Midbrain and contralateral labyrinth influences on brain stem vestibular neurons in the cat

312

BRAIN RESEARCH

M I D B R A I N A N D C O N T R A L A T E R A L L A B Y R I N T H I N F L U E N C E S ON B R A I N STEM V E S T I B U L A R N E U R O N S I N T H E CAT

CHARLES H. MARKHAM UCLA School of Medicine, Division of Neurology and the Brain Research Institute, I~)s Angele~ Calif. 90024 (U.S.A.)

(Accepted January 29th, 1968)

INTRODUCTION The contralateral labyrinth has been shown to influence neural activity in the ipsilateral vestibular complex 6,13.19,22. Especially pertinent to the present work has been the demonstration that vestibular neurons which were physiologically connected to the horizontal semicircular canal on the same side were almost invariably inhibited by stimulation of the contralateral vestibular nerve 2z. One of the questions considered in the present study is whether vestibular neurons in the brain stem related to the anterior canal are similarly influenced by the opposite labyrinth. The second part of the study concerns itself with midbrain modulation of vestibular activity and compares this with that mediated by the commissural inhibitory pathway connecting the two labyrinths. This aspect of the study rests on past work showing that stimulation of the interstitial nucleus of Cajal inhibited vestibular neurons in the brain stem which were physiologically connected to the horizontal canaP 6. This phenomenon is found ipsilaterally, is conducted via the medial longitudinal fasciculus, and may be mediated through an intercalated inhibitory neuron in the vestibular complex. It is important to find out if this effect has neural pathways in c o m m o n with the commissural inhibitory effect and whether there is a similar action on brain stem neurons primarily connected with the anterior semicircular canal. Answers to these questions may clarify the role of the interstitial nucleus of Cajal in rotating and vertical head and eye movements 9.l°.26. METHODS The experiments were performed on 42 adult cats weighing between 2.5 and 3.5 kg. Under ether anesthesia, a tracheal cannula was inserted, the head was fixed in a stereotaxic frame (Trent Wells. Jr., Southgate. Calif.) and the cervical spinal cord was transected at about Ca. Artificial respiraticn was maintained for the duration of the experiment. The head was first rotated so that the inner ear could be surgically Brain Research, 9 (1968) 312-333

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approached from a ventrolateral direction. The mastoid bulla and bone posterior to the tympanic membrane were exposed and an opening made into the tympanic cavity. The branches of the vestibular nerve innervating the utricle and the anterior and horizontal semicircular canals were exposed by fracturing or drilling away the bone between the round and oval windowsL A stimulating electrode consisting of an insulated silver wire 0.1 mm in diameter and bared at the tip was placed proximally on one of these nerves in order to deliver stimulating currents as effectively as possible to the branch from the posterior canal. An indifferent electrode was inserted into the basal turn of the cochlea. The electrodes were held in place with dental cement, and the tympanic cavity was filled with petroleum jelly. Only infrequently was damage to the end organ indicated by the development of spontaneous nystagmus. After placement of the vestibular nerve-stimulating electrodes, the cat was rotated to a prone position and bilateral burr holes were made for the insertion of the midbrain-stimulating electrode. The cerebellum was exposed and left intact in most instances. In 12 cats, the medial portion of the cerebellum was sucked away to expose the floor of the fourth ventricle. After the surgical procedures, the ether anesthesia was discontinued and the scalp incisions and pressure points infiltrated with 1}Jo Xylocaine. The local anesthesia was renewed periodically throughout the experiment. The stereotaxic frame holding the cat was then secured to a power-driven gimbal (Trent Wells, Jr.), a device which allowed the cat's head to be rotated about the intraaural and naso-occipital axes. q-he gimbal was in turn placed on a TSnnies turntable (Dr. Ing. J. F. Tannies, Freiburg/Br.) so that the cat's head was at the center of rotation of the turntable. Vestibular units were recorded extracellularly with glass microelectrodes filled with 2 M sodium chloride and fast green FCF dye 3°. The DC resistance of the electrodes was from 2 to 6 M-Q. The action potentials from the vestibular complex were led to one channel of a Tektronix 502 oscilloscope via an input cathode follower mounted on the TSnnies turntable, and a TSnnies capacity-coupled amplifier. The oscilloscope trace was photographed with a Grass C-4 kymograph. A time constant of 10 msec was used. The vestibular complex was located with the aid of a stereotaxic atlas 23 in the cats with an intact cerebellum and by direct observation when the fourth ventricle was exposed. The glass microelectrode was directed approximately perpendicularly to the floor of the fourth ventricle. Vestibular units were identified physiologically by their responses to rotational acceleration (see below) and by their responses to stimulation of the contralateral vestibular nerve. The immediate vicinity of many units was marked by electrolytic or fast green FCF lesions and the lesions later localized histologically. In stimulating the midbrain, particular attention was directed to the interstitial nucleus of Cajal. Also frequently stimulated were structures up to 5 mm dorsal and ventral, and 1 mm medial and 2 mm lateral to the interstitial nucleus. Included in this area was the medial part of the superior colliculus, the posterior commissure, the nucleus of the posterior commissure, the central gray, the nucleus of Darkschewitsch, the medial longitudinal bundle and the upper part of the red nucleus. The area stimuBrain Research, 9 (1968) 312-333

314

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lated extended to the fasciculus retroflexus region rostrally and caudally to the middle of the third nerve complex. Midbrain stimulation was performed with a monopolar steel electrode with a D C resistance of about 0.5 M~2. The indifferent electrode was placed in the temporalis muscle. The stimulus originated from Tektronix pulse and wave form generators, passed through an isolation transformer and was monitored for voltage and current. Square waves of 0.1 msec duration and 0.1-10 V were used. Frequencies of stimulus ranged from 1 to 100/sec. The sites of the midbrain stimulation were marked with electrolytic lesions which were later located histologically. The vestibular nerve was stimulated with parameters comparable to those used in the midbrain. About 2 h after ether had been discontinued, the cat was positioned with the naso-occipital axis depressed 30 '~' so the horizontal canals were in the plane of the turntable's rotation. Field responses in the vestibular complex on stimulating the ipsilateral vestibular nerve were usually first examined to determine threshold of the first negative wave (N1) z0. The thresholds of firing of individual units on ~timulating the vestibular nerve was then divided by the N~ threshold; this relative threshold was used since this tended to circumvent the variations in threshold produced by different degrees of contact of the stimulating electrode with the vestibular nerve. After examining one or more units in relation to the horizontal canals, the anterior canal on the same side as the recording electrode and the contrataterat posterior canal were brought into the plane of rotation of the turntable. This was done by rotating the cat's head 70 ° about th.~ naso-occipitat axis and pointing the cat's nose upward with the naso-oceipital axis angled 45 ° fi'om the horizontal. Vestibular units in relation with the underlying anterior canal were then identified by their responses to angular acceleration. Only tonic vestibular units, i.e. spontaneously firing neurons with a low threshold of response to angular acceleration ~1 were used in the present experiment. RESULTS

