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The tonotopic map in the embryonic chick cochlea
Abstract
The purpose of the present study was to record frequency tuning curves (FTC's) from primary afferents in the embryonic (E19) chick cochlea, inject biocytin to determine the neural projections to the basilar papilla, and document the tonotopic map. Seventy-three domestic chick embryos (Gallus domesticus) were used. FTC's were recorded from the cochlear ganglion via the recessus scala tympani in 44 animals. Biocytin was injected iontophoretically or with hydraulic pressure. Cochleae were processed as wholemounts with an avidin-HRP conjugate and DAB. Definitive labeling of neural projections to the papilla was then documented and distance of labelled neurons was measured from the apex in seven animals. Neural characteristic frequency (CF) was then displayed as a function of the distance from the apex expressed in percent. The majority of FTC's exhibited the typical 'V' shape described for older birds and appeared strikingly mature in thresholds (34 to 97 dBSPL), low (61.9 $\pm$ 37) and high (64.6 $\pm$ 33) frequency slopes (dB/octave), and tip sharpness (Q$\sb{\rm 10dB}$ = 5.2 $\pm$ 3). CF's ranged from 188 to 1623 Hz. Some tuning curves displayed an unusual "sawtooth" shaped pattern. The most significant sign of immaturity was the limited expression of high frequencies that in part may be due to the developing middle ear transfer function, and to immature basal regions of the papilla. The tonotopic map for the E19 chick cochlea was comparable to maps reported by Manley et al. (1987) for P2 and P21 chicks at least for 200 to 1600 Hz. This frequency range corresponds to the apical 65% of the papilla. An hypothesis is offered which proposes that all regions of the cochlea are functional at the onset of hearing. Best frequencies likely shift from low to higher frequencies during ontogeny, an idea that is consistent with previous theory. However, the apex manifests the smallest frequency shifts and becomes mature first, whereas basal regions undergo the greatest shifts in frequency and mature last. In this case, maturation is seen as proceeding from the apex to the base, exactly opposite of that previously proposed.
Subject Area
Anatomy & physiology|Animals|Audiology
Recommended Citation
Jones, Sherri M, "The tonotopic map in the embryonic chick cochlea" (1994). ETD collection for University of Nebraska-Lincoln. AAI9416917.
https://digitalcommons.unl.edu/dissertations/AAI9416917