Determinants of semicircular canal afferent response dynamics in fish

Numerical simulations to determine the stimulation of the crista ampullaris during the Head Impulse Test

The Head Impulse Test, the most widely accept test to assess the vestibular function, comprises rotations of the head based on idealized orientations of the semicircular canals, instead of their individual arrangement specific for each patient. In this study, we show how computational modelling can help personalize the diagnosis of vestibular diseases. Based on a micro-computed tomography reconstruction of the human membranous labyrinth and their simulation using Computational Fluid Dynamics and Fluid-Solid Interaction techniques, we evaluated the stimulus experienced by the six cristae ampullaris under different rotational conditions mimicking the Head Impulse Test. The results show that the maximum stimulation of the crista ampullaris occurs for directions of rotation that are more aligned with the orientation of the cupulae (average deviation from alignment of 4.7°, 9.8°, and 19.4° for the horizontal, posterior, and superior maxima, respectively) than with the planes of the semicircular canals (average deviation from alignment of 32.4°, 70.5°, and 67.8° for the horizontal, posterior, and superior maxima, respectively). A plausible explanation is that when rotations are applied with respect to the center of the head, the inertial forces acting directly over the cupula become dominant over the endolymphatic fluid forces generated in the semicircular canals. Our results indicate that it is necessary to consider cupulae orientation to ensure optimal conditions for testing the vestibular function.

A mathematical model for mechanical activation and compound action potential generation by the utricle in response to sound and vibration

Introduction

Calyx bearing vestibular afferent neurons innervating type I hair cells in the striolar region of the utricle are exquisitely sensitive to auditory-frequency air conducted sound (ACS) and bone conducted vibration (BCV). Here, we present experimental data and a mathematical model of utricular mechanics and vestibular compound action potential generation (vCAP) in response to clinically relevant levels of ACS and BCV. Vibration of the otoconial layer relative to the sensory epithelium was simulated using a Newtonian two-degree-of-freedom spring-mass-damper system, action potential timing was simulated using an empirical model, and vCAPs were simulated by convolving responses of the population of sensitive neurons with an empirical extracellular voltage kernel. The model was validated by comparison to macular vibration and vCAPs recorded in the guinea pig, in vivo.

Results

Transient stimuli evoked short-latency vCAPs that scaled in magnitude and timing with hair bundle mechanical shear rate for both ACS and BCV. For pulse BCV stimuli with durations <0.8 ms, the vCAP magnitude increased in proportion to temporal bone acceleration, but for pulse durations >0.9 ms the magnitude increased in proportion to temporal bone jerk. Once validated using ACS and BCV data, the model was applied to predict blast-induced hair bundle shear, with results predicting acute mechanical damage to bundles immediately upon exposure.

Discussion

Results demonstrate the switch from linear acceleration to linear jerk as the adequate stimulus arises entirely from mechanical factors controlling the dynamics of sensory hair bundle deflection. The model describes the switch in terms of the mechanical natural frequencies of vibration, which vary between species based on morphology and mechanical factors.

Voltage-gated Ca2+ channels in accessory lobe neurons of the chick

Birds have ten pairs of protrusions, "accessory lobes", on the lateral sides of the lumbosacral spinal cord. It has been proposed that accessory lobes act as a sensory organ of equilibrium and neurons in accessory lobes transmit sensory information to the motor center. We have reported that cells in chick accessory lobes express functional voltage-gated Na(+) and K(+) channels and generate action potentials. In this study, we examined properties of voltage-gated Ca(2+) channels (VGCCs). The amplitude of voltage-gated Ca(2+) channel currents carried by Ca(2+) and Ba(2+) increased gradually during 10 min rather than showing the usual run-down. The current-voltage relationship of Ba(2+) currents was consistent with that of the high-voltage-activated Ca(2+) channel. The proportion of Ba(2+) currents inhibited by ω-conotoxin GVIA was larger than 80%, indicating that the major subtype is N type. Amplitudes of tail currents of Ca(2+) currents evoked by repetitive pulses at 50 Hz are stable for 1 s. If the major subtype of VGCCs at synaptic terminals is also N type, this property may contribute to the establishment of stable synaptic connections between accessory lobe neurons, which are reported to fire at frequencies higher than 15 Hz, and postsynaptic neurons in the spinal cord.

