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Peter M. Narins
with the overlying otoconia! mass which can weigh 20 mg in adult frogs (Lewis
and Lombard 1988). When the animal undergoes an acceleration, the vibrations
reach the saccular macula, where they result in a shearing of the stereocilia of the
sensory hair cells, due to the differential motion between the sensory epithelium
and the otoconia! mass. This leads directly to a modulation of the resting
discharge rate in the afferent fibers innervating the saccular hair cells.
To investigate the neural basis of the remarkable behavior of L. albilabris, the
responses of single fibers in the eighth (acoustico-vestibular) cranial nerve to
whole-body vibrations were studied (Narins and Lewis 1984). A male frog was
placed on a platform which was sinusoidally vibrated over a wide range of
frequencies and amplitudes. To properly isolate the animal from ambient
microseismic vibrations, it was necessary to enclose the entire recording apparatus
and the animal in a specially designed vibration-damped room, in which the
vibration noise floor was reduced to at least an order of magnitude lower than any
stimulus being applied. In the case of the white-lipped frog, a vibration noise floor
of 0.0001 cms- 2 was obtained, enabling accurate measurements of accelerations as
low as 0.001 cms- 2 •
Single saccular fibers in the eighth cranial nerve were impaled with KCI-filled
glass micropipettes and the responses to whole-body vibrations were recorded.
Vibration-sensitive fibers show a characteristic tuned response; that is, each fiber
responds most vigorously (highest discharge rate) to a best vibratory frequency
(BVF), analogous to the best excitatory frequency of auditory fibers_ Two
populations of fibers were found that responded selectively to whole-body
vibrations: a highly sensitive group with BVFs below 160 Hz, and a second, lesssensitive group with BVFs between 200-300 Hz.
The most sensitive fibers in this species respond to peak whole-body accelerations on the order of about 0.001 cm/s- 2 (Narins and Lewis 1984). This
represents seismic sensitivity of about an order of magnitude greater than saccular
fibers in the North American bullfrog, Rana catesbeiana (Koyama et al. 1982),
and two orders of magnitude more sensitive than has been reported for
mammalian inner ear organs.
3.1.2 Bimodal Fiber Response Properties
In fact, most low-frequency axons in the frog inner ear are acutely sensitive to
both substrate-borne vibrational (seismic) and sound stimuli (Yu et al. 1991;
Christensen-Dalsgaard and Narins 1993; Christensen-Dalsgaard and J0rgensen
1996a); these are called bimodal fibers. Lewis eta!. (1982a) dye-filled fibers from
various regions of the eighth nerve in the bullfrog, and found that a fiber's distal
origin could often be predicted based on its location within the nerve.
Peter M. Narins
with the overlying otoconia! mass which can weigh 20 mg in adult frogs (Lewis
and Lombard 1988). When the animal undergoes an acceleration, the vibrations
reach the saccular macula, where they result in a shearing of the stereocilia of the
sensory hair cells, due to the differential motion between the sensory epithelium
and the otoconia! mass. This leads directly to a modulation of the resting
discharge rate in the afferent fibers innervating the saccular hair cells.
To investigate the neural basis of the remarkable behavior of L. albilabris, the
responses of single fibers in the eighth (acoustico-vestibular) cranial nerve to
whole-body vibrations were studied (Narins and Lewis 1984). A male frog was
placed on a platform which was sinusoidally vibrated over a wide range of
frequencies and amplitudes. To properly isolate the animal from ambient
microseismic vibrations, it was necessary to enclose the entire recording apparatus
and the animal in a specially designed vibration-damped room, in which the
vibration noise floor was reduced to at least an order of magnitude lower than any
stimulus being applied. In the case of the white-lipped frog, a vibration noise floor
of 0.0001 cms- 2 was obtained, enabling accurate measurements of accelerations as
low as 0.001 cms- 2 •
Single saccular fibers in the eighth cranial nerve were impaled with KCI-filled
glass micropipettes and the responses to whole-body vibrations were recorded.
Vibration-sensitive fibers show a characteristic tuned response; that is, each fiber
responds most vigorously (highest discharge rate) to a best vibratory frequency
(BVF), analogous to the best excitatory frequency of auditory fibers_ Two
populations of fibers were found that responded selectively to whole-body
vibrations: a highly sensitive group with BVFs below 160 Hz, and a second, lesssensitive group with BVFs between 200-300 Hz.
The most sensitive fibers in this species respond to peak whole-body accelerations on the order of about 0.001 cm/s- 2 (Narins and Lewis 1984). This
represents seismic sensitivity of about an order of magnitude greater than saccular
fibers in the North American bullfrog, Rana catesbeiana (Koyama et al. 1982),
and two orders of magnitude more sensitive than has been reported for
mammalian inner ear organs.
3.1.2 Bimodal Fiber Response Properties
In fact, most low-frequency axons in the frog inner ear are acutely sensitive to
both substrate-borne vibrational (seismic) and sound stimuli (Yu et al. 1991;
Christensen-Dalsgaard and Narins 1993; Christensen-Dalsgaard and J0rgensen
1996a); these are called bimodal fibers. Lewis eta!. (1982a) dye-filled fibers from
various regions of the eighth nerve in the bullfrog, and found that a fiber's distal
origin could often be predicted based on its location within the nerve.