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Figure 1. Location of the spiral-shaped cochlea in the human head. It is found in a cavity that forms deep in the skull during development, behind and below the level of the eyes. The cavity also contains the sensory organs of balance (vestibular organs) that have their own sensory hair cells and communicate to the brain via a bundle of nerve fibers that terminate in different parts of the brain than do the au- ditory nerve fibers that emerge from the cochlea. The left and right cochleas are mirror images of one another. They spiral around their central axes in opposite directions. The direction of the spiral of the right cochlea is the same as for a right-handed screw while the spiral of the left cochlea is left-handed. The drawing shows the left ear.
1561 publication. Two centuries later, Domenico Cotugno, a Neapolitan surgeon and humanist, published an anatomical dissertation (Cotugno 1775)2 based on his scientific obser- vations. He expanded on the findings of two 17th-century inner ear anatomists, Antonio Maria Valsalva and Guichard Joseph Duverney, correcting speculation by the latter about where high and low frequencies were processed in the co- chlea. Cotugno found fluid in the inner ear of freshly har- vested skulls, in contrast to Fallopio and earlier anatomists who examined desiccated cadaver skulls and assumed that the inner ear was filled with air. The fluid environment is relevant to understanding the role of OHC electromotility in hearing. But first, a greater understanding about the cellular organization of the inner ear and identification of its hair cells was needed.
Improvements in microscopic methods during the 19th cen- tury led to Alfonso Corti’s detailed description of the cel- lular organization of the mammalian hearing organ. Albert
2 Available online at https://goo.gl/PtvBFW.
Figure 2. Pathway of sound to the cochlea in humans. The external ear collects sound and directs it to the eardrum. Sound vibrations are conveyed across the middle ear by three tiny middle ear bones that match the acoustic impedance of air with the impedance of the fluids that fill the inner ear. The three middle ear bones and the outer hair cells (OHCs) are found in mammals. The bones and the OHCs are both required for high-frequency hearing.
Kölliker had pioneered techniques to harden, section, and stain body tissue before looking at it through the micro- scope. Corti used these techniques in Kölliker’s laboratory to examine tissue he dissected from the inner ear of humans and other mammals (Corti, 1851). He described the sensory epithelium in the cochlea, which was soon named after him (the organ of Corti).3 His drawings depicted the sensory cells that became known as “hair cells” because each had a tuft of enlarged microvilli at one end (see Figure 4 for greater detail of the stereociliary tuft). More than a century later, the func- tion of the tuft was identified, and it was demonstrated that bending the bundle of stereocilia modulates the flow of ions into the cell (Wersäll et al., 1965; Harris et al., 1970). This, in turn, regulates the release of neurotransmitters that activate nerve fibers making contact at synapses located at the op- posite end of the hair cell.
At the beginning of the 20th century, it had been established that the organ of Corti contained two types of hair cells. They were named based on their location relative to the cen- tral axis of the cochlear spiral. Inner hair cells (IHCs) make up a single spiraling row of mechanoreceptor cells located over the thin bony plate to which the axial margin of the bas- ilar membrane is attached (Figure 3). Three rows of OHCs are located further away from the central axis. OHCs have a distinctly cylindrical shape and are not in intimate contact with supporting cells for most of their length. The large fluid spaces surrounding the lateral walls of the OHCs are bio- logically unusual. Cells in most organs (heart, brain, kidney,
3 Book available online at https://goo.gl/WAIsnS.
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