Edwin R. Lewis

Silicon-substrate microelectrode arrays for parallel recording of neural activity in peripheral and cranial nerves

A new process for the fabrication of regeneration microelectrode arrays for peripheral and cranial nerve applications is presented. This type of array is implanted between the severed ends of nerves, the axons of which regenerate through via holes in the silicon and are thereafter held fixed with respect to the microelectrodes. The process described is designed for compatibility with industry-standard CMOS or BiCMOS processes (it does not involve high-temperature process steps nor heavily-doped etch-stop layers), and provides a thin membrane for the via holes, surrounded by a thick silicon supporting rim. Many basic questions remain regarding the optimum via hole and microelectrode geometries in terms of both biological and electrical performance of the implants, and therefore passive versions were fabricated as tools for addressing these issues in on-going work. Versions of the devices were implanted in the rat peroneal nerve and in the frog auditory nerve. In both cases, regeneration was verified histologically and it was observed that the regenerated nerves had reorganized into microfascicles containing both myelinated and unmyelinated axons and corresponding to the grid pattern of the via holes. These microelectrode arrays were shown to allow the recording of action potential signals in both the peripheral and cranial nerve settings, from several microelectrodes in parallel.<<ETX>>

Acoustically induced call modification in the white-lipped frog, Leptodactylus albilabris

The vocalization behaviour of Leptodactylus albilabris was investigated using field playback experiments. To assess the response of males to pre-recorded natural ‘chirp’ (advertisement call) and natural ‘chuckle” (aggressive call) stimuli of gradually increasing broadcast intensity, three parameters (intensity, dominant frequency and repetition rate) of the chirp call were analysed. Of the males tested, 69% showed a significant increase in chirp intensity with increased levels of both stimulus types. In response to playback of the chirp stimulus, males actively modified the dominant frequency of their chirp calls over a mean range of 91·42 Hz, and in one case as much as 400 Hz. Moreover, 12 of 17 males shifted the frequency of their call towards the dominant frequency of the chirp stimulus (2175 Hz) by either increasing or decreasing the dominant frequency of their chirp calls. In response to the natural chuckle stimulus, 83% of the males showed either a decrease or no significant change in the dominant frequency of their chirps. All eight males for which both the chirp frequency and intensity were analysed and that showed an increase in chirp intensity also showed a concomitant increase in chirp dominant frequency. These results are the first to document quantitatively the plasticity of advertisement call intensity and dominant frequency in an anuran. The possible effects of advertisement call modification on male mating success in L. albilabris is discussed.

Hair cell types and distributions in the otolithic and auditory organs of the bullfrog

Abstract Based on surface morphology, 6 hair cell types have been identified on the sensory epithelia of the bullfrog otolithic and auditory organs. One of these types apparently is the morphogenetic precursor of vibrational and auditory hair cells. Another type, with a kinocilium several times as long as the longest stereocilia, apparently mediates gravistatic sensitivity. A third type, with a bulbed kinocilium no longer than the longest stereocilia, apparently mediates vibrational and auditory sensitivities. The other types may serve a variety of functions, depending on their location. One suggested function is that of an anchor for the gelatinous superstructure.

The tonotopic organization of the bullfrog amphibian papilla, an auditory organ lacking a basilar membrane

SummaryIntracellular dye-injection studies have revealed tonotopic organization of the bullfrog (Rana catesbeiana) amphibian papilla, an auditory organ lacking a basilar membrane or its equivalent. The best excitatory frequency (BEF) for auditory stimuli was identified in each of twenty-nine VIIIth-nerve afferent axons that subsequently were traced to their peripheral terminations at the sensory surface. Among those axons, the five with BEFs greater than 550 Hz all terminated in the caudalmost region of the papilla, the ten with the BEFs greater than 300 Hz and less than or equal to 550 Hz all terminated in the central region of the papilla, and the fourteen with BEFs equal to or less than 300 Hz all terminated in the rostralmost region of the papilla (Fig. 4). The tectorium is very much larger and presumably more massive under the low-frequency region of the papilla than it is under the high-frequency region (Fig. 1). Higher-frequency axons tended to innervate few (one to four) receptor cells, and low-frequency axons tended to innervate many (six or more). Higher-frequency axons often terminated in large claw-like structures that engulfed the basal portions of individual hair cells and in this way were morphologically similar to type I terminals in the inner ears of higher vertebrates.

Acute seismic sensitivity in the bullfrog ear

Single axons in the auditory/vestibular nerve of the American bullfrog exhibit by far the most exquisite sensitivity to substrate-borne vibration yet reported for a quadruped vertebrate. Earlier dye-injection studies revealed that these axons, which are relatively insensitive to airborne sound, originate at the saccular and lagenar maculae of the bullfrog inner ear. The more sensitive axons exhibited clear responses to vibratory sinusoids with peak accelerations as low as 0.005 cm/s2.

Do frogs communicate with seismic signals

Male white-lipped frogs exhibit conspicuous behavioral responses to calling conspecific males that are nearby but out of view. Since the calls often are accompanied by strong seismic signals (thumps), and since the male white-lipped frog exhibits the most acute sensitivity to seismic stimuli yet observed in any animal, these animals may use seismic signals as well as auditory signals for intraspecific communication.

