Responses of semicircular canal and otolith afferents to small angle static head tilts in the Gerbil

Brain Research, 280 (1983) 287-298 287 Elsevier

Responses of Semicircular Canal and Otolith Afferents to Small Angle Static Head Tilts

in the Gerbil

ADRIAN A. PERACHIO and MANNING J. CORREIA

Departments of Otolaryngology, and Physiology and Biophysics, The University of Texas Medical Branch, Galveston, TX 77550-2 778 (U.S.A.)

(Accepted April 5th, 1983)

Key words: vestibular afferents - - anesthesia - - decerebration - - linear forces - - gerbils

The discharge activity of first-order vestibular neurons was recorded in anesthetized or decerebrated gerbils from the post-ganglionic fibers of the vestibular nerve. Semicircular canal afferents were distinguished from otolith afferents on the basis of their responses to linear and angular head acceleration. In decerebrated preparations, canal afferents exhibited significantly faster discharge activity (average = 87.8 impulses/s) than that of canal afferents in anesthetized preparations (average = 66.2 impulses/s), when the head was held to position the lateral semicircular canals coplanar with the earth horizontal plane (standard position).

The effects of changes in linear forces on vestibular afferent activity were determined by statically tilting the head + 10 ° about either the fore-aft and/or left-right head axes. A change in activity, from that recorded in the standard position, of 10% or greater was considered significant. Using this criterion, significant changes in the tilt response in anesthetized animals were observed in both anterior (23 of 48 neurons, 48%) and lateral (22 of 31, 71%) canal afferents as well as otolith (18 of 25, 72%) afferents. In decerebrated preparations for tilts around the pitch (left-right) axis, comparable effects were measured in (19 of 36, 53%) anterior and (17 of 30, 57%) lateral canal afferents. Neurons with irregular firing activity were more likely than regularly firing canal afferents to change their average discharge rate during static tilt. No significant differences in response magnitude to + 10 ° head tilt were found between canal and otolith afferents in anesthetized animals. Mechanisms to account for the responses to linear acceleration of canal afferents are discussed.

INTRODUCTION

Several lines of evidence have been deve loped that

support the proposa l that the semicircular canals may

respond to l inear accelerat ion of the head. In a vari- ety of species and types of preparations2,10,16,22, 25, it

has been noted that fibers within the ampul lary

nerves exhibit a l terat ions in their firing pat terns as a

consequence of changes in static head position.

Among the earl iest of such findings were those re-

por ted by Ledoux 22 for recordings made from semi-

circular canal afferents in the frog. In those studies,

the prepara t ion was viable for a l imited t ime since the

head was severed and the otic capsule was opened to

gain access to the nerves. Al though the effects were

observed ear ly in the course of the exper iments , they

could have been due to a pos t -mor tem phenomenon.

Lowenstein 25 observed similar effects in semicircular

canal afferent activity in the isolated e lasmobranch

labyrinth but qualif ied his in terpre ta t ion of the data

since in the exper imenta l p repara t ion the b lood sup-

ply to the labyrinth had been in ter rupted and the per-

i lymphatic space was opened. Ei ther of these manip-

ulations could have produced changes in the ionic

composi t ion of the labyrinthine fluids or a physical

change in the cupulal6, 2s. Subsequent to those stud-

ies, similar findings were repor ted for mammal ian

semicircular canal afferents in the cat 11 and squirrel

monkey 16. In both investigations, the post-ganglionic

fibers of the vest ibular nerve were exposed by aspi-

ration of overlying brain tissue. In a set of control ex-

per iments , Goldberg and Fernandez 16 demonst ra ted

in squirrel monkeys that in such prepara t ions , a ther-

mal gradient could be created across the labyrinth

which was of sufficient magni tude to produce a sensi-

tivity of the canals to l inear accelerat ion due to ther-

mally induced density differences. In those same ex-

periments , when the anter ior semicircular canal was

plugged, responses of anter ior canal ampul lary nerve

fibers to angular accelerat ions in the plane of that ca-

nal were abolished; however , they remained respon-

sive to l inear accelerat ion. Thus, al though much of

0006-8993/83/$03.00 © 1983 Elsevier Science Publishers B.V.

288

the data concerning the responses of semicircular canal primary afferents to linear acceleration must be

interpreted with caution in light of several sources of

experimental artifacts, there remains an intriguing

possibility that such phenomena can be appropriately

considered as physiological in nature.

