Abstract
The semicircular canals are responsible for sensing angular head motion in three-dimensional space and for providing neural inputs to the central nervous system (CNS) essential for agile mobility, stable vision, and autonomic control of the cardiovascular and other gravity-sensitive systems. Sensation relies on fluid mechanics within the labyrinth to selectively convert angular head acceleration into sensory hair bundle displacements in each of three inner ear sensory organs. Canal afferent neurons encode the direction and time course of head movements over a broad range of movement frequencies and amplitudes. Disorders altering canal mechanics result in pathological inputs to the CNS, often leading to debilitating symptoms. Vestibular disorders and conditions with mechanical substrates include benign paroxysmal positional nystagmus, direction-changing positional nystagmus, alcohol positional nystagmus, caloric nystagmus, Tullio phenomena, and others. Here, the mechanics of angular motion transduction and how it contributes to neural encoding by the semicircular canals is reviewed in both health and disease.
Highlights
The inner ear vestibular organs are phylogenetically ancient mechanosensory transducers that provide amniotes with the ability to sense movement and orientation of the head relative to gravity
In some species these five primary vestibular organs are augmented by a papilla neglecta that supplements angular motion sensation by the canals (Brichta and Goldberg 1998) and by a lagena that supplements function of the otolith organs and can play a role in magnetoreception (Fritzsch et al 1990; Khorevin 2008; Lu et al 2003; Wu and Dickman 2011; Zakir et al 2012)
This review describes the role of semicircular canal biomechanics in angular motion sensation and how common disorders altering the biomechanics can lead to transmission of inappropriate signals to the central nervous system (CNS)
Summary
The inner ear vestibular organs are phylogenetically ancient mechanosensory transducers that provide amniotes with the ability to sense movement and orientation of the head relative to gravity. In all cases, encoding varies between individual afferent neurons within nerve branches arising from each ampulla, with some units modulating their action potential discharge rate in direct proportion to angular head velocity in the sensitive canal plane over a broad range of oscillatory head frequencies, and other units exhibiting frequency-dependent sensitivity and timing of peak discharge relative to the stimulus The origin of this diversity arises primarily from morphophysiology and cellular biophysics of hair-cell/afferent complexes rather than diversity in macromechanical hair bundle inputs. Displacement of the cupula depends on macromechanics and exhibits a simple relaxation behavior, while temporal modulation of afferent neurons is more complex and diverse across individual units This diversity in signal processing is responsible for parsing angular head movement signals into parallel channels encoding various aspects of the stimulus and transmitted simultaneously to the CNS (Sadeghi et al 2007a). Double arrow vectors indicate directions perpendicular to anatomical plane that maximally excite the left AC and right PC,
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