Transmitting sound waves to sound (Introduction)

by David Turell @, Tuesday, June 05, 2018, 19:38 (167 days ago)

We humans have an accurate complex evolved system:

"In the 19th century there was one really important physiological insight from the German scientist Hermann von Hemholtz that endures today. He recognized that the cochlea—the receptive organ of the ear—is, in essence, an inverse piano. In the piano, each of the strings represents a single tone and the output is stirred together into a harmonious whole. The ear basically undoes that work. It takes the harmonious whole, separates out the individual tones and represents each of them at a different position along the spiral cochlea. Each of the 16,000 hair cells that line the cochlea is a receptor that responds to a specific frequency. And those hair cells are in a systematic order, just as the piano strings are.

"The common currency of the nervous system is electrical. It is action potentials—streams of 1’s and 0’s, in effect—much like those in a computer. But the currency of the external sensory world is very different. We have photons—that is sight. We have pressure—that is touch. We have molecules—that is smell or taste. And finally we have vibrations in the air—that is the essence of sound. Each of those different types of physical stimulus must somehow be converted into the electrical signals that the brain is then capable of interpreting. That’s the transduction process. The thing that motivated me, and took the first 20 years of my 40-year career to really understand, is how that is accomplished. How the mechanical vibration, as it strikes the upper part of the hair cell—the so-called hair bundle—how that energy is converted into an electrical response.

"The sound going in didn’t simply evoke a response. Instead, the ear has a so-called active process. The ear has a built-in amplifier, and that amplifier is unlike any of our other senses. It would be as if light going into the eye produced more light inside the eye, or smell going into the nose produced more smell molecules. In the case of our ears, the sound that goes into the ear is actually mechanically amplified by the ear, and the amplification is between 100- and 1,000-fold. It’s quite profound. And the active process also sharpens the tuning of hearing, so that we can distinguish frequencies that are only about 0.1 percent apart. By comparison, two keys on a piano are 6 percent apart.

"One of the others is trying to understand how the hair bundle—the mechanically sensitive upper part of the hair cell—is assembled. It’s a real problem in developmental biology how you put something that complicated together. And another is the attempt to help regenerate hair cells. One of the biggest challenges in the field is that hair cells in mammals are not replaced when they die. That is why all of us tend to get progressively harder and harder of hearing, and eventually significantly deaf. One of the approaches we’re taking is to screen drugs to try to find a molecule that will allow hair cells to begin to regenerate again. We’ve screened 80,000 drugs so far and we have two compounds in particular that look promising. We’re now trying to learn in more detail how they operate, and whether they or related compounds could be used for regeneration in humans."

Comment: the complexity of the auditory system is amazing. We still don't know how the hair cells make hairs, perhaps as complexly as bacterial flagella. How did evolution develop this preciseness? Not by chance. It requires design.

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