May 17, 2018 | David F. Coppedge

“Y” Your Inner Ear Hears So Well

A new paper about the inner ear shows an additional level of organization and architecture that increases sensitivity and frequency discrimination.

The “organ of Corti” is not a keyboard instrument in a European cathedral, but it might as well be. It’s a keyboard of sorts, composed of rows of ‘hair cells” inside that tiny snail-like coil in the inner ear, the cochlea. Physiologists have known for a long time that hair cells along its coiled-up track, called the basilar membrane, respond to different frequencies. There are two rows of hair cells, inner and outer. According to Massachusetts Eye and Ear Infirmary, “the ‘outer hair cells’ act as micromotors that amplify incoming sound, and the ‘inner hair cells’ act to sense and transmit information about the sound to the brain.” The hair cells, therefore, play the starring roles in hearing. But do they work alone or in concert? (Pun intended.)

Amazing FactsThe field of cochlear mechanics has been undergoing a revolution due to recent findings made possible by advancements in measurement techniques. While it has long been assumed that basilar-membrane (BM) motion is the most important determinant of sound transduction by the inner hair cells (IHCs), it turns out that other parts of the sensory epithelium closer to the IHCs, such as the reticular lamina (RL), move with significantly greater amplitude for weaker sounds. It has not been established how these findings are related to the complex cytoarchitecture of the organ of Corti between the BM and RL, which is composed of a lattice of asymmetric Y-shaped elements, each consisting of a basally slanted outer hair cell (OHC), an apically slanted phalangeal process (PhP), and a supporting Deiters’ cell (DC). Here, a computational model of the mouse cochlea supports the hypothesis that the OHC micromotors require this Y-shaped geometry for their contribution to the exquisite sensitivity and frequency selectivity of the mammalian cochlea.

In a piano, it’s not just the keys and hammers that make sound. The whole instrument is involved: the sounding board, the pedals, the dampers, and everything else. The keys and action are tied into this vibrating framework. In a similar way, the hair cell bundles do not act in isolation. A new paper in PNAS (quoted above) explains that “Cochlear amplification and tuning depend on the cellular arrangement within the organ of Corti.” In short, the whole inner architecture is involved in hearing, not just the hair cells.

Credit: PNAS, Massachusetts Eye & Ear

While the near-crystalline structure of the organ-of-Corti cytoarchitecture in the mammalian cochlea has been known for some time, its functional consequences on hearing remain to be established. The present computational-modeling studies show that individual outer hair cells (OHCs) can work together to produce high hearing sensitivity and frequency selectivity because of the overlapping asymmetrical Y-shaped structures that they form with the Deiters’ cells (DCs) and phalangeal processes (PhPs). Altering the geometry and material properties of these structures reveals that all three components have a profound effect on basilar-membrane and reticular-lamina amplification and tuning. One implication is that the DCs and PhPs are not just supporting structures, but that they must also be properly restored in emerging therapies to regenerate OHCs.

An illustration in the press release shows a diagram of the model of this Y-shaped structure, looking like a row of “Y” letters arranged along their sides. Physiologists had hoped that techniques to regenerate just the hair cells might restore hearing. Now, however, this team has demonstrated that the “sounding board” needs to be restored as well. Animations within the PNAS paper show waves traveling along this architecture, looking like waves along a series of closely-spaced power poles. Alterations to this structure compromise the sensitivity of hearing.

In addition, there are two domains of fluid inside the organ of Corti that play critical roles in hearing. Their viscous properties and pressures must be finely tuned to the architecture. The organization extends all the way down to individual proteins within the hair cells: “Prestin proteins in the OHC plasma membrane provide a piezoelectric effect, such that depolarization causes axial contraction and hyperpolarization causes axial expansion,” they write. The senior author says, “Our work suggests that, in order for humans to hear well, the outer hair cells in the cochlea not only need to be in good working order, but also need to be connected to other nearby cochlear structures in a very particular way.

The inner ear demonstrates a remarkable example of irreducible complexity. As masterfully organized as the cochlea is, it’s part of another irreducibly complex system in the ear, starting with the outer ear, the ear canal, the eardrum, the bones of the middle ear, the cochlea, and the auditory nerve. Even then, signals to the brain signify nothing unless the brain has the “software” to interpret the sounds.

The authors never mentioned “evolution” in the paper or in the press release. The closest word was “revolution,” as in “The field of cochlear mechanics has been undergoing a revolution” in understanding the remarkable architecture by which we hear pressure waves emanating in the world around us.

 

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