**2. Hearing system qualities**

The human cochlea consists of about two and a half turns and is about 9 cm in length from the oval window to the helicotrema, corresponding to a frequency gradient starting with high frequencies at the base and proceeding to low frequencies at the apex.

The middle ear main function is to achieve mechanical gain by the ossicles as a lever and the tympanic membrane and oval window as a plane focuser. Besides, the middle ear makes the stimulus processable by harmonizing the sound wave resistance (impedance) of air and perilymph and the hair cells, respectively (**Figure 1**).

Adaptation mechanisms are characteristical for a sense organ, and as the cochlea has to process travelling waves continuously, it involves all molecular structures and biochemical processes of the inner ear. Adaptation lowers the system requirements, protects from overstimulation and reflects the environmental necessities of stimulus perception. In addition, it characterizes tone and music perception. Contrastingly, to the chemical receptors of the olfactory and gustatory system and the dermal mechano-, and thermoreceptors, sound adaptation does not lead to no stimulus perception at all. Generally speaking, the adaptation time constant is faster in hair cells at the high-frequency end than at the low-frequency end, what probably contributes to frequency selectivity [3].

Different adaptation mechanisms contribute to inner ear function, namely voltage-dependent hair-cell properties, structural hair-bundle characteristics and afferent transmitter release. Fast adaptation operates around the most sensitive portion of the hair cell activation, whereas larger displacements of the hair bundle induce slow adaptation [4]. Fast adaptation has been identified in both cochlear and vestibular hair cells, but is the main form of adaptation in cochlear hair cells. Slow adaptation has been identified in all but mammalian auditory hair cells, and is the predominant adaptation mechanism found in vestibular hair cells [5–7]. (Comparison of the adaptation mechanisms of the human sense organs and receptor cells is described at the end of the article in **Table 1**. Comparison of the perception qualities of the human sense organs and further comparison are listed in **Table 2**. Comparison of the structural characteristics of the human sense organs.)

**Figure 1.** Schematic illustration of the human ear. (**a**) The ear consists of the outer, middle and inner ear. (**b**) A section through the cochlear duct illustrates the fluid-filled compartments of the inner ear. (**c**) The organ of Corti resides in the scala media, with sensory hair cells surrounded by supporting cells that include Deiters, Hensen and pillar cells. (**d**) Immunohistochemistry with the inner ear hair cell marker myosin VI, marking the cytoplasm of inner and outer hair cells, and 4,6-diamidino-2-phenylindole (DAPI), marking the nuclei. (**e**) Scanning electron microscopy image of the top view of the sensory epithelium reveals the precise arrangement of one row of inner hair cells and three rows of outer hair cells, separated by the pillar cells (with permission from Ref. [75]. Copyright © 2009, Annual Reviews. All rights reserved).