Infrared photostimulation of the crista ampullaris

The present results show that the semicircular canal crista ampullaris of the toadfish, Opsanus tau, is sensitive to infrared radiation (IR) applied in vivo. IR pulse trains (∼1862 nm, ∼200 μs pulse⁻¹) delivered to the sensory epithelium by an optical fibre evoked profound changes in phasic and tonic discharge rates of postsynaptic afferent neurons. Phasic afferent responses to pulsed IR occurred with a latency of <8 ms while tonic responses developed with a time constant (τ) of 7 ms to 10 s following the onset or cessation of the radiation. Afferents responded to direct optical radiation of the sensory epithelium but did not respond to thermal stimuli that generated nearly equivalent temperature increases of the whole organ. A subset of afferent neurons fired an action potential in response to each IR pulse delivered to the sensory epithelium, at phase-locked rates up to 96 pulses per second. The latency between IR pulses and afferent nerve action potentials was much greater than synaptic delay and spike generation, demonstrating the presence of a signalling delay interposed between the IR pulse and the action potential. The same IR stimulus applied to afferent nerve axons failed to evoke responses of similar magnitude and failed to phase-lock afferent nerve action potentials. The present data support the hypothesis that pulsed IR activates sensory hair cells, thus leading to modulation of synaptic transmission and afferent nerve discharge reported here.

Transformation of vestibular signals into motor commands in the vestibuloocular reflex pathways of monkeys

Parallel pathways mediate the rotatory vestibuloocular reflex (VOR). If the VOR undergoes adaptive modification with spectacles that change the magnification of the visual scene, signals in one neural pathway are modified, whereas those in another are not. By recording the responses of vestibular afferents and abducens neurons for vestibular oscillations at frequencies from 0.5 to 50 Hz, we have elucidated how vestibular signals are processed in the modified versus unmodified VOR pathways. For the small stimuli we used (+/- 15 degrees/s), the afferents with the most regular spontaneous discharge fired throughout the cycle of oscillation even at 50 Hz, whereas afferents with more irregular discharge showed phase locking. For all afferents, the firing rate was in phase with stimulus head velocity at low frequencies and showed progressive phase lead as frequency increased. Sensitivity to head velocity increased steadily as a function of frequency. Abducens neurons showed highly regular spontaneous discharge and very little evidence of phase locking. Their sensitivity to head velocity during the VOR was relatively flat across frequencies; firing rate lagged head velocity at low frequencies and shifted to large phase leads as stimulus frequency increased. When afferent responses were provided as inputs to a two-pathway model of the VOR, the output of the model reproduced the responses of abducens neurons if the unmodified and modified VOR pathways had frequency-dependent internal gains and included fixed time delays of 1.5 and 9 ms. The phase shifts predicted by the model provide fingerprints for identifying brain stem neurons that participate in the modified versus unmodified VOR pathways.

Responses of irregularly discharging chinchilla semicircular canal vestibular-nerve afferents during high-frequency head rotations

Mammalian vestibular-nerve afferents innervating the semicircular canals have been divided into groups according to their discharge regularity, gain at 2-Hz rotational stimulation, and morphology. Low-gain irregular afferents terminate in calyx endings in the central crista, high-gain irregular afferents synapse more peripherally in dimorphic (bouton and calyx) endings, and regular afferents terminate in the peripheral zone as bouton-only and dimorphic endings. The response dynamics of these three groups have been described only up to 4 Hz in previous studies. Reported here are responses of chinchilla semicircular canal vestibular-nerve afferents to rotational stimuli at frequencies up to 16 Hz. The sensitivity of all afferents increased with increasing frequency with the sensitivity of low-gain irregular afferents increasing the most and matching the high-gain irregular afferents at 16 Hz. All afferents increased their phase lead with respect to stimulus velocity at higher frequencies with the highest leads in low-gain irregular afferents and the lowest in regular afferents. No attenuation of sensitivity or shift in phase consistent with the presence of a high-frequency pole over the range tested was noted. Responses were best fit with a torsion-pendulum model combined with a lead operator (tau(HF1)s + 1)(tau(HF2)s + 1). The discharge regularity of individual afferents was correlated to the value of each afferent's lead operator time constants. These findings suggest that low-gain irregular afferents are well suited for encoding the onset of rapid head movements, a property that would be advantageous for initiation of reflexes with short latency such as the vestibulo-ocular reflex.