Identification and classification o[ vestibular unit responses Vestibular neurons were classified by their responses to angular acceleration. Those neurons which were accelerated by angular acceleration inducing ampullopetal flow in the horizontal canal and were slowed by acceleration in the opposite direction, were called 'type I neurons of the horizontal canal'. Units which showed reciprocal behavior to the type I cells were called 'type II neurons of the horizontal canal'. The few units excited by rotatory acceleration to either side were called 'type 111 neurons of the horizontal canal'. The phrase ' o f the horizontal canal' is added to the previous designations of type I (refs. 4, 7, 21), type II (refs. 4, 21), and type I11 (refs. 4, 21). Units which had their resting discharge rate increased by angular acceleration which induced amputlofugal endolymph flowin the anterior canal and decreased by acceleration in the opposite direction were called 'type I neurons of the anterior canal'. Neurons which responded in an inverse fashion were designated 'type I I neurons

Brain Research, 9 (1968) 312-333

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31 5

Fig. 1. Klfiver stain of a section passing t h r o u g h the rostral part o f the medial vestibular nucleus. A r r o w shows a lesion in the inferior part of the medial nucleus, m a d e at the site of a type II n e u r o n o f the anterior canal. A b o u t 50/~ higher a type II n e u r o n of the horizontal canal was also found.

of the anterior canal'. This was done even though the opposite posterior canal seems to play a role in their responses (see below). Neurons which increased their resting rate by rotatory acceleration in either direction were called 'type 111 neurons of the anterior canal'. in the course of the experiments, 35 type 1, 18 type II and 3 type Iil units of the horizontal canal were found. Most of these were found in the initial 30 rain of the recording period. Further, there were usually many other units sensitive to horizontal canal stimulation in the same fields. Thirty-four type I, 11 type II and 3 type Ill neurons of the anterior canal were physiologically identified. Many of these vestibular units were in the same electrode track as, or were only several hundred microns away from units responsive to horizontal canal physiological stimulation. These neurons were found over several hours of searching in most of the animals. During the hunt for units responsive to rotatory acceleration in the plane of the anterior canal, more units were unresponsive to this stimulus than when the horizontal canal was being similarly stimulated. While an accurate count was not kept, it seems likely there are at least 2 times as many vestibular neurons responsive to angular acceleration of the horizontal canals as compared to the anterior canals. The type I neurons of the anterior canal had resting discharges ranging from 0 to 30/sec and the amplitudes of the spikes were in the same range as type l neurons of the horizontal canal, namely 300 1000/~V (ref. 21). The thresholds of response to Brain Research, 9 (1968) 312-333

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Fig. 2. To show the principal kinds of unit response observed in the experiment. A, shows a small type I unit of the anterior canal, located in the rostral-inferior part of the medial nucleus, being antidromically activated by stimulation of the interstitial nucleus of Cajal. It follows 5/sec single shocks and double shocks with 1.2 msec interval at nearly fixed latency. The intensity was 2:5 times threshold. Each tracing had about 15 sweeps. The positive component (downward) was also large in the resting state (not shown); this was not uncommonly seen in apparently healthy, spontaneously firing vestibular neurons. B, shows a type I1 neuron being transsynaptically facilitated by stimulation of the ipsilateral interstitial nucleus of Cajal at 2.5 V, 5/sec frequency. Stimulation is shown in the right picture and resting activity on the left. Ten sweeps were superimposed in each. C, shows a type I unit of the anteriOr canal, histologically located in the lateral part of the medial nucleus, being inhibited on stimulation of the ipsilateral interstitial nucleus of CajaL About 50 sweeps were superimposed in the resting (on left) and stimulus records. Stimulus was at 5/sec and 5 V.

rotational acceleration seemed to be higher than thresholds of type I neurons of the horizontal canal; this will be the subject of a future report. Of the type I neurons related to the horizontal semicircular canals, 10 were histologically located (Fig. 1). Four were in the superior nucleus, 5 in the rostral half of the medial nucleus and 1 in the lateral nucleus. Two type II cells of the horizontal canal were located histologically, both being in the inferior part of the superior nucleus. Type 1 neurons of the anterior canal were located histologically in 19 instances. Brain Research, 9 (1968) 312-333

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317

Three were in the superior nucleus and 16 in the rostral part of the medial nucleus. Seven type 11 neurons were similarly located; 5 were in the inferior and rostral part of the medial nucleus and 2 were in the superior nucleus (the depth was not clear). The locations of these neurons of the anterior canal correspond quite closely to the terminal degeneration studies of Stein and Carpenter 24 following destruction of the primary neurons in serving both the horizontal and anterior canals. On electrical stimulation of the structures outside of the brain stem vestibular complex, vestibular units responsive to angular acceleration gave 3 types of responses : (1) responses which were considered antidromic on the basis of relatively short latencies that fluctuated by no more than 0.5 msec, the ability to respond to slow and fast frequency stimulation and the ability to follow 2-3 msec interval double shock stimulation (Fig. 2A). No attempt was made to observe collisions between orthodromically fired action potentials and the antidromic spikes al. (2) Neurons were considered to be transsynaptically activated when they showed fluctuating and longer latencies (Fig. 2B) and failed to follow double shocks at 2-3 msec intervals. (3) Neurons were defined as inhibited when their resting rate was decreased on stimulation. This was most evident with 50-100/sec stimulation but occasionally occurred with slower rates (Fig. 2C). Type I neuron inhibition on stimulation o f the contralateral vestibular nerve

Weak stimulation of the contralateral vestibular nerve inhibited 12 of 16 type I neurons of the horizontal canal, as has previously been described by Shimazu and Precht 2'~. The resting discharges of the remaining 4 neurons were not affected. The inhibition took place with stimulation frequencies from 2 to 50/sec, being best demonstrated at the higher frequencies. In 4 instances in which the N1 threshold of the ipsilateral evoked potential was measured, the relative voltage thresholds determined by dividing the voltage which inhibited the unit by the N1 threshold averaged 1.7 ± 0.2 (standard deviation). Stimulation with voltages of more than 4 times the N1 threshold value, or roughly 3 times the threshold of the unit itself increased the firing rate of these neurons. This excitation presumably occurs through reticulovestibular connections, crossing the midline deep to the commissural fibers between the two vestibular complexes z2. The suppression of unit activity, as determined by multiple superimposition of oscilloscope traces before and during stimulation, occurred during a period from 4 to 6 msec to 20-30 msec (Fig. 3A). When the stimulation voltage or frequency was increased, the inhibition became more prolonged. These values of threshold of stimulation and durations of inhibitions are in the range observed by Shimazu and Precht zz. Type I neurons of the anterior canal also showed inhibition on stimulation of the contralateral vestibular nerve. As with the type l neurons of the horizontal canal, suppression of resting activity could be obtained with frequencies of from 2 to 50/sec with the higher frequencies being most effective. Sixteen of 24 units showed distinct suppression, but even with high frequency stimulation, it was rarely complete. Six showed no response to voltages up to 4 times the N1 threshold, even with high freBrain Research, 9 (1968) 312 333

