Why Is the Cochlea A Spiral?

The cochlea is the snail-shaped organ that mammals use to perceive and transduce sound, and is located deep in the inner ear. Hair cells lie on a membrane in the cochlea's interior, and sound waves disperse through the fluid-filled spaces which contact the hair cells (for more on this go here). Many people in the field have wondered over the year why the cochlea is shaped the way it is. Is it to conserve space? To better preserve the integrity of sound waves? Recently it has been discovered that the unique coiled shaped of the cochlea boosts its sensitivity to low frequencies, as reported in a March issue of Physical Review Focus by Chadwick et al.

The fluid-filled space inside the cochlea (the scala media) coils from base to apex continuously, but the properties of this space gradually change the further "up" the cochlea sound travels. The sound waves reach their "sweet spot" at different places along the cochlea, which allows the cochlea to distinguish frequency, however this phenomenon would not be altered if the cochlea was a straight tube. But what the spiral shape DOES facilitate is the transmitted vibrations that translate into nerve signals. Calculations show that as a sound wave progresses, the energy increasingly accumulates near the outside edge of the spiral, where the sensory cells of the ear sit (hair cells). The spiral shape ensures that sound will focus close to the "wall" of the cochlea, towards the sensory cells. Low frequencies travel the farthest, so the effect is greatest for those ranges.

This is especially interesting given that not all species of hearing animals have spiral-shaped organs for sound detection. Birds, whose equivalent of the cochlea is called the basilar papilla, is a flat tube lined with hair cells. It tapers slightly but is mostly 2-dimensional. Birds' hearing is variable across species, some have wider frequency ranges that humans, some have narrower. Owls even have asymmetrical ears, with one being slightly higher up on the head that the other increasing their ability to detect the direction of sound.