Functional and anatomical correlation of afferent responses from the isolated semicircular canal

Abstract

THE semicircular canal is considered classically to be a sensor of angular head motion during animal navigation; however, the functional significance of the canal's morphology and innervation for transduction and encoding of specific head trajectories is unclear. Neural responses from single afferent fibres innervating the crista of isolated elasmobranch labyrinths were used by Groen et al. to determine linear system coefficients of an overdamped pendulum equation, considered to reflect directly the dynamics of the motion of the cupula–endolymphatic fluid in the ampulla¹. Similar studies on other preparations revealed certain neural response characteristics that were at variance with simple pendulum models, and these differences were ascribed variously to differences in hair cell physiology and afferent or efferent innervation properties, whose effects were superimposed on the fundamental fluid pendulum dynamics^2–4. But in addition to these hypotheses based on cellular influences, the topological organisation of the receptor could result in specific afferent response characteristics deriving from different anatomical regions within the crista. As evidence for the latter, we report diverse classes of afferent responses, describable by distinctly different linear system equations, that are correlated with the position of separate bundles innervating specific areas of the crista of the guitarfish, Rhinobatos productus (Fig. 1).