Normal performance and expression of learning in the vestibulo-ocular reflex (VOR) at high frequencies

The rotatory vestibulo-ocular reflex (VOR) keeps the visual world stable during head movements by causing eye velocity that is equal in amplitude and opposite in direction to angular head velocity. We have studied the performance of the VOR in darkness for sinusoidal angular head oscillation at frequencies ranging from 0.5 to 50 Hz. At frequencies of > or = 25 Hz, the harmonic distortion of the stimulus and response were estimated to be <14 and 22%, respectively. We measured the gain of the VOR (eye velocity divided by head velocity) and the phase shift between eye and head velocity before and after adaptation with altered vision. Before adaptation, VOR gains were close to unity for frequencies < or = 20 Hz and increased as a function of frequency reaching values of 3 or 4 at 50 Hz. Eye velocity was almost perfectly out of phase with head velocity for frequencies < or = 12.5 Hz, and lagged perfect compensation increasingly as a function of frequency. After adaptive modification of the VOR with magnifying or miniaturizing optics, gain showed maximal changes at frequencies <12.5 Hz, smaller changes at higher frequencies, and no change at frequencies larger than 25 Hz. Between 15 and 25 Hz, the phase of eye velocity led the unmodified VOR by as much as 50 degrees when the gain of the VOR had been decreased, and lagged when the gain of the VOR had been increased. We were able to reproduce the main features of our data with a two-pathway model of the VOR, where the two pathways had different relationships between phase shift and frequency.

Convergence of excitatory and inhibitory hair cell transmitters shapes vestibular afferent responses

The vestibular semicircular canals respond to angular acceleration that is integrated to angular velocity by the biofluid mechanics of the canals and is the primary origin of afferent responses encoding velocity. Surprisingly, some afferents actually report angular acceleration. Our data indicate that hair-cell/afferent synapses introduce a mathematical derivative in these afferents that partially cancels the biomechanical integration and results in discharge rates encoding angular acceleration. We examined the role of convergent synaptic inputs from hair cells to this mathematical differentiation. A significant reduction in the order of the differentiation was observed for low-frequency stimuli after gamma-aminobutyric acid type B receptor antagonist administration. Results demonstrate that gamma-aminobutyric acid participates in shaping the temporal dynamics of afferent responses.

The three-dimensional vestibulo-ocular reflex evoked by high-acceleration rotations in the squirrel monkey

The aim of this study was to determine if the angular vestibulo-ocular reflex (VOR) in response to pitch, roll, left anterior-right posterior (LARP), and right anterior-left posterior (RALP) head rotations exhibited the same linear and nonlinear characteristics as those found in the horizontal VOR. Three-dimensional eye movements were recorded with the scleral search coil technique. The VOR in response to rotations in five planes (horizontal, vertical, torsional, LARP, and RALP) was studied in three squirrel monkeys. The latency of the VOR evoked by steps of acceleration in darkness (3,000 degrees /s(2) reaching a velocity of 150 degrees /s) was 5.8+/-1.7 ms and was the same in response to head rotations in all five planes of rotation. The gain of the reflex during the acceleration was 36.7+/-15.4% greater than that measured at the plateau of head velocity. Polynomial fits to the trajectory of the response show that eye velocity is proportional to the cube of head velocity in all five planes of rotation. For sinusoidal rotations of 0.5-15 Hz with a peak velocity of 20 degrees /s, the VOR gain did not change with frequency (0.74+/-0.06, 0.74+/-0.07, 0.37+/-0.05, 0.69+/-0.06, and 0.64+/-0.06, for yaw, pitch, roll, LARP, and RALP respectively). The VOR gain increased with head velocity for sinusoidal rotations at frequencies > or =4 Hz. For rotational frequencies > or =4 Hz, we show that the vertical, torsional, LARP, and RALP VORs have the same linear and nonlinear characteristics as the horizontal VOR. In addition, we show that the gain, phase and axis of eye rotation during LARP and RALP head rotations can be predicted once the pitch and roll responses are characterized.

