Now Hear This! (Part I)

Of all of the great inventors, engineers, designers, and architects, Nature is by far my favorite. She designs an enormous diversity of elegant solutions to very complicated environmental challenges based on only a few simple concepts. Life on earth adapts by accumulating mutations, or changes, in the genetic sequence. Over enormous periods of time and many generations, these slight changes enable our bodies to build better and better structures for specific uses. Often times, the outcome of this process is astoundingly simple.

One of the great challenges faced by nature in the design of us, human beings, is the ability to detect and understand the differences between sounds around us. We are surrounded by objects and other organisms each creating their own sounds, and the ability to process sound is an ability that has greatly aided our survival as a species. The ability to hear, in combination our other senses, is used for orientation as well as discovery and avoidance of dangerous predators or hazards.

But what is sound really? From a physics standpoint, sound travels as vibrations in the molecules of the air around us. These vibrations are understood as waves, where air molecules bunch up and spread out alternatively like ripples over water. Sound waves of higher pitch, or frequency, have peaks occurring in much closer groupings than low base-like sounds with peaks much more spread out. What you know as volume or loudness, is interpreted as the Amplitude, or the maximum height of each wave (interpreted from the maximum density of air particles, but this is beyond what you need to know).Pressure Wave

Understanding the physical basis of sound now, you can begin to ponder a device to detect it. Not only would you need a way to capture the range of sounds, but you need to transform them into understandable signals. Our brains integrate signals from sensory receptors in the form of neural firing. The most impressive piece of the story is what happens in the space of time between sound reaching our ears and our brains perceiving it.

What you probably already know is that the cuplike shape of mammalian ears is for focusing sound into your ear canal, where it is amplified up to twenty-two times the original intensity and funneled towards the eardrum. Tiny changes in the position of this membrane coordinated by the frequency of sounds hitting the eardrum push a series of three bones, each connected to the next to confer the eardrum’s movement to one of the most astounding sensory organs in the human body, the cochlea.

Choclea Diagram - Dollos, 1992The cochlea resembles a snail shell, spiraling inwards to its end, the apex. When the last of the three inner ear bones makes contact with the base at what’s called the oval window, mechanical waves mimicking the original sound waves from the ear canal travel the length of the cochlea.

This is where the story gets really interesting. Here in this organ is the site of transduction from mechanical waves to neural pulsing. The natural process of evolution has rendered a method for determining both of the two qualities of sound (frequency and amplitude) that is so perfectly simple I can’t help but be in awe.

Imagine if we were able to unroll the cochlea from its packed spiral into a straight line. At the base, the middle ear bones press a copy of the mechanical sound waves onto the outer membrane of the cochlea. These representative sound waves then travel the length of the cochlea all the way to the apex. Now, if we cut across the width of the unrolled organ, you would notice that the inside of our “shell” has three compartments. The outer two fluid-filled compartments oppose one another and are separated from the center by thin membranes. The center chamber is lined with rows small hair-like cells responsible for signaling to the underlying neurons (more on this in the next installment).

The bottom, or basilar membrane running the length of the cochlea actually changes shape to match the movements of the incoming sound wave. It’s the thinness of this membrane that is the key. When pressure at the peak of the wave builds, the basilar membrane responds by forming a physical wave!

Now you have to wonder why the cochlea is divided into so many compartments, right? Well, the separation between these compartments is the secret to dissecting the qualities of sound. Recall that the frequency of sound is marked by the distance separating peaks in air pressure. The hair cells of the cochlea can actually determine the frequency of sound by the distance from the base to the peak of the wave formed by the basilar membrane. High-frequency sounds, such as screaming, have short distances separating peaks and will peak closer to the base of the cochlea while low baritone sounds with long separations between peaks will peak further towards the apex. This peak remains vibrating up and down in its location along the basilar membrane until the sound eventually dies out.

Since the hair cells in the middle chamber of the cochlea are rooted on top of the basilar membrane and situated under the other, tectorial membrane, they transmit signals to neurons when they are compressed or stretched between the two. This change in compression between the two membranes, called “shear stress”, gives the brain an accurate picture of the location of sound wave, therefore, the frequency, and the height between the two membranes interpreted as loudness.

In essence, our bodies have engineered an incredibly simple device to first modify sound waves into mechanical waves and then allow for interpretation of the qualities of the incoming sound. It does so by creating a tonographic (“sound map”) image of incoming sound waves. This double membrane system situated in the spiraled cochlea of the inner ear is the remarkably elegant design rendered by nature to address the problem of deciphering auditory information.

To be continued in Part II…