The primary purpose of the present investigation was to re-examine this problem utilizing a prepara-

tion in which the integrity of the labyrinth and inter-

nal auditory meatus was maintained along with that

of surrounding neural structures. We report here the

finding that a significant number of semicircular ca-

nal primary afferents in both the anesthetized and the

unanesthetized decerebrated gerbil exhibit a tonic

alteration of their discharge pattern during small an-

gle static head tilts. The resting discharge of each

identified afferent was determined in order to char-

acterize the variability of its firing rate. Brief and

preliminary descriptions of a portion of these results have been reported previously 6,2s.

MATERIALS AND METHODS

In the first set of experiments, recordings were

made from the post-ganglionic portion of the vestibu-

lar nerve in 17 anesthetized male gerbils (Meriones unguiculatus). Anesthesia consisted of a combination

of ketamine hydrochloride (40 mg/kg, i.m.) and ure-

thane (500 mg/kg, i.p,) As required, supplementary

doses of ketamine (20 mg/kg) were administered

throughout the experiment. The animals were placed in a stereotaxic frame with the incisor bar secured 8

mm below the zero, ear bar plane of the instrument. This placed the head in a position, which will be re-

ferred to as the standard position, that aligned the major plane of the lateral (horizontal) semicircular

canals approximately coplanar with the horizontal

stereotaxic plane. Alignment of the lateral canals was verified by opening the bulla and exposing the labyrinth following the end of most experiments.

In the second set of experiments, an additional group of 13 male gerbils were used to control for the potential effects of anesthesia and thermal changes in the temporal bone due to the presence of stereotaxic ear bars placed in the external auditory canal. The

animals were decerebrated by a precollicular tran- section according to the method of Grill and Nor- gren TM. First the gerbils were anesthetized either with

a short-acting barbiturate (Brevital, 50 mg/kg, i.p.) alone or in combination with ketamine (25 mg/kg,

i.m.). After placement in the sterotaxic frame all in-

cisions and pressure points were infiltrated with a lo-

cal anesthetic (xylocaine, 1.0%). An L-shaped

bracket, secured to an electrode carrier, was ce-

mented to the cranium at bregma. This allowed the ear bars to be removed with no displacement of the

animal from the standard position.

Micropipettes filled with 2 M NaCL, (10-18 Mr2 im- pedance) were driven through an opening (maximum

2 mm anterior-posterior by 4 mm medial/lateral) in the cranium into the intact cerebellum at an angle of

25 ° off the sagittal plane. In the decerebrated prepa-

rations, additional openings of 1 × 5 mm were made

over each hemisphere for passage of the spatula.

There was no retraction of muscle tissue or exposure

of the bullae of the temporal bone. Dental cement

covered the exposed cranium up to the opening over

the cerebellum. Prior to penetration of the post-gan-

glionic vestibular nerve fibers, neurons responsive to sound stimuli were usually encountered in the coch-

lear nucleus which lies dorsal to the root fibers of the

vestibular nerve. Verification of the recording site

was obtained in several animals by deposition of A1- cian blue dye ionophoreticaily23 (see Fig. 1) through

the recording pipettes at a locus where responsive

neurons were detected. In virtually all preparations,

primary afferents were identified at nearly identical stereotaxic coordinate locations.

Identification criteria Each vestibular afferent was identified according

to a set of physiological response characteristics. These were based on a modification of criteria de- rived originally by Estes and co-workers for cat vesti-

bular primary afferents u~. This procedure involved a

measurement of the firing activity of each neuron during a sinusoidai head angular acceleration at approximately 1.0 Hz about an earth vertical axis while the animal's head was held in each static tilt position. The turntable to which the stereotaxic frame had been affixed was rotated manually or by a velocity- servo-controlled DC torque motor coupled to the central axis of the base of the table. Afferents innervating the vertical canals of the gerbil are responsive to angular acceleration about the yaw head axis when the head is held in the standard position27. Thus, af-

Fig. 1. Transverse section through the medulla illustrating the electrode tract (ET) of a micropipette that was driven through the intact cerebellum and brainstem into the post-ganglionic fibers of the vestibular nerve medial to Scarpa's ganglion (SG). Inset: magnified view of a portion of the same section con- taining SG cell bodies and fibers of the vestibular nerve. The site from which the activity of a semicircular canal afferent responsive to static head tilt was recorded is marked by an Alcian blue dye spot (DS) ionophoretically deposited through the recording pipette.

ferents of the anterior semicircular canal could be

distinguished from other canal afferents by their response of decreased firing activity when the head was accelerated toward the recording side. Both lateral

and posterior canal afferents increased firing activity

with ipsilateral head acceleration. Further distinction among canal afferents was made possible by retesting

and noting each afferent 's response to acceleration

about the yaw axis after the head had been reposi- tioned to a roll tilt angle of +20 ° or greater. With a static roll tilt of the head to the ipsilateral side, a re- versal in the polarity of the response of vertical canal afferents was observed, i.e. anterior canal afferents were excited by ipsilateral yaw acceleration and posterior canal afferents were inhibited. Although re-

289

duced in magnitude, the response of lateral canal af-

ferents when the head was tilted +20 ° was unaltered

in polarity. An additional group of fibers were classi-

fied as otolith afferents since they exhibited a system-

atic alteration in activity that was related to head tilt

position, but did not respond at any static head tilt

position to yaw angular acceleration t6.

