The Basilar Membrane* (Osseous Spiral Lamina/Spiral Diagram/Tonal Reception/Frequency Limit) ERNEST GLEN WEVER, JAMES G
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Proc. Nat. Acad. Sci. USA Vol. 68, No. 11, pp. 2708-2711, November 1971 Cochlea of the Dolphin, Tursiops truncatus: The Basilar Membrane* (osseous spiral lamina/spiral diagram/tonal reception/frequency limit) ERNEST GLEN WEVER, JAMES G. MCCORMICKt, JERRY PALIN, AND S. H. RIDGWAYt Auditory Research Laboratories, Princeton University, Princeton, New Jersey 08540 Contributed by Ernest Glen Wever, August 6, 1971 ABSTRACT In the dolphin Tursiops truncatus, the and thus at right angles to its course, measurements of the basilar membrane over its course from basal to apical width of the basilar membrane may be made directly in the ends shows a systematic variation in width and in the manner and rigidity of its suspension. microscope field. At all other points the sectioning is oblique, The suspension is by bony laminae on both the outer and the true width may be obtained only by taking the and inner edges in the basal region, and by progressively obliquity into account. A method for doing this was worked less rigid attachments in the more apical regions, until out by Wever (3), and is illustrated in Fig. 1. The observed near the apex the membrane is held only by connective tissue. width is first plotted on the spiral diagram (as, for example, The basilar membrane shows an unusual variation in the width d-e), then arcs are drawn from the center of width of 14 times, ranging from an astonishingly small curvature (o) of the cochlear region concerned, with radii value of 25 pm at the basal end to a value toward the apex passing through the two ends of the observed membrane of about 350 pm. This structural variation is in harmony The radial distance between these arcs is the with the extension of sensitivity of the dolphin ear into (d,e). (d-f) the very high frequencies, and suggests unusual capa- corrected width of the basilar membrane. (A trigonometric bilities of pitch discrimination in this high range. correction can also be made, but the graphic procedure as described is sufficiently accurate if performed with care.) The In a preceding article (1) we outlined the present status of our method works satisfactorily at all positions, except those knowledge concerning the detailed anatomy of the dolphin close to the tangents of the spiral turns, where errors of cochlea, and reported some recent observations on the measurement become large. general morphology of the inner-ear structures of Tursiops truncatus. Here we present results on the form and suspension RESULTS of the basilar membrane. Suspension of the basilar membrane METHOD It was observed by early students of the whale ear that the basilar membrane is suspended between bony supports along As described in our earlier article, the specimens were perfused both edges. In addition to the internal osseous spiral lamina, intravitally with a fixative that in the more successful experi- which anchors the inner border of the basilar membrane in ments produced an excellent preservation of cochlear ele- these and other mammalian ears (at least in the basal turn), ments. In all specimens, the basilar membrane was well there is an external osseous spiral lamina, which supports the 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 pre- ceding report (1). in two It This cochlear reconstruction is useful ways. _g_ 0d ________-R shows for the different serial sections the portions of the cochlea that they represent, and (because the spiral can be graduated along its course) it provides the equivalent of uncoiling the cochlea, so that its features can be related to the linear position. Comparisons between different ears are thereby facilitated. At those points on the spiral cochlea where the basilar membrane has been sectioned along its radius of curvature, FIG. 1. A portion of the basilar membrane, with a few of the * This is the second paper of a series on: "The Cochlea of the lines of sectioning indicated by the horizontal broken lines. A Dolphin". The first paper is ref. 1. tangential section is indicated by T, radial (or transverse) see- t Present address: Section of Otolaryngology, Bowman Gray tions by R. All other sections in the region shown are oblique. School of Medicine, Winston-Salem, N.C. The distances g-h and i-j on radial sections represent true I Naval Undersea Research and Development Center, San widths of the basilar membrane, and require no correction. The Diego, Calif. other distances a-b and d-e must be corrected as indicated. 