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Fig. 3. To show inhibition of type I units on stimulation of the contralateral vestibular nerve. Resting records are on the left and stimulus on the right. EaCh record is made up of ab0ut 50 sweeps. A, shows a type I unit of the horizontal canal histologically located in the rostral part of the medial nucleus, being stimulated at 5/sec, 2.0 V (2 times N1 threshold). Inhibition begins at about 4 msec, typical of such neurons. B, shows a type I unit of the anterior canal being stimulated at 3 V (3 times NI threshold), 10/sec. Partial inhibition starts at about 12 msec; at the stimulus parameter necessary to produce this partial inhibition, some earlier excitation may be seen. C, shows a type I unit of the anterior canal being partially inhibited at about 8 msec latency by stimulation at 5/sec; 3.5 V (3.5 times N1 threshold). quency stimulation. Two showed n o response o n increasing voltage until they became facilitated at 2.8 and 3 times the N1 threshold voltage. The r a t i o of thresholds of u n i t i n h i b i t i o n a n d the N1 evoked response averaged 2,0 ± 0.8 (n ~- 9); this is c o m p a r a b l e to the values o b t a i n e d from type I units of the h o r i z o n t a l canal in the present study a n d the more extensive investigation of S h i m a z u a n d Precht sz. I n contrast to the similar thresholds a n d resting frequencies of the type I u n i t s of the horizontal a n d a n t e r i o r canals, the onset o f i n h i b i t i o n seems to be different: The type 1 units of the anterior canal were occasionally suppressed a s early as a b o u t Brain Research, 9 (1968) 312-333

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Fig. 4. Three vestibular units in the medial nucleus of one cat and their responses to stimulation at different depths in the same electrode path in the midbrain. The ordinates of the graphs represent midbrain depth with '0' representing the interstitial nucleus of Cajal (Int.). The transverse section of the midbrain from the same cat shows the electrode track and lesion (arrow) in the interstitial nucleus. All stimulations were performed at 0.1 msec duration, 10/sec. Open circles represent inhibition of vestibular unit, x facilitation and filled circles represent no response. Unit 6 was histologically located by a fast green FCF lesion in the lateral part of the medial nucleus.

5 msec, but often did not begin until a delay o f 9-11 msec (Figs. 3B and C). The inhibition then lasted 2 0 - 3 0 m s e c f r o m the time o f stimulus. Possible explanations for this difference will be discussed later. Before dealing with the effect on type II neurons w h i c h seem to be intercalated in the inhibitory activity o f type I neurons on contralateral vestibular nerve stimulation '~2, I w a n t to consider the influences on type 1 cells exerted by m i d b r a i n stimulation.

Type I vestibular neuron responses to midbrain stimulation As noted in the M e t h o d s section, the medial midbrain and pretectal region Brain Research, 9 (1968) 312 333

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surrounding the interstitial nucleus of Cajal was stimulated. This included Hassler's nucleus prestitialis, which lies medial, rostral and inferior to the interstitial nucleus, mostly at the anterior-posterior plane of the fasciculus retroflexus 1'-'. Type I vestibular neurons of the horizontal canal have been previously shown to be inhibited by stimulation of the ipsilateral interstitial nucleus of Cajal~L This work was partially repeated (and confirmed)in the present study. Of 21 type I units of the horizontal canal examined while the midbrain was being stimulated, 15 were inhibited, 5 were uninfluenced and 1 was antidromically activated. Of the 15 which were inhibited, 14 were suppressed by stimulation of the ipsilateral interstitial nucleus of Cajal and i by stimulation of the posterior commissure. The voltage thresholds in the interstitial nucleus of Cajal producing vestibular inhibition averaged 1.5 ~ 0.7 (tl 5). In the area surrounding the interstitial nucleus on all sides, the voltage threshold rose sharply and the suppression was occasionally replaced by excitation (Fig. 4, upper diagram). Frequencies of 10-50/sec were used to produce the inhibition; occasionally lower frequencies were also effective. The discharge of the type I units of the horizontal canal was suppressed as early as 5-6 msec. The inhibition then lasted 20-40 msec. The thresholds of inhibition and the beginning of suppression of spike activity are very similar to that which we have previously seenl~L Of 10 type 1 neurons of the horizontal canal tested by stimulation of both the ipsilateral interstitial nucleus of Cajal and the contralateral vestibular nerve, 6 were inhibited from both sources. Three showed inhibition by one or by the other and one unit was unaffected. The type I units of the anterior canal reacted differently in several respects on midbrain stimulation. When the i psilateral interstitial nucleus of Cajal was stimulated, only 6 of 25 units were inhibited (Figs. 4 and 5). Inhibition was not seen~on stimulation of the nucleus of Darkschewitsch or the area of the nucleus prestitialis. These neurons were occasionally inhibited by stimulation within a millimeter of the midbrain site from which a type I unit of the horizontal canal was inhibited (Fig. 5). Histologically this phenomenon has been localized to the ipsilateral interstitial nucleus of Cajal. The inhibition threshold voltages averaged 2.1 r- I.I (n ==16). These values are not significantly different from those of the horizontal canal. The onset of suppression of unit firing ranged from 5 to I1 msec I Fig. 1C). Further. the suppression of unit firing was less often as complete as that seen with horizontal canal type I units. Lastly, of the 6 neurons which were inhibited on stimulation of the ipsilateral interstitial nucleus of Cajal, 5 were also inhibited on contralateral vestibular nerve stimulation. No attempt was made to summate the inhibitory effects from these two stimulus sites. Thus, one can say that as compared to type ! units of the horizontal canal. midbrain stimulation less often inhibited those of the anterior canal, inhibition was less complete and seemed to have a longer latency of onset. However. both varmties of type I were inhibited from the same midbrain site. the interstitial nucleus of Cajal : and both had convergence of inhibition from the ipsitateral midbrain and the opposite vestibular nerve. Of the remaining 19 type 1 neurons of the anterior canal which were examined Brain Research, 9 ('1968) 312-333

