Hair cells of the mammalian cochlea: extraordinary nanomachines

Abstract

In mammals, the sense of hearing relies on the normal function of two types of specialized cells: inner hair cells (IHC) and outer hair cells (OHC). They both possess the capacity to detect and convert mechanical movements within the cochlea, associated with sounds, into electrical potentials. A set of stereocilias in their apical end is where the mechano-electrical transduction actually occurs. IHC and OHC have very different functions within the complex process of hearing. IHC are responsible for transmitting the electrical information to the brain, for which they possess a specialized glutamatergic synapse with very unique properties. Neurotransmitter is released without pause and with high temporal precision, taking advantage of a synaptic organelle called the ribbon. OHC are exquisite piezoelectric devices, as changes in their membrane potential produce measurable changes in length. This capacity of OHC provides refined frequency selectivity and extra sensitivity to low intensity sounds. Interestingly, OHC function is regulated by an inhibitory innervation that descends from the brainstem and is mediated by acetylcholine. A very special type of nicotinic receptor, α9α10, participates in this synapse. Recent advances have shed light on the importance of this efferent control on acoustic trauma and higher hearing capabilities.