research report spatial distribution of semicircular canal

Brain Research 880 (2000) 70–83www.elsevier.com/ locate /bres

Research report

Spatial distribution of semicircular canal nerve evoked monosynapticresponse components in frog vestibular nuclei

*Hans Straka , Stefan Biesdorf, Norbert DieringerPhysiologisches Institut, Pettenkoferstrasse 12, 80336 Munich, Germany

Accepted 25 July 2000

Abstract

Most second-order vestibular neurons receive a canal-specific monosynaptic excitation, although the central projections of semicircularcanal afferents overlap extensively. This remarkable canal specificity prompted us to study the spatial organization of evoked fieldpotentials following selective stimulation of individual canal nerves. Electrically evoked responses in the vestibular nuclei were mappedsystematically in vitro. Constructed activation maps were superimposed on a cytoarchitectonically defined anatomical map. The spatialactivation maps for pre- and postsynaptic response components evoked by stimulation of a given canal nerve were similar. Activationmaps for monosynaptic inputs from different canals tended to show a differential distribution of their peak amplitudes, although theoverlap was considerable. Anterior vertical canal signals peaked in the superior vestibular nucleus, posterior vertical canal signals peakedin the descending and in the dorsal part of the lateral vestibular nucleus, whereas horizontal canal signals peaked in the descending and inthe ventral part of the lateral vestibular nucleus. A similar, differential but overlapping, spatial organization of the canal inputs wasdescribed also for other vertebrates, suggesting a crude but rather conservative topographical organization of semicircular canal nerveprojections within the vestibular nuclei. Differences in the precision of topological representations between vestibular and other sensorymodalities are discussed. 2000 Elsevier Science B.V. All rights reserved.

Theme: Motor systems and sensorimotor integration

Topic: Vestibular system

Keywords: Field potential; Topographical mapping; Vestibular nerve afferent fiber; Organotopic organization

1. Introduction neurons that imply a convergence of multiple canal[3,4,6,10,14] or canal-otolith inputs [1,5]. At variance with

The projection areas of afferent fibers from individual these reports, however, intracellular studies employinglabyrinthine organs in the ipsilateral vestibular nuclei selective electrical stimulation of individual semicircularoverlap to a large extent in non-mammalian [8,17,24– canal nerves demonstrated that monosynaptic responses of26,30,37] as well as in mammalian species [11,31,32]. 28VN are typically evoked from only one of the ipsilateralEach of the vestibular nuclei receives inputs from most semicircular canal nerves (cat [19,29], pigeon [42], froglabyrinthine sense organs, but the density of the innerva- [34]). Therefore, 28VN select among the available afferenttion from different vestibular sense organs varies between canal inputs and the convergence of multiple canal signalsdifferent regions. A considerable convergence of inputs onto 28VN might be mediated oligosynaptically.from different sense organs onto second-order vestibular The absence of a clear topographic order in the ana-neurons (28VN) might be expected from these overlapping tomical projection zones of afferent fibers from differentprojection areas. In fact, a number of single unit studies semicircular canal nerves, however, does not imply theemploying natural stimuli have reported best response absence of such an order at the synaptic or at the somaticorientation vectors of second- or higher-order vestibular level. The density of synaptic contacts between afferent

fibers and 28VN for instance could exhibit regional differ-ences such that zones with high synaptic densities from*Corresponding author. Fax: 149-89-5996-216.

E-mail address: [email protected] (H. Straka). one semicircular canal nerve alternate with similar zones

0006-8993/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0006-8993( 00 )02768-2

H. Straka et al. / Brain Research 880 (2000) 70 –83 71

from other semicircular canal nerves. Since the dendrites the sylgard floor. The temperature of the bath was elec-of 28VN are long and select their appropriate semicircular tronically controlled and maintained at 1460.28C.canal input it is also conceivable that the cell bodies of 28 For electrical stimulation of individual semicircularcanal nerves with an input from a particular semicircular canal nerve branches we used single constant currentcanal are clustered and form modular zones that represent pulses (0.2 ms; 8–12 mA) across suction electrodes (innerdifferent vestibular endorgans in a manner similar to diameter 120–150 mm; see [34]). For stimulation of theretinotopic, somatotopic or tonotopic maps of other sen- VIIIth nerve we used single constant current pulses (0.2sory systems. ms; 5–30 mA) across a concentric bipolar electrode (tip

We analyzed these possibilities first with evoked field diameter 25 mm) that was located about 2 mm morepotentials recorded in vitro in the isolated frog brain. proximal than the electrodes stimulating the individualSeparate electrical stimulation of individual semicircular semicircular canal nerves [34]. Constant current pulsescanal nerves activated field potentials that consisted of two were produced by a stimulus isolation unit (WPI A 360) ator more negative components. The spatial distribution of a repetition rate of 0.33 Hz. For extracellular recordingsvestibular nerve evoked N and N field potentials, glass microelectrodes were fabricated with a horizontal0 1

representing regional presynaptic (N ) and postsynaptic puller (P-87 Brown/Flaming), beveled (308, 20 mm tip0

(N ) response components [28], was mapped systematical- diameter) and filled with 2 M sodium chloride (1–3 MV).1

ly. We compared these results with the location of 28VN, Electrodes for intracellular recordings were filled with 2 Midentified by their monosynaptic responses following elec- potassium acetate (90–120 MV) or 2 M potassium chlo-trical stimulation of the horizontal, anterior vertical or ride (80–100 MV). Vertical displacements of the recordingposterior vertical semicircular canal nerve branch. The electrodes were controlled with a nanostepper. Horizontalphysiological response properties of these neurons had displacements were performed with a two axes micro-been described elsewhere [34]. manipulator.