2708 Downloaded by guest on September 27, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) Cochlea of the Dolphin 2709 outer border. This external lamina is an extension of the bony cochlear capsule. As shown in Fig. 2, which represents a cross section of the C cochlea of a Tursiops specimen in the lower basal region, the bony shelf tapers rapidly as it runs inward from the cochlear capsule along the tympanic surface of the spiral ligament. Br >*r -f 4 ......b#~.L It combines with the dense connective tissue of this ligament to form the external support of the basilar membrane. The o _~~~~~~ two dense layers are separated by loose and cell-containing E, connective tissue that closely resembles the main constituent of the spiral ligament. The rigidity of the support provided by the external osseous spiral lamina varies along the cochlea. Only at the basal end does this bony layer extend all the way to the basilar membrane, and thus directly fix its outer edge. The extent over which this condition was found in one of our specimens was about 8 mm at the basal end. Thereafter in the basal turn, the bony lamina stops short FIG. 2. Photomicrograph of a section from a specimen of of the edge of the basilar membrane, and ligamentary strands Tursiops truncatus cochlea, at a point 4.6 mm from the basal continue to make the connection as shown in Fig. 3. The end. C, indicates the wall of the cochlear capsule; E, its inward bony layer of course adds substantially to the rigidity of the extension as the external osseous spiral lamina; S, the spiral the In ligament; B, the basilar membrane; L, the limbus; N the series connective tissue layer of spiral ligament. the specimen of bundles of nerve fibers in the internal osseous spiral lamina. just referred to, this condition prevailed over a distance of The obliquity of the sectioning is considerable at this level. The about 18 mm, or from nearly the middle of the lower basal arrow points to the junction between the end of the external half-turn to about the end of the upper basal half-turn. osseous spiral lamina and the basilar membrane. Scale X30. Near the beginning of the apical turn, and over a distance of 3 or 4 mm, the external osseous lamina undergoes a rapid reduction in size. Its width becomes less and less, until about lamina, which is a bony shelf that carried within it the series two-thirds of the distance along the lower apical half-turn, of channels for the cochlear dendrites. In Tursiops, this where it disappears altogether. From this location to the support also varies along the cochlea. In the lower basal turn apical end, the basilar membrane is supported on its external (Figs. 2 and 3), the two layers of the bony plate in the region edge only by the spiral ligament. below the middle of the Jimbus are relatively thick, of the The inner edge of the basilar membrane (as usual in order of 20 Am, and then they taper off and join in a point at mammalian ears) is anchored to the internal osseous spiral the edge of the basilar membrane. More apically, the thickness Limbus Ce! is Tectorial membrane Ligament Intermediate tissue Internal osseous spiral larniria Nerve fibers FIG. 3. Drawing of the cochlea of Tursiops truncatus in cross section, at a position near the middle of the lower basal half-turn, 10.7 mm from the basal end. Scale X 240. Downloaded by guest on September 27, 2021 2710 Physiology: Wever et al. Proc. Nat. Acad. Si. USA 68 (1971) 400 300 E :Z. 200 4- 31 100 1 _11111111 0 5 10 15 20 25 30 35 40 Distance from basal end, mm FIG. 4. The width of the basilar membrane in Tursiops truncatus as a function of distance from the basal end. Results are shown for four ears, two of them (triangles) represent ears of the same animal. of these bony plates undergoes a progressive decrease, and in The general trend of the functions is much the same, and the middle of the lower apical turn both have been reduced to most of the variations no doubt come from errors of measure- 6-8 Am. Beyond, in the upper apical turn, this bone dis- ment. The basilar membrane is narrow, around 25 Mum in appears, and the basilar membrane is supported only by the width, at the basal end, and the width increases at a uniform fibrous lip of the limbus. rate over the first 20 mm, and then at a somewhat greater rate over the remaining distance. The maximum width near the Width of the basilar membrane apical end of the cochlea is around 350 /Am. The variation of The width of the basilar membrane varies greatly along the width over the basilar membrane thus amounts to 14 times.