VESTIBULAR-MIDBRAIN MECHANISMS

32 1

d u r i n g m i d b r a i n s t i m u l a t i o n , 15 w e r e unaffected by m i d b r a i n s t i m u l a t i o n in the s a m e sites, e v e n to h i g h e r v o l t a g e ( 7 - 1 0 V), 5 0 - 1 0 0 / s e c stimulus. F o u r units w e r e facilitated, 2 o f t h e m f r o m s t i m u l a t i n g the s a m e site in the interstitial nucleus f r o m w h i c h a t y p e I unit o f the h o r i z o n t a l c a n a l h a d b e e n inhibited. T h e f a c i l i t a t i o n o c c u r r e d at l o w threshold v o l t a g e s o f 1.8 and 2.0 (n - - 2) a n d w i t h a f l u c t u a t i n g l a t e n c y o f 4 - 9 msec. W e h a v e +3

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Fig. 5. Shows response to different depths of ipsilateral midbrain stimulation in a type 1 unit of the anterior canal histologically located in the rostral and lateral part of the medial nucleus. '0' was histologically located in the center of the interstitial nucleus of Cajal; the other numbers on the right represent millimeter changes in depth. Stimulus at 20/sec, 5 V, resulted in clear, partial inhibition at '0' and ' 1'. Onset and cessation of stimulation in each trace are indicated by upward and downward triangles respectively. Higher frequency and voltage caused excitation at these depths. The interruptions in traces at ÷ 3, ~ 2 and ~ 1 were due to the film sticking together in development. Brain Research, 9 (1968) 312 333

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Fig. 6. To show facilitation of type 1I units. A, shows a type 11 unit of the horizontal canal, located histologically in the inferior part of the superior nucleus, responding to stimulation of the contralateral vestibular nerve at 2/sec, 1.2 V (2 times N1 threshold). B, shows a type II unit of the anterior canal, histologically shown to be in the rostral part of the medial nucleus, being facilitated by contralateral veStibular nerve stimulation at 2/sec, 2 V (3 times N1 potential). C, shows the same unit in 'B' being facilitated on stimulation of the ipsilateral interstitial nucleus of Cajal at 2/sec, 5 Y. About 10 sweeps are superimposed in each record. not seen such facilitation o f t y p e I units o f the h o r i z o n t a l canal f r o m s t i m u l a t i o n with such low values within the interstitial nucleus o f Cajal in the present or previous studies. Type II neuron responses on contralateral vestibular nerve stimulation

S h i m a z u and Precht 2z have shown that a b o u t h a l f o f t y p e I1 n e u r o n s o f the h o r i z o n t a l canal are excited by s t i m u l a t i n g the c o n t r a l a t e r a l vestibular nerve b u t n o t the ipsilateral one a n d c o n t i n u e to r e s p o n d to r o t a t i o n a l a c c e l e r a t i o n after d e s t r u c t i o n o f the ipsilateral l a b y r i n t h . It is thus clear these t y p e II units derive their l a b y r i n t h i n e i n p u t from the o p p o s i t e side. A f t e r a n a l y z i n g the evoked field responses, the earlier onset o f t y p e II unit facilitation by c o n t r a l a t e r a l nerve s t i m u l a t i o n as c o m p a r e d to t y p e I i n h i b i t i o n a n d o t h e r s u p p o r t i n g d a t a , they c o n c l u d e d t h a t these type II n e u r o n s were intercalated n e u r o n s t h a t i n h i b i t ipsilateral t y p e I activity. In the present experiment, a few type II n e u r o n s o f the h o r i z o n t a l canal were examined a n d stimulated via the c o n t r a l a t e r a l v e s t i b u l a r nerve in o r d e r to c o m p a r e with similar activation o f t y p e 11 n e u r o n s o f the a n t e r i o r canal. Eighteen t y p e II units o f the h o r i z o n t a l c a n a l were observed. All fired s p o n t a n e ously with rates o f 2-30/sec. The spike a m p l i t u d e w a s rarely over 200 ,tW. O f 9 such units tested with c o n t r a l a t e r a l vestibular nerve stimulation, 8 were facilitated. T h e r a t i o o f the unit firing t h r e s h o l d to the N~ t h r e s h o l d a v e r a g e d t.7 :z: 0.4 (n -= 5). These ratios are in the same range as the t h r e s h o l d o f t y p e I i n h i b i t i o n p r o d u c e d on Brain Research, 9 (1968) 312-333

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contralateral vestibular nerve stimulation 22. The facilitated ranged from 3.4 (the earliest) to 8.6 (n is thus somewhat less than the earliest onset of contralateral vestibular nerve stimulation. This that these type I1 neurons could be intercalated

latencies of onset of the first spikes -- 5) (Fig. 6A). The shortest latency type I inhibition (about 4 msec) on supports previous work indicating inhibitory neurons acting on type I

n e u r o n s 22.

Consider now the type II neurons of the anterior canal. All fired spontaneously from 2 to 30/sec and the amplitudes of the spikes were the same as those of their counterparts of the horizontal canal. In 3 cats the ipsilateral labyrinth was destroyed and the contralateral labyrinth left intact. As compared to the cats in which the ipsilateral labyrinth was preserved, the type II units fired spontaneously in the same range, were no more difficult to find, and responded briskly to rotational acceleration. (On the other hand, no spontaneously firing type I neurons could be found or could be facilitated by angular acceleration.) On stimulation of the contralateral vestibular nerve in cats with intact ipsilateral labyrinths, 6 of 7 type 1I neurons of the anterior canal were facilitated. The ratios of the unit thresholds averaged 1.3 ± 0.1 (n -- 5). The average thresholds value is somewhat less than for the type II neurons of the horizontal canal, but this difference is not significant. The latencies of the earliest spikes of type II neurons of the anterior canal on contralateral vestibular nerve stimulation were 3.6-6.3 msec (n = 5) (Fig. 6B). The latencies fluctuated over several milliseconds; a slight increase in stimulation intensity reduced the fluctuation somewhat. It appears that the type II neurons of the anterior and horizontal canals have very similar patterns of spontaneous firing, size of spikes, responses on stimulating the contralateral vestibular nerve in terms of threshold and latency of firing. A very strong circumstantial case has been made for a certain group of type II cells of the horizontal canal being intercalated inhibitory neurons acting on type I neurons of the horizontal canal 22. It is reasonable to assume type I1 neurons of the anterior canal play a similar role in the inhibition of type 1 neurons of the anterior canal on contralateral vestibular nerve stimulation. Type I1 vestibular neuron responses on midbrain stimulation