Test paradigm The activity of each afferent was recorded while

the head of the animal was held in standard position

and after the head had been statically tilted through a

10 ° angle around either the longitudinal (roll) or

transverse (pitch) cardinal head axes. Recordings

were made both during repositioning of the head and

for a minimum of one-and-a-half minutes following

the tilt. The head was tilted slowly from one position

to another to minimize the cross coupling effects of

angular acceleration. The analyzed data were taken

from samples of neuronal activity that occurred from

30 to 60 s following cessation of head movement . In

decerebrated preparations, an abbreviated paradigm

was used. Static head tilts were restricted to the pitch axis and only canal afferents were systematically

tested.

Data analysis Magnetic tape recordings of the vestibular afferent

activity were analyzed to determine the average fir-

ing activity and the variability of discharge rate for

each neuron. Segments of data including 512 to 1024

interspike intervals were used for these calculations.

The coefficient of variation (CV = standard devia- tion/mean) was used as a measure of the regularity of

neuronal firing. A post-hoc analysis of all neurons classified as semicircular canal afferents (Fig. 2) indi-

cated that approximately half of the sample from

anesthetized preparations was composed of neurons that were highly regular in their firing rate with CVs

less than 0.1. The remainder of the sample could be

further divided in two roughly equal categories with

CVs falling into an intermediate range from 0.1 to 0.5 or in the range of highly irregularly firing neurons with CVs greater than 0.5. These classifications were based solely on data obtained from anesthetized gerbils under the standard position test condition. Addi- tional first-order statistical properties of the discharge activity of all afferent neurons included the

290

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coeff icients of skewness , kurtosis and excess, which

have been used prev ious ly to charac ter ize the dis-

charge activity of ves t ibular af ferents in the gerb iP 0

and the p igeon 7.

Statistical tests (t-test for d i f ferences be tween

means) were used to c o m p a r e segment s of discharge

activity r eco rded when the head was held in the stand-

ard posi t ion to that occur r ing af ter a 10 ° head tilt. In

addi t ion, changes in firing rate were t r ans fo rmed into

percen tage d i f ference scores re la t ive to the s tandard

posi t ion rate. All scores g rea te r than 10% for neu-

rons in each g roup (e.g. la teral canal i r regulars)

were used in a statistical test for s ignif icant differ-

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RESULTS

Anes the t i zed preparat ions

Record ings were m a d e f rom 109 ident i f ied vesti-

bular afferents whi le the head was he ld in the s tand-

ard posi t ion. Since v i r tua l ly no da ta were ob t a ined

f rom pos te r io r semic i rcu lar canal af ferents , the re-

cordings were apparen t ly m a d e f rom the super io r

branch of the ves t ibular ne rve . T h e sample was divid-

ed as follows: o to l i th af ferents , n = 25; la tera l canal

afferents , n = 35; an te r io r canal af ferents , n = 49

(see Tab le I). The ave rage discharge ra te for all af-

ferents was 62.2 + 26.8 (mean + S .D . ) impulses/s .

The average firing rate of all canal af ferents was 66.2

+ 33.0 impulses/s (n = 84).

Regula r ly firing canal and o to l i th af ferents exhib-

ited a significantly faster ave rage d ischarge ra te than

that of the most i r regular ly firing neurons (t-test for

means , 2- tai led, P < 0.02). Canal af ferents with in-

t e rmed ia t e class firing pa t te rns were found to dis-

charge at a significantly s lower rate than regular ly fir-

ing canal neurons (see Tab le I A , t = 4.04, df = 61, P

< 0.01). A similar s ignif icant d i f fe rence was found

be tween in t e rmed ia t e and i r regular o to l i th af ferents

(t = 3.08 df = 11, P < 0.01). In compar i sons b e t w e e n

canal and otol i th af ferents , s ignificant d i f ferences

TABLE I

Average discharge rate ( impulses/s + S. D. ) of identified vestibular afferents for the standard head tih position

(A ) Anesthetized preparations

Coefficient of variation classification

Afferent type Regular Intermediate Irregular

Otolith 71.0 + 19.8 (n = 12) 65.3 + 20.2 (n = 6) Lateral 96.1 + 23.1 (n = 15) 69.2 + 38.8 (n = 7) Anterior 96.3+ 19.2 (n =28) 72.9+ 16.1 (n = 13) Lateral + anterior 96.2 + 20.4 (n = 43) 71.6 + 25.4 (n = 20)

