in withd r a w a l of b a c k g r o u n d facilitation f r o m o e u l o m o t o r neurons, i.e., disfacilitation, which m a y cooperate w i t h t h e p o s t s y n a p t i c i n h i b i t i o n in o c u l o m o t o r neurons~ I n two p r e p a r a t i o n s , a l o n g i t u d i n a l cut was m a d e along t h e midline of t h e medulla, 3 m m deep from t h e dorsal surface of t h e medulla. A n t e r i o r l y , t h e cut r e a c h e d t h e level of t h e locus coeruleus a n d p o s t e r i o l y covered t h e r o s t r a l a r e a in one p r e p a r a t i o n a n d t h e entire s t r e t c h in t h e other, of t h e m e d i a l v e s t i b u l a r nucleus. Slow muscle p o t e n t i a l s o b s e r v e d in i-II~ a n d c-SO muscles of these p r e p a r a t i o n s d i d n o t show a n y a p p r e c i a b l e difference before a n d after t h e cut.
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The Canal-Muscle Relationship/or the Slow Muscle Potential B i p o l a r s t i m u l a t i o n o f t h e a n t e r i o r c a n a l r e g u l a r l y p r o d u c e d p r o m i n e n t slow m u s c l e p o t e n t i a l s in t w o p a r t i c u l a r m u s c l e s , i.e., i-It~ a n d c-SO, as i l l u s t r a t e d in F i g . 5 ; in t h e s e m u s c l e s , t h e slow m u s c l e p o t e n t i a l was p r o d u c e d w i t h a s i m i l a r l y i-IR
i-IO
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Fig. 5. Slow muscle potentials evoked by stimulation of the anterior canal. Records taken simultaneously from 5 muscles with various stimulus intensities indicated (as inultiple of the threshold T). Note that reflex discharges instead of slow muscle potentials were induced in c-IO muscle. Voltage scale of 2 mV applies to c-IO and that of 0.5 mV to others A
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I0,2mV 10msec Fig. g. Slow muscle potentials evoked by stimulation of the posterior and horizontal canals. (A) Selective stimulation of the posterior canal. (B) That of the horizontal canal. (C) ~onopolar stimulation with an electrode placed in the horizontal canal. Muscles from which potentials were recorded are indicated. In A and B stimulus intensities are given as the multiple of the threshold for i-IO and i-LR, respectively. In C the strength of stimulating currents is indicated in #A
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low threshold and developed with a similar stimulus-response relationship. I n i-IO and e-SR muscles illustrated for comparison, stimulation of the anterior canal produced virtually no potential change. The slow muscle potential in i-SR in some cases was of significant amplitude, yet it was much smaller than those in i - I g and c-SO muscles. Figure 5 also shows the reflex discharges evoked in e-IO muscle, as described in the preceding paper (Ito et al., 1976). I t was regularly seen that the threshold for evoking the slow muscle potential was considerably lower than that for initiating reflex excitation (Discussion). Similar stimulation of the posterior canal revealed that two muscles, i-IO and c-SI~, exhibited prominent slow muscle potentials as shown in Fig. 6A, with equally low thresholds. The posterior canal stimulation sometimes induced a small slow muscle potential also in i-IR muscle with the threshold similar to those for i-IO and c-SR muscles. This effect for i-It~ muscle, however, was not regularly observed. The stimulation of the horizontal canal always evoked a prominent slow muscle potential in i-LR muscle, as illustrated in Fig. 6B. A prominent slow muscle potential was induced also in e-MR muscle but with a threshold about twice as high as that for i-LR muscles. Two to five-fold differences of the threshold between i-LR and c-MR were consistently observed in 4 experiments where the stimulation of the horizontal canal was performed with great care. Figure 6C illustrates the effects of monopolar stimulation in the horizontal canal which is presumed to activate all branches of the vestibular nerve non-selectively (Ito et al., 1976). The slow muscle potentials were in fact induced in all of the i-IR, c-SO, i-IO, c-SI~ and i-LR muscles with comparable threshold values as exemplified by i-LR and i-IR. Yet, the threshold for e-Mt~ muscle was about three times as high as those for other muscles. Similar observation was made in 3 preparation. Implication of these observations will be discussed later (Discussion).
