The human ear, with its ability to hear sound, is possibly the most well-developed sensory organ in our body. It is even more sophisticated than vision. We hear sounds between the frequency range of 20 Hz and 20,000 Hz. In comparison, the sensitivity range of our eyes is very limited.
Our ear can distinguish between sounds that are produced within 0.1 milliseconds of each other. At a concert, for example, the ear can identify sounds generated from individual instruments being played simultaneously. In comparison, the eye does not break down white light into its individual colours (VIBGYOR).
Despite the high sensitivity of our ears, they are also robust. In general, the more sensitive an organ is, the more delicate its structure or functioning becomes.
We have a fair understanding of the hearing process. Sound produces oscillations in air. These oscillations, when they reach the human ear, travel through the auditory canal, hit the ear drum, and make the ear fluid inside it oscillate. The ear fluid has cells called ‘hair cells’. These hair cells are fixed at their base and bundled at the top; any two adjacent hair-cells are connected at their tips through spring-like molecules that are referred to as tip-links.
When the ear fluid oscillates, the top of the hair-cells deflect, creating a tensile force in the tip-links. One end of these tip-links is also connected to ‘ion-channels’, that are like gates or doors for metal ions to pass. When the tip-link experiences tensile force by the oscillation in ear fluid due to sound stimuli or head-movements, it pulls the gate open.
The metal ions can now enter the hair-cells through the ion-channel and change the cell-polarity. The change in cell-polarity, an electrical signal, is measured by neurons attached to the other end of hair-cells. Neurons convey this electrical signal to the brain and brain decodes it into sound and completes the hearing process.
Tip-links are also responsible for restoring the structural and functional integrity of hair-cells after each stimulation, and retain sensitivity for next transient stimuli.
But after this process there is another stage, which is not very well understood. The minutest sound that the human ear can clearly hear and identify is roughly what is made by a mosquito buzzing about a metre away. In terms of sound intensity, this is about 5 decibels. The maximum that humans can hear without damaging the ear is the starting sound of a jet engine parked 3 metres away. This has an intensity of about 120 decibels.
Now, if a 5 decibel sound stretches tip-links enough to open the ‘ion channel’ or the gate fully, what happens when a 120 decibel sound is generated? How is the extra force absorbed, or disseminated by tip-links? Because, decibel is a logarithmic scale, this extra force is to the order of 10 raised to the power 7. How does the ear cope with this extra force? My team at IISER Mohali is now working to find answers to these questions.
The tip-links are made of two proteins, Cadherin-23 and Protocadherin-15. We have extracted these two proteins in our laboratory and are trying to study their interactions, elasticity, when they are subjected to different kinds of force as generated by sound waves. We begin the experiment with a weak force and slowly increase it to record the reaction of the tip-links. We monitor one tip-link at a time as it occurs during the hearing process. The understanding of the behaviour of these two proteins under tensile force will not only unveil the secret of how tip-links deal with force, but would also lead to new therapeutic approaches for treatment and cure of hearing impairments.
As of now, nearly 10 per cent of the total human population, including newborns, healthy adults and ageing people, suffer from hearing impairments and imbalance mainly due to a defect in their tip-links.
Prof Sabyasachi Rakshit & team IISER, Mohali
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