27.4 + 20.3 (n = 7) 31.6 + 22.8 (n = 13) 29.8 + 12.2 (n = 8) 30.9 ___ 18.6 (n = 21)

(B) Decerebrated preparations

Lateral Anterior Lateral + anterior

101.1 _+ 10.1 (n =24) 100.6 + 32.3 (n = 22) 100.8 + 26.2 (n = 46)

86.9 + 52.1 (n = 13) 94.6 + 53.9 (n = 20) 91.5 --- 52.5 (n = 33)

21.7 _+ 13.4 (n = 8) 34.3 + 19.4 (n = 4) 25.9 + 16,0 (n = 12)

were found for regularly firing neurons when tested

in the s tandard posi t ion; otol i th afferents fired at a

significantly slower rate than canal afferents (otoli th

vs lateral , t = 2.87, df = 25, P < 0.01; otoli th vs ante-

rior, t = 3.68, df = 38, P < 0.01). Fur the rmore , post-

hoc analyses comparing average discharge rate and

coefficients of skewness and excess for samples of af-

ferent activity for the s tandard posi t ion, be tween ca-

nal neurons that were subsequent ly found to respond

to l inear accelerat ion and the rest of the canal affer-

ents (within each CV category) , revea led no signifi-

cant differences.

A f f e r e n t activity - - static head tilt

Not all neurons could be tested for all head tilt con-

ditions; however , except for 5 canal afferents, data

were obta ined for each afferent (n = 104) for the con-

ditions of + 10 ° head tilt a round ei ther the pitch or roll

cardinal head axis. Neurons that were classified as

otol i th afferents (i.e. those that did not respond to

angular head accelerat ion for any head tilt posi t ion)

were usually tested by head tilts in whichever of the

two vertical planes was closest to their plane of maxi-

mal sensitivity. There was no a t tempt to maximize

the o to l i tha f fe ren t responses by defining their polar-

ization vectors 11.

A large percentage of the total tested sample of ca-

nal afferents exhibi ted a change in average firing rate

~ I ~ NOSE UP I0 °

STANDARD POSI TION

Fig. 3. Response of a regular (CV = 0.03) lateral canal after- ent, in an anesthetized gerbil, to static pitch tilt. Top trace: ex- tracellular recording of action potentials during 10 ° nose up pitch, average interspike interval = 10.5 ms. Middle trace: recording at standard position, average interspike interval = 9.8 ms. Bottom trace: recording during 10 ° nose down pitch, average interspike interval = 8.7 ms. Each trace composed of ten superimposed traces.

291

from that recorded while the head was held in the

s tandard posi t ion following a static head tilt a round

ei ther the transverse or longitudinal head axis (see

Fig. 3). Since the sample of neuronal firing was rela-

tively large, a simple statistical test for differences in

average discharge rates in almost every case proved

to be significant. Differences in activity rate for regu-

larly firing afferents represented by a change of less

than 0.5 ms in average interspike interval were, for

samples of 512-1024 intervals, statistically signifi-

cant, e.g. lateral canal regular afferent , unit

GR6410, s tandard posit ion (mean interspike interval

_+ S.D.) = 8.25 _+ 0.27 ms, 10 ° nose down pitch =

8.62 + 0.26 ms; t = 31.6, df = 2046, P < 0.02. It was

de te rmined both in this study and by o ther investiga-

tors 12 that otoli th afferents can exhibit as much as a

5% change in average firing activity when the head is

t i l ted and then re turned to the original or s tandard

position. Therefore , we imposed an addit ional crite-

rion to the t-test for statistical difference in firing ac-

tivity: a significant response following static head tilt

was defined as a minimum difference of 10% or

greater between the firing rate under the s tandard

posit ion condit ion as compared to the average firing

rate following a 10 ° head tilt.

In Table II , the responses of identif ied canal affer-

ents to static head tilt are summarized. Significant re-

sponses were found in a large percentage of both an-

ter ior (23/48, 48%) and lateral (22/31, 71%) canal af-

ferents of all types. The highest percentage of re-

sponsive neurons from ei ther the lateral or anter ior

semicircular canal were among those in the interme-

diate and irregularly firing classifications. However ,

the magni tude of the response, measured either as a

difference in impulses per second or as a percentage

change relat ive to the mean firing rate for the stand-

ard posit ion, was statistically equivalent for all types

of lateral canal afferents (see Table III) . Change in

average firing rate in impulses per second following

head tilt for anter ior canal neurons was significantly

less for i rregular afferents as compared to the other

two classifications. An te r io r i rregular afferents also

were less sensitive to head tilt than lateral canal irreg-

ulars as measured by the percentage change in their

discharge rates (t = 2.84, df = 41, P < 0.02).

