Abstract
The semicircular ducts (SCDs) of the vestibular system play an instrumental role in equilibration and rotation perception of vertebrates. The present paper is a review of quantitative approaches and shows how SCDs function. It consists of three parts. First, the biophysical mechanisms of an SCD system composed of three mutually connected ducts, allowing endolymph to flow from one duct into another one, are analysed. The flow is quantified by solving the continuity equations in conjunction with the equations of motion of the SCD hydrodynamics. This leads to mathematical expressions that are suitable for further analytical and numerical analysis. Second, analytical solutions are derived through four simplifying steps while keeping the essentials of the coupled system intact. Some examples of flow distributions for different rotations are given. Third, the focus is on the transducer function of the SCDs. The complex structure of the mechano-electrical transduction apparatus inside the ampullae is described, and the consequences for sensitivity and frequency response are evaluated. Furthermore, both the contributions of the different terms of the equations of motion and the influence of Brownian motion are analysed. Finally, size limitations, allometry and evolutionary aspects are taken into account.
Highlights
Introduction and motivationThe semicircular ducts (SCDs) are the vertebrate sensors for three-dimensional rotation
A first group are statocyst-like organs formed by the maculae of the utriculus and the sacculus, using heavy particles—small stones, so to speak which are currently called otoliths or otoconia—that exert inertial forces on the hair bundles forming the top of the sensory cells
For a step-like stimulus of a single duct SCD system, as described above, the endolymph moves from its resting position to a maximal position, following a time course characterized by T 2
Summary
The semicircular ducts (SCDs) are the vertebrate sensors for three-dimensional rotation. A first group are statocyst-like organs formed by the maculae of the utriculus and the sacculus (both small sacs that form the principal compartments of the labyrinth), using heavy particles—small stones, so to speak which are currently called otoliths or otoconia—that exert inertial forces on the hair bundles forming the top of the sensory cells. These organs sense gravitational and linear inertial accelerations.
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