An ear for sensitivity

Print edition : February 15, 2019

This image, taken through an optical microscope, shows a cross section of the tectorial membrane, a gelatinous structure that lies atop the tiny hairs that line the inner ear. Photo: Jonathan Sellon, MIT micromechanics group

The human ear is so extraordinarily sensitive that it can detect soundwave-induced vibrations of the eardrum that move by less than the width of an atom. Researchers at MIT, led by Jonathan Sellon, have discovered how the ear achieves this extraordinary ability to pick up faint sounds. The results appear in “Physical Review Letters”.

Both the ear’s sensitivity and its selectivity depend crucially on the behaviour of a minuscule gelatinous structure in the inner ear called the tectorial membrane. The MIT group has found that the way the gel membrane gives our hearing its extreme sensitivity has to do with the size, stiffness, and distribution of nanoscale pores in that membrane, and the way those nanopores control the movement of water within the gel. The tectorial is a saturated sponge-like structure made mostly of water. But, “if you squeeze it as hard as you can, you can’t get the water out. It’s held together by electrostatic forces,” explains Dennis Freeman. Although there are many gel-based materials in the body, including cartilage, elastin and tendons, the tectorial membrane develops from a different set of genetic instructions and any defects in its structure caused by gene variations can degrade a person’s hearing.

After detailed tests of the microscopic structure, the team found that the size and arrangement of pores within it, and the way those properties affect how water within the gel moves back and forth between pores in response to vibration, makes the ear’s response highly selective. “What we’re finding is that the tectorial membrane is less solid than we thought,” says Freeman. The key finding was that “for middle frequencies, the structure moves as a liquid, but for high and low frequencies it only behaves as a solid.” The researchers hope that a better understanding of these mechanisms may help in devising ways to counteract various kinds of hearing impairment.