Asymmetric responses to rotation at high frequencies in central vestibular neurons of the alert cat

The horizontal rotatory vestibulo-ocular reflex (VOR) stabilizes gaze by moving the eyes at an angular velocity proportional to head velocity, and can accomplish this for a broad range of frequencies and amplitudes of head motion. Rotation at 5 Hz and above may be processed differently than lower frequencies by the VOR network. We recorded discharges and calculated spike densities of a small sample of vestibular neurons in alert cats during low-velocity rotation at frequencies up to 8 Hz. At high frequencies, we found both vestibular-only (V-only) and eye-movement-sensitive (EM) cells that generated asymmetric output signals. Asymmetry was primarily of the cutoff type, i.e., changes in spike density were smallest for rotation in the inhibitory direction. Most cells were identified as secondary neurons. The mean spike density was 23 sp/s, which was lower than previously reported in vestibular neurons of monkeys. A few neurons had very high sensitivities, associated with phase-locking, to rotation at high frequencies. In general, vestibular neurons carried a high-pass-filtered version of rotational signals. When synaptic inputs from the vestibular commissure were quantified, we found that the immediate change in probability of firing due to commissural vestibular input was inversely correlated with the degree of high-pass filtering. At high frequencies, increased asymmetry and phase-locking occurred in some neurons. A small number of neurons responded with increased probability of firing to both directions of rotation. Together, these observations suggest that high frequencies of rotation may be encoded differently than low frequencies by central vestibular neurons in alert animals.

Vestibuloocular reflex dynamics during high-frequency and high-acceleration rotations of the head on body in rhesus monkey

For frequencies >10 Hz, the vestibuloocular reflex (VOR) has been primarily investigated during passive rotations of the head on the body in humans. These prior studies suggest that eye movements lag head movements, as predicted by a 7-ms delay in the VOR reflex pathways. However, Minor and colleagues recently applied whole-body rotations of frequencies < or =15 Hz in monkeys and found that eye movements were nearly in phase with head motion across all frequencies. The goal of the present study was to determine whether VOR response dynamics actually differ significantly for whole-body versus head-on-body rotations. To address this question, we evaluated the gain and phase of the VOR induced by high-frequency oscillations of the head on the body in monkeys by directly measuring both head and eye movements using the magnetic search coil technique. A torque motor was used to rotate the heads of three Rhesus monkeys over the frequency range 5-25 Hz. Peak head velocity was held constant, first at +/-50 degrees /s and then +/-100 degrees /s. The VOR was found to be essentially compensatory across all frequencies; gains were near unity (1.1 at 5 Hz vs. 1.2 at 25 Hz), and phase lag increased only slightly with frequency (from 2 degrees at 5 Hz to 11 degrees at 25 Hz, a marked contrast to the 63 degrees lag at 25 Hz predicted by a 7-ms VOR latency). Furthermore, VOR response dynamics were comparable in darkness and when viewing a target and did not vary with peak velocity. Although monkeys offered less resistance to the initial cycles of applied head motion, the gain and phase of the VOR did not vary for early versus late cycles, suggesting that an efference copy of the motor command to the neck musculature did not alter VOR response dynamics. In addition, VOR dynamics were also probed by applying transient head perturbations with much greater accelerations (peak acceleration >15,000 degrees /s(2)) than have been previously employed. The VOR latency was between 5 and 6 ms, and mean gain was close to unity for two of the three animals tested. A simple linear model well described the VOR responses elicited by sinusoidal and transient head on body rotations. We conclude that the VOR is compensatory over a wide frequency range in monkeys and has similar response dynamics during passive rotation of the head on body as during passive rotation of the whole body in space.

Relationship between inner-ear fluid pressure and semicircular canal afferent nerve discharge

The present study was designed to determine (1) the transcupular fluid pressure (deltaP) generated across the semicircular canal cupula in response to sinusoidal head rotation, (2) the translabyrinthine dilational pressure (P0) generated across the membranous labyrinth in response to an increase in endolymph fluid volume (hydrops), (3) afferent nerve discharge patterns generated by these distinct pressure stimuli and, (4) threshold values of deltaP and P0 required to elicit afferent neural responses. The experimental model was the oyster toadfish, Opsanus tau. Micromechanical indentation of the horizontal canal (HC) duct and utricular vestibule was used to simulate sinusoidal head rotation and fluid volume injection. Single-unit neural spike trains and endolymph pressure within the ampulla, on both sides of the cupula, were recorded simultaneously. deltaP averaged 0.013 Pa per 1 degrees/s of sinusoidal angular head velocity and P0 averaged 0.2 Pa per 1 nL of endolymph volume injection. The most responsive afferents had a threshold sensitivity to deltaP of 10(-3) Pa and to P0 of 5 x 10(-2) Pa based on a discharge modulation criterion of 1 impulse/s per cycle for 2 Hz pressure stimuli. Neural sensitivity to AP was expected on the basis of transverse cupular and hair bundle deflections. Analysis of mechanics of the end organ, neuronal projections into the crista, and individual neural firing patterns indicates that P0 sensitivity resulted from pressure-induced distension of the ampulla that led to a nonuniform cupular deformation pattern and hair bundle deflections. This explanation is consistent with predictions of a finite element model of the end organ. Results have implications regarding the role of deltaP in angular motion transduction and the role of P0 under transient hydropic conditions.