Preliminary parts of this study had been published in In order to standardize our results field potentials wereabstract form [35]. recorded at the beginning of each experiment at the same

standard recording site (0.4 mm caudal to the caudal endof the entry of the VIIIth nerve at a depth of 0.4 mm below

2. Material and methods the top of the brainstem). The stimulus threshold for theevoked N component of the field potential was similar for1

In vitro experiments were performed on the isolated each of the three semicircular canal nerves and rangedbrains of grass frogs (Rana temporaria) and comply with usually between 2 and 3 mA. Stimulus intensities are giventhe Principles of Animal Care of the National Institutes of as multiples of these threshold values (3T) and wereHealth, publication no. 86-23, revised 1985. Permission for restricted to intensities of maximally 43T.these experiments was granted by Regierung von Ober- In order to cast our electrophysiological results into abayern (211-2531-31/95). Grass frogs were deeply anes- normalized geometry of the vestibular nuclei we used thethetized (0.1% 3-aminobenzoic acid ethyl ester; MS-222) same reference frame for a three-dimensional coordinateand perfused transcardially with iced Ringer solution (75 system as in a parallel anatomical study [24]. Distances inmM NaCl; 25 mM NaHCO ; 2 mM CaCl ; 2 mM KCl; the rostro-caudal direction refer to the caudal end of the3 2

0.5 mM MgCl ; 11 mM glucose; pH 7.4 [33]). The skull entry of the VIIIth nerve, in dorso-ventral direction to the2

and the bony labyrinth were opened by a ventral approach top of the brainstem and in medio-lateral direction to theand the three semicircular canals on either side were medial surface of the brainstem at the particular rostro-sectioned. Then, the brain was removed with the labyrin- caudal level (Fig. 1B–D). For the latter measurement thethine endorgans attached to the VIIIth nerve. The isolated midline of the brainstem was not practical because of thebrain was submerged in iced Ringer and the dura, the variability in the shape of the alar plate after the choroidlabyrinthine endorgans and the choroid plexus above the plexus had been removed. Measurements rostral to theIVth ventricle were removed. The forebrain was discon- caudal end of the entry of the VIIIth nerve and medial tonected and in part of the experiments also the cerebellum the top of the brainstem were denoted with a negative signwas removed. Brains were stored overnight at 68C in (see Fig. 1B,D). The mean distance between the caudal endcontinuously oxygenated Ringer solution with a pH of of the cerebellum and the obex was 3.7460.07 mm (n5

7.560.1 and were used up to 5 days after their isolation. 15). The distance between the left and right top of theFor some experiments the brainstem was glued with brainstem at the level of the VIIIth nerve was 2.6860.07cyanoacrylate glue to a plastic net with the ventral side mm (n515) and the distance between the top of thedown. This net was fixed with insect pins to the sylgard brainstem and the midline (floor of the IVth ventricle atfloor of a chamber (volume 2.4 ml) which was continuous- the level of the VIIIth nerve) was 0.7860.04 mm (n515).ly perfused with oxygenated Ringer solution at a rate of These measurements were used to define a standard size1.3–2.1 ml /min. In other experiments the brainstem was and to normalize the brains of different individuals. Thesectioned along the midline and fixed with insect pins to normalized location of the vestibular nuclei of grass frogs

72 H. Straka et al. / Brain Research 880 (2000) 70 –83

Fig. 1. Schematic presentation of the vestibular nuclei in the brainstem and how the spatial distribution of semicircular canal nerve evoked field potentialswas explored. (A,C,E) Frontal (A), parasagittal (C) and oblique medio-lateral views of the vestibular nuclei as reconstructed from data published by Matesz[23] and Kuruvilla et al. [22]. Dashed lines in A indicate the orientation of the parasagittal (1) and oblique medio-lateral (2) planes shown in C and E,respectively. (B,D,F) Electrode tracks (from surface to 0.8 mm in depth) through the vestibular nuclei. Arrows indicate the laterality (B) or therostro-caudal position (D,F) of electrode tracks. Zero laterality (B) refers to the top of the brainstem, zero rostro-caudal position (D,F) refers to the caudalend of the entry of the VIIIth nerve in the brainstem. Same calibrations for A and B and for C–F, respectively. CB, cerebellum; DN, dorsal nucleus; DVN,descending vestibular nucleus; LVN, lateral vestibular nucleus; MVN, medial vestibular nucleus; s.l., sulcus limitans; SVN, superior vestibular nucleus.

in a frontal (Fig. 1A,B), parasagittal (Fig. 1C) and in an In the latter case electrodes were advanced perpendicularlyoblique medio-lateral plane (Fig. 1E) was used to specify to the surface of the medial wall of the brainstem (dashedthe search area for this mapping study. line 2 in Fig. 1A). The touch point was about 20.4 mm

