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PubMed2000

Assessing vestibular function: which tests, when?

Vertigo and dizziness are common complaints encountered in clinical practice. The patient's history and a thorough otoneurological evaluation are essential for identifying the specific pathology behind the patient's complaints. If the patient reports an illusion of movement (vertigo), this most likely indicates an imbalance within the vestibular system. A sensation of rotatory movement together with a spontaneous nystagmus suggests a lesion involving the semicircular canals, while an illusion of linear movement indicates a disturbance of the otoliths. Nystagmus of central origin or caused by a peripheral vestibular lesion can usually be distinguished by other features in the history or on clinical examination. While peripheral vestibular lesions usually lead to a mixed horizontal-torsional or vertical-torsional nystagmus, a pure vertical or pure torsional nystagmus is always caused by a central lesion. With simple bedside tests such as head-shaking nystagmus and rapid head impulses deficits in labyrinthine function can clearly be detected. For a more thorough investigation of vestibular function at the level of individual semicircular canals and the otoliths, modern techniques are now available such as three-dimensional eye movement vector analysis for the evaluation of individual semicircular canal function, measurement of the subjective visual vertical for utricular, and click-evoked myogenic potentials for saccular testing.

Dizziness↗
PubMed2004

Vertical canal function in normal subjects and patients with benign paroxysmal positional vertigo.

OBJECTIVES: To assess the dynamics of the vertical semicircular canal (VSCC)-ocular reflex in normal subjects and then to compare their gain in VSCC-ocular reflex with that of patients with benign paroxysmal positional vertigo (BPPV). MATERIAL AND METHODS: Subjects were sinusoidally rotated around the earth-vertical axis with their head tilted 60 degrees backward and turned 45 degrees to the right or left side from the sagittal plane at frequencies of 0.1, 0.3, 0.5, 0.7 and 1.0 Hz with a maximum angular velocity of 50 degrees/s. Head rotation to the right side on the right anterior semicircular canal (SCC)-left posterior SCC plane or to the left side on the left anterior SCC-right posterior SCC plane stimulated the pair of VSCCs. Eye movements were recorded on a video imaging system with an infrared charge-coupled device camera, using our new technique for analyzing the rotation vector of eye movements in three dimensions. RESULTS: The mean gains in left posterior SCC-ocular reflex in normal subjects ranged from 0.44 at 0.1 Hz to 0.79 at 1.0 Hz, while the mean gains in right anterior SCC-ocular reflex ranged from 0.45 at 0.1 Hz to 0.73 at 1.0 Hz. The mean gains in right posterior SCC-ocular reflex in normal subjects ranged from 0.53 at 0.1 Hz to 0.89 at 1.0 Hz, while the mean gains in left anterior SCC-ocular reflex ranged from 0.53 at 0.1 Hz to 0.88 at 1.0 Hz. Thus, the gains in VSCC-ocular reflex did not differ among the four VSCCs in normal subjects. Similarly, vestibulo-ocular reflex (VOR) gains of the four VSCCs in patients with right- or left-sided BPPV were almost the same at all frequencies compared to those of normal subjects. CONCLUSION: In patients with BPPV, gains in VOR in the four VSCCs were not changed in comparison with those of normal subjects. It is suggested that the mass of free-floating otoconial debris associated with canalolithiasis was too small compared to that of the endolymph to change the canal dynamics.

Adult↗
PubMed2002

Three-dimensional analysis of nystagmus in benign paroxysmal positional vertigo. New insights into its pathophysiology.

