Anatomical Differences in the Peripheral Auditory System of Mammals and Man

The major anatomical differences among animal models and man are briefly reviewed. Differences are described in the length and width of the basilar membrane, the number of inner (IHCs) and outer hair cells (OHCs), and the length of cilia on both cell types. Significant differences in the innervation pattern of the IHCs among these species include the number of afferent nerve terminals per IHC, the degree of branching of afferent fibers and the number of synapses per afferent nerve terminal. At the OHCs, the number of afferent and efferent nerve terminals, the presence or absence of presynaptic bodies, reciprocal synapses and the presence of dendrodendritic synapses in the outer spiral bundles may have important physiological functions. In the cochlear nerve, significant differences are described in the number of spiral ganglion cells (SGCs) and cochlear nerve fibers. Furthermore the percentage of myelinated SGCs and the presence of synapses on SGCs varies enormously.