Hair cell shape fine-tunes sound signals

26 September 2008

Scientists from the UK and Germany have recently discovered the first evidence that hair cells in the inner ear that process high-pitched tones are different to the cells that process lower pitches.

The research, which was reported recently in The Journal of Neuroscience, was conducted with support from Deafness Research UK by Dr Walter Marcotti at the University of Sheffield, with colleagues in Tübingen, Germany and at the UCL Ear Institute in London.

The ability to function with precision-timing is essential for the cochlea to represent the patterns in sound waves accurately. In mammals this ability depends on the release of neurotransmitters, the chemical signals that brain cells use to communicate with each other, from inner hair cells to the neurons that carry the sound signal onward to the brain. The new research shows that the way neurotransmitters are released varies along the length of the cochlea.

“An understanding of the underlying physiological characteristics of hair cells is essential for the success of future methods intending to regenerate or re-implant these cells into abnormal cochleae. This type of fundamental study could aid the generation of hair cells with characteristics that are specific to a defined region of the cochlea,” said Dr Marcotti.

It has long been known that high-pitch tones are processed by hair cells at the base of the cochlea and low pitches are processed at the apex. Previous thinking held that in mammals this difference was just due to their position along the cochlea rather than any differences in the type of cell.

However, it turns out that the point, known as the ribbon synapse, from which the neurotransmitters are released has a spherical shape for higher pitches and an ellipsoid shape for the lower. It seems that this difference in shape may affect the hair cell's ability to keep on signalling a repetitive sound, with low-frequency cells giving up sooner that the higher pitched ones..