The spatial distribution of the evoked field potentials medial to the top of the dorsal brainstem. Typically,was mapped systematically with recording tracks from the records were taken every 0.1 mm in depth. However, in thesurface of the brainstem to a depth of 0.8 mm in three oblique medio-lateral plane we recorded every 0.05 mm updifferent planes. Electrode tracks in the frontal plane were to a depth of 0.3 mm because of the relatively thin strippositioned rostral (20.4 mm) or caudal (0.4 mm or 0.7 that represents the medial vestibular nucleus (Fig. 1A,E).mm) to the caudal end of the entry of the VIIIth nerve and Deeper measurement points were separated by 0.1 mm upconsisted in each case of five depth tracks separated by to a depth of 0.8 mm (see grid in Fig. 1F).0.15 mm in laterality (arrows in Fig. 1B). In the parasagit- Averages from 20 single sweeps of the evoked fieldtal plane, seven depth tracks explored the dorsal brainstem potentials at a given recording site were digitized (CEDat a laterality of 0 mm (dashed line 1 in Fig. 1A). The 1401, Cambridge Electronic Design), stored on computerrecording sites were located 20.7, 20.4 or 0 mm rostral and analyzed off-line (SIGAVG, Cambridge Electronicand 0.4, 0.7, 1.0 or 1.5 mm caudal to the caudal end of the Design). From the averaged field potentials separate depthentry of the VIIIth nerve (arrows in Fig. 1D). The oblique profiles were constructed for N and N potentials for each0 1

medio-lateral plane was explored by another seven elec- recording track and for each stimulated nerve branch.trode tracks (see arrows in Fig. 1F) at the same rostro- Initially, we searched for the largest N and N field0 1

caudal locations as used for parasagittal recording tracks. potential component in the rostro-caudal extension of the


vestibular nuclear complex after stimulation of the VIIIthnerve or of one of its canal nerve branches. To minimizethe variability of results we averaged the amplitudesrecorded in a given depth track separately for eachstimulated canal nerve, compared these averaged valuesbetween different parasagittal depth tracks and normalizedthe averaged values according to the largest values evokedby a given canal nerve branch in a particular track (see Fig.3). This procedure facilitated a comparison of the resultsfollowing stimulation of different semicircular canal nervesor of the VIIIth nerve and a comparison of data fromdifferent experiments. From the analysis of data obtainedin parasagittal depth tracks we obtained evidence for adifferential spatial distribution of the largest evoked fieldpotential amplitudes following stimulation of differentsemicircular canal nerves (Fig. 3). In later experiments weestimated the maximal response amplitude of N or N in0 1

each series of depth tracks for a given nerve branch andnormalized all other responses evoked by the same nervebranch. In order to include depth information in ouranalysis (field potentials are continuous functions andrecording tracks were relatively close to each other inspace) isopotential surface plots were calculated throughlinear interpolation between maximal response amplitudes(Stanford graphics, 3-D visions, Torrance, CA). Fourgroups of relative response amplitudes (up to 25%, 50%,75% or 100%) were represented by different intensities ofgray tones.

Statistical analyses were performed with the aid ofFig. 2. Field potentials in the vestibular nuclear complex evoked bycommercially available computer software (INSTAT;stimulation of the VIIIth nerve or its semicircular canal nerve branches onGraphpad, San Diego, CA). Statistical differences inthe ipsilateral side. (A–D) Pre- (N ) and postsynaptic (N ) field potential0 1latencies, amplitudes and areas were calculated accordingcomponents evoked by stimulation of the VIIIth nerve (A), the horizontal

to the Wilcoxon signed rank test (test for paired parame- canal (HC) nerve (B), the anterior canal (AC) nerve (C) or the posteriorters). Graphical presentations were performed with the aid canal (PC) nerve branch (D). Each of the responses in A–D were

recorded at the same site. Dashed lines indicate baseline and arrow headof commercially available computer software (Origin,the onset of stimulus. Calibration bars in B apply also for C and D. EachMicrocal Software, Northampton, MA; Designer, Microg-record represents an average of 20 responses.rafx, Richardson, TX).

differed between nerve branches and recording sites.3. Results Horizontal canal (HC) nerve evoked field potentials were

characterized by a negativity (Fig. 2B) that lasted longer3.1. Field potentials following stimulation of the VIIIth than that of the anterior canal (AC), posterior canal (PC)nerve or of individual semicircular canal nerve or VIIIth nerve evoked field potentials (Fig. 2A,C,D).branches. Here, areas instead of peak amplitudes of evoked N field1

potentials were compared (Table 2). This comparisonCanal nerve evoked presynaptic N and postsynaptic N indicated that HC nerve evoked postsynaptic field po-0 1

field potential components recorded at the standard record- tentials were characterized by a significantly (P#0.0001)ing site had significantly longer onset latencies and smaller larger relative area than AC, PC or VIIIth nerve evokedamplitudes than the corresponding VIIIth nerve evoked field potentials recorded at the same site (Table 2).potentials (Fig. 2; Tables 1 and 2). This difference in onsetlatency was expected, since the site of canal nerve 3.2. Topography of vestibular field potential componentsstimulation was further away from the recording site thanthe site of VIIIth nerve stimulation. N and N components In a parasagittal plane,VIIIth nerve or semicircular canal0 1

were separated by a synaptic delay of 1.8–1.9 ms at a bath nerve evoked field potentials were recorded in dorso-temperature of 148C as in an earlier study [33]. The shapes ventrally oriented tracks at a laterality of zero mm (see Fig.of the canal nerve evoked N field potentials (Fig. 2B–D) 1B). The most rostral recording site was at 20.7 mm and1


Table 1Parameters of presynaptic N field potential components evoked by electrical stimulation of the VIIIth nerve or of an individual semicircular canal nerve on0

athe ipsilateral side at the standard recording site

VIIIth nerve Horizontal semicircular canal Anterior semicircular canal Posterior semicircular canal