Not all identif ied otoli th afferents responded to

changes in head posit ion within the limits of the test

paradigm, which confined head tilts to +10 ° about

292

TABLE II

(A) Semicircular canal afferents in anesthetized animals that responded* to linear acceleration

Afferent type CV class Pitch Roll

No. tested % respond. No. tested % respond.

Anterior

Lateral

Regular 27 15% 13 23% Intermed. 13 62% 8 100% Irregular 7 86% 5 80%


(B) C•mparis•n •f the percentage •f latera• and anteri•r semi•ircu•ar cana• afferents in anesthetized and decerebrated preparati•ns that responded to pitch tilt

Decerebrated Anesthetized

CV class No. tested % respond. No. tested % respond.


* Difference of 10% or greater in mean firing rate after 10 ° static tilt from standard position.

TABLE III

Afferent type Regular Intermediate Irregular

(A ) Average percentage change (+ S.D.) in discharge rate of responsive neurons, in anesthetized animals following 10 ° static head tilt Otolith 15.3 + 7.4 (13)* 20.4 + 13.0 (8) 26.6 + 14.2 (9) Lateral 11.2 + 0.4 (4) 20.5 + 12.0 (12) 36.2 + 26.1 (29) Anterior 13.2 + 3.0 (8) 16.3 _+ 6.0 (20) 19.4 + 13.9 (14)

(B) Average change in discharge rate of responsive neurons in anesthetized animals in impulses per second (+ S.D.) Otolith 10.0 + 2.9 9.8 + 3.1 6.1 + 3.0 Lateral 9.0 + 1.9 9.8 + 8.0 7.4 + 4.9 Anterior 9.9 + 3.4 10.1 + 4.3 4.8 + 3.1

* Total number of responses for all tilt positions in parentheses; some neurons represented for multiple tests.

the pitch and roll head axes. No signif icant differ-

ences were found be tween the 18 responsive otol i th

afferents and the responsive lateral and an te r ior ca-

nal afferents in te rms of the magn i tude of the change

of firing rate (see Tab le III) . A m o n g otol i th neu rons ,

regularly firing afferents differed significantly from

irregularly firing neu rons . In te rms of percentage

change, i r regular otol i th afferents exhibi ted a signifi-

cantly greater response to static head tilt; however ,

the difference in firing rate measu red in impulses per

second was significantly grea ter for regular ly firing

otoli th neurons .

Directional sensitivity

Al though a n u m b e r of canal afferents exhibi ted

systematic responses re la ted to the direct ion of head

tilt, no single rule appl ied to all neurons . This was in

part because the full test pa rad igm could no t be ap-

plied to the ent i re sample. The fol lowing results per-

tain to anes thet ized animals : of the lateral canal af-

ferents that were responsive to backward pitch tilt

(15/31), all exhibi ted a decrease in discharge activity.

More were affected by forward pitch; of these, 70%

(12/17) r e sponded with an increase in discharge rate.

Of the lateral canal neu rons that r e sponded to roll tilt

to the contralateral side, approximately 80% (5/6)

were inhibited; however, ipsilateral roll tilts were ap-

proximately equally effective for inducing excitatory

or inhibitory effects. Less consistent response pat-

terns were observed for anterior canal afferents.

One-third of the tested anterior canal neurons

(16/48) exhibited a decrease in their firing rate for

any tested direction of head tilt. Of all anterior canal

afferents tested by head pitch, approximately one- half (18/37) responded. Most of these neurons

(11/18) showed inhibitory responses. Ten anterior

canal afferents were tested with ipsilateral roll head

tilt; all exhibited inhibitory responses.

In most cases, the magnitudes of the responses for

any given plane of head tilt were asymmetric. Ap-

proximately 32% (7/22) of responsive lateral canal afferents were sensitive to both forward and back-

ward 10 ° head tilts, increasing their discharge rate in

the nose down position and decreasing their dis-

charge rate in the nose up position, Among this small

number, there was no significant difference in the av-

erage response or percentage change in neuronal ac-

tivity for forward as compared to backward pitch;

however, for individual neurons, the response mag-

nitude was in some cases as much as 3-fold more sen-

sitive for the nose up tilt condition. Similarly, of all

responsive anterior canal afferents, approximately

30% (7/23) had a significant bidirectional response to

roll or pitch static head tilt. However , unlike the bidi- rectionally responsive lateral canal afferents, no con-

sistency was found in response directionality across

anterior canal neurons. Most canal afferents were

found to be sensitive for only one direction of tilt for a given head plane. Thus, for the tilt planes and angles

used in this study, a vector of maximum sensitivity could not be defined for the directionality and magni-

tude of the responsive canal afferents to linear acceleration.