Nystagmus is a rhythmical rotation of the eyeball. Its characteristics can be defined mathematically by the axis of rotation and the angular velocity around this axis. We analysed the axis of rotation for the nystagmus in benign paroxysmal positional vertigo (BPPV) to elucidate its pathophysiology. Thirteen patients with typical unilateral BPPV participated in the present study. The axis of rotation for the nystagmus was calculated from images recorded on digital videotape with an infrared CCD camera, using an algorithm that we developed. The patients' responsiveness to Semont's liberatory maneuver was also assessed.The results showed that patients could be assigned to one of two groups based on the rotation axis of the nystagmus. In one group of patients (n = 7; Group A), the axis of rotation was almost vertical to the plane containing the posterior semicircular canal (PSC) on the presumed affected side. In the other group of patients (n = 6; Group B), it was clustered around the naso-occipital axis. These results suggest that in the patients in Group A the responsible lesion is confined to the PSC, whereas the pathogenesis underlying the conditions in those in Group B is considered to be more complicated. This speculation was reinforced by the difference in responsiveness to Semont's liveratory maneuver. All of the patients in Group A showed remission of vertigo within 10 days (mean, 2.4 days) after onset of treatment using Semont's liberatory maneuver, whereas the patients in Group B required much longer periods of time for remission to occur (mean, 25.7 days). Our findings lead to the conclusion that by considering the spatial orientation of individual semicircular canals, in the patients in Group B the pathophysiology is not confined to the PSC, but most probably all three semicircular canals are involved.

Adult↗
PubMed1996

Three-dimensional orientation of the eye rotation axis during the Purkinje sensation.

The otolith-semicircular canal interaction during postrotatory nystagmus was studied in six normal human subjects by applying fast, short-lasting, passive head and body tilts (90 degrees in the roll or pitch plane) 2 s after sudden stop from a constant velocity rotation (100 degrees/s) about the earth-vertical axis in yaw. Eye movements were measured with 3-D magnetic search coils. Following the head tilt, activity in the semicircular canal primary afferents continues to reflect the postrotatory angular velocity vector in head-centered coordinates, whereas otolith primary afferents signal a different orientation of the head relative to gravity. Pitch (roll) tilts away from upright during postrotatory nystagmus after yaw rotation elicited a transient vertical (torsional) VOR. Despite the change in head orientation relative to gravity, postrotatory eye velocity decayed closely along the axis of semicircular canal stimulation (horizontal in head coordinates). These results suggest that postrotary nystagmus is largely organized in head-centered rather than gravity-centered coordinates in humans as suggested by the Purkinje-sensation.

Eye Movements↗
PubMed1999

Vestibular function in children with the CHARGE association.

BACKGROUND: Histopathological examinations and computed tomographic scans of the temporal bone in patients with the CHARGE association (a malformative syndrome that includes coloboma, heart disease, choanal atresia, retarded development, genital hypoplasia, and ear anomalies, including hypoplasia of the external ear and hearing loss) have shown an absence of semicircular canals and a Mondini form of cochlear dysplasia. Until recently, no information was available concerning a possible loss of vestibular function, which could be a factor in retarded posturomotor development. To our knowledge, this is the first report of otolith tests done on patients with the CHARGE association. OBJECTIVE: To test residual vestibular function in patients with the CHARGE association. STUDY DESIGN: In 7 patients with the CHARGE association, we made electro-oculographic recordings of vestibulo-ocular responses to earth-vertical and off-vertical axis rotations to evaluate the function of the canal and the otolith-vestibular systems. RESULTS: None of the 7 patients had semicircular canals in the computed tomographic scan, and none had canal vestibulo-ocular responses to earth-vertical axis rotation, but all had normal otolith vestibulo-ocular responses to the off-vertical axis rotation test. CONCLUSIONS: These results support the hypothesis of a residual functional otolith organ in the hypoplastic posterior labyrinth of children with the CHARGE association. The severe delays in psychomotor development presented by these children are more likely a consequence of multiple factors: canal vestibular deficit, visual impairment, and environmental conditions (long hospital stays and breathing and feeding problems). The remaining sensitivity of the otolith system to gravity and linear acceleration forces in these children could be exploited in early education programs to improve their posturomotor development.

Abnormalities, Multiple↗
PubMed1995

Responses to vertical vestibular stimulation of neurons in the nucleus reticularis gigantocellularis in rabbits.