The Acoustic Periphery of Amphibians: Anatomy and Physiology

According to current classification, the living amphibians are distributed among three orders—Caudata (newts and salamanders, or urodeles), Gymnophiona (caecilians), and Anura (frogs and toads)—which often are grouped in a single subclass—Lissamphibia. A current summary of the biology of the Lissamphibia is found in Duellman and Trueb (1994). Among the morphological features common to the three orders of Lissamphibia, but lacking in fish, are four evidently related to acoustic sensing (see Bolt and Lombard 1992; Fritzsch 1992 for recent reviews): (1) a hole (the oval window) in the bony wall of the otic capsule; (2) the insertion of one or two movable skeletal elements, the columella and the operculum, into that hole from its lateral side; (3) a periotic labyrinth, part of which projects into the hole from its medial side; and (4) two extraordinarily thin membranes (contact membranes), comprising locally fused epithelial linings of the periotic and otic labyrinths, each contact membrane forming part of the wall of a separate papillar recess in the otic labyrinth. The two papillae themselves may be homologues of two sensors found in fish—the macula neglecta and the basilar papilla. In amphibians, the putative homologue of the macula neglecta is called the amphibian papilla. Among fish, the basilar papilla has been found only in the coelacanth fish, Latimeria (Fritzsch 1987).

The vertebrate ear as an exquisite seismic sensor.

The neotropical frog Leptodactylus albilabris exhibits the greatest sensitivity to substrate-borne vibrations (seismic stimuli) reported to date for any terrestrial animal. Nerve fibers from the source of this extraordinary sensitivity in the ear show clear stimulus-evoked modulations of their resting discharge rates in response to sinusoidal seismic stimuli with peak accelerations less than 0.001 cm/s2 (10(-6) g). Evidence indicates that its source is the saccule, an organ of hearing in fish and of balance in man. We report that single vibration-sensitive fibers in the white-lipped frog saturate at (whole animal) displacements of 10 A peak to peak [Fig. 1(b)]. Assuming a conservative 20-dB dynamic range for these fibers, the in vivo frog saccule and the mammalian cochlea exhibit roughly equal sensitivities to displacement.

Correspondences between afferent innervation patterns and response dynamics in the bullfrog utricle and lagena

Otoconial afferents in the bullfrog were characterized as gravity or vibratory sensitive by their resting activity and their responses to head tilt and vibration. The responses of gravity afferents to head tilt were tonic, phasic-tonic, or phasic. A few afferents, termed vibratory/gravity afferents, had gravity as well as vibratory sensitivity. Functionally identified otoconial afferents were injected with Lucifer Yellow and subsequently traced to their peripheral arborizations. Morphological maps, previously constructed with the scanning electron microscope, were used to identify microstructural features of the sensory maculae associated with the peripheral arborizations of dye-filled afferents. The utricular and lagenar macula each is composed of a specialized central band surrounded by a peripheral field. The central bands are composed of densely packed medial rows and more sparsely packed lateral rows of hair cells. Hair cells exhibit a variety of surface topographies which correspond with their macular location. The response dynamics of afferents in the utricle and lagena correspond with the macular locations of their peripheral arborizations. Tonic afferents were traced to hair cells in the peripheral field. Phasic-tonic and phasic afferents innervated hair cells in the lateral rows of the central band, the former innervating hair cells at the edges of the central band and the latter innervating hair cells located more medially. Afferents with vibratory sensitivity were traced to hair cells in the medial rows of the lagenar central band. The response dynamics of afferents corresponded with the surface topography of their innervated hair cells. Tonic and phasic-tonic gravity afferents innervated hair cells with stereociliary arrays markedly shorter than their kinocilium (Lewis and Li types B and C) while phasic gravity and vibratory afferents innervated hair cells with stereociliary arrays nearly equal to their kinocilium (Lewis and Li types E and F). Vibratory sensitivity was uniquely associated with hair cells possessing bulbed kinocilium (Lewis and Li type E) while afferents sensitive to both gravity and vibration innervated hair cells from both of the above groups. We argue that afferent response dynamics are determined, at least in part, at the level of the sensory hair bundle and that morphological variations of the kinocilium and the otoconial membrane are dictated by specialization of sensitivity. We propose that morphological variations of the kinocilium reflect variations in its viscoelastic properties and that these properties determine the nature of the mechanical couple between the stereociliary array and the otoconial membrane.

The use of seismic signals by fossorial southern african mammals : A neuroethological gold mine

Behavioral adaptations exhibited by two African fossorial mammals for the reception of vibrational signals are discussed. The Namib Desert golden mole (Eremitalpa granti namibensis) is a functionally blind, nocturnal insectivore in the family Chrysochloridae that surface forages nightly in the Namib desert. Both geophone and microphone recordings in the substrate suggest that the golden mole is able to detect termite colonies and other prey items solely using seismic cues. This animal exhibits a hypertrophied malleus, an adaptation favoring detection of low-frequency signals. In a field study of the Cape mole-rat (Georychus capensis), a subterranean rodent in the family Bathyergidae, both seismic and auditory signals were tested for their propagation characteristics. This solitary animal is entirely fossorial and apparently communicates with its conspecifics by drumming its hind legs on the burrow floor. Auditory signals attenuate rapidly in the substrate, whereas vibratory signals generated in one burrow are easily detectable in neighboring burrows. The sensitivity to substrate vibrations in two orders of burrowing mammals suggests that this sense is likely to be widespread within this taxon and may serve as a neuroethological model for understanding the evolution of vibrational communication. Neuroethological implications of these findings are discussed.