Decerebrated preparations Following experiments on anesthetized animals, a

second set of experiments was conducted on decerebrated animals, specifically to test whether the presence of centrally acting anesthetics and/or the place-

ment of restraining ear bars in the external auditory canal have any significant influence on vestibular afferents' spontaneous activity or the response of canal afferents to static head tilt. Data were obtained on

293

the spontaneous activity of lateral (n = 45) and ante-

rior (n = 46) canal afferents from 13 decerebrated

gerbils. The average discharge rate compared to that

of canal afferents in anesthetized animals was signifi-

cantly faster, i.e. 87.6 + 44.2 impulses/s in decere-

brated gerbils vs 66.2 + 33.0 impulses/s in anesthetized animals (t = 3.50, df = 173, P < 0.001). In both

anesthetized and decerebrated preparations, a sig-

nificant inverse correlation existed between the CV

and average discharge rate of canal neurons: lateral and anterior canal afferents in anesthetized gerbils, r

+ ~0 .83 , n = 84, P < 0.001; lateral and anterior ca-

nal afferents in decerebrated gerbils, r = ----0.48, n =

91, P < 0.01. The reduced correlation for decere-

brated preparations was related in part to the in-

crease of the average firing activity of intermediate

neurons, e.g. 0.1 ~< CV ~< 0.5. In contrast to the anesthetized preparations, there was no significant differ-

ence in the average discharge rate of regular vs inter-

mediate canal neurons (see Table IB, t = 1.02, df =

77, P > 0.05). The response of canal neurons to static head tilt

was tested in the decerebrated preparations by pitch

tilts about the left-right head axis alone. A compara-

ble percentage of canal afferents in decerebrated ani-

mals to that found in anesthetized gerbils responded

with a change of 10% or greater in discharge rate

when the head was pitched +10 ° nose up or down from the major plane of the lateral semicircular canal

(see Table liB). As in anesthetized animals, there

was no consistent directional sensitivity that typified those responses.

A post-hoc assessment of the relationship between percentage change in firing activity relative to that of the standard position, for +10 ° pitch tilt, was made

for data obtained from all tested canal afferents in

both types of preparations. A significant correlation between CV and percentage change (independent of

sign) was found for both anesthetized and decerebrated samples of canal afferents with the exception

of anterior canal afferents in decerebrated animals

(see Fig. 4). Upon inspection of the data, it appears that the a priori criterion of a 10% change in firing activity as a minimum limit for defining a significant

change compares favorably with the results of a linear regression analysis for the middle CV range. For regular and irregular neurons, a 10% criterion may have been an over or under estimation, respectively,

294

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Om*O [ "m~ r *]4 ' . '6 ' . 'e ' i '.o

÷ * , 28 r ,O .g3" A % , 2 5 . 0 C V + 4 2

o { ~ ; . . . , • o . . • 22 .3CV + 3. l

6 0 . 0

5 0 .I

4 0 J

3 0 . 0

2 0 . 0

I 0 . 0

0 . 0

0 . 0

ANTERIOR

o o

o

. I 2 I . ' 4 I 1 6 I r 8 I i 1,0

~ 135 [ I 0 Z7 + & % i 1 4 I O C V + 5 , (

n ,33 r "010 ] 0 { ~ , % , 5 . 3 C V + I I . 5

COEFFICIENT OF VARIATION

Fig. 4. Relationship, in canal afferents in anesthetized and decerebrated gerbils, between coefficient of variation and percentage change (A%) in average interspike interval relative to the standard position for 10% nose up (+) or nose down (O) static head tilt. Criterion level of 10% change (see text) is indicated by long dashed line. Correlation coefficients (r) were computed for each canal type for each condition (*P < 0.01, **P < 0.05). Note that the 4 highest values for lateral canal afferents in anesthetized preparations were greater than 60% change in interspike interval. Linear least-squares best-fit lines and equations are shown for each test condition: solid line for nose up tilt, short dashed line for nose down tilt.

of significant changes. Nevertheless, the overall per-

centage of responsive neurons would not have been

different from that determined on the basis of the

original criterion. That is 48% (23 of 38 neurons) of

anterior afferents in anesthetized preparations and

53% (19 of 36 neurons) in decerebrated preparations

responded with a 10% change. Comparable figures

for lateral canal afferents in anesthetized gerbils

were 71% (22 of 31 neurons) and 57% (17 of 30 neu-

rons) in decerebrated gerbils.

