Semicircular Duct Dimensions and Sensitivity of the Vertebrate Vestibular System

Architecture and dimensions of the labyrinth of vertebrates are explained as optimizations towards a maximal sensitivity of the system. The flow circuit of a single semicircular duct is composed of a very regularly shaped ductal part, an ampulla containing the mechano-electrical sensor and an utricular part of irregular shape. The relatively wide external shape of the ampulla would lead to a considerable attenuation of the flow and a decrease in sensitivity. However, one would expect a functional design of the semicircular duct system in which flow amplification in the ampulla would lead to an increased sensitivity, or at least the flow would be equal to that in the duct. dy this problem of the morphological design of the semicircular ducts, the endolymph motion has been determined as a function of the lengths and the cross-sectional areas of the labyrinth parts mentioned above. The functions that were studied are: (i) the initial endolymph velocity, (ii) the fast time constant, indicating the response speed and (iii) the maximal endolymph displacement which is a measure for the sensitivity. ampulla, the space between the sensory epithelium and the cupula is called the "subcupular space". The relation between the ratio of ampullar and ductal lengths and the ratio of the cross-sectional areas of subcupular space and duct for which the maximal endolymph displacement was maximized was calculated. It is proved that the maximal sensitivity of the system is achieved for a very short and narrow subcupular space. A narrow subcupular space must be coupled with a very short length. Otherwise, fluid friction will overshade flow amplification. The endolymph motion in the subcupular space is then large compared to the motion in other parts of the system. Furthermore, the response speed is increased by a factor of about 2. ove results support the view that the endolymph movement in the subcupular space is the relevant mechanical quantity in the transduction process and that the stereovilli of the hair cells are the structures that transmit this mechanical stimulus to the sensory system. ng the subcupular space to ductal and utricular dimensions gave rise to two optimal solutions: a relatively narrow and short, and a relatively wide and long utriculus. These solutions correspond to actual vestibular designs in higher and lower vertebrates respectively. This was verified with data about dimensions and shape of different labyrinths from the literature. been argued that the semicircular duct system does not only fulfill the biological function of an angular velocity transducer but that it senses angular displacement, velocity and even acceleration simultaneously.