Mechanically facilitated ion transportation in the mammalian hearing organ

Catherine Knox, MS Defense

Hopeman 224

The inner ear is comprised of two fluid filled spaces, the cochlea and the vestibule. The primary function of the cochlea is to convert mechanical signals to electrical signals. Low frequency sounds are encoded at the apical end while high frequency sounds are encoded at the basal end. The organ of Corti (OoC) is a structure within the cochlea and is the primary mechanism for this conversion. The OoC separates two lymphatic fluids, the endolymph and the perilymph, which are comprised of different ion concentrations. The difference in ion concentration produces an electro-chemical gradient between the two fluid spaces, driving a mechano-transduction (MET) current, which is mostly carried by K+. This current is greater toward the basal end of the cochlea, which implies that it takes greater effort to clear K+ toward the base and could result in a buildup of K+ disrupting ion homoeostasis and preventing proper function of the OoC. It is believed that ion transportation in the extracellular fluid space in the OoC is diffusion limited. With a width of 50 μm and a length of 12 mm, it would take a long time for the fluid to homogenize only due to diffusion. This assumption is challenged using a finite element model that demonstrates longitudinal fluid flow produced in the fluid spaces in the OoC advectively transports ions, allowing the fluid space to homogenize more quickly. The model shows that exposure to sound can locally mix ions and that this process is the most effective in low frequency locations.