Type II neurons of the horizontal canal showed transsynaptic excitation from almost every site of stimulation in the ipsilateral and contralateral midbrain and pretectum. Paralleling our previous study 16, the lowest thresholds of facilitation were obtained from the ipsilateral interstitial nucleus of Cajal (average 0.9 ~- 0.2 V, n 5). The opposite interstitial nucleus and the rest of the medial midbrain tegmentum had 1 2 V higher threshold. The transsynaptic excitation was most easily produced by high frequency stimulation of the ipsilateral interstitial nucleus of Cajal. A minority of units followed low frequency (2-5/sec) stimulation with complete security. In such units it could be seen that the units followed the stimulus with fluctuating latencies, with the earliest Brain Research, 9 (1968) 312-333

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onset ranging from 2.2 to 8.6 msec (n : 6). One unit (Fig. 7A) had a nearly fixed latency of 1.7 msec. failed to follow 2.4 msec interval double shocks, and may be monosynaptically activated. These values compare closely with those of our previous study 16 except that such possibly monosynaptic responses were not found. Type II units of the horizontal canal were facilitated by both ipsilateral nucleus of Cajal in the midbrain and the contralateral vestibular nerve. Four units were observed in which this was demonstrated. Fig. 7B shows a unit which was facilitated with a fluctuating latency beginning at 6.1 rnsec on contralateral vestibular nerve stimulation and responding to ipsilateral midbrain stimulation with a considerably shorter and nearly fixed latency (Fig. 7A). This convergence of influence from these two sites on single cells of presumed inhibitory function will be considered further in the discussion section. The type II cells of the anterior canal behaved very similarly on mid braha stimulation. Most could be transsynaptically activated by stimulation of almost any point in the medial midbrain and pretectal area. The lowest threshold values using 20/see stimulation were also obtained from the ipsilateral interstitial nucleus of Cajal (average 0.9 ~ 0.5 V. n 101. In Fig. 8. the stimulating electrode passed vertically through the midbrain tegmentum, superimposed traces were made just above threshold, stimulating at 5/sec. At 4 m m above the interstitial nucleus (4-4 in the figure) Brain Research, 9 11968) 312-333

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Fig. 8. Shows a type II neuron of the anterior canal, histologically shown to be in the middle (in the rostrocaudal sense) of the medial nucleus, on stimulation at different depths in the ipsilateral midbrain. The numbers represent depth in millimeters, with '0' being in the middle of the interstitial nucleus of Cajal. All stimulations were done at 5/sec. Five sweeps were superimposed except in 'A, 3', where about 10 sweeps were used. Some spikes were retouched. In 'A', all stimuli were at threshold: 17.4 V at + 4 ; 8.0 V at + 3 ; 3.2 V at ÷ 2; 1.7V at + 1 ; 1.2V at0; 1.2 V at --1; 1.7Vat --2; 3.2 V at --3 and 8.0 V at --4. In addition to the unit facilitation, at --3 an antidromic field response is seen (its ability to follow double shock is not shown). In 'B' and 'C' stimulation at ÷ 1 at 2.7 V, about 2 times threshold, shortened the latency to 1.2 msec and rendered it nearly fixed. This response failed to follow double shock (C).

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/3OOpV 1 rn s e c Fig. 9. A type II unit of the anterior canal is facilitated by stimulation of the ipsilateral interstitial nucleus of Cajal. All traces were made with stimulus of 2/see and 2.5 V, just above threshold. Five to 6 sweeps were superimposed in each record. 'A' was obtained, and the stimulating electrode was lowered by about 0.2 mm. where "B' and 'C' were recorded• In 'C' the unit failed to follow double shock of 3.7 msec interval. 17 V stimulus strength was necessary to facilitate the unit. 1 m m lower. 8 V were needed. A t 1 the threshold was 1.7 V: increasing the voltage to 2.7 V changed the fluctuating latency of 3.3-4 msec to a fixed latency of 1.2 msec. This failed to follow double shock. At '0', later shown histologically to be in the interstitial nucleus of Cajal, the threshold was 1.7 V. 3 m m below, a fixed latency (0.6 msec) a n t i d r o m i c unit response (its ability to follow double shock at 3 msec interval is not illustrated) was also seen, while the type II u n i t i n question was facilitated with a fluctuating latency by a threshold stimulus of 3.2 V. Parenthetically, it was usual to find such an a n t i d r o m i c response, both ipsi- a n d contralaterally, 2-3 m m below the interstitial nucleus of Cajal. 4 m m below the interstitial nucleus the units were facilitated at 8 V threshold. The type II units of the anterior canal could be facilitated with fluctuating latencies with the earliest spikes r a n g i n g from 1.2 to 6.0 msec (n - - 8) (Fig. 6C). In addition. 2 units had nearly fixed latencies of 1.2 a n d 1.4 msec. Fig. 8 shows one of these short. fixed latency responses which was activated a b o u t 1 m m away from the center of the Brain

Research,

9 (1968) 312-333

327

VESTIBULAR-MIDBRAIN MECHANISMS

interstitial nucleus of Cajal. This took place with a somewhat higher strength of stimulus as compared with the threshold stimulus in or slightly below the interstitial nucleus (see Fig. 8, '0' and '--1'). Another unit (Fig. 9) had a nearly fixed latency response at 4.6 msec which shifted to an almost fixed latency response at 1.4 msec after a downward displacement of the midbrain electrode by about 0.2 mm. The stimulating voltage remained the same. This response failed to follow double shock and may represent a monosynaptic connection. The type II neurons of the anterior canal showed convergence of influence from the contralateral vestibular nerve and the ipsilateral interstitial nucleus of Cajal in 5 out of the 6 instances it was looked for (see Figs. 6B and C for such a unit). No attempt was made to test the interaction of these two converging forces. While the role of a similar convergence on type II cells of the horizontal canal may be understood in terms of inhibition of type I cells of the horizontal canal, the role of type 11 cells of the anterior canal is less clear. DISCUSSION

One of the most obvious findings was not quantitated: both type 1 and type II neurons of the anterior canal seemed distinctly fewer in number than those of the horizontal canal. This conclusion is based on the difficulty in finding physiologically identifiable units when the cat was rotated with the anterior semicircular canal in the plane of the turntable and on the greater number of units in the vestibular nucleus which were not sensitive to angular acceleration with the cat in this position. The paucity of type 1 units of the anterior canal is indirectly supported by the observation of Suzuki and Cohen ~5 that less total muscle tension in the appropriate eye muscle was evoked by stimulation of the vertical semicircular canal nerves than the horizontal canal nerve. These workers did not comment on such a difference on direct stimulation of the motor nerves of the eye muscles so the weaker response on stimulating the vertical canal may be explained by some difference in the vestibulo-oculomotor pathways before the eye motor axons leave the brain stem. Further, Ledoux 1 observed that the changes in frequency of action potentials recorded from the vestibular nerve had a shorter duration and did not seem equal for both directions of rotation when the vertical canals were physiologically stimulated as compared to the horizontal canals. Relations between type I and H vestibular neurons and the opposite labyrinth

