| The hydrodynamics of the semicircular canal generally acts to transform a head rotation acceleration stimulus into a cupula-level stimulus that follows head rotation velocity. Experimental measurement of the primary afferent response over the physiological range, however, reveals a diversity of neural responses. Some afferent responses follow the head rotation velocity and have a sensitivity that is unchanging with stimulus frequency. Other afferents have a response with less phase lag relative to acceleration and a sensitivity that increases with frequency. The source of this diversity in afferent response dynamics could lie in the micromechanics of the cupula or in post-mechano-transduction neural mechanisms. The goal of this work was to determine whether cupular micromechanics are a contributing source of unexplained afferent response behavior such as the diverse response dynamics during physiologically relevant stimuli, as well as unusual responses during pathological conditions. An experimental approach was used to shed light on the biomechanical role of the cupula in vestibular sensory transduction. The differential pressure across the cupula and the dilational pressure acting throughout the canal were measured using extremely sensitive custom-built pressure sensors. Fluorescent microsphere markers were used to track the displacement of the cupula. Measurement of dilational pressure did reveal that cupular deformations could underlie abnormal neural responses seen in some pathological conditions. The differential pressure response was found to vary between animals, but on average showed only a slight increase in sensitivity with frequency and was closely aligned with the phase of head rotation velocity. Cupula motion was found to follow head rotational velocity and had a flat frequency sensitivity, matching predictions of 1-D mathematical models for canal macromechanics. Preliminary work also found no significant variation in the displacement response with different locations on the cupula. From these results, it was concluded that cupular micromechanics are not a primary source of the diversity in afferent response dynamics during normal physiological conditions. |