Latency 0.960.2 ms 1.760.6 ms* 1.860.4 ms* 1.760.4 ms*Amplitude at 43T 5026202 mV 161684 mV* 109672 mV* 147662 mV*a n529 for all instances. *P#0.0001, significance of difference with respect to values evoked by VIIIth nerve stimulation (Wilcoxon signed-rank test).

the most caudal site was at 1.5 mm with respect to the maxima evoked by stimulation of the AC nerve werecaudal end of the entry of the VIIIth nerve (see Fig. 1D). located rostral with respect to the entry of the VIIIth nerveThe stimulus intensity was 4-times threshold (43T) of the (0 mm; Fig. 5A,B). In the medio-lateral direction theN component recorded at the standard recording site. maximal amplitudes of the N and N components were1 0 1

The spatial distribution of the normalized average observed in the intermediate and lateral part of theamplitudes of N and of N components evoked at a given vestibular nuclear complex. Only very small field potential0 1

stimulation site was not uniform along the rostro-caudal amplitudes were recorded in the medial vestibular nucleus.search area but exhibited peaks that differed in their In frontal plane recordings the rostro-caudal location andlocation (Fig. 3). Stimulation of each of the different nerve the depth of the local maxima following stimulation ofbranches evoked potentials that peaked at a different but each of the three different canal nerves were again similarfor this nerve branch characteristic site in the vestibular for N and for N components (Figs. 6 and 7). The spatial0 1

nuclear complex (Fig. 3A–D for the N and Fig. 3E–H for distribution of these potentials was consistent with those0

the N components). N and N response components observed in the parasagittal and in the oblique medio-1 0 1

evoked by a given nerve branch had rather similar spatial lateral planes (Figs. 4 and 5). The depth of the localdistributions along the rostro-caudal extent of the vestibu- maxima of the N as of the N components ranged0 1

lar nuclear complex. between 0.2 and 0.6 mm. Differences in depth of the localTo include depth information, the data shown in Fig. 3 maxima were observed between the rostral plane i.e. at

are presented in Fig. 4 as isopotential surface plots. The 20.4 mm (AC nerve, Figs. 6B and 7B ) and the two1 1

largest relative amplitudes (75–100%) of the N and N caudal planes i.e. at 0.4 and 0.7 mm (HC nerve, Figs.0 1

field potential components following stimulation of an 6A ,A and 7A ,A ; PC nerve, Figs. 6C ,C and 7C ,C ).2 3 2 3 2 3 2 3

individual semicircular canal nerve were recorded at a Differences in the laterality of the local maxima betweendepth between 0.4 and 0.6 mm from the top of the N and N components were observed in the frontal planes0 1

brainstem at a laterality of 0 mm. This location corre- following stimulation of all three semicircular canalsponds to the anatomical center of the vestibular nuclear nerves. Independent of the stimulated semicircular canalcomplex [22–24]. Differences in the spatial distribution of nerve the maximal amplitudes of the N components were0

the peak amplitudes of N and N field potentials evoked located more laterally (Fig. 6) than the corresponding N0 1 1

by different canal nerves were now more apparent. components (Fig. 7).Semicircular canal nerve evoked N and N field0 1

potential components recorded in an oblique medio-lateral 3.3. Location of the maxima of semicircular canal nerveplane (see dashed line 2 in Fig. 1A and F) and evoked by evoked N components in the vestibular nuclei1

stimulation of a given nerve branch had rather similartopographies, but exhibited different spatial distributions The maximum of the HC nerve evoked N component1

for different canal nerves. The rostro-caudal extent of the representing the monosynaptic excitation of 28 HC neuronslargest relative amplitudes (75–100%) of the N and N was located in the lateral vestibular nucleus and more0 1

field potential components (Fig. 5A,B) were rather similar caudally in the descending vestibular nucleus (Figs. 8A2,3

to those observed in the parasagittal plane (Fig. 4A,B). and 9A,D). The maximum of the AC nerve evoked N1

The maxima of the field potentials evoked by the HC nerve component was observed in the central part of the superioror by the PC nerve were located caudal whereas the vestibular nucleus (Figs. 8B and 9B,E). After PC nerve1

Table 2Parameters of N field potentials evoked by electrical stimulation of the VIIIth nerve or of an individual semicircular canal nerve on the ipsilateral side at1

athe standard recording site

VIIIth nerve Horizontal semicircular canal Anterior semicircular canal Posterior semicircular canal

Latency 2.660.4 ms 3.660.7 ms* 3.760.6 ms* 3.560.6 ms*Amplitude at 43T 10316379 mV 2696153 mV* 2086105 mV* 3006247 mV*Normalized area 13.164.4 mV3ms 22.068.1 mV3ms* 11.864.3 mV3ms* 11.464.2 mV3ms*a n529 for all instances. *P#0.0001, significance of difference with respect to values evoked by VIIIth nerve stimulation (Wilcoxon signed-rank test).


