The innervation of the bat cochlea
Bijdragen tot de dierkunde , Volume 40 - Issue 1 p. 30- 32
For different species of bats, fixed in 5 % formaldehyd, an estimation of the number of neurons in the spiral ganglion was made. The cochleae were decalcified in EDTA and embedded in paraffin. The complete series of sections were stained with hematoxylin. On the sections through the ganglion, the number of ganglion cell nuclei was determined, with the help of an eye-piece micrometer. A 200 times magnification was used. The figures obtained with this method were corrected, with the help of Floderus’ formula: N = n. T/T + d—2k (N = the corrected estimate of the number of nuclei centres within the section; n = the observed count; T = section thickness; d = diameter of nuclei; k = vertical thickness of the smallest visible fragment). This formula helps to avoid double counting of split nuclei. After this correction the following results were obtained (table I). The numbers of Cavia, Felis and man are taken from Firbas, Wicke & Volavsek (in press), Schuknecht (1960), and Guild (1932). After comparing these numbers, it can be inferred, that the number of nerve cells in the spiral ganglion depends on the dimension of the cochlea, and on the range of perceived sounds. The neurons in the basal coil are responsible for the perception of high frequency sounds. It follows that in both the mole and the bat, the number of cells in this coil corresponds to the frequency range. The Talpidae are placed by Henson in the low frequency group. Our own investigation of the mole’s cochlea also indicates the special sensitivity of this animal to low sounds. A study of the distribution of the nerve cells to the different coils revealed interesting details. It was seen, that in the Rhinolophidae the percentage of cells belonging to the basal turn is a little lower than in Plecotus, but nevertheless much higher than in the guinea pig. In Rhinolophus 38 % of the total number of cells belongs to the basal coil, in Plecotus 48 % and in Cavia 22 %. These percentages depend on the size of the basal coil, on the arrangement of cells, and on the number of turns. Nevertheless the highest absolute values are found in bats for cochlea of that size. As usual the spiral ganglion does not extend to the apex of the cochlea. Therefore in Rhinolophus the ganglion has three turns instead of three and a half in Corti’s organ.
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Firbas, Wilhelm. (1970). The innervation of the bat cochlea. Bijdragen tot de dierkunde, 40(1), 30–32.