1. Because the nucleus reticularis gigantocellularis (NRGc) receives a substantial descending projection from the caudal vestibular nuclei, we used extracellular single-unit recording combined with natural vestibular stimulation to examine the possible peripheral origins of the vestibularly modulated activity of caudal NRGc neurons located within 500 microns of the midline. Chloralose-urethan anesthetized rabbits were stimulated with an exponential "step" and/or static head-tilt stimulus, as well as sinusoidal rotation about the longitudinal or interaural axes providing various combinations of roll or pitch, respectively. Recording sites were reconstructed from electrolytic lesions confirmed histologically. 2. More than 85% of the 151 neurons, in the medial aspect of the caudal NRGc, responded to vertical vestibular stimulation. Ninety-six percent of these responded to rotation onto the contralateral side (beta responses). Only a few also responded to horizontal stimulation. Seventy-eight percent of the neurons that responded to vestibular stimulation responded during static roll-tilt. One-half of these neurons also responded transiently to the change in head position during exponential "step" stimulation, suggesting input mediated by otolith and semicircular canal receptors or tonic-phasic otolith neurons. 3. Seventy-five percent of the responsive neurons had a "null plane." The planes of stimulation resulting in maximal responses, for cells that responded to static stimulation, were distributed throughout 150 degrees in both roll and pitch quadrants. Five of these cells responded only transiently during exponential "step" stimulation and responded maximally when stimulated in the plane of one of the vertical semicircular canals. 4. The phase of the response of the 25% of medial NRGc neurons that lacked "null planes" gradually shifted approximately 180 degrees during sinusoidal vestibular stimulation as the plane of stimulation was shifted about the vertical axis. These neurons likely received convergent input with differing spatial and temporal properties. 5. The activity of neurons in the medial aspect of the caudal NRGc of rabbits was modulated by both otolithic macular and vertical semicircular canal receptor stimulation. This vestibular information may be important for controlling the intensity of the muscle activity in muscles such as neck muscles where the load on the muscle is affected by the position of the head with respect to gravity. Some of these neurons may also shift muscle function from an agonist to an antagonist as the direction of head tilt changes.

Animals↗
PubMed2001

Vertical divergence and counterroll eye movements evoked by whole-body position steps about the roll axis of the head in humans.

In healthy human subjects, a head tilt about its roll axis evokes a dynamic counterroll that is mediated by both semicircular canal and otolith stimulation, and a static counterroll that is mediated by otolith stimulation only. The vertical ocular divergence associated with the static counterroll too is otolith-mediated. A previous study has shown that, in humans, there is also a vertical divergence during dynamic head roll, but this report was not conclusive on whether this response was mediated by the semicircular canals only or whether the otoliths made a significant contribution. To clarify this issue, we applied torsional whole-body position steps (amplitude 10 degrees, peak acceleration of 90 degrees /s(2), duration 650 ms) about the earth-vertical (supine body position) and earth-horizontal (upright body position) axis to healthy human subjects who were monocularly fixating a straight-ahead target. Eye movements were recorded binocularly with dual search coils in three dimensions. The dynamic parameters were determined 120 ms after the beginning of the turntable movement, i.e., before the first fast phase of nystagmus. The static parameters were measured 4 s after the beginning of the turntable movement. The dynamic gain of the counterroll was larger in upright (average gain: 0.48 +/- 0.10 SD) than in supine (0.36 +/- 0.10) position. The static gain of the counterroll in the upright position (0.21 +/- 0.06) was smaller than the dynamic gain. Divergent eye movements (intorting eye hypertropic) evoked during the dynamic phase were not significantly different between supine (average vergence velocity: 0.87 +/- 0.51 degrees /s) and upright (0.84 +/- 0.64 degrees /s) positions. The static vertical divergence in upright position was 0.32 +/- 0.14 degrees. The results indicate that the dynamic vertical divergence in contrast to the dynamic ocular counterroll is not enhanced by otolith input. These results can be explained through the different patterns of connectivity between semicircular canals and utricles to the eye muscles. Alternatively, we hypothesize that the small dynamic vertical divergence represents the remaining vertical error necessary to drive an adaptive control mechanism that normally maintains a vertical eye alignment.

Adult↗
PubMed1998

Hypothesis for shared central processing of canal and otolith signals.

