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

Abstract

AbstractThe 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 at its apex. The average length of the nerve cell body is 7.4 μ (range: 5–15 μ) and it is 6.4 μ wide (5–10 μ). The average number of ganglion cells enumerated was 63.2 × 103 (range: 60.6 × 103–65.2 × 103) and following correction for “split cell error,” the total number was 55.3 × 103 ganglion cells. The ganglion cell density curve increases gradually to a maximum value of 11.7 × 103 cells/mm2 at 2.4–3.2 mm from the basal end. The two and one‐half turn cochlea is 6.9 mm long (6.8–7.1 mm) with an 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 at 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.