Fig. 3. Normalized average amplitudes of the VIIIth nerve or semicircular canal nerve evoked N and N field potential components recorded in0 1

rostro-caudal depth tracks along the vestibular nuclear complex at a laterality of 0 mm. Symbols connected by dashed lines represent results from fourdifferent experiments, bold lines connect mean values of these data. HC, AC and PC stand for horizontal, anterior and posterior semicircular canal,respectively. Zero in rostro-caudal extension is the caudal end of the entry of the VIIIth nerve in the brainstem.

stimulation the largest amplitudes were observed in the the descending vestibular nucleus (Figs. 8 and 9). Interest-dorsal part of the lateral vestibular nucleus and to a ingly, the N component was absent or was less than 25%1

somewhat lesser degree more caudally in the descending of the maximal amplitude (Figs. 8 and 9) in the medialvestibular nucleus (Figs. 8C and 9C,F). Although, there vestibular nucleus throughout its rostro-caudal extent.2,3

were distinct areas with maximal amplitudes of the N1

components following stimulation of each one of the three 3.4. Correlation between the location of identifiedsemicircular canal nerves areas of overlap were observed second-order canal neurons and the topography of canalas well. However, these areas of overlap consisted pre- nerve evoked N field potentials1

dominantly of areas in which the amplitudes of the N1

components from at least one or two semicircular canal In an earlier study [34] we had investigated second-nerves were submaximal. These areas included the su- order vestibular neurons intracellularly at a lateralityperior vestibular nucleus, the central and dorsal part of the between the top of the brainstem (zero) and about 0.2 mmlateral vestibular nucleus and caudally the dorsal part of more medially between 0.5 mm rostral and 1.0 mm caudal


Fig. 4. Amplitude distribution plot of semicircular canal nerve evoked field potential components recorded in a parasagittal plane through the vestibularnuclear complex at a laterality of 0 mm. (A,B) Normalized amplitudes of semicircular canal nerve evoked N (A) and N (B) field potential components0 1

from four experiments. HC, AC and PC stand for horizontal, anterior and posterior semicircular canal, respectively. Zero in rostro-caudal extension refersto the caudal end of the entry of the VIIIth nerve. Relative amplitude magnitudes are represented by gray tones.

to the caudal end of the VIIIth nerve in tracks that were Rather, the amplitudes of these potentials were prominentseparated by 0.1 mm. These neurons were identified by in distinct subregions of the vestibular nuclear complex.their monosynaptic excitation from one of the three The spatial differences between the maxima of pre- andipsilateral semicircular canal nerves. Second-order canal postsynaptic components of field potentials evoked byneurons were searched systematically in depth tracks that stimulation of a particular semicircular canal nerve werewere regularly spaced over the entire rostro-caudal exten- only minor when compared with the spatial differencession of the vestibular nuclear complex. The rostro-caudal between the maxima of field potentials evoked by differentand dorso-lateral location of these second-order semicircu- semicircular canal nerves. However, the overlap of theselar canal neurons is shown in Fig. 10A–C. Regional potentials representing the afferent inputs from differentdifferences in the distribution of these second-order canal semicircular canals in the ipsilateral labyrinth was exten-neurons are expressed by the histograms in Fig. 10D–F sive. Large field potentials following stimulation of each of(gray tone). Comparison of these distributions with those the three semicircular canal nerves were encountered in allof the canal nerve evoked N field potential amplitudes vestibular nuclei with the notable exception of the medial1

(dashed lines in Fig. 10D–F) revealed compatible local vestibular nucleus.maxima. Anatomical tracer studies with selective labeling of

individual vestibular nerve branches demonstrated in vari-ous species that the central projections of afferent fibers

4. Discussion from different semicircular canals, as from different otolithorgans are diffuse and innervate all vestibular nuclei, even

Field potentials evoked by separate semicircular canal though the densities of these projections exhibit regionalnerve stimulation and recorded in the ipsilateral vestibular variations [8,11,17,25,37]. Moreover, the projections ofnuclear complex were not homogeneously distributed. afferent fibers from different vestibular endorgans overlap


Fig. 5. Amplitude distribution plot of semicircular canal nerve evoked field potential components recorded in an oblique medio-lateral plane through thevestibular nuclear complex. (A,B) Normalized amplitudes of semicircular canal nerve evoked N (A) and N (B) field potential components from four0 1

experiments. HC, AC and PC stand for horizontal, anterior and posterior semicircular canal, respectively. Zero in rostro-caudal extension refers to thecaudal end of the entry of the VIIIth nerve.

extensively in the superior, lateral and descending vestibu- of canal–canal signal convergence is more common forlar nuclei but spare part of the medial vestibular nucleus in polysynaptic than for monosynaptic inputs, particularlyanurans [24,37], pigeon [8] and cat [11]. The activation during natural stimulation. It should be noted, however,maps composed in this study for semicircular canal inputs that the spatial orientation of the vector of maximalare consistent with these anatomical descriptions. On the responses during natural stimulation is composed by theone hand was the medial vestibular nucleus almost devoid convergence of major and minor canal input componentsof evoked pre- and postsynaptic semicircular canal signals and not by equally strong inputs from different canals.within the central area investigated here. Large evoked Therefore, major input components might correlate withpotentials were recorded in this region, however, following the peaks and minor input components with the extensiveelectrical stimulation of the VIIIth nerve on the contralater- overlap in our maps of evoked responses, respectively.al side [21]. On the other hand, pre- and postsynaptic Topographic, organotypical maps of the vestibular sen-components of semicircular canal nerve evoked field sory organs akin to retinotopic, somatotopic or tonotopicpotentials were not limited to narrow, alternating zones or representations in the neocortex or in the brainstem ofthin, longitudinal columns but overlapped extensively. A mammals are apparently not existing, neither in amniotestendency towards a crude representation, however, was nor in anamniotes [8,11,24,42]. Positive examples forpresent since the maxima of pre- and postsynaptic field faithful representations of afferent sensory information,potential components evoked by stimulation of different however, exist for fish, frog and bird (e.g. electrosensorysemicircular canal nerves were localized in different maps in the hindbrain of weakly electric fish [15]; auditoryregions of the vestibular nuclear complex. These maps are and/or visual maps in the tectum opticum of owl and frognot necessarily in conflict with the fact that the incidence [12,20]). Organotypical maps are therefore not an achieve-


