It’s fascinating to understand how our bodies work , from how our eyes work, to what makes our hearts beat. No less interesting is how we listen, that is, what is the journey of the sound from the outside until it is transformed into an electrical impulse that is interpreted by our brain .
It has always been fascinating to me that we all have a different fingerprint (by the way, our tongue also has a unique fingerprint). Along these same lines, another miracle of nature is that we are capable of distinguishing the differences between so many sounds and the voices of so many different people, with the same organ.
The usual explanation
The process is not simple and is usually explained from “outside to inside” , that is, from the sound that travels in the form of vibrations to our tympanic membrane, through our external auditory canal. In turn, the membrane vibrates when sound hits it, transmitting these vibrations to a chain of 3 small bones that lead to a snail-shaped structure, the cochlea. The cochlea has a liquid substance inside and allows sound to pass through to the auditory nerve, the nerve that is responsible for carrying the information to our brain, which interprets it.
This is the fastest way to explain it, but honestly, it doesn’t do justice to the sheer beauty of this process.
The explanation as you have never been told before
If we had to choose a structure of this process as “the most important” it would be, without a doubt, the hair cells . These cells are sensory receptors, which have a somewhat unfortunate name. Why? Because despite the word “ciliated”, they have nothing to do with the type of hair that usually comes to mind. They were initially named for their shape under the light microscope, but with the progress brought by the electron microscope, we have seen that the structure is not what it seemed, but rather a group of cylindrical rods. In other words, what was initially thought to be a kind of “hair” is actually around 20 grouped cylindrical structures.
To give you a better idea of what these sensory cells are, we are talking about the same cells that bats use as “sonar”. Hair cells are found even in the most primitive fish. These cells are a machine that convert sound vibrations into electrical impulses .
At the top of each bundle of cilia, there is a fine filament that connects each of the tiny hairs to each other. This filament is called a stereocilia . Well, at the base of this filament, the electron microscope has allowed us to see a pair of iron channels, which are structures that span the membrane.
Before continuing, it is worth clarifying a couple of things
You see, ions are atoms or groups of atoms that have an electrical charge . Ions with a positive charge are called cations. Those with a negative charge are called anions. But together, they are all ions. Many normal substances exist in the body in the form of ions. Common examples include sodium, potassium, calcium, chloride, and bicarbonate. These substances are called electrolytes.
Why am I telling you this? Because the ions constitute an electrical current, and in the cilia we also see other ion channels, such as potassium or calcium. Basically, hair cells have ion channels that can be open or closed, and whether they are one way or the other depends on that fine filament.
Continuing with this amazing explanation
This is already fascinating, but wait because it gets even better. You see, when the sound hits the bundle of “hairs” , they transmit the sound to their highest edge, which produces tension in that filament that connects the cilia. The tension, like a string being pulled taut, causes the structures it connects to move in one direction, and that movement in turn causes ion channels to open (and ions to enter the cell). Movement in the opposite direction causes the channels to close.
Ok, so far so good, but as you may already know, sound travels in the form of waves , so when they hit these structures they do not do so continuously (it is not a continuous line, it is a succession of waves), this It causes the “hairs” to oscillate , to dance from one side to the other, opening and closing the ion channels when they come into contact with sound waves.
Each opening involves the entry of millions of ions into the cells. The electrical excitation this produces passes to a nerve fiber (a nerve) and this spreads to the brain .
Now it is easier to understand that the intensity of the sound is represented by the magnitude of this response. A louder sound pushes the bundle of “hairs” farther, which opens the channel longer, allows more ions to pass through, and results in a greater response.
Fascinating, right? Let’s keep digging!
Hair cells , in case you haven’t spotted them yet, are found in the cochlea . If we remember, surely you were once told that the organ of hearing is a small structure (in fact, it is really small, the size of a chickpea) in the shape of a snail. Well, the cochlea is capable of dividing sounds into their component frequencies . Thus, if we play different notes on a piano, different parts of the cochlea will be stimulated. Let’s get straight to the point: the lowest frequencies (bass sounds) go all the way to the top of the cochlea. The highest frequencies (sharp sounds), up to 20,000 hertz, reach the bottom. All other frequencies are somewhere in between.
And this is how we can differentiate so many different voices! Quite simply, different voices assume that the sound vibrates at different frequencies, and this in turn stimulates different parts of a highly specialized organ capable of discriminating both the direction from which the sound comes, and its differences with other sounds. In my opinion, one of the most beautiful designs of our body.
And all this with the size of a chickpea!
Referencias:
Hudspeth, A. J. (2014, August 6). Integrating the active process of hair cells with cochlear function. Nature Reviews Neuroscience, 15(9), 600–614. https://doi.org/10.1038/nrn3786
Johns Hopkins medicine (s.f.) How the Ear Works. Hopkinsmedicine.org. https://www.hopkinsmedicine.org/health/conditions-and-diseases/how-the-ear-works#:~:text=The%20Outer%20Ear&text=It%20collects%20sound%20waves%20and,cause%20the%20eardrum%20to%20vibrate
López-Schier, H., Starr, C. J., Kappler, J. A., Kollmar, R., & Hudspeth, A. J. (2004, September 1). Directional Cell Migration Establishes the Axes of Planar Polarity in the Posterior Lateral-Line Organ of the Zebrafish. Developmental Cell, 7(3), 401–412. https://doi.org/10.1016/j.devcel.2004.07.018
National Institute on Deafness and Other Communication Disorders (2018, January 3). How Do We Hear? NIDCD. https://www.nidcd.nih.gov/health/how-do-we-hear
UCSF Benioff Children’s Hospital (2020, December 22). How the Ear Works. UCSF Health. https://www.ucsfbenioffchildrens.org/education/how-the-ear-works
Withnell, R. H., Shaffer, L. A., & Lilly, D. J. (2002, February). What Drives Mechanical Amplification in the Mammalian Cochlea? Ear and Hearing, 23(1), 49–57. https://doi.org/10.1097/00003446-200202000-00004
