Click-evoked responses in vestibular afferents in rats

Abstract

Sound activates not only the cochlea but also the vestibular end organs. Research on this phenomenon led to the discovery of the sound-evoked vestibular myogenic potentials recorded from the sternocleidomastoid muscles (cervical VEMP, or cVEMP). Since the cVEMP offers simplicity and the ability to stimulate each labyrinth separately, its values as a test of human vestibular function are widely recognized. Currently, the cVEMP is interpreted as a test of saccule function based on the assumption that clicks primarily activate the saccule. However, sound activation of vestibular end organs other than the saccule has been reported. To provide the neural basis for interpreting clinical VEMP testing, we employed the broadband clicks used in clinical VEMP testing to examine the sound-evoked responses in a large sample of vestibular afferents in Sprague-Dawley rats. Recordings were made from 924 vestibular afferents from 106 rats: 255 from the anterior canal (AC), 202 from the horizontal canal (HC), 177 from the posterior canal (PC), 207 from the superior vestibular nerve otolith (SO), and 83 from the inferior nerve otolith (IO). Sound sensitivity of each afferent was quantified by computing the cumulative probability of evoking a spike (CPE). We found that clicks activated irregular afferents (normalized coefficient of variation of interspike intervals >0.2) from both the otoliths (81%) and the canals (43%). The order of end organ sound sensitivity was SO = IO > AC > HC > PC. Since the sternocleidomastoid motoneurons receive inputs from both the otoliths and the canals, these results provide evidence of a possible contribution from both of them to the click-evoked cVEMP.

Frequency Tuning of the Cervical Vestibular-Evoked Myogenic Potential (cVEMP) Recorded from Multiple Sites along the Sternocleidomastoid Muscle in Normal Human Subjects

“…al. 2009;Zhu et al 2011Zhu et al , 2012. Because sound stimulation offers simplicity, selectivity, and the capacity to stimulate each labyrinth separately, its potential value in the diagnosis of vestibular disorders has been widely recognized (Colebatch 2001(Colebatch , 2010Curthoys 2010;Halmagyi et al 2005;Minor et al 2001;Rauch 2006;Rosengren et al 2010;Streubel et al 2001;Welgampola and Carey 2011;Colebatch 2001, 2005;Zhou and Cox 2004).…”

“…Because previous studies suggested that sound primarily activates the vestibular afferents that innervated the saccule, the cVEMP is presently used to assess the saccule function (Murofushi and Curthoys 1997;Kushiro et al 1999;Colebatch and Rothwell 2004). However, the saccule hypothesis of sound activation of the vestibular system has been challenged by several studies (Carey et al 2004;Curthoys and Vulovic 2011;Todd et al 2009;Young et al 1977;Zhou et al , 2005Zhou et al , 2007Xu et al 2009;Zhu et al 2011). In particular, Zhu et al (2011) recently quantitatively examined sound-evoked responses in vestibular afferents that innervated the five vestibular end organs.…”

The Cervical Vestibular-Evoked Myogenic Potentials (cVEMPs) Recorded Along the Sternocleidomastoid Muscles During Head Rotation and Flexion in Normal Human Subjects

“…For example, the origins of the cVEMP responses have yet to be elucidated. In contrast to the earlier reports that identified the saccule as the sole source of the cVEMPs (Murofushi et al 1995;Todd et al 2000), recent animal studies have demonstrated sound sensitivity in vestibular afferents from the semicircular canals and the utricle (Zhou et al 2004(Zhou et al , 2005(Zhou et al , 2007Xu et al 2009;Zhu et al 2011 and2014). Our recent human study (Wei et al 2013) also showed that cVEMP frequency tuning curves are better modeled as a summation of two mass spring systems with resonance frequencies of 300 and 1000 Hz.…”

“…In particular, we have focused on factors related to the sensory components and the motor components of the VCR pathways. On the one hand, we have conducted neurophysiological studies in monkeys and rats and showed that acoustic stimulation similar to that used in clinics activates multiple vestibular end organs (Zhou et al 2004(Zhou et al , 2005(Zhou et al , 2007Xu et al 2009;Zhu et al 2011Zhu et al , 2014. Consistent with the animal studies, our recent human study showed that frequency tuning of the cVEMPs is better described as a summation of at least two massspring-damper systems, rather than a single mass spring model (Wei et al 2013).…”

The effect of alcohol on cervical and ocular vestibular evoked myogenic potentials in healthy volunteers

“…As sound and vibration have been shown to preferentially activate the otoliths, with the semicircular canals activated to a lesser extent (Curthoys et al, 2006;Murofushi and Curthoys, 1997;Zhu et al, 2011), VEMPs are thought to reflect the integrity of the otolith organs. The particular otolith organ responsible for each type of VEMP evoked by different stimuli is still controversial (e.g.…”