Discussion I n the present investigation, the reflex inhibition was monitored by depression of spontaneous discharges from oeulomotor neurons as reflected in transient decrease of the muscle tension as well as in appearance of the slow muscle potential, while the reflex excitation was evaluated by synchronized discharges from oculomotor neurons. Spontaneous discharges from oculomotor neurons may readily be influenced by their postsynaptic inhibition, in a more or less proportionate manner, whereas synchronized reflex discharges may be evoked only when the postsynaptic excitation rises over a certain critical value. This may explain, at least partly, why the reflex inhibition was induced with a lower threshold intensity of stimulating currents than the reflex excitation and without apparent threshold for current duration such as obtained for the reflex excitation (Ito et al., 1976). The possibility, however, still remains that the reflex inhibition and excitation differ from each other in the ease of impulse transmission from the primary to second-order vestibular neurons. The present investigation demonstrated that with stimulation of each canal postsynaptic inhibition occurred in oeulomotor neurons of two muscles antagonis-
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c-SO i-LR
9 HC
VN
| I
Fig. 7. Schematic drawing of the inhibitory vestibulo-ocular reflex pathways. III IIIrd nucleus. IV Trochlear nucleus. VI Abducens nucleus. VN Vestibular nuclear complex, M V Medial vestibular nucleus, S V Superior vestibular nucleus. Other symbols are defined in the text. Vertical interrupted lines indicate the mid sagittM line. Inhibitory second-order neurons were located according to Baker et al. (1969), Highstein et al. (1971), Precht and Baker (1972), Highstein (1973a, b). In A and C it is assumed that pathways to the two muscles influenced by a canal are relayed by the same inhibitory second-order neuron. This is simply because there is no special reason to postulate separate groups of inhibitory neurons bound to different muscles, except for i-LR and c-MR muscles in B. Note that those muscles excited by canal signals are drawn by dotted lines
tie to those which were excited with the same stimulation (Ito et al., 1975). Figure 7 illustrates the pathways and the specific pattern of distribution of the vestibuloocular reflex inhibition thus determined. The pattern for c-SO and i-LR muscles is in good agreement with microelectrode studies on IVth (Baker et al., 1973) and VIth (Baker et al., 1969; Highstein, 1973b) nucleus motoneurons. Concerning the inhibitory pathway to c-MR motoneurons, the threshold for evoking the slow muscle potential in this muscle from the horizontal canal is exceptionally high (Fig. 6B), and therefore it is necessary to exclude the possibility that this effect is derived by current spread out of the horizontal canal to other endorgans. Spread to the anterior and posterior canals does not account for this effect, because selective stimulation of these canals did not produce the slow muscle potentials in c-MR muscle. Direct stimulation of the utricle produces contraction in c-MR muscle instead of its relaxation (Suzuki et al., 1969). And, there is no evidence indicating that the c-MR muscle is influenced by stimulation of the saccule (Owada and Shiizu, 1960; Fluur and MellstrSm, 1970). The present observation that the threshold for evoking the slow muscle potential in c-MR was high even with monopolar stimulation of all vestibular nerve branches (Fig. 6C) indicates that the high threshold is not due to remoteness of the responsible endorgan to the stimulating electrode, but that the reflex pathway leading to the postsynaptic inhibition of c-Ml• motoneurons from the horizontal canal has a high threshold locus in its course. Weak effects obtained besides the prominent inhibition of Fig. 7 would also be a matter of argument, just as with the weak reflex excitation dealt with in the preceding paper (Ito et al., 1976) ; whether it is due to spread of currents from the stimulated canal to its surround or alternatively due to the existence of a pathway