DISCUSSION

Responses of vestibular primary afferents to linear acceleration

A large percentage of vestibular semicircular canal

afferents responded to changes in linear acceleration produced by static head tilts. The results do no t appear to be artifactual since, in the types of preparations used in this study, the labyrinth and brain remained intact and vestibular nerve and petrous bone were not surgically exposed, thus presumably avoid- ing both ionic imbalances that might produce fluid/ tissue density differences and thermal gradients across exposed or deeper lying portions of the lab- yrinth15, 25. Moreover, in the decerebrated prepara-

tions, the ear bars were removed. Although decerebrated preparations differed from the anesthetized animals, in terms of spontaneous activity of the vestibular afferents, the response of canal afferents to static head tilt was still observed. It also appears un- likely that the findings can be accounted for by a mechanical effect produced by movement of the nervous tissue with respect to the electrode tip. Within the same preparation and for the same electrode penetration, successively encountered neurons were either unresponsive or responsive to head tilting with different directional sensitivities, i.e. anterior canal neurons differed in the directionality of their responses while responsive lateral canal neurons exhibited a fairly consistent bidirectional response. Aside from action potentials, the only other monitored physiological parameter was body core temperature; thus, it was not possible to definitively determine whether other physiological functions varied over the course of the experiment. Such events are also un- likely to account for the responses to linear acceleration since positive and comparable effects were obtained in all preparations, at all stages of the experiments.

Possible mechanisms for canal neuron responses to

linear acceleration

Four features of the labyrinth suggest possible mechanisms that may account for responses of physi- ologically identified semicircular canal neurons to changes in static head tilt position. First, Caston and GribenskP. 4 have proposed, on the basis of physiological studies, that the saccule and semicircular canals in the frog are directly related by receptor-to-receptor fibers that apparently form connections between neurons within Scarpa's ganglion 4. Their findings suggest that the mechanism for the responses of canal neurons to gravity might act through an addi-

295

tional input afferent from otolith receptors. Al-

though this proposal offers an interesting possibility, Caston and GribenskP, 4 reported that 21% (12/57 of tested neurons) of anterior canal afferents exhibited an inhibitory response to electrical stimulation of the saccule. This figure is smaller than our sample of tilt- sensitive anterior canal afferents (23/48, 48%). Moreover, there is limited anatomical evidence for intrinsic connections between neurons within Scar- pa's ganglion 13.

A second anatomical feature of primary vestibular neurons that may imply a mechanism for connection of individual neurons to different receptors in the labyrinth is the recently reported finding of a number (39/928) of multipolar cells with unmyelinated dendritic processes in the vestibular ganglion of the cat 5. At present, the receptor innervation of multipolar neurons has not been identified; however, it has been established that the multipolar neurons are primary vestibular afferents that project to the vestibular nuclei. The extent of dendritic branching in those neurons, particularly with regard to their distribution to more than one receptor within the labyrinth, is not known; nor has it been established whether intrinsic synaptic connections exist between those neurons. However, such arrangements could offer a morphological basis for preganglionic or intraganglionic in- teractions between the labyrinthine neurons of linear (otolith organs) and angular (semicircular canal) acceleration sensors.

A third characteristic of the anatomy of the labyrinth was identified by Estes and colleagues10 as a possible mechanism to account for the gravity sensitivity of semicircular canal afferent neurons. They noted that the base of the crista of all 3 canals pro- jects into the vestibule and is supported by the endo- lymphatic membrane which also underlies the ultri- cular macula. They suggested that the mass of the utricule can exert varying degrees of tension, de- pending on the head tilt angle, along the membrane thus producing movement of the crista with respect to the cupula. This morphological feature was also cited to account for the polarization of the response of anterior and lateral canal afferents to roll tilt. Our findings, based on a large sample of neurons, were more equivocal concerning the directionality of the canal roll tilt response. However, one finding consistent with the prediction of the proposal of Estes et

296

al. m was that static roll head tilt toward the ipsilateral

ear produced an inhibition in all responsive anterior

canal neurons. It is more difficult to determine the

mechanical effects of the membranous coupling be-

tween the crista and the utricle during pitching mo-

tions of the head. However, assuming that a back-

ward head pitch causes the crista to be pulled toward

the utricle, decreased firing activity in lateral canal

afferents would be predicted, a result we observed.

The centrally originating efferent innervation of

the vestibular receptors is a fourth anatomical entity

which may act as a potential mechanism to modify

the activity of the primary vestibular afferents in re-

sponse to a change in head tilt position. In mamma-

lian species, electrical stimulation of the efferent sys-

tem has been found to produce either inhibitory 29 or

excitatory ~7 effects on the resting activity of vestibu-

lar primary afferents. The excitatory effect, meas-

ured in squirrel monkeys, was of markedly greater

magnitude in the irregularly discharging afferents 17.