The vestibular end organ of the horizontal canal responds in a 'type I' fashion as recorded from the vestibular nervea4, is. Thus, rotational acceleration in one direction induces ampullopetal endolymph flow in one horizontal semicircular canal and increases the firing rate from its ampulla and also causes ampullofugal flow and consequent reduction of discharge from the contralateral horizontal canal. Rotation in the opposite direction reverses these effects. The resting activity of the type I neurons Brain Research, 9 (1968) 312-333

328

c. iq. MARKHAM

in the brain stem of the normal cat is largely maintained by input from th e labyrinth on the same side, for if this end organ is destroyed, ipsilateral type I responses are almost completely abolished2L Considering the influences exerted by the opposite vestibular system, one may conclude on the basis of the work of Shimazu and Precht 22 that the axons of some type I vestibular neurons of the horizontal canal cross the brain stem in the floor of the fourth ventricle and act on some type II neurons, which then inhibit type 1 units on the same side. The present work has confirmed this to the extent of showing that type 1 neurons of the horizontal canal were almost all inhibited (and none excited at comparable stimulus strengths) by contralateral vestibular nerve stimulation and that many type lI neurons were facilitated with similar stimulus strengths, with the shortest latencies being less than the earliest type I inhibition. Functionally the following events probably take place. Angular acceleration induces ampullopetal flow in one horizontal canal which facilitates ipsilateral type ! neurons; and ampullofugal flow induced simultaneously in the contralateral horizontal canal inhibits contralateral type I neurons, including the ones with commissural axons. Such reduction in commissural influence would reduce synaptic activation of ipsilateral type II neurons which in turn would result in lessened inhibitory effect on ipsilateral type I neurons. The precise time relationships of the impingement on type ! neurons of ipsilateral labyrinthine excitatory influence and commissural reduction of inhibition is not known. However. this would seem to offer a method of enhancing or sharpening ipsilateral type I activity. Similarly, rotatory acceleration in the opposite direction would cause enhancement of commissural inhibitory influence and enhance the reduction of ipsilateral labyrinthine-induced type 1 activity. The anterior semicircular canal on one side is approximately in the same plane as the posterior canal on the other side. In the present experiment, the cat's nose was angulated upward 45 ° and rotated 70 ° about the naso-occipital axis. This maneuver brought the underlying anterior canal and the superiorly placed posterior canal into the plane of the horizontally rotating turntable. In this position the other pair of anterior and posterior canals were perpendicular and the horizontal canals nearly perpendicular to the plane of the turntable. It is very unlikely these other canals were stimulated by rotational acceleration of the turntable 13. Vestibular nerve fibers connected to the vertical semicircular canals increase their discharge rates on ampullofugal and reduce them on ampullopetal endolymph flow 13,14. Type I neurons on the side of the underlying anterior semicircular canal are thus facilitated by rotation which produces ampullofugal endolymph flow and inhibited by rotation in the other direction. Type II units on the same side as the underlying anterior canal responded to physiological stimulation in an inverse manner to type I units of the anterior canal. The type II units with connections from the opposite labyrinth (we have not considered those type II units which derive influence from the ipsilateral labyrinth in this paper. see Shimazu and Prechff'~) have been demonstrated by low intensity stimulation of the contralateral vestibular nerve in the present experiment. Further, type I1 neurons were easily demonstrable by rotational acceleration after the ipsilateral labyrinth was Brain Research, 9 (1968) 312-333

VESTIBULAR-MIDBRAIN MECHANISMS

329

destroyed. Their principal, though not necessarily only, physiological input from the opposite labyrinth must be from the posterior canal. Type [I neurons of the anterior canal which were examined in this work were activated by stimulation of the contralateral vestibular nerve at voltage threshold and latencies which were comparable to type II neurons of the horizontal canal. Similarly, these units were histologically located in the ventral and rostral part of the medial nucleus and in the superior nucleus as were the type !I units of the horizontal canal. The main difference is that the type 11 units of the anterior canal seemed to be fewer in number. Type 1 neurons of the anterior canal were also fewer than their horizontal canal counterparts. While stimulation of the contralateral vestibular nerve produced inhibition of resting activity, the inhibition was rarely as complete. Further, the inhibition of action potentials of many units did not start at 4-5 msec (as was the case with type I units of the horizontal canal), but began anywhere from 4 to l l msec after the stimulus. Even in a single type I unit of the anterior canal, the onset of inhibition was more variable than in the typical type I neurons of the horizontal canal. It seems reasonable that some type I1 neurons of the anterior canal are intercalated inhibitory neurons receiving input from the contralateral labyrinth, thus being similar to the type |I neurons of the horizontal canal. Yet, the inhibition of the type I cells of the anterior canal is, as noted above, not as marked or as early in onset. This difference from the horizontal canal system may be explained by the relative paucity of type Ii cells of the anterior canal. Placing the cat's head so the anterior semicircular canal is parallel to the turntable also alters the position of the otoliths in relation to gravity. The anatomical position of the utricle in the head, and the augmented vestibular response on parallel swing testing observed by Bos et al. 3 when the test animal was on its side as compared to prone position, suggest the overall activity of the utricle may be increased when the head is turned on its side. Since there is some convergence of gravity receptors on type I (and type II) neurons of the horizontal canaU, ,~, it is probable there is similar convergence on type I cells of the anterior canal. Thus, it is possible that with the cat's head placed as it was in the present experiment, increased input from the utricle exerts excitatory influence on type I neurons of the anterior canal and thus renders them less sensitive to commissural inhibitory effects. On the other hand, the resting discharge of the type I neurons of the anterior canal (tested with the anterior canal in the plane of the turntable) was not different from the resting discharge type l units of the horizontal canal (tested with that canal parallel to the turntable). All in all, there are insufficient data to properly evaluate this explanation for a weak inhibitory response of type I neurons of the anterior canal on contralateral vestibular nerve stimulation. Functionally the contralateral labyrinth exerts a less powerful commissural inhibitory influence on the type I cells of the anterior canal than on those of the horizontal canal. Type I cells of all the canals probably have to do with eye movements or tonus, possibly serving as second-order neurons in a three-neuron chain from the ampullae of the semicircular canals to the eye muscles zv. Each semicircular canal Brain Research, 9 (1968) 3 ! 2-333

