Basilar membrane and its anchoring system in the cochlea of the greater horseshoe bat.

Abstract

The auditory system of Rhinolophus ferrumequinum is sharply tuned to 83 kHz, the frequency of the constant-frequency segment of the bat's echolocation signal. On the basis of morphological observations the cochlea of the horseshoe bat can be divided into three regions. (A) In the apical region (8 to 16 mm from the basal end of the organ of Corti; frequency response below 40 kHz) the structure is comparable to that of other mammals sensitive to high frequencies. (B) The basal region (0 to 4.5 mm) which responds to frequencies around 83 kHz is characterized by the following unusual features. (1) The basilar membrane (BM) is 35 μm thick. This thickening sharply decreases to 10 μm at 4.5 mm. (2) The filaments in the BM thickening mainly run spirally rather than exclusively in a radial direction as in other parts of the cochlea and in other mammals. (3) The outer segment of the BM is connected to the secondary spiral lamina by filaments from the inner margin of the spiral ligament. The specialized thick secondary spiral lamina is connected to the outer bony wall only by a thin curved lamella. (4) The large spiral ligament consists mainly of hyaline tissue and is almost devoid of fibers. The inner margin contains fibers in a spirally oriented bundle. (5) The tunnel of Corti is reduced and the first space of Nuel is enlarged by the inward displacement of the feet of the outer pillar cells. (M) In the middle region (4.5 to 8 mm; frequency response 40 to 80 kHz) the outer anchoring system (secondary spiral lamina and spiral ligament) is specialized as in the basal region; whereas all other elements correspond to those of the apical region.