You can hear a pin drop - why our senses are sensitive
A new study into hearing has uncovered the secret to our extraordinary ability to perceive a range of sounds, from leaves rustling in the breeze to the roar of a jet engine. The findings also shed light on other sensory systems, such as smell and sight, and could pave the way to a better understanding of deafness and hearing loss.
Funded by the Wellcome Trust, Deafness Research UK and the Royal Society, Dr Walter Marcotti, of the University's Department of Biomedical Science, has discovered how a particular calcium sensor present in highly specialised sensory cells allows us to hear with such remarkable sensitivity across a wide range of sound intensities.
Working with researchers in 4 other institutions, including Keele University and the Tübingen Hearing Research Centre in Germany, Dr Marcotti and his research assistant Dr Stuart Johnson have found that a calcium sensor present in auditory sensory cell synapses allows them to encode graded sound stimuli. Their findings have been published in the journal Nature Neuroscience .
The human ear can process an impressive range of sounds, from a pin dropping to a jet engine on take-off. This remarkable achievement depends upon the ability of these sensory receptors to respond to graded signals across a wide range of sound intensity. A similar phenomenon exists in other sensory systems, including the eye. The system depends on the properties of specialised ribbon synapses that convey sensory information from the receptors to the brain.
Our auditory system works thanks to unique hair cells (Fig. 1a), which convert sound into electrical signals. These electrical signals are then relayed to the brain, via auditory nerve fibres, by neurotransmitters packaged in small vesicles that, different from the central nervous system, attach to synaptic structures called ribbons (Fig. 1b, c).
The release of neurotransmitters from hair cells occurs due to specialised calcium sensors known as synaptotagmins. It was previously believed that other molecules were involved, but Dr Marcotti and his team have shown how synaptotagmin IV (Fig. 2), one of 17 synaptotagmins in the human body, is integral to our processing of sound across a wide range of volumes and intensities.
Dr Walter Marcotti and Dr Stuart Johnson explain: "The function of this specific calcium sensor is to extend the dynamic range of sensory synapses in order to increase hearing sensitivity across such a wide spectrum of sound intensities. We are now studying how the calcium sensors, or synaptotagmins, interact to produce our remarkably sensitive auditory, visual and olfactory systems".
By revealing the main determinants of normal cochlear synaptic development, they hope that the information gathered could bring us closer to an understanding of mechanisms behind deafness, and improve methods aimed at repairing hearing loss due to damage or genetic defects.
Vivienne Michael, Chief Executive of Deafness Research UK, said: "These findings are incredibly exciting. The notion that we may be able to repair hearing loss at some level in the future is one that will give hope to millions of deaf people and sufferers of other hearing conditions across the world. It is of course early days and with all such scientific breakthroughs we need to be cautious; however we believe that the findings are significant and we may indeed be closer to understanding deafness than at any point previously".
For further information, please contact Dr Walter Marcotti at:
email : w.marcotti@sheffield.ac.uk