This finding may have a bearing on the finding of our

study that the greatest percentage of canal neurons responsive to static head tilt were irregularly firing.

The vestibular efferent system consists of relatively few neurons 14,17 that appear to branch extensively

and innervate all receptor organs of the labyrinth 24.

These physiological and anatomical findings suggest that the efferent vestibular system may operate in a

feedback manner to modify the discharge rate of ca-

nal afferents as the head is statically tilted. Since rela-

tively little is known about the efferent inputs and

their response characteristics to natural vestibular stimulation, especially in warm-blooded animals 2l,

the role of the efferents in the response of canal after- ents to linear forces remains speculative.

Relative sensitivity of semicircular canal and otolith afferents

Semicircular canal afferents are, according to our criteria for response, as sensitive as otolith afferents

to small angle head tilts restricted to the cardinal head axes. This restriction is an important qualifica- tion since head tilts only in the two orthogonal planes are not sufficient to determine the actual linear force sensitivity of vestibular neuronslJ. Nevertheless,

within the limits of our test procedures, there was a remarkable overlap in the range of sensitivity for all types of afferents and no significant average differ-

ence between the responses of presumed otolith af-

ferents and identified canal afferents. The most im-

portant distinction in the canal afferent response to

static tilt as compared to that observed in the tested otolith afferents was a lack of symmetry or direction-

ality of response related to the functional and mor-

phological polarization of the receptors. In lieu of

any direct explanation for this finding, it may only be

speculated that in addition to the possible mech-

anisms discussed above, the vectors of sensitivity of

canal neurons responsive to linear forces are difficult

to define because of the complex geometry of the

neuroepithelial surface of the crista ampullaris. Important issues for future study include the deter-

mination of: (1) whether linear acceleration-sensitive

canal afferents exhibit a polarization vector; (2) the

degree of consistency of their response directionality; and (3) the symmetry of the tilt response meas-

ured over greater head tilt angles. Estes et al.10 re-

ported results from a few gravity-sensitive canal neu-

rons tested over larger head tilt angles, e.g.

180°-360 °, that exhibited a systematic and replicable

change in the magnitude and directionality of their

responses to changes in head position, especially with roll tilt. In our study, two lateral canal neurons,

tested with pitch tilt of up to +20 ° , similarly exhibited a significant relationship between head tilt angle and

discharge rate. Moreover, a significant inverse corre-

lation was found between their mean firing rate and

their CV. These responses to static linear force imply that canal neurons may also exhibit responses to dy-

namic linear acceleration. In addition, their dynamic response characteristics to angular acceleration may be altered (other than by changes in gain or sensitivi-

ty) as a function of head position with respect to the

gravity vector, since it has been demonstrated that the transfer characteristics of canal neurons are cor- related with the CV of their firing rates15, 30.

Implication for vestibular function The finding that the semicircular canals respond to

linear forces requires a re-interpretation of a number of experimental findings on labyrinthine function and may have important implications for clinical phenomena. Goldberg and Fernandez 16 have suggested

that this property of the canals may account for the clinically observed phenomenon of a positional nystagmus observed in some apparently normal sub-

297

jects 1. In squirrel monkeys ablation of the maculae

did not abolish a pre-existing idiopathic positional

nystagmus that can be reliably observed in individual

animals of that species t9. These and other phenome-

na that are sequelae to head t rauma 31 may be re-in-

terpreted .in terms of a contr ibut ion from a canal

gravity-sensitive mechanism.

A number of investigators have reported that neu-

rons within the vestibular nuclei apparently receive

converging inputs from the canals and the otolith or-

gans since these neurons respond to both angular and

linear accelerationS, 9. An assumption common to

both those studies was that slow or static tilting of the

head or other forms of l inear force would stimulate

only otolith related inputs to the vestibular nuclei

while angular head rotation would affect only canal

related afferents. Although canal/otolith conver-

gence on second- or higher-order vestibular neurons

is likely, it cannot be definitely proven by such lim-

ited neurophysiological criteria.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the expert tech-

nical assistance of H. Hughes and S. Steffens and to

thank S. Rainey for typing the manuscript. The col-

laboration of Dr. T. Clegg on early portions of this

study and the helpful suggestions of Dr. D. J. Ander-

son concerning the post-hoc correlational analysis

are gratefully acknowledged. This research was sup-

ported by the National Aeronautics and Space Ad-

ministration through Grant NAG2-26 and Contracts

NAS9-14641 and NAS2-10502.

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Documents

Responses of semicircular canal and otolith afferents to small angle static head tilts in the Gerbil