A new study by the researchers at Linköping University, Sweden reveals why human ears feel numb or you feel a loss of hearing for a while on exposure to loud sounds.
This study was published in the journal Proceedings of the National Academy of Sciences on 5th March 2019.
It is commonly seen that human ears get numb or temporarily lose the sense of hearing after they are exposed to loud sounds. Hence, researchers made an attempt to reveal the reason behind this phenomenon. This new study was conducted on the inner ear of guinea pigs as it is very similar to the human ear.
The major task of conversion of sound waves into nerve impulses is done by the spiral shell-shaped part of the inner ear called cochlea. This cochlea contains the tectorial membrane that stores the calcium ions and affects the functions of inner ear sensory cells.
Calcium ions play a vital role in the entire process of hearing. When the calcium store inside the ear is exhausted, the sound-evoked responses of the sensory cells go down. When the same calcium store is regained, the sensory functions get back to normal.
The depletion of calcium ions inside the ear may happen because of brief exposure to rock concert-level sounds or by the introduction of calcium chelators etc.
The fluids around the ear that surround sensory cell lack in calcium content whereas the tectorial membrane is rich in calcium ions. When a substance that vanished the calcium from the inside of the ear was added by the scientists, the sensory cells stopped working.
At further stages of the study, scientists exposed the sensory cells to loud sounds and noises. They found the same results, i.e. the cells again stopped working temporarily. This led them to deduce that exposure to loud sounds or noises can cause numbness or temporary hearing loss because of lack of calcium ions inside the ear.
The lead author of the study Anders Fridberger said, “We have discovered that a tiny structure in the cochlea known as the tectorial membrane plays an important role in this process, by acting as a storage depot for calcium ions. These calcium ions contribute to regulating the function of the sensory cells.”
Fridberger added, “When we expose isolated inner ears to loud noise in the lab, the level of calcium in the tectorial membrane falls, and the sensory cells cease to function. After a while, however, the calcium ion concentration returns to its previous level, and the sensory cells start to function again.”
Pierre Hakizimana, one of the researchers behind the study said, “We knew that the tectorial membrane is necessary for hearing and that it must be intact and correctly located, but it has been unclear why damage to this membrane contributes to impaired hearing.”
Further studies would include finding the relevance of the same phenomenon in age-related hearing impairments.