The vestibular system is phylogenetically ancient. Its development was an important evolutionary event, enabling vertebrates (and invertebrates) to maintain equilibrium and spatial orientation while moving freely in their environments. Optimization of the vestibular end-organ plan in vertebrates occurred rapidly, as evidenced by the close similarity between the labyrinths of the fossil record, the earliest extant species, such as myxine and lamprey, and those of human and nonhuman primates. Thus the original design was successful in accomplishing its goals and has changed little throughout phylogeny.
The vestibular labyrinths sense and report angular and linear accelerations of the head. There are 5 paired labyrinthine end organs, consisting of 3 angular and 2 linear accelerometers. The angular accelerometers, the semicircular canals, are the subject of this review. In primates, the lateral (horizontal) canal is positioned in the head to align roughly with the head-based yaw axis of rotation, whereas the anterior and posterior canals are situated in the orthogonal planes, each oriented approximately 45 deg from the pitch and roll axes of rotation. Each semicircular canal is an endolymphatic fluid-filled tube with an enlarged sac called an ampulla at one end. The receptor sheet, the crista ampullaris, is found inside the sac and is anchored to the ampullary wall. The crista itself is an inverted saddle-shaped structure whose topmost layer consists of a single sheet of sensory hair cells. The apical hairs or cilia of these cells can reach 100 microns in length, and they project into a gelatinous vane or cupula that closes the ampullary space to transcupular fluid flow. Vestibular afferent nerves are in contact with the hair cells, and emerge from the base of the ampulla to travel toward the brain.
Because the labyrinths are tethered to the skull, the canals move with the head. Angular acceleration of the head generates an inertial force within the endolymph. Displacement of the endolymph produces a viscous drag force on the canal walls and a compensatory displacement of the cupula and ciliary bundles, which generates an elastic restorative force. These forces balance the inertial force in dynamic equilibrium (Curthoys and Oman 1987
; Damiano and Rabbitt 1996
; Lorenté de Nó 1927
; Steinhausen 1933
; Van Buskirk et al. 1976
). Deflection of the stereocilia leads to the generation of a receptor potential in the hair cell, ultimately resulting in the modulation of transmitter release from the hair cells. Individual primary vestibular afferent fibers typically receive and summate input from multiple hair cells (Boyle et al. 1991
; Goldberg 2000
; Lysakowski and Goldberg 1997
; Lysakowski et al. 1995
). Summated depolarizations generate action potentials in the afferents that encode parameters of angular head velocity and acceleration. These parameters are transmitted to the brain stem by the frequency and relative timing of the action potentials.
The endolymphatic fluid displacement resulting from angular head acceleration can be reproduced by mechanical indentation of the long and slender canal, an experimental strategy that Ewald (1892)
invented, applying a pneumatically actuated tapered rod to compress the membranous canal duct mechanically. More recently, Dickman et al. (Dickman and Correia 1989a
; Dickman et al. 1988
) applied a piezoelectrically driven rod to the pigeon canal, and demonstrated that the neural response dynamics attributed to rotational stimuli are generally reproduced by mechanical indentation. Rabbitt et al. (1995)
further documented the utility of this method by quantifying and modeling the results of indentation. In these studies, a single flat-ended glass rod of about the same dimension as the canal diameter was placed in contact with the canal limb, about 3–7 mm from the sensory epithelium, at a site where the canal is backed by bone. A static preload was set by stimulating with a sinusoidal profile of indentation (peak-to-peak displacement) and lowering the rod until the afferent nerve response indicated continuous contact by the indenter rod. This procedure serves to linearize subsequent stimuli about the preload indentation. Theory, verified by experimental results, indicates that the responses of semicircular canal afferents to head rotation can be mimicked by canal limb mechanical indentation over a broad range of physiological frequencies (Rabbitt et al. 1995