330

('. H. MARKHAM

exerts a primary influence on two eye muscles: the horizontal canal acting on the ipsilateral medial rectus and the contralateral lateral rectus; the anterior canal acting on the ipsilateral superior rectus and the contralateral inferior oblique; and the posterior canal on the ipsilateral superior oblique and the contralateral inferior rectus27, ~8. The weaker influence of the contralateral labyrinth on type I neurons of the anterior canal may imply there is a less precise coordination of activity for control of eye muscles mediating vertical and torsion movements than for lateral eye movements. Midbrain-vestibular nucleus relations

The type I neurons of the horizontal canal have previously been shown to be inhibited by stimulation of the ipsilateral interstitial nucleus of Cajal. Because type It neurons of the horizontal canal were transsynaptically facilitated at lower thresholds than the type I cells on stimulating the same area, and because the earliest facilitation of type II neurons preceded the earliest inhibition of type 1 action potentials, it was suggested the type II neurons might be intercalated inhibitory neurons acting on the type I cells 16. These effects were demonstrated to pass down the medial longitudinal bundle, probably by the pathway identified anatomically by Pompeiano and Walberg is. This pathway originates in the interstitial nucleus of Cajal, passes down the medial border of the medial longitudinal bundle and terminates in the ipsilateral medial vestibular nucleus. The present study extends this work by demonstrating: (1) On stimulation of the ipsilateral interstitial nucleus of Cajal an occasional type 11 unit was found which was facilitated at a very short (1.2-1.7 msec) and nearly fixed latency, followed single shock stimulus with excellent security and tailed to follow double shock at 2-3 msec intervals. These connections might be via antidromic paths to recurrent collaterals or through orthodromic connections. The antidromic pathway seems less likely since the few antidromic unit responses found in the vestibular nuclei on stimulation of the interstitial nucleus of Cajal were seen bilaterally, and since the c o m m o n site for inducing antidromic field responses was 2-3 m m inferior to the interstitial nucleus on both sides. It seems likely these responses are orthodromic and they may be monosynaptlc. (2) There is frequent convergence on type I cells of inhibition from both the ipsilateral interstitial nucleus of Cajal and the contralateral vestibular nerve; and there is also convergence on type I1 cells of facilitatory influences fi'om these two sites. In both instances the earliest facilitation of the type II neurons precedes the earliest inhibition of type | neurons. This suggests the inhibition of type I units from the contralateral labyrinth and from the ipsitateral interstitial nucleus of Cajal shares a c o m m o n path within the vestibular complex, and this c o m m o n path includes some intercalated inhibitory type II neurons. It seems likely these inhibitory effects should interact, but this was not tested. Type I neurons of the anterior canal, unlike those of the horizontal canal, were infrequently affected by stimulation of the interstitial nucleus of Cajal and other parts of the midbrain and pretectum. A minority of such units was inhibited by stimulation of the interstitial nucleus, but the inhibition of' action potentials was rarely Brain Research, 9 (1968) 312-333

VESTIBULAR-MIDBRAIN MECHANISMS

331

complete even with strong, high frequency stimulus. Further, its onset was more variable than that seen with the type I neurons of the horizontal canal. The interstitial nucleus of Cajal appears to be a center for rotational movements of the head 9 and for rotational and vertical movements of the eyes 26. This is supported by the observation that the interstitial nucleus of Cajal has synaptic endings in all the eye motor nuclei, except the abducens nucleus and the medial rectus part of the oculomotor nucleus 26. It was previously suggested 16 that the ipsilateral inhibitory influence from the interstitial nucleus of Cajal to type I neurons of the horizontal canal might allow the suppression of the vestibular substrate for horizontal eye movements and may thus aid in the performance of vertical and rotatory eye movements. The comparatively few type I neurons of the anterior canal showing inhibition from the interstitial nucleus of Cajal and the weaker quality of the inhibition support this view. The role of the interstitial nucleus of Cajal in powerfully inhibiting ipsilateral type 1 neurons of the horizontal canal and weakly inhibiting ipsilateral type 1 units of the anterior canal may have another function. The interstitial nucleus receives projections from the frontal 29 and striate cortex17, 32 and globus pallidus tl. Influences from these higher centers, acting via the interstitial nucleus of Cajal, could over-ride, or modulate the brain stem pathway concerned with laterally directed eye movements. Eye movements primarily controlled by the anterior canal would appear to be less influenced by this mechanism. Similarly, reciprocal modification of monosynaptic flexor and extensor reflexes by angular acceleration s might be controlled by higher centers. SUMMARY

Brain stem vestibular neurons were examined by extracellular recording in cats with high cervical spinal cord transsections and were physiologically identified by their response to angular acceleration. Type I units of the horizontal canal were strongly inhibited by stimulating both the contralateral vestibular nerve and the ipsilateral interstitial nucleus of Cajal in the midbrain. On stimulation of both these sites, type II neurons of the horizontal canal were transsynaptically facilitated. Type I neurons of the anterior canal showed less frequent and weaker inhibitory responses than those of the horizontal canal on stimulation of both the contralateral vestibular nerve and the interstitial nucleus of Cajal. However, units which were inhibited from one site were usually inhibited from the other. Type 11 neurons of the anterior canal were facilitated by stimulation of the opposite vestibular nerve, and responded to angular acceleration acting on the opposite posterior semicircular canal. The difference in strength of inhibition of type 1 units of the horizontal and the anterior canals on contralateral vestibular nerve stimulation suggests that precise coordination is more important for horizontal than for vertical eye movements. The stronger suppression, on interstitial nucleus of Cajal stimulation, of type 1 neurons of the horizontal canal as compared to those of the anterior canal may better Brain Researchf9 (1968) 312-333

332

~. [!. MARKHAM

c o n t r i b u t e t o s u p p o r t t h e i n t e r s t i t i a l n u c l e u s in its role in v e r t i c a l a n d r o t a t o r y h e a d a n d eye m o t i o n s . ACKNOWLEDGEMENTS I t is a p l e a s u r e t o a c k n o w l e d g e t h e l a b o r a t o r y a s s i s t a n c e o f M r . N o r m a n S i m p s o n and the secretarial help of Mrs. Marianne Lyden. This investigation was supported

by Public

Health Service Grant

No.

NB

06658-02.

