Abstract
Objective New tests of otolithic function use air conducted sound (ACS) and bone conducted vibration (BCV) as otolithic stimuli. These stimuli generate vestibular evoked myogenic potentials (VEMPs) from tensed neck muscles or extraocular muscles which differentiate utricular and saccular function in each labyrinth. Methods New anatomical and physiological results show why these are valid otolithic stimuli: that within each otolithic macula there are effectively two co-existing sensing systems – that in addition to the classical response of some otolith afferents to low frequency linear accelerations like tilts and translations by what is called the sustained system, other afferent neurons from a specialized band of receptors in each macula are activated by ACS and BCV (the transient system). These afferents are activated and synchronized (phase-locked) to every cycle of an ACS or BCV stimulus up to frequencies above 1000 Hz. In each otolithic macula these two systems are subserved by neurons with very different anatomical structure and physiological characteristics arising from different regions of the maculae because there is unique specialization of receptors in different regions – the maculae are not uniform homogenous receptor structures. Otolithic afferents with regular resting ischarge, originating mainly from type II receptors in the extrastriolar region constitute the low frequency sustained system, whereas afferents with irregular resting activity originating mainly from type I receptors at the striola constitute the high frequency transient system and it is the transient system which is activated by ACS and BCV and so is responsible for VEMPs. Results Reduced or absent function of one otolithic macula results in asymmetric VEMP responses to bilaterally equal ACS or BCV stimuli. Conclusion VEMPs provide evidence of unilateral and bilateral otolithic loss and even more remarkably, evidence of the status of each saccular and each utricular macula separately.
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