). Linearity of the first harmonic afferent response to indentation was also established. However, the fluid dynamics induced by indentation begin to diverge from those induced by head rotation at stimulus frequencies >2 Hz. This is because at high frequencies inertial forces on the fluid become dominant, causing a cutoff in the biomechanical response to head rotation, the classical upper corner. In contrast, at higher frequencies of mechanical stimulation, the indenter forces endolymph flow, simply overpowering the additional inertial force. Therefore when using mechanical indentation stimuli at frequencies >2 Hz, the amplitude of indentation must be decreased and the phase must be retarded to achieve the same movements of the cupula and endolymph as are produced by head rotation (Rabbitt et al. 1995
Substituting canal indentation for angular rotation, a strategy that we have used extensively, facilitates experiments requiring stable intracellular recordings from hair cells or nerve fibers. indicates the placement of the indenter on the long and slender limb of the horizontal canal in the oyster toadfish Opsanus tau
(, HCI). In this preparation, one micron of canal indentation is equivalent to approximately 4°/s angular velocity, at least at low frequencies. As noted, mechanical indentation stimuli, like physiological rotation, modulate hair cell transduction currents by inducing micromechanical stereociliary bundle displacements and corresponding nanomechanical gating of ciliary bundle ion channels. The open probability of hair cell transduction channels is a function of bundle deflection and determines the ion flow through these channels. Based on data from other hair cell organs, about 13% of the transduction channels are open when the bundle is not moving, resulting in a resting rate of current flow (Hudspeth 1989a
). Hair cell receptor potentials and afferent responses can be modulated experimentally by applying transepithelial electrical stimuli or by voltage clamping the endolymph relative to the perilymph (, Ie
) (Damiano and Rabbitt 1996
; Highstein and Politoff 1978
; Norris et al. 1998
). The latter manipulation modulates the apical-face membrane potential and causes current flow through open transduction channels in the absence of an applied mechanical force on the hair bundles. Although hair bundle motion in the otolith organs is known to be elicited by transepithelial electrical polarization (Bozovic and Hudspeth 2003
), in the canals the mechanical movement is relatively small and a majority of the induced afferent responses appear to arise from alterations in the current flow through open transduction channels by modulation of the apical face membrane potential (Highstein et al. 1996
). This electrical stimulus also activates synaptic transmitter release from hair cell basal synapses. We have used this strategy to activate hair cells in the absence of biomechanical stimulation of the canal [cf. illustrating the loci for measurement of translabyrinthine potentials (Ve
), recordings of single canal afferents (Vn
), and hair cell voltage- and current-clamp recordings (HCVC/CC
FIG. 1 General experimental setup. Top: expanded view of the labyrinth. Bottom: overview of the fish. A current-passing electrode (Ie) is placed in the posterior limb of the anterior canal and a second voltage-measuring electrode (Ve) placed in the anterior (more ...)
To record hair cell potentials and currents in vivo, quartz microelectrodes can be introduced by a small fistula or opening in the utricular side of the horizontal canal ampulla (, HCVC/CC
) to impale hair cells during canal indentation. Control experiments have verified that the mechanical stimulus is dominated by the long and slender limb of the canal (Rabbitt et al. 1995
), which is on the canal side of the cupula, so that the presence of the fistula (on the utricular side) has no impact on the effectiveness of the stimulus. Indeed, recordings from afferents remain stable and unchanged after fistulation and after resealing of the fistula (S. M. Highstein and R. D. Boyle, unpublished data). Further, the fistula is effectively sealed in these experiments by the interface pressure between endolymph and FC-75 (3M), a fluorocarbon that is used to fill the perilymphatic space. It should be noted that the toadfish is the only vertebrate preparation used to date that allows intracellular recordings from vestibular hair cells in vivo. This offers the experimental potential to directly relate the intracellular hair cell recordings to data from the vestibular nerve, which provides a reference regarding the total output of the vestibular peripheral sensory apparatus, including all signal processing performed by the labyrinthine canal end organ.