A common goal of the translational vestibuloocular reflex (TVOR) and the rotational vestibuloocular reflex (RVOR) is to stabilize visual targets on the retinae during head movement. However, these reflexes differ significantly in their dynamic characteristics at both sensory and motor levels, implying a requirement for different central processing of canal and otolith signals. Semicircular canal afferents carry a signal proportional to angular head velocity, whereas primary otolith afferents modulate approximately in phase with linear head acceleration. Behaviorally, the RVOR exhibits a robust response down to approximately 0.01 Hz, yet the TVOR is only significant above approximately 0.5 Hz. Several hypotheses were proposed to address central processing in the TVOR pathways. All rely on a central filtering process that precedes a "neural integrator" shared with the RVOR. We propose an alternative hypothesis for the convergence of canal and otolith signals that does not impose the requirement for additional low-pass filters for the TVOR. The approach is demonstrated using an anatomically based, simple model structure that reproduces the general dynamic characteristics of the RVOR and TVOR at both ocular and central levels. Differential dynamic processing of otolith and canal signals is achieved by virtue of the location at which sensory information enters a shared but distributed neural integrator. As a result, only the RVOR is provided with compensation for the eye plant. Hence canal and otolith signals share a common central integrator, as in previous hypotheses. However, we propose that the required additional filtering of otolith signals is provided by the eye plant.

Animals↗
PubMed2000

Variables affecting treatment in benign paroxysmal positional vertigo.

OBJECTIVE: To identify variables affecting outcome in patients with benign paroxysmal positional vertigo (BPPV) treated with canalith repositioning maneuvers. STUDY DESIGN: Retrospective review of patients at a tertiary vestibular rehabilitation center. METHODS: Variables identified for statistical analysis included method of diagnosis, age, sex, onset association with trauma, semicircular canal involvement, presence of bilateral disease, treatment visits, and cycles of canalith repositioning maneuvers per treatment visit. Multivariate statistical analysis using Pearson chi2, likelihood ratio, linear-by-linear association, and cross-tabulation tests were performed. RESULTS: Two hundred fifty-nine patients with BPPV who received treatment were identified from 1996 to 1998. Average follow-up time was 16.9 months. 74.8% required one treatment visit, 19.0% required a second treatment visit, and 98.4% were successfully treated after three treatment visits. The remainder required up to seven treatment visits for relief of symptoms. Variables affecting the number of treatment visits included bilateral disease or location of disease other than in the posterior semicircular canal. Patient age, sex, method of diagnosis, and onset association with trauma had no statistically significant impact. CONCLUSION: Patients with benign paroxysmal positional vertigo not located in a single posterior semicircular canal are more likely to require multiple visits for canalith repositioning.

Adult↗
PubMed2001

Human experience with canal plugging.

Canal occlusion/plugging is a very effective technique with a low risk to hearing. The series of posterior semicircular canal occlusions described in this review now becomes the largest in the literature to date. It continues to support this procedure as the treatment of choice for intractable benign paroxysmal positional vertigo (BPPV). All 44 operated ears were relieved of BPPV, with one patient having an atypical late recurrence. Of the 40 ears with normal preoperative hearing, one had a delayed (3-month) sudden and permanent profound loss, while one other had a mild (20 dB) loss. Six patients had protracted courses of imbalance and motion sensitivity. Canal plugging has led to several new and innovative developments including the partial labyrinthectomy for difficult-to-access skull base lesions and superior semicircular canal plugging for dehiscence. These new procedures and their development are reviewed in this paper.

Humans↗
PubMed1978

A comparison of fenestration of the horizontal canal and stapedectomy in the opposite ear.

The Lempert one stage horizontal semicircular canal fenestration was the surgical procedure of choice for otosclerosis from 1939 to 1954. Subsequently, mobilization and stapedectomy replaced fenestration. In the offices of Drs. Day, Jordan, and Caparosa, the horizontal semicircular canal fenestration was performed from 1940 to 1955; the mobilization, two years; and, thereafter, the stapedectomy. A significant number of patients have had fenestration on one side and stapedectomy on the opposite ear. Fifty patients seen consecutively were reviewed. Reliable office audiological testing has been available only in the more recent years; and, therefore, audiological statistical comparison has limited reliability. It is felt, however, that a review of these patients and the study of their surviving hearing results is a worthwhile inferential comparison.