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m e d i a t i n g a weak, b u t genuine, effect from t h a t canal. W e a k i n h i b i t i o n was indueed from the anterior canal to i-SR a n d from the posterior canal to i-It~ muscle. Since i-SR muscle receives strong i n h i b i t i o n from the posterior canal a n d i-It~ muscle from the anterior canal, it is possible t h a t spread of a small fraction of currents between these two canals causes the weak inhibition. T h a t the weak i n h i b i t i o n often occurred with a threshold comparable to t h a t for the p r o m i n e n t ones does n o t necessarily exclude the possibility of the c u r r e n t spread, as the threshold does n o t d e p e n d only on the accessibility of n e r v e branches to the stimulating electrode, b u t also on the ease for impulse transfer t h r o u g h the reflex pathways. Hence, it seems wise to limit out present conclusion only to the prom i n e n t , u n q u e s t i o n a b l e effects of canal stimulation, u n t i l a f u r t h e r i n v e s t i g a t i o n is made to elucidate the cause of the weak inhibition. To summarize, the present i n v e s t i g a t i o n d e m o n s t r a t e s t h a t the t r a n s i e n t tension decrease a n d the slow muscle p o t e n t i a l c o n j o i n t l y provide a c o n v e n i e n t a n d reliable measure of the p o s t s y n a p t i e i n h i b i t i o n of oculomotor n e u r o n s produced b y l a b y r i n t h i n e impulses. Usefulness of these indices was obvious n o t only in revealing the specific canal-muscle relationship, as reported i n this paper, b u t also i n d e t e r m i n i n g the specific projection from the cerebellum to vestibulo-ocular reflex arcs (Ito et at., 1973b).
References Baker, R. G., Mano, N., Shimazu, H.- Postsynaptic potentials in abducens motoneurons induced by vestibular stimulation. Brain Res. 15, 577--580 (1969) Baker, I~., Precht, W., Berthoz, A. : Synaptic connections to trochlear motoneurons determined by individual vestibular nerve branch stimulation in the cat. Brain Res. 64, 402--406 (1973) Cohen, B., Suzuki, J., Bender, M. B. : Eye movement from semicircular canal nerve stimulation in the cat. Ann. oto-rhino-laryng. 73, 153--170 (1964) Fluur, E., MellstrSm, A. : Saecular stimulation and oculomotor reactions. Laryngoscope (St. Louis) 80, 1713--1721 (1970) Highstein, S.M.: The organization of the vestibulo-ocular and trochlear reflex pathways in the rabbit. Exp. Brain Res. 17, 285--300 (1973a) Higtlstein, S.M. : Synaptic linkage in the vestibulo-ocular and cerebello-vestibular pathways to the VIth nucleus in the rabbit. Exp. Brain Res. 17, 301--314 (1973b) Highstein, S.M., Ito, M., Tsuchiya, T.." Synaptic linkage in the vestibulo-ocular reflex pathway of rabbit. Exp. Brain Res. 13, 306--326 (1971) Ito, M., Nisimaru, N., Yamamoto, M. : The neural pathways relaying reflex inhibition from semicircular canals to extraocular muscles of rabbits. Brain Res. 55, 189--193 (1973a) Ito, M., Nisimaru, N., u M. : Specific neural connections for the cerebellar control of vestibulo-ocular reflexes. Brain Res. 66, 238--247 (1973b) Ito, M., Nisimaru, N., u M. : Pathways for the vestibulo-ocular reflex excitation arising from semicircular canals of rabbits. Exp. Brain Res. 24, 257--271 (1976) Kato, T. : [~ber histologische Untersuchungen der Augenmuskeln yon Menschen und SiiugeR tieren. Okajimas Folia anat. jap. 16, 131--145 (1938) Mano, N., Oshima, T., Shimazu, H. - Inhibitory commissural fibers interconnectingthe bilateral vestibular nuclei. Brain Res. 8, 378--382 (1968) Owada, K., Shiizu, S. : The eye movement as a saccular function. Acta oto-laryng. (Stockh.) 52, 63--71 (1960) Precht, W., Baker, R." Synaptic organization of the vestibulo-trochlear pathway. Exp. Brain Res. 14, 158--184 (1972)
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Received July 14, 1975 Prof. M. Ito Department of Physiology Faculty of Medicine University of Tokyo Bunkyo-ku, Tokyo
Japan
20 Exp. Brain l~es~Vol~24
