A neuroanatomical study of the cochlea of the little brown bat (Myotis lucifugus)

Abstract

The population and density of the bipolar ganglion cells and sensory hair cells were determined from serial sections and graphic reconstructions of the cochlea and spiral ganglion of four little brown bats. The spiral ganglion lacks a bony canal and is a one and one-half turn spiral of almost uniform thickness which ends in a broad flat nodule a t its apex. The average length of the nerve cell body is 7.4 p (range: 5-15 p ) and it is 6.4 p wide (5-10 p). The average number of ganglion cells enumerated was 63.2 x lo3 (range: 60.6 x 103-65.2 x lo3) and following correction for "split cell error," the total number was 55.3 x lo3 ganglion cells. The ganglion cell density curve increases gradually to a maximum value of 11.7 x lo3 cells/mm2 at 2.4-3.2 mm from the basal end. The two and onehalf turn cochlea is 6.9 mm long (6.8-7.1 mm) with a n estimated total of 700 inner and 2,800 outer sensory hair cells. The densities of outer and inner hair cells increase from the base to the apex by 21% and 32%, respectively. However, the density of sensory cells per unit of sensory epithelium decreases from base to apex. The ratio increases to a maximum value of 22: 1 a t 2.4 mm and then declines toward the apex. The significance of these findings is discussed and compared to similar studies on other high-frequency hearing mammals.Most anatomical studies of the mammalian ear include inferences about the function of that organ. Nowhere is this practice more prevalent than in anatomical studies on the ear of mammals having high-frequency hearing. Structural variations found within the ear of these mammals have been interpreted as adaptations for high-frequency hearing as in Cetacea (Reysenbach de Haan, '57; Wever e t al., '71a1, Phocida (Ramprashad e t al., '72; Ramprashad, '751, and Chiroptera (Pye, '66a,b, '67). More precisely, these variations are structural adaptations which augment stimulation of the sensory cells of the cochlea.Neuroanatomical studies of the cochlea of the Pacific white-sided dolphin (Lagenorhynchus obliquidens), bottlenose dolphin (Tursiops truncatus) and the harp seal (Pagophilus groenlandicus) have indicated that the total number of ganglion cells, and the ratio of ganglion cells to the sensory cells exceed those of the human (Wever e t al., '71b, '72; Ramprashad, '76). Since these aquatic mammals are associated with high-frequency hearing, it

A modeling approach to explain pulse design in bats

“…6 should simply be interpreted as a process that stops working relatively abruptly above a certain frequency f l . The signal emerging from all manipulations y(t) can be regarded as the probability function for the release of transmitter from vesicles into the synaptic cleft to the estimated 70 fibres (in Myotis; Ramprashad et al 1978) innervated there. In the simulation, each frequency channel has its own hair cell and probability function of transmitter release.…”

Hearing in Bats: An Overview

“…In bats using CF components, there is dense innervation of the basilar membrane in regions corresponding to the representations of the second and third harmonics, with sparse innervation in between (Zook and Leake 1989). The density of innervation at these sites is close to the maximum observed for other mammals (25-35: 1); in the FM bat Myotis, interestingly, the density is about twice that high (70:1) (Ramprashad, Money, Landolt and Laufer 1978).…”

Morphometric study of the cochlea of the rabbit (Oryctulagus cuniculus)

“…8). Morphometric studies on the Microchiroptera and two-toes sloth cochleae have suggested that the best frequencies coincided within regions where the stiffness gradient was relatively uniform (Bruns 1976~1, 1976bRamprashad et al 1978;Ramprashad et ul. 1979;Ramprashad et ul.…”

“…Recent studies of the mammalian cochlea have attempted to correlate psychoacoustic and neuroanatomic data with pertinent cochlear morphometric dimensions (Bruns 1976a, 19766;Ehret and Frankenreiter 1977;Ramprashad et al 1978;Ramprashad et ul. 1979).…”