Inner-ear mystery solved

14 March 2006
Belle Dumé is science writer at PhysicsWeb

Why is the cochlea in our ears shaped like a spiral? According to new work by scientists in the US, the spiral shape makes us more sensitive to low frequency sounds. Daphne Manoussaki of Vanderbilt University in Nashville, Tennessee, together with Emilios Dimitriadis and Richard Chadwick at the National Institutes of Health in Bethesda, Maryland, have calculated that the spiral shape can affect the wave mechanics that take place inside the cochlea. It increases the strength of vibrations produced by sound waves, especially at low pitch (Phys. Rev. Lett. 96 088701).

The cochlea is a small seashell-shaped organ in the inner part of the ear where sound vibrations are converted into nerve impulses. These are then sent to the brain as electrical signals. The human cochlea occupies a volume of about 1 cubic centimetre and operates at frequencies between 20 hertz and 20 kilohertz. It can detect sounds over a range of 120 decibels.

Scientists have long wondered whether the cochlea's coiled-up shape plays an important role in how it processes sound. Although intuition said it should, theoretical models did not support the idea. The new work by Manoussaki, who is an applied mathematician, and colleagues has now shown that the spiral shape probably increases hearing sensitivity to low frequency sounds by as much as 20 decibels.

When we hear something, the incoming sounds are transmitted via the tympanic membrane (or eardrum) to the basilar membrane, which runs along the length of the cochlea. The sound waves then cause the basilar membrane to vibrate. Manoussaki and co-workers looked at the equations that describe the mechanical interaction between this membrane and its surrounding fluid. In this way, they were able to describe how the amplitude of the membrane's movement changes across the width in each section of the spiral tube.

The scientists found that the spiral shape makes the membrane deflect more towards the outside of the cochlea wall and less towards the inside. This difference in the membrane's motion, which is effectively a tilt about a line running along its centre, increases with increasing radius of curvature and so enhances hearing sensitivity. "Since low frequency sounds are processed where the spiral curvature is greatest – at the apex of the cochlea - the effect will be more important for low-frequency sound waves," explains Manoussaki.

The spiral structure may have developed because animals use low-frequency sounds, which travel the furthest of all sound waves, for communication and survival. The US team will now try to confirm its results by comparing cochlea in different mammals.