Bone Conduction↗
PubMed1985

Central projections of primary vestibular fibers in the bullfrog: I. The vestibular nuclei.

The central projections of the vestibular end organs in the bullfrog Rana calesbeiana were analyzed by using horseradish peroxidase labeling of the primary vestibular afferents. Separate extracellular injections were made of the anterior branch, the posterior branch, the ampullary nerve of each of the three semicircular canals, and the branch to the saccule. The anterior and posterior branches of the bullfrog eighth nerve, each containing both vestibular and auditory fibers, merge and enter the brain stem as a single nerve root. The thin caliber fibers of the anterior branch enter the brain stem on the ventral-posterior aspect of the nerve and immediately divide dichotomously into ascending (rostral) and descending (caudal) branches. The thick caliber fibers of the anterior branch enter the brain stem on the ventral-anterior aspect of the eighth nerve and traverse medially into the alar plate before dividing into ascending and descending branches. The primary afferent fibers of the vestibular nerve innervate an ipsilateral area of the brain stem which extends caudally from the rhombencephalon at the level of the twelfth nerve nucleus and rostrally up to and including the cerebellar nucleus and the cerebellum. The following vestibular nuclei can be identified by the fact that they receive primary vestibular afferents: the ventral vestibular nucleus, medial vestibular nucleus, descending vestibular nucleus, superior vestibular nucleus, and cerebellar nucleus. The dorsal (acoustic) nucleus, which receives primary auditory input, also receives afferents from the saccule. In addition to these nuclei, the cerebellum and the reticular formation receive significant primary input from the various vestibular receptors. The primary vestibulo-cerebellar fibers terminate mainly among the granular cells of the lobus auricularis and of the corpus cerebelli on the ipsilateral side. Each of the three semicircular canals projects into the cerebellum, while no such projection was observed in the saccular nerve preparations. Fibers from each of the three semicircular canals project to all of the vestibular nuclei. Heavily labeled large neurons, presumably vestibular efferents, are seen in the ipsilateral reticular formation, adjacent to the seventh motor nucleus.

Animals↗
PubMed1999

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. II. Responses after canal plugging.

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in four squirrel monkeys after unilateral plugging of the three semicircular canals. During the period (1-4 days) that animals were kept in darkness after plugging, the gain during steps of acceleration (3, 000 degrees /s(2), peak velocity = 150 degrees /s) was 0.61 +/- 0.14 (mean +/- SD) for contralesional rotations and 0.33 +/- 0.03 for ipsilesional rotations. Within 18-24 h after animals were returned to light, the VOR gain for contralesional rotations increased to 0. 88 +/- 0.05, whereas there was only a slight increase in the gain for ipsilesional rotations to 0.37 +/- 0.07. A symmetrical increase in the gain measured at the plateau of head velocity was noted after animals were returned to light. The latency of the VOR was 8.2 +/- 0. 4 ms for ipsilesional and 7.1 +/- 0.3 ms for contralesional rotations. The VOR evoked by sinusoidal rotations of 0.5-15 Hz, +/-20 degrees /s had no significant half-cycle asymmetries. The recovery of gain for these responses after plugging was greater at lower than at higher frequencies. Responses to rotations at higher velocities for frequencies >/=4 Hz showed an increase in contralesional half-cycle gain, whereas ipsilesional half-cycle gain was unchanged. A residual response that appeared to be canal and not otolith mediated was noted after plugging of all six semicircular canals. This response increased with frequency to reach a gain of 0.23 +/- 0.03 at 15 Hz, resembling that predicted based on a reduction of the dominant time constant of the canal to 32 ms after plugging. A model incorporating linear and nonlinear pathways was used to simulate the data. The coefficients of this model were determined from data in animals with intact vestibular function. Selective increases in the gain for the linear and nonlinear pathways predicted the changes in recovery observed after canal plugging. An increase in gain of the linear pathway accounted for the recovery in VOR gain for both responses at the velocity plateau of the steps of acceleration and for the sinusoidal rotations at lower peak velocities. The increase in gain for contralesional responses to steps of acceleration and sinusoidal rotations at higher frequencies and velocities was due to an increase in the gain of the nonlinear pathway. This pathway was driven into inhibitory cutoff at low velocities and therefore made no contribution for rotations toward the ipsilesional side.

