A Biomechanical Model of the Inner Ear: Numerical Simulation of the Caloric Test

Whether two vertical semicircular canals can receive thermal stimuli remains controversial. This study examined the caloric response in the three semicircular canals to the clinical hot caloric test using the finite element method. The results of the developed model showed the horizontal canal (HC) cupula maximally deflected to the utricle side by approximately 3 μm during the hot supine test. The anterior canal cupula began to receive the caloric stimuli about 20 s after the HC cupula, and it maximally deflected to the canal side by 0.55 μm. The posterior canal cupula did not receive caloric stimuli until approximately 40 s after the HC cupula, and it maximally deflected to the canal side by 0.34 μm. Although the endolymph flow and the cupular deformation change with respect to the head position during the test, the supine test ensures the maximal caloric response in the HC, but no substantial improvement for the responses of the two vertical canals was observed. In conclusion, while the usual supine test is the optimum test for evaluating the functions of the inner ear, more irrigation time is needed in order to effectively clinically examine the vertical canals.

Sensory transduction of head velocity and acceleration in the toadfish horizontal semicircular canal

1. Sinusoidal mechanical indentation of the long-and-slender limb of the horizontal semicircular canal and/or utricle was used to produce adequate stimulation of the labyrinth. Indentation of the canal increased, while indentation of the utricle decreased the afferent discharge rate. This follows because indentation of the canal and utricle produce oppositely directed mechanical stimuli as defined by endolymph flow, transcupular pressure, and cupular deflection. Simultaneous in-phase indentations of both the canal and utricle, with amplitudes adjusted to produce equal (but opposite) magnitudes of afferent response modulation, generate destructive interaction that minimizes the afferent modulation, whereas sinusoidal indentation 180 degrees out-of-phase generates constructive interaction that maximizes the afferent modulation. This observation correlates directly with analysis of the labyrinthine elasto-hydrodynamics which predicts that balanced in-phase indentations minimize macromechanical endolymph flow through the ampullary cross section and maximize the dilatational pressure within the ampulla acting equally on both sides of the cupula and across the labyrinthine wall. 2. Two groups of afferents are identified according to their response to balanced sinusoidal indentation of the canal limb and the utricle. In one group there is complete destructive interaction and the afferent response can be effectively nulled by adjusting the relative amplitude and phase of the two stimuli. In the second group a residual afferent response remains that cannot be nulled. The residual is described in the model as unit-specific sensitivity to dilatational pressure acting equally on both sides of the cupula.

The angular acceleration receptor system of diverse cephalopods

The system for monitoring angular acceleration is described in 59 genera of cephalopods. The dimensions are measured and volumes calculated. The volume of the statocyst is always small relative to the size of the adult animal but is smallest in animals that move fast, such as ommastrephids, loliginids and sepiids; it is larger in neutrally buoyant squids and very large in cirroctopods. In many genera the flow of endolymph is restricted by knobs, the anticristae, usually arranged on a standard plan. These reach their greatest extent in non- buoyant squids and sepiids, where some of them join to form incomplete semicircular canals in the horizontal plane, providing the lower sensitivity appropriate to rapid turning to right or left. In buoyant squids and octopods the cavity is less obstructed. The system for adjusting sensitivity is not the same in all three planes: there are not three canals as in vertebrates. In the pitching and rolling planes the channels for endolymph flow are wide, presumably providing the higher sensitivity appropriate to slower movements. Sensitivity is perhaps also adjusted by variations in the cupulae, as it is in Octopus . The crista of pelagic octopods such as Argonauta and Japetella is divided into nine sections, as in the typical benthic forms. In cirroctopods the crista is not divided in this way and indeed shows no interruption of the rows of hair cells, even at the turns. In some cirroctopods and in Vampyroteuthis there are several anticristae, an apomorphic feature that they share with the decapods; but they have perilymph- and endolymph-like octopods. In the more active cranchiid squids the anticristae are arranged on a helical course, perhaps serving to provide appropriate sensitivity during turns from the head-down position. The statocyst of Spirula differs from both sepiids and teuthids.