REFERENCES 1 ADRIAN, E. D., Discharges from vestibular receptors in the cat, J. Physiol. (LomL), 101 (1943) 389-407. 2 ANDERSSON, S., AND GERNANDT, B. E., Cortical projection of vestibular nerve in cat, Acta otolaryng. (Stockh.), 116, Suppl. (I 954) l 0-18. 3 Bos, J. H., JONGKEES, L. B. W., ANO PHILIPSZOON, A. J., On the action of linear accelerations upon the otolith, Acta oto-laryng. (Stockh.), 56 (1%3) 477-489. 4 DtrENSING, F., UND SCHAEFER, K. P., Die Aktivit~it einzelner Neurone im Bereich der Vestibulariskerne bei Horizontalbeschleunigungen unter besonderer Berticksichtigung des vestibul~iren Nystagmus, Arch. Psychiat. Nervenkr., 198 (1958) 225-252. 5 D~NSING, F., trND SCHAEFER, K. P., ~ b e r die Konvergenz verschiedener labyrinth/irer Afferenzen auf einzelne Neurone des Vestibulariskerngebietes, Arch. Psychiat. Nervenkr., 199 0959) 345-371. 6 DEVITo, R. V., BROSA, A., ANt) ARDUINI, A., Cerebellar and vestibular influences on Deitersian units, J. Neurophysiol., 19 (1956) 241-253. 7 GERNANDT, B. E., Response of mammalian neurons to horizontal rotation and caloric stimulation, J. Neurophysiol., 12 (1949) 173-184. 8 GERNANDT, B. E.. AND THULIN. C. A., Vestibular mechanisms of facilitation and inhibition of cord reflexes, /liner. J. Physiol.. 172 (1953) 653-660. 9 HASSLER. R., UNr) HESS. W. R., Experimentelle und anatomische Befunde fiber die Drehbewcgungen und ihre nerv6sen Apparate, Arch. Psychiat. Nervenkr., 192 (1954) 448-526. 10 HYDE, J. E.. AND TOCZEK, S., Functional relation of interstitial nucleus to rotatory movements evoked from zona inserta stimulation. J. Neurophysiol., 25 (1962) 455-466. 1 l JOHNSON, T. N., AND CLEMENTE, C. D., An experimental study of the fiber connections between the putamen, globus pallidus, ventral thalamus, and midbrain tegmentum in cat. J. comp. Neurol.. 133 (1959) 83-101 12 JUNG, R., AND HASSLER, R., The extrapyramidal motor system. In J. F1ELO. H. W. MAGOUN AND V. E. HALL (Eds.), Handbook o f Physiology, Sect. I. Neurophysiology, VoL 2. Amer. Physiol. Soc.. Washington, D.C.. 1960, pp. 863-927. 13 LEDOUX, A.. Les canaux s6mi-circulaires, l~tude ~lectrophysiologique. Contribution /l l'effort d'uniformisation des 6preuves vestibulaires. Essai d'interpr6tation de la s6miologie vestibulaire. Acta oto-rhino-laryng, belg., 12 (1958) 109-348. 14 LOWENSTE~N. O.. Peripheral mechanisms of equilibrium, Brit. med. Bull., 12 (1956) 114-118. 15 LOWENSTEIN,O., AND SAND, A., The mechanism of the semicircular canal. A study of the responses of single-fibre preparations to angular accelerations and to rotation at constant speed. Proc. roy. Soc. B, 129 (1940) 256-275. 16 MARKHAM, C. H., PRECrrr. W., AND SHIMAZU, H., Effect of stimulation of interstitial nucleus of Cajal on vestibular unit activity in the cat, J. NeurophysioL, 29 (1966) 493-507. 17 ME'rrLER, F. A., Cortifugal fiber connections of the cortex of the Macaea rnulatta. The occipital region, J. eomp. Neurol.. 61 (1935) 221-256. 18 POMPEIANO, O.. AND WALBERG. F., Descending connections to the vestibular nuclei. An experimental study in the cat. J. comp. Neurol., 108 (1957) 465-503. 19 PRECa-IT. W., SmMAZU, H.. AND MARKHAM, C. H.. A mechanism of central compensation of vestibular function following bemilabyrinthectomy, J. Neurophysiol.. 29 (1960, 996-1010. Brain Research, 9 (1968) 312-333

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20 PRECHT, W., AND SHIMAZU, H., Functional connections of tonic and kinetic vestibular neurons with primary vestibular fibers, J. Neurophysiol., 28 (1965) 1014-1028. 21 SH~MAZU, H., AND PRECH7, W., Tonic and kinetic responses of cat's vestibular neurons to horizontal angular acceleration, J. Neurophysiol., 28 (1965) 991 1013. 22 SHIMAZU, H., AND PRECHT, W., Inhibition of central vestibular neurons from the contralateral labyrinth and its mediating pathway, J. Neurophys&l., 29 (1966) 467-492. 23 SNIDER, R. S., ANt) NIEMER, W. T., A Stereotaxic Atlas of the Cat Brain, Univ. Chicago Press, Chicago, I11., 1961. 24 STEIN, B. M., AND CARPENTER, M. B,, Central projections of portions of the vestibular ganglia innervating specific parts of the labyrinth in the Rhesus monkey, Amer. J. Anat., 120 (1967) 281-318. 25 SUZUKI, J.-l., AND COHEN, B., Integration of semicircular canal activity, J. Neurophysiol., 29 (1966) 981 995. 26 SZENTAGOTHA1, J., Die zentrale Innervation der Augenbewegungen, Arch. Psychiat. Nervenkr., 116 (1943) 721-760. 27 SZENTAGOTHAI,J., The elementary vestibulo-ocular reflex arc, J. Neurophysiol., 13 (1950) 395 407. 28 SZENTAGOTHA1,J., Pathways and synaptic articulation patterns connecting vestibular receptors and oculomotor nuclei. In M. B. B~NDER (Ed.), The Oculomotor System, Harper and Row, New York, 1964, pp. 205-223. 29 SZENTXGOTHM, J., UND RAJKOWTS, K., Der Hirnnervenanteil der Pyramidenbahn und der pr~imotorische Apparat motorischer Hirnnervenkerne, Arch. Psychiat. Nervenkr., 197 (1958} 335-354. 30 THOMAS, R. C., AND WILSON, V. J., Precise localization of Renshaw cells with a new marking technique, Nature (Lond.), 206 (1965) 211 213. 31 WILSON, V. J., KATO, M., PETERSON, B. W., AND WYLIE, R. M., A single-unit analysis of the organization of Deiters' nucleus, J. Neurophysiol., 30 (1967) 603-619. 32 WOODBURNE, R. T., CROSBY, E. C., AND McCOTTER, R. E., The mammalian midbrain and the isthmus regions. Part II. The fiber connections. A. The relations of the tegmentum of the midbrain with the basal ganglia in Macaca mulatta, J. comp. Neurol., 85 (1946) 67-92.

Brain Research, 9 (1968) 312-333