Acceleration↗
PubMed2005

Paroxysmal positional vertigo as a complication of osteotome sinus floor elevation.

Paroxysmal positional vertigo (PPV) is a high prevalence, vestibular end organ disorder due to the detachment of the utricular otoconia floating in the posterior or lateral semicircular canal. Even though in the majority of cases the etiology of PPV is unknown, it may follow viral infection, vascular disorders and head trauma after different surgical procedures. The aim of this study was to investigate the correlation between PPV and the surgical trauma induced by the vibratory and percussive forces on the upper maxilla during the osteotome sinus floor elevation procedure. We performed a complete otoneurological examination on 146 patients affected by atrophic ridges before and after upper maxilla surgery. Four patients showed a PPV of the posterior semicircular canal controlateral to the implanted side 1 or 2 days after the surgical procedure, which promptly was solved with the Epley re-positioning maneuver. We hypothesize that the surgical trauma, and specifically the pressure exerted by the osteotomes, determines the detachment of the otoliths from the utricular macula while the patient head position, hyper-extended and tilted opposite to the side where the surgeon is working, favors the entry of these free-floating particles in the posterior semicircular canal of the implanted side. Although this disease is rather frequent in the normal population and it is a benign, self-limiting peripheral disorder, it should be considered by the oral surgeon as a possible complication of pre-prosthetic upper maxilla surgery, and the patient should be informed before undergoing surgery.

Aged↗
PubMed2000

Vestibular hypersensitivity to sound (Tullio phenomenon): structural and functional assessment.

OBJECTIVES: To establish the role of high-resolution CT imaging and tests of vestibulocollic reflexes in diagnosing and understanding the pathogenesis of the Tullio phenomenon. BACKGROUND: The Tullio phenomenon is a syndrome in which acoustic stimulation produces symptoms and signs of vestibular activation. It has previously been associated with an abnormally low threshold for click-evoked vestibulocollic responses and also with dehiscence of the roof of the anterior (superior) semicircular canal on high-resolution CT scans of the temporal bones. METHODS: High-resolution CT scans of the temporal bones and vestibulocollic responses in sternocleidomastoid to both clicks and transmastoid galvanic stimulation (3 mA/2 msec) were studied in four patients with the Tullio phenomenon (one bilateral). RESULTS: Click-evoked thresholds were low for all affected ears (four at 65 dB nHL, one at 55 dB nHL) and normal (>70 dB nHL) for the three unaffected ears. In contrast, galvanic-evoked vestibulocollic responses were symmetric and of normal size in all patients. The bony roof of the anterior (superior) semicircular canal was thin, possibly absent, on CT of all affected ears and also in two out of three unaffected ears. CONCLUSIONS: The normal galvanic vestibulocollic responses indicate that sound sensitivity in patients with the Tullio phenomenon is likely to occur distal to the vestibular nerve, probably at the level of the receptors. Both click hypersensitivity and dehiscence of the anterior (superior) semicircular canal are associated with the Tullio phenomenon but as the CT scan abnormality can occur in clinically unaffected ears, click testing is important for specific diagnosis. Abnormal sound sensitivity, as demonstrated by click responses, confirms that the radiologic abnormality is function significant.

Adult↗
PubMed1989

Surgical management of labyrinthine fistulae in chronic otitis media with cholesteatoma by a one-stage closed technique.

