title: Determinants of spatial and temporal coding by semicircular canal afferents.
abstract: The vestibular semicircular canals are internal sensors that signal the magnitude, direction, and temporal properties of angular head motion. Fluid mechanics within the 3-canal labyrinth code the direction of movement and integrate angular acceleration stimuli over time. Directional coding is accomplished by decomposition of complex angular accelerations into 3 biomechanical components-one component exciting each of the 3 ampullary organs and associated afferent nerve bundles separately. For low-frequency angular motion stimuli, fluid displacement within each canal is proportional to angular acceleration. At higher frequencies, above the lower corner frequency, real-time integration is accomplished by viscous forces arising from the movement of fluid within the slender lumen of each canal. This results in angular velocity sensitive fluid displacements. Reflecting this, a subset of afferent fibers indeed report angular acceleration to the brain for low frequencies of head movement and report angular velocity for higher frequencies. However, a substantial number of afferent fibers also report angular acceleration, or a signal between acceleration and velocity, even at frequencies where the endolymph displacement is known to follow angular head velocity. These non-velocity-sensitive afferent signals cannot be attributed to canal biomechanics alone. The responses of non-velocity-sensitive cells include a mathematical differentiation (first-order or fractional) imparted by hair-cell and/or afferent complexes. This mathematical differentiation from velocity to acceleration cannot be attributed to hair cell ionic currents, but occurs as a result of the dynamics of synaptic transmission between hair cells and their primary afferent fibers. The evidence for this conclusion is reviewed below.
authors.affiliation: Washington University School of Medicine, Department of Otolaryngology, St. Louis, MO 63110, USA.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Highstein
authors.forename: Richard D
authors.initials: RD
authors.lastname: Rabbitt
authors.forename: Gay R
authors.initials: GR
authors.lastname: Holstein
authors.forename: Richard D
authors.initials: RD
authors.lastname: Boyle
mesh_terms: D000344:Afferent Pathways / Q000033:anatomy & histology / Q000502:physiology; D000818:Animals; D001696:Biomechanical Phenomena; D018069:Hair Cells, Vestibular / Q000502:physiology; D019416:Head Movements / Q000502:physiology; D006801:Humans; D007761:Labyrinthine Fluids / Q000502:physiology; D009039:Motion Perception / Q000502:physiology*; D016571:Neural Networks, Computer; D012027:Reflex, Vestibulo-Ocular / Q000502:physiology*; D012665:Semicircular Canals / Q000166:cytology / Q000502:physiology*; D015398:Signal Transduction; D013028:Space Perception / Q000502:physiology*; D013569:Synapses