Experiencing Acoustic Trauma (a very loud noise presented abruptly or over a long period of time) can-and usually does-result in permanent hearing loss. Yes, we’ve all been told this for years, and it doesn’t stop us from going to that rock concert, standing directly infront of the speakers, and loving it, but, what if I told you it actually REALLY was a bad idea. Why? Well, our hair cells-the little sensory epithelial cells that pick up sound information in our ears-die. Earlier this week, I presented on a very interesting article-published in January-that suggests inhibiting a particular developmental mechanism after acoustic trauma actually creates new Hair Cells. I’ll talk about the article next week, but for now, I’m going to tell you how this whole sound process works, and why you reaaally might want to reconsider your standing location.
Hair cells are special
Located in the Cochlea-a spiral structure in our Inner Ear-hair cells sit with supporting cells on top of the Basilar Membrane, and
below the Tectorial membrane, in a region we like to call the organ of Corti. When sound enters the ear, the Basilar Membrane vibrates, causing the hair cells to vibrate up and down with it, which pushes the stereocilia-little hairs on top of the hair cell that can bend back-and-forth- into the Tectorial membrane, thus bending them. The direction of bending either hyperpolarizes or depolarizes the hair cell, thus transducing sound to the afferents.
Now, what I said above is actually only half true. There are actually two types of hair cells, outer hair cells and inner hair cells-arranged as 3 rows of outer and 1 row of inner, and only the inner hairs transduce sound to the auditory nerve. Outer hair cells detecting low-level sound and amplify it so that only inner hair cells that specifically respond to the respective sound frequency
activate. How is this specificity achieved? Hair cells are MAJORLY organized to respond to specific sound frequency based on their location. That spirally Cochlea structure is actually arranged tonotopically, with the base-the wider end-responding to high frequencies, and the apex-the narrow tip in the center-responding to low frequency sounds. So, when a sound of specific frequency causes the basilar basilar membrane that picks up that frequency-based on the length/width-to vibrate and the outer hair cells to amplify that vibration, allowing only the specific hair cells in that location to pick up the sound.
The important part
When acoustic trauma occurs the overstimulation of the hair cells results in major, and usually fatal problems. Hair cells can experience oxidative cell death causing them to die. Furthermore, with a loud abrupt noise (a gunshot, explosion, firecrackers etc.) the outer hair cells can become structurally deformed causing them and their inner structures to degenerate. For some reason acoustic trauma from a loud abrupt noise (ex. explosion, firecrackers, gunshot) destroy outer hair cells, leaving almost all the inner hair cells
intact. Therefore, although we might be able to hear a sound, our ability to detect differences in the respective frequencies is severely attenuated.
In Mammals, almost all of our cells participate in Cell Turnover-the act of replacing cells with new ones generated from the old ones for optimal functioning, but for unknown reasons, our hair cells don’t. In fish, birds, and amphibians, hair cells actually regenerate after damage, but again, Mammalian hair cells dont. Therefore, I hope you can see just why these hair cells are PIVOTAL and that finding a way to replace them after damage would be quite a great accomplishment.
What is To Come
As I mentioned in the beginning, an article was posted in January suggesting a possible solution to this hair cell problem. In the study, the researchers discovered that inhibiting a specific mechanism that determines the fate of epithelial progenitor cells can actually turn supporting cells into outer hair cells. Next week I’ll explain to you just how they discovered this very interesting mechanism, why it works, and what it really means for long-term hair cell regeneration and regaining the ability to hear after trauma.