Twenty cases of labyrinthine fistulae have been reviewed. From 1979 to 1987, 234 patients with cholesteatomatous ears were operated on by one surgeon, 20 of these had a labyrinthine fistula (incidence 8.6%). In 75% only the lateral semicircular canal was involved and in 25% the other semicircular canals and/or the cochlea were involved. The fistula test was positive in 50% of the cases. The surgical technique used in all 20 cases was a one-stage closed tympanoplasty, i.e. removal of the cholesteatoma matrix, covering of the fistula with fascia and bone, obliteration of the cavity and reconstruction of the meatal wall. 76% of the patients achieved a hearing improvement, while 1 patient had anacusis (5%). The results indicate that it is possible in one stage to remove the cholesteatoma matrix from the fistula and still be able to restore useful hearing in the majority of the patients.

Cholesteatoma↗
PubMed1996

The delayed rectifier, IKI, is the major conductance in type I vestibular hair cells across vestibular end organs.

Hair cells were dissociated from the semicircular canal, utricle, lagena and saccule of white king pigeons. Type I hair cells were identified morphologically based on the ratios of neck width to cuticular plate width (NPR < 0.72) as well as neck width to cell body width (NBR < 0.64). The perforated patch variant of the whole-cell recording technique was used to measure electrical properties from type I hair cells. In voltage-clamp, the membrane properties of all identified type I cells were dominated by a predominantly outward potassium current, previously characterized in semicircular canal as IKI. Zero-current potential, activation, deactivation, slope conductance, pharmacologic and steady-state properties of the complex currents were not statistically different between type I hair cells of different vestibular end organs. The voltage dependence causes a significant proportion of this conductance to be active about the cell's zero-current potential. The first report of the whole-cell activation kinetics of the conductance is presented, showing a voltage dependence that could be best fit by an equation for a single exponential. Results presented here are the first data from pigeon dissociated type I hair cells from utricle, saccule and lagena suggesting that the basolateral conductances of a morphologically identified population of type I hair cells are conserved between functionally different vestibular end organs; the major conductance being a delayed rectifier characterized previously in semicircular canal hair cells as IKI.

4-Aminopyridine↗
PubMed1992

Neural basis for eye velocity generation in the vestibular nuclei of alert monkeys during off-vertical axis rotation.

Activity of "vestibular only" (VO) and "vestibular plus saccade" (VPS) units was recorded in the rostral part of the medial vestibular nucleus and caudal part of the superior vestibular nucleus of alert rhesus monkeys. By estimating the "null axes" of recorded units (n = 79), the optimal plane of activation was approximately the mean plane of reciprocal semicircular canals, i.e., lateral canals, left anterior-right posterior (LARP) canals or right anterior-left posterior (RALP) canals. All units were excited by rotation in a direction that excited a corresponding ipsilateral semicircular canal. Thus, they all displayed a "type I" response. With the animal upright, there were rapid changes in firing rates of both VO and VPS units in response to steps of angular velocity about a vertical axis. The units were bidirectionally activated during vestibular nystagmus (VN), horizontal optokinetic nystagmus (OKN), optokinetic after-nystagmus (OKAN) and off-vertical axis rotation (OVAR). The rising and falling time constants of the responses to rotation indicated that they were closely linked to velocity storage. There were differences between VPS and VO neurons in that activity of VO units followed the expected time course in response to a stimulus even during periods of drowsiness, when eye velocity was reduced. Firing rates of VPS units, on the other hand, were significantly reduced in the drowsy state. Lateral canal-related units had average firing rates that were linearly related to the bias or steady state level of horizontal eye velocity during OVAR over a range of +/- 60 deg/s. These units could be further divided into two classes according to whether they were modulated during OVAR. Non-modulated units (n = 5) were VO types and all modulated units (n = 5) were VPS types. There was no significant difference between the bias level sensitivities relative to eye velocity of the units with and without modulation (P > 0.05). The modulated units had no sustained change in firing rate in response to static head tilts and their phases relative to head position varied from unit to unit. The phase did not appear to be linked to the modulation of horizontal eye velocity during OVAR. The sensitivities of unit activity to eye velocity were similar during all stimulus modalities despite the different gains of eye velocity vs stimulus velocity during VN, OKN and OVAR. Therefore, VO and VPS units are likely to carry an eye velocity signal related to velocity storage.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