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. I. Normal responses

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in five squirrel monkeys with intact vestibular function. The VOR evoked by steps of acceleration in darkness (3,000 degrees /s(2) reaching a velocity of 150 degrees /s) began after a latency of 7.3 +/- 1.5 ms (mean +/- SD). Gain of the reflex during the acceleration was 14.2 +/- 5.2% greater than that measured once the plateau head velocity had been reached. A polynomial regression was used to analyze the trajectory of the responses to steps of acceleration. A better representation of the data was obtained from a polynomial that included a cubic term in contrast to an exclusively linear fit. For sinusoidal rotations of 0.5-15 Hz with a peak velocity of 20 degrees /s, the VOR gain measured 0.83 +/- 0.06 and did not vary across frequencies or animals. The phase of these responses was close to compensatory except at 15 Hz where a lag of 5.0 +/- 0.9 degrees was noted. The VOR gain did not vary with head velocity at 0.5 Hz but increased with velocity for rotations at frequencies of >/=4 Hz (0. 85 +/- 0.04 at 4 Hz, 20 degrees /s; 1.01 +/- 0.05 at 100 degrees /s, P < 0.0001). No responses to these rotations were noted in two animals that had undergone bilateral labyrinthectomy indicating that inertia of the eye had a negligible effect for these stimuli. We developed a mathematical model of VOR dynamics to account for these findings. The inputs to the reflex come from linear and nonlinear pathways. The linear pathway is responsible for the constant gain across frequencies at peak head velocity of 20 degrees /s and also for the phase lag at higher frequencies being less than that expected based on the reflex delay. The frequency- and velocity-dependent nonlinearity in VOR gain is accounted for by the dynamics of the nonlinear pathway. A transfer function that increases the gain of this pathway with frequency and a term related to the third power of head velocity are used to represent the dynamics of this pathway. This model accounts for the experimental findings and provides a method for interpreting responses to these stimuli after vestibular lesions.

Relationship of the utriculus and sacculus to the stapes footplate: anatomic implications for sound-and/or pressure-induced otolith activation

One hundred thirty human temporal bones that were sectioned in the vertical plane were examined to evaluate the relationship between the stapes footplate and the otolith organs. The shortest distance between the footplate and the utriculus was 0.58+/-0.10 mm in the posterior third of the oval window, 1.04+/-0.20 mm in the middle third, and 1.51+/-0.20 mm in the anterior third. The distance from the sacculus to the footplate was 1.33+/-0.20 mm in the middle third of the oval window and 1.31+/-0.18 mm in the anterior third. Membranous connections extending between the utriculus and the footplate were found in 26% of temporal bones. These membranous connections in coexistence with additional anatomic factors such as stapes hypermobility and/or dehiscence of bone within labyrinthine structures may predispose patients to sound- and/or pressure-induced otolith activation. The findings may have implications for different causes of the Tullio phenomenon.

Three-dimensional reconstruction of the membranous vestibular labyrinth in the toadfish, Opsanus tau

Membranous vestibular labyrinths from the oyster toadfish, Opsanus tau, were fixed, dissected from the animal, stained, and embedded in rectangular blocks of clear histological resin. Photomicrographs of complete embedded labyrinths were taken from six orthogonal directions and used to construct three-dimensional (3D) geometrical models of the semicircular canals, ampullae, utricular vestibule and common crus. Membraneous ducts and ampullae were modeled using a set of cross-sectional elliptical curves laced together to generate curved tubular models of each structure. The ensemble of these curved tubes was used to generate a complete 3D reconstruction of the outside surface of the membranous labyrinth. When viewed from six orthogonal directions, reconstructions closely matched the embedded tissue. Dimensions of the reconstruction and histological sections were compared to measurements of fresh tissue taken from the same animals prior to fixation and used to correct the reconstructions for tissue shrinkage. Results provide estimates of the endolymphatic volumes, local cross-sectional areas and elliptical eccentricities as well as 3D orientations of the geometric canal planes relative to the skull. Ten micrometer histological sections of the material were also prepared to measure wall thickness in various regions of the labyrinth.