Ian S. Curthoys, Juno Kim, Samara K. McPhedran and Aaron J. Camp (2006):
Bone conducted vibration selectively activates irregular primary otolithic
vestibular neurons in the guinea pig. Exp. Brain Res., 175(2), 256-267
Abstract: The main objective of this study was to determine whether
bone-conducted vibration (BCV) is equally effective in activating both
semicircular canal and otolith afferents in the guinea pig or whether there
is preferential activation of one of these classes of vestibular afferents.
To answer this question a large number (346) of single primary vestibular
neurons were recorded extracellularly in anesthetized guinea pigs and were
identified by their location in the vestibular nerve and classed as regular
or irregular on the basis of the variability of their spontaneous discharge.
If a neuron responded to angular acceleration it was classed as a
semicircular canal neuron, if it responded to maintained roll or pitch tilts
it was classified as an otolith neuron. Each neuron was then tested by BCV
stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone
bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction
oscillator cemented to the guinea pig's skull. All stimulus intensities were
referred to that animal's own auditory brainstem response (ABR) threshold to
BCV clicks, and the maximum intensity used was within the animal's
physiological range and was usually around 70 dB above BCV threshold. In
addition two sensitive single axis linear accelerometers cemented to the
skull gave absolute values of the stimulus acceleration in the rostro-caudal
direction. The criterion for a neuron being classed as activated was an
audible, stimulus-locked increase in firing rate (a 10% change was easily
detectable) in response to the BCV stimulus. At the stimulus levels used in
this study, semicircular canal neurons, both regular and irregular, were
insensitive to BCV stimuli and very few responded: only nine of 189
semicircular canal neurons tested (4.7%) showed a detectable increase in
firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level
we delivered to the B-71 oscillator (which produced a peak-to-peak skull
acceleration of around 6-8 g and was usually around 60-70 dB above the
animal's own ABR threshold for BCV clicks). Regular otolithic afferents
likewise had a poor response; only 14 of 99 tested (14.1%) showed any
increase in firing rate up to the maximum BCV stimulus level. However, most
irregular otolithic afferents (82.8%) showed a clear increase in firing rate
in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48
were activated, with some being activated at very low intensities (only
about 10 dB above the animal's ABR threshold to BCV clicks). Most of the
activated otolith afferents were in the superior division of the vestibular
nerve and were probably utricular afferents. That was confirmed by evidence
using juxtacellular injection of neurobiotin near BCV activated neurons to
trace their site of origin to the utricular macula. We conclude there is a
very clear preference for irregular otolith afferents to be activated
selectively by BCV stimuli at low stimulus levels and that BCV stimuli
activate some utricular irregular afferent neurons. The BCV generates
compressional and shear waves, which travel through the skull and constitute
head accelerations, which are sufficient to stimulate the most sensitive
otolithic receptor cells.
Keywords: Vestibular - Otolith - Labyrinth - Semicircular canal - Sound -
Vibration - Utricular - Saccular
DOI: 10.1007/s00221-006-0544-1
Available at: http://www.springerlink.com/content/x71516864l48l641/
Regards
Sonja Amoser
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Sonja Amoser, PhD Student
University of Vienna, Dept. of Neurobiology and Behavior
Althanstrasse 14, 1090 Vienna, Austria
Phone: +43-1-4277-54467 oder +43-664-5006106 (private)
Fax: +43-1-4277-54506
email:
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