Cochlea of the Dolphin, Tursiops truncatus: The Basilar Membrane

Abstract

In the dolphin Tursiops truncatus, the basilar membrane over its course from basal to apical ends shows a systematic variation in width and in the manner and rigidity of its suspension.The suspension is by bony laminae on both the outer and inner edges in the basal region, and by progressively less rigid attachments in the more apical regions, until near the apex the membrane is held only by connective tissue.The basilar membrane shows an unusual variation in width of 14 times, ranging from an astonishingly small value of 25 pm at the basal end to a value toward the apex of about 350 pm. This structural variation is in harmony with the extension of sensitivity of the dolphin ear into the very high frequencies, and suggests unusual capabilities of pitch discrimination in this high range.In a preceding article (1) we outlined the present status of our knowledge concerning the detailed anatomy of the dolphin cochlea, and reported some recent observations on the general morphology of the inner-ear structures of Tursiops truncatus. Here we present results on the form and suspension of the basilar membrane. METHODAs described in our earlier article, the specimens were perfused intravitally with a fixative that in the more successful experiments produced an excellent preservation of cochlear elements. In all specimens, the basilar membrane was well maintained. Seven of these specimens were studied in detail for this report.The first step in the examination of these ears was a graphic reconstruction of the cochlear spiral by Guild's method (2), which provides a representation to scale of the structures as projected on a plane perpendicular to the cochlear axis. Such a spiral diagram was shown in Fig. 2 of the preceding report (1).

The spiral ganglion and Rosenthal's canal in beluga whales

“… Delphinapterus data is from this study. (1) Hyrtl, ; (2) Pilleri et al ; (3) Reysenbach de Haan, ; (4) Ketten, ; (5) Wever et al, ; (6) Wever et al, ; (7) Ketten, .…”

A Review of Some Current Concepts of the Functional Evolution of the Ear in Terrestrial Vertebrates

“…It was demonstrated by plotting the highest frequency analysed in each ear against a basilar membrane value which took the length of each membrane and its width gradient into account ( 1971). Since this figure first appeared, data from the dolphin and the sheep have been added with entirely consistent values from these large mammals (Bullock et al, 1968;Keen, 1939Keen, , 1940Kolmer, 1908;Wever et al, 1971;Wollack, 1963). In birds, the basilar papilla resembles that of reptiles, although in general it is more elongated (Schwartzkopff, 1955(Schwartzkopff, , 1957Schwartzkopff and Winter, 1960).…”

Shaping sound in space: the regulation of inner ear patterning

“…The organs rest on the basilar membrane (see Glossary, Box 1), which is ~25 mm long in humans and extends to over 60 mm in elephants and whales (Parks et al, 2007;Ulehlova et al, 1987;West, 1985). By contrast, the width of the sensory epithelium is narrow, ranging from only ~0.1-0.3 mm in mammals and birds (Gleich et al, 2004;Wever et al, 1971). The dimensions of the orthogonal axes of the cochlea thus place specific temporal demands on molecular diffusion as a patterning mechanism owing to theoretical limits on the size of effective morphogen gradients (see Box 2).…”