Ear, ear: the inner life of the cochlea
Recording technology in action
Professor Jonathan Ashmore FRS, Professor Andy Forge, Dr Jonathan Gale and Professor David Kemp.
University College London.
The inner ear contains a finely tuned piece of hearing machinery- the cochlea. Its protected location within the temporal bone on each side of the head means that it has been hard to study how the cochlea works in any detail. Hearing research has benefited enormously from recent advances in genomics, where researchers have started to identify what genes influence how the cochlea develops.
Almost 30 genes have now been linked to the genetic disorders that account for deafness in approximately 1 in every 1,000 births. Finding ways to detect and manage this pre-lingual deafness has a significant effect on how children develop their social skills. Deafness also affects a large proportion of the adult population - the combined effects of environmental noise and ageing mean that more than 50% of the population aged over 75 has significant hearing loss. Understanding the genetic causes of this disability will have significant implications for finding ways of prevention and treatment.
The cochlea has the job of transforming the sound energy delivered to the cochlear fluid by the middle ear. The elastic basilar membrane, which runs the length of the cochlea, starts to ripple, and each frequency causes a wave that peaks at a different point on the membrane. The cochlea separates out different frequencies in the same way that a prism splits up light, and this is crucial for analysing complex sounds. Each sound is then translated into a pattern of electrical signals by the sensory hair cells found along the length of the basilar membrane. The stimulus that prompts the hair cells to work is measured on a molecular scale, meaning that understanding single molecules, and the genes that encode them, is particularly important in this research.
Quiet sounds need to be strengthened before they can be transformed into the neural activity that leads to perception. For these sounds, the healthy cochlea acts like a hearing aid. A subset of hair cells reacts physically to increase the strength of the ripples as they pass by. A small proportion of this wave energy escapes, and on its way out of the ear causes secondary vibrations of the middle ear and eardrum - creating 'otoacoustic emissions' (OAEs). This feature of a healthy ear to produce sounds provides a useful mechanism for screening for how well the cochlea is working.
'We can use the fact that a healthy ear makes sounds,' explains Jonathan Ashmore.