Fig. 6. Amplitude distribution plot of semicircular canal nerve evoked field potential components recorded in a frontal plane through the vestibular nuclearcomplex at three different rostro-caudal locations. (A–C) Normalized amplitudes of semicircular canal nerve evoked N field potential components0

following stimulation of the horizontal canal (HC), anterior canal (AC) or posterior semicircular canal (PC) nerve in four animals. The rostral (20.4 mm)and the caudal (0.4 or 0.7 mm) locations of the frontal planes with respect to the caudal end of the entry of the VIIIth nerve are indicated on top. Arrowspoint towards the sulcus limitans in the medial wall of the brainstem.

ment that is restricted in its appearance to mammals. hypothesis of the fundamental importance of topographicRather, the absence of a representational map of the maps to sensory processing in general. Rather, maps mayvestibular sense organ appears to be particular for this be especially well-suited for particular aspects of sensorysensory modality. This particularity is of interest in the information processing and may exist wherever thesecontext of the controversy whether topographic maps are particular aspects are of relevance. Kaas [18] suggestedfundamental to sensory processing [18] or not [41], but has that the requirements for metabolically costly connectionsso far not been considered. could be reduced by grouping together those neurons that

The existence of a sensory system that lacks an or- most commonly interact, for instance by the constructionganotypical map of its receptor surfaces tends to negate the of local neural circuits that make center surround com-


Fig. 7. Amplitude distribution plot of semicircular canal nerve evoked field potential components recorded in a frontal plane through the vestibular nuclearcomplex at three different rostro-caudal locations. (A–C) Normalized amplitudes of semicircular canal nerve evoked N field potential components1

following stimulation of the horizontal canal (HC), anterior canal (AC) or posterior semicircular canal (PC) nerve in four animals. The rostral (20.4 mm)and the caudal (0.4 or 0.7 mm) locations of the frontal planes with respect to the caudal end of the entry of the VIIIth nerve are indicated on top. Arrowspoint towards the sulcus limitans in the medial wall of the brainstem.

parisons. The central processing of semicircular canal that differ among each other in addition with respect tosignals differs in this respect, however, from the typical their axonal diameter and regularity of their spontaneousprocessing of, e.g., somatosensory, visual or acoustic discharge and converge with a differential contributionafferent information. All hair cells in the crista of a onto second-order vestibular neurons [16]. However, mostsemicircular canal encode the same direction of angular (about 90%) second-order vestibular neurons receive aacceleration but differ in their response dynamics in monosynaptic canal input from only one of the threerelation to their central or peripheral location in the crista ipsilateral semicircular canal nerves [19,34,42]. Therefore,[2]. These differences are mediated by afferent nerve fibers local neuronal circuits are neither required for a tuning of


Fig. 8. Distribution of the semicircular canal nerve evoked N components with respect to the vestibular nuclei in frontal planes at three different1

rostro-caudal levels as indicated on top. The amplitude distribution plots shown in Fig. 7 for four animals were superimposed on frontal sections showingthe outlines of the vestibular nuclei as in Fig. 1. (A ) Distribution of N amplitudes following horizontal canal (HC) nerve stimulation. (B )1–3 1 1–3

Distribution of N amplitudes following anterior canal (AC) nerve stimulation. (C ) Distribution of N amplitudes following posterior canal (PC) nerve1 1–3 1

stimulation. The length of the calibration arrows on the bottom corresponds to 0.3 mm. DN, dorsal nucleus; DVN, descending vestibular nucleus; LVN,lateral vestibular nucleus; MVN, medial vestibular nucleus; s.l., sulcus limitans; SVN, superior vestibular nucleus.

the spatial response characteristics nor for a contrast less than for other sensory systems and the encounteredenhancement of the dynamic response characteristics of crude representation could be a relict, more related tosecond-order vestibular neurons. In essence, the demands development than to function.for an orderly topographic representation of the receptor Part of the second-order vestibular neurons are premotorsurfaces of the cristae of semicircular canals appear to be neurons and innervate extraocular and/or spinal


Fig. 9. Distribution of the semicircular canal nerve evoked N components with respect to the vestibular nuclei in parasagittal or oblique medio-lateral1

planes. The amplitude distribution plots of the N field potential components shown in Figs. 4 and 5 for four animals were superimposed on the outlines of1

the vestibular nuclei. HC, AC and PC stand for horizontal, anterior and posterior semicircular canal, respectively. The length of the calibration arrows onthe bottom corresponds to 0.3 mm. CB, cerebellum; DN, dorsal nucleus; DVN, descending vestibular nucleus; LVN, lateral vestibular nucleus; MVN,medial vestibular nucleus; SVN, superior vestibular nucleus.

