Mechanics of the Inner Ear
The mammalian cochlear partition.
- The cochlea houses the sensory cells in the mammalian auditory system (see figure above). An active process boosts vibrations in the cochlea arising from sound, which largely determine the signal sent to the brain, more than a thousandfold. Although absence of the active process results in profound hearing loss, there is no consensus as to its mechanism.
- The active process in nonmammals likely arises from active hair-bundle motility. The outer hair cells of mammals possess a second form of motility, somatic motility, in which the cell body contracts (elongates) in response to membrane-potential depolarization (hyperpolarization) (see figure below). Modelling reveals that somatic motility can increase the characteristic frequency and sensitivity of the cochlea by providing feedback to active hair bundles1. The effects of hair-bundle motility and somatic motility depend, however, on their environment in vivo2.
- New experimental techniques have additionally revealed that the cochlea vibrates in a complex manner in response to sound. We use computational models to interpret these observations and to form hypotheses about how the cochlea works1,4
- Owing to the active process of the inner ear, your ears produce sounds known as otoacoustic emissions. Some animals emit sounds spontaneously from both ears at the same frequency and have ears that are coupled by the air in their head cavities. By describing the ears as active oscillators acoustically coupled by air in the head cavity, it is possible to explain why some spontaneous otoacoustic emissions are at the same frequency5. The extensive agreement between theory and experiment implies that the inner ear’s active process allows the ear to act as a whole like an active oscillator.
A model of the mammalian cochlear partition.
- The interplay between active hair bundle motility and electromotility in the cochlea, Ó Maoiléidigh, D and Jülicher F, J Acoust Soc Am 128:1175 (2010) Review
- Effects of cochlear loading on the motility of active outer hair cells, Ó Maoiléidigh, D and Hudspeth AJ, Proc Natl Acad Sci USA 110:5474 (2013)
- Comparison of nonlinear mammalian cochlear-partition models, Szalai R, Homer M, Champneys A, Ó Maoiléidigh, D, Cooper N, and Kennedy H, J Acoust Soc Am 133:323 (2013)
- Vibrational modes and damping in the cochlear partition, Ó Maoiléidigh D and Hudspeth AJ, AIP Conference Proceedings 1703:050003 (2015)
- Bilateral spontaneous otoacoustic emissions show coupling between active oscillators in the two ears, Roongthumskul Y†, Ó Maoiléidigh D†, and Hudspeth AJ, Biophys J 116:2023 (2019) (†Equal contribution)