A review of mechanical and synaptic processes in otolith transduction of sound and vibration for clinical VEMP testing.

Older studies of mammalian otolith physiology have focused mainly on sustained responses to low-frequency (<50 Hz) or maintained linear acceleration. So the otoliths have been regarded as accelerometers. Thus evidence of otolithic activation and high-precision phase locking to high-frequency sound and vibration appears to be very unusual.

Otolithic Receptor Mechanisms for Vestibular-Evoked Myogenic Potentials: A Review.

Air-conducted sound and bone-conduced vibration activate otolithic receptors and afferent neurons in both the utricular and saccular maculae, and trigger small electromyographic (EMG) responses [called vestibular-evoked myogenic potentials (VEMPs)] in various muscle groups throughout the body. The use of these VEMPs for clinical assessment of human otolithic function is built on the following logical steps: (1) that high-frequency sound and vibration at clinically effective stimulus levels activate otolithic receptors and afferents, rather than semicircular canal afferents, (2) that there is differential anatomical projection of otolith afferents to eye muscles and neck muscles, and (3) that isolated stimulation of the utricular macula induces short latency responses in eye muscles, and that isolated stimulation of the saccular macula induces short latency responses in neck motoneurons. Evidence supports these logical steps, and so VEMPs are increasingly being used for clinical assessment of otolith function, even differential evaluation of utricular and saccular function.

Otoliths - Accelerometer and seismometer; Implications in Vestibular Evoked Myogenic Potential (VEMP).

Vestibular otolithic organs are recognized as transducers of head acceleration and they function as such up to their corner frequency or undamped natural frequency. It is well recognized that these organs respond to frequencies above their corner frequency up to the 2-3 kHz range (Curthoys et al., 2016).

Biomechanical measurement of kinocilium.

Vestibular hair cell bundles in the inner ear each contain a single kinocilium that has the classic 9+2 axoneme microtubule structure. Kinocilia transmit movement of the overlying otoconial membrane mass and cupula to the mechanotransducing portion of the hair cell bundle. Here, we describe how force-deflection techniques can be used to measure turtle utricle kinocilium shaft and base rotational stiffness.

Quantifying utricular stimulation during natural behavior.

The use of natural stimuli in neurophysiological studies has led to significant insights into the encoding strategies used by sensory neurons. To investigate these encoding strategies in vestibular receptors and neurons, we have developed a method for calculating the stimuli delivered to a vestibular organ, the utricle, during natural (unrestrained) behaviors, using the turtle as our experimental preparation. High-speed digital video sequences are used to calculate the dynamic gravito-inertial (GI) vector acting on the head during behavior.

Biomechanics of hair cell kinocilia: experimental measurement of kinocilium shaft stiffness and base rotational stiffness with Euler-Bernoulli and Timoshenko beam analysis.

Vestibular hair cell bundles in the inner ear contain a single kinocilium composed of a 9+2 microtubule structure. Kinocilia play a crucial role in transmitting movement of the overlying mass, otoconial membrane or cupula to the mechanotransducing portion of the hair cell bundle. Little is known regarding the mechanical deformation properties of the kinocilium.

A virtual hair cell, I: addition of gating spring theory into a 3-D bundle mechanical model.

We have developed a virtual hair cell that simulates hair cell mechanoelectrical transduction in the turtle utricle. This study combines a full three-dimensional hair bundle mechanical model with a gating spring theory. Previous mathematical models represent the hair bundle with a single degree of freedom system which, we have argued, cannot fully explain hair bundle mechanics.

A virtual hair cell, II: evaluation of mechanoelectric transduction parameters.

The virtual hair cell we have proposed utilizes a set of parameters related to its mechanoelectric transduction. In this work, we observed the effect of such channel gating parameters as the gating threshold, critical tension, resting tension, and Ca(2+) concentration. The gating threshold is the difference between the resting and channel opening tension exerted by the tip link assembly on the channel.

Mechanical properties and consequences of stereocilia and extracellular links in vestibular hair bundles.

Although knowledge of the fine structure of vestibular hair bundles is increasing, the mechanical properties and functional significance of those structures remain unclear. In 2004, Bashtanov and colleagues reported the contribution of different extracellular links to bundle stiffness. We simulated Bashtanov's experimental protocol using a three-dimensional finite element bundle model with geometry measured from a typical striolar hair cell.

Computational models of hair cell bundle mechanics: III. 3-D utricular bundles.

Six utricular hair bundles from a red-eared turtle are modeled using 3-D finite element analysis. The mechanical model includes shear deformable stereocilia, realignment of all forces during force load increments, and tip and lateral link inter-stereocilia connections. Results show that there are two distinct bundle types that can be separated by mechanical bundle stiffness.

Computational models of hair cell bundle mechanics: II. Simplified bundle models.

Simplified versions of hair cell bundles are mechanically modeled. The influence of various geometric and material combinations on bundle stiffness, link tensions and deformation shape are examined. Three models are analyzed within this paper: two stereocilia connected by one link, two stereocilia connected by a biologically realistic set of links, and a column of stereocilia connected by realistic links.

Computational models of hair cell bundle mechanics: I. Single stereocilium.

A distributed parameter model for describing the response of a stereocilium to an applied force is presented. This model is based on elasticity theory, plus the geometry and material properties of the stereocilium. The stereocilia shaft above the taper is not assumed to be perfectly rigid.