motoneurons. In the monkey vestibulo-ocular neurons survive to some extent in frogs into adulthood (Straka,initiating eye movements of distinct directions were more unpublished results). The group of vestibulo-spinal neu-or less separated in different parts of the vestibular nuclei rons, for instance, has about the same location with respect[39]. The location of these premotor neurons for upward to external landmarks in tadpoles [36] as in adult frogsand downward rotatory and horizontal eye movements was [7,9,27,40]. A more detailed motor representation for theconsistent with the distribution of semicircular canal nerve innervation of, e.g., cervical, brachial or lumbaractivated second-order vestibular neurons reported here for motoneurons or for the innervation of synergistic motorthe frog or with results obtained in the pigeon [42]. It is pools is so far unknown.interesting to note such a similarity in distribution oversuch a wide range of vertebrate species. During develop-ment second-order vestibular neurons form pools confined Acknowledgementsto distinct rhombomeric segments and are more (zebrafish[38]; tadpole [36]) or less well (chicken [13]) segregated Thanks are due to Lucia Schindler for technical assis-into neural groups premotor for ocular and/or spinal tance and to Dr F.P. Kolb for the help with the two-motoneurons. This differential distribution appears to dimensional surface plots. This research was supported by


Fig. 10. Location of identified second-order semicircular canal neurons and comparison with the distribution of N field potential amplitudes evoked by the1

same semicircular canal nerve branch. (A–C) Parasagittal view of the location of neurons that received a monosynaptic EPSP from the horizontal canal(HC) nerve (A), from the anterior canal (AC) nerve (B) or from the posterior semicircular canal (PC) nerve (C). Neurons were recorded between 0.5 mmrostral and 1.0 mm caudal to the entry of the VIIIth nerve in depth tracks 0.1 mm apart from each other. (D–F) comparison of the locations of recordedsingle neurons (histogram bars) with the distributions of N field potential amplitudes evoked by the same semicircular canal nerve (dashed lines). Data for1

N field potentials were taken from Fig. 3F–H.1

[4] J. Baker, J. Goldberg, G. Hermann, B. Peterson, Spatial andSonderforschungsbereich 462 (Sensomotorik) dertemporal response properties of secondary neurons that receiveDeutschen Forschungsgemeinschaft and by the Friedrich-convergent input in vestibular nuclei of alert cats, Brain Res. 294Baur-Stiftung 44/95.(1984) 138–143.

[5] G.A. Bush, A.A. Perachio, D.E. Angelaki, Encoding of headacceleration in vestibular neurons. I. Spatiotemporal response prop-erties to linear acceleration, J. Neurophysiol. 69 (1993) 2039–2055.

References [6] I.S. Curthoys, C.H. Markham, Convergence of labyrinthine in-fluences on units in the vestibular nuclei of the cat. I. Natural

[1] D.E. Angelaki, G.A. Bush, A.A. Perachio, Two-dimensional stimulation, Brain Res. 35 (1971) 469–490.spatiotemporal coding of linear acceleration in vestibular neurons, J. [7] P. D’Asciano, N. Corvaja, Spinal projections from the rhombence-Neurosci. 13 (1993) 1403–1417. phalon in the toad, Arch. Ital. Biol. 119 (1981) 139–150.

´[2] R.A. Baird, G. Desmadryl, C. Fernandez, J.M. Goldberg, The [8] J.D. Dickman, Q. Fang, Differential central projections of vestibularvestibular nerve of the chinchilla: II. Relation between afferent afferents in pigeons, J. Comp. Neurol. 367 (1996) 110–131.response properties and peripheral innervation patterns in the [9] V.V. Fanardjian, L.R. Manvelyan, V.L. Zakarian, V.I. Pogossian, A.M.semicircular canals, J. Neurophysiol. 60 (1988) 182–203. Nasoyan, Electrophysiological properties of the somatotopic organi-

[3] J. Baker, J. Goldberg, G. Hermann, B. Peterson, Optimal response zation of the vestibulospinal system in the frog, Neuroscience 94planes and canal convergence in secondary neurons in vestibular (1999) 845–857.nuclei of alert cats, Brain Res. 294 (1984) 133–137. [10] K. Fukushima, S.I. Perlmutter, J.F. Baker, B.W. Peterson, Spatial


properties of second-order vestibulo-ocular relay neurons in the alert [28] W. Precht, A. Richter, S. Ozawa, H. Shimazu, Intracellular study ofcat, Exp. Brain Res. 81 (1990) 462–478. frog’s vestibular neurons in relation to the labyrinth and spinal cord,

[11] R.R. Gacek, The course and central termination of first order Exp. Brain Res. 19 (1974) 377–393.ˆneurons supplying vestibular endorgans in the cat, Acta. [29] A. Sans, J. Raymond, R. Marty, Projections des cretes ampullaires et

´Otolaryngol. Suppl. 254 (1969) 1–66. de l’utricle dans les noyaux vestibulaires primaires. Etude mi-´[12] R.M. Gaze, The representation of the retina on the optic lobe of the crophysiologique et correlations anatomo-fonctionelles, Brain Res.

frog, Q. J. Exp. Physiol. 43 (1958) 209–214. 44 (1972) 337–355.[13] J.C. Glover, G. Petursdottir, Regional specificity of developing [30] D.W.F. Schwarz, I.E. Schwarz, Projection of afferents from in-

reticulospinal, vestibulospinal, and vestibulo-ocular projections in dividual vestibular sense organs to the vestibular nuclei in thethe chicken embryo, J. Neurobiol. 22 (1991) 353–376. pigeon, Acta Otolaryngol. 102 (1986) 463–473.

[14] W. Graf, J. Baker, B.W. Peterson, Sensorimotor transformation in the [31] J. Siegborn, K. Yingcharoen, G. Grant, Brainstem projections ofcats vestibuloocular reflex system. I. Neuronal signals coding spatial different branches of the vestibular nerve: An experimental study bycoordination of compensatory eye movements, J. Neurophysiol. 70 transganglionic transport of horseradish peroxidase in the cat, Anat.(1993) 2425–2441. Embryol. 184 (1991) 291–299.

[15] W. Heiligenberg, Electrosensory maps form a substrate for the [32] B.M. Stein, M.B. Carpenter, Central projections of portions of thedistributed and parallel control of behavioral responses in weakly vestibular ganglia innervating specific parts of the labyrinth in theelectric fish, Brain Behav. Evol. 31 (1988) 6–16. rhesus monkey, Am. J. Anat. 120 (1967) 281–317.

´[16] S.M. Highstein, J.M. Goldberg, A.K. Moschovakis, C. Fernandez, [33] H. Straka, N. Dieringer, Electrophysiological and pharmacologicalInputs from regularly and irregularly discharging vestibular nerve characterization of vestibular inputs to identified frog abducensafferents to secondary neurons in the vestibular nuclei of the motoneurons and internuclear neurons in vitro, Eur. J. Neurosci. 5Squirrel monkey: II. Correlation with output pathways of secondary (1993) 251–260.neurons, J. Neurophysiol. 58 (1987) 714–739. [34] H. Straka, S. Biesdorf, N. Dieringer, Canal-specific excitation and

[17] S.M. Highstein, R. Kitch, J. Carey, R. Baker, Anatomical organiza- inhibition of frog second order vestibular neurons, J. Neurophysiol.tion of the brainstem octavolateralis area of the oyster toadfish, 78 (1997) 1363–1372.Opsanus tau, J. Comp. Neurol. 319 (1992) 501–518. [35] H. Straka, S. Biesdorf, A. Birinyi, N. Dieringer N, Spatial organiza-

[18] J.H. Kaas, Topographic maps are fundamental to sensory process- tion of semicircular canal inputs in the frog, Soc. Neurosci. Abstr.ing, Brain Res. Bull. 44 (1997) 107–112. 23 (1997) 508/2.

[19] M. Kashara, Y. Uchino, Bilateral semicircular canal inputs to [36] H. Straka, E. Gilland, R. Baker, Rhombomeric organization ofneurons in cat vestibular nuclei, Exp. Brain Res. 20 (1974) 285– hindbrain vestibular neurons in larval frog, Soc. Neurosci. Abstr. 25296. (1999) 266/2.

[20] E.I. Knudsen, Auditory and visual maps of space in the optic tectum [37] C. Suarez, A. Kuruvilla, S. Sitko, I. Schwartz, V. Honrubia, Centralof the owl, J. Neurosci. 2 (1982) 1177–1194. projections of primary vestibular fibers in the bullfrog. II. Nerve

[21] A.W. Kunkel, N. Dieringer, Morphological and electrophysiological branches from individual receptors, Larygoscope 95 (1985) 1238–consequences of unilateral pre- versus postganglionic vestibular 1250.lesions in the frog, J. Comp. Physiol. A 174 (1994) 621–632. [38] H. Suwa, E. Gilland, R. Baker, Otolith ocular reflex function of the

[22] A. Kuruvilla, S. Sitko, I.R. Schwartz, V. Honrubia, Central projec- tangential nucleus in teleost fish, Ann. N. Y. Acad. Sci. 871 (1999)tions of primary vestibular fibers in the bullfrog: I. The vestibular 1–14.nuclei, Laryngoscope 95 (1985) 692–707. [39] K. Tokumasu, K. Goto, B. Cohen, Eye movements from vestibular

[23] C. Matesz, Central projection of the VIIIth cranial nerve in the frog, nuclei stimulation in monkeys, Ann. Otol. Rhinol. Larnygol. 78Neuroscience 4 (1979) 2061–2071. (1969) 1105–1119.

´ ´ ´[24] C. Matesz, A. Birinyi, H. Straka, N. Dieringer, Location of dye- [40] P. Toth, G. Csank, G. Lazar, Morphology of the cells of origin ofcoupled second order canal and otolith neurons and of efferent descending pathways to the spinal cord in Rana esculenta. A tracingvestibular neurons in the frog, Neurobiology 6 (1998) 226–227. study using cobaltic-lysine complex, J. Hirnforsch. 26 (1985) 365–

[25] C.A. McCormick, M.R. Braford Jr, Organization of inner ear 383.endorgan projections in the goldfish, Carassius auratus, Brain [41] R.J. Weinberg, Are topographic maps fundamental to sensoryBehav. Evol. 43 (1994) 189–205. processing?, Brain Res. Bull. 44 (1997) 113–116.

[26] G.E. Meredith, B. Butler, Organization of eighth nerve afferent [42] V.J. Wilson, L.P. Felpel, Specificity of semicircular canal input toprojections from individual endorgans of the inner ear of the teleost, neurons in the pigeon vestibular nuclei, J. Neurophysiol. 35 (1972)Astonotus ocellatus, J. Comp. Neurol. 220 (1983) 44–62. 253–254.

[27] N.M. Montgomery, Projections of the vestibular and cerebellarnuclei in Rana pipiens, Brain Behav. Evol. 31 (1988) 82–95